PowerApplications

byCurtis Sterud

PowerApplications

byCurtis Sterud

w 2 w 3

Applications of CCI valves in a B&W once-through unit upgraded for cycling

VALVE NO. & PURPOSE

205 LP Superheater Stop

205C LP Throttle Control

207 Secondary Bypass

210 Turbine Bypass

218 SSH Outler Steam Attemporator

219 Reheat SH Steam Attemperator

220 HP HTR Steam Level

221 Flash Tank to HTR

230 DEA Steam Pegging

231 Flash Tank Level

240 Flash Tank Pressure

241 Flash Tank Level

401 HP Throttle Control Ahead of the SSHTR

w 2 w 3

CCI Control Valves in nuclear plants

PRESSURIZED WATER REACTOR

BOILING WATER REACTOR

w 4 w 5

n Drum boilers vary in size, from small boilers used to generate steam

industrial heating up to the large public utility boilers which produce

enough steam to generate up to 900 MW of power, such as the Lake

Cayuga Plant of New York Electric and Gas.

n We supply the Bailey thermo-hydraulic feedwater regulators for small

boilers. The cogenerating units are between 50 MW to 200 MW in size.

In these units, we supply feedpump recirc. valves, and auxiliary steam

valves. Vents are another application.

n Combined-cycle plants often require turbine bypass valves of 50–100%

capacity.

n Large utility drum boilers have more severe service applications which

require DRAG® valve technology. These boilers also require turbine

bypass valves.

Shown on the following pages are the typical schematics and diagrams of areas

which require a DRAG valve. These applications see either the potential for

cavitation, flashing, or a combination of high-pressure drop and low flow rate.

There are four areas that we will discuss:

I. CONDENSATE SYSTEM

a) Condensate Pump Recirculation Valve

b) Deaereator Level Control Valve

II. FEEDWATER SYSTEM

a) Boiler Feedwater Pump Recirculation Valve

b) Boiler Feedwater Regulator Valve (B&W 100)

III. MAIN STEAM SYSTEM

a) Start-Up System Valves (B&W)

i. 501

ii. 502

iii. 518

iv. 519

b) Superheater Attemperator Spray Valve

c) Reheater Attemperator Spray Valve

d) Turbine Bypass Valve (B&W 510—This is a small bypass)

e) Deaereator Steam Pegging Valve

f) Soot Blower Valve

IV. HEATER DRAIN SYSTEM

a) High Pressure Heater Drain Valves, Emergency Heater Drain

b) Low Pressure Drain Valves

Drum Boilers

w 4 w 5

Diagram of a typical B&W Drum Boiler which

incorporates control valves for dual pressure operation

A Feed Water Regulator

B Feed Pump Recirculation

C D.A. Level Control

D Booster Pump Recirculation

E Main Steam Attemperation

F Reheat Steam Attemperation

G Heater Drains

H Auxiliary Steam

J Sampling—Various Locations

Typical schematics and diagrams of areas which require a DRAG® valve

w 6 w 7

I. Condensate System

This is the portion of the plant where condensate is taken from the

condenser hotwell, circulated through the low pressure heaters, and to the

deaereator.

The condenser acts as a heat exchanger that serves the purpose of creating

a vacuum which increases the efficiency of the turbine and for recovery of

quality feedwater (condensate).

Shown below is a schematic of a typical condensate system:

w 6 w 7

Condensate System (continued)

a) Condensate Pump Recirculation Valve:

The condensate pump must have a minimum amount of flow through it at all

times to prevent it from overheating and to protect it from cavitation. Therefore,

a recirculation valve and line runs from the pump outlet line back to the

condenser. When the boiler load is low the flow of condensate required is less

than the pump minimum flow requirement. The recirc. valve is used to allow

the additional flow required through the pump. The pump outlet pressure varies

from 300 psi to 600 psi with fluid temperature from 100° to 150°F.

The recirc. line dumps into the condenser which is at vacuum. If a

conventional valve were used here, there would be severe cavitation. (The

outlet pressure at the valve is higher than condenser vacuum because of pipe

friction, elevation and sparger back pressure.

An 800D is the valve for this service. This valve must have positive shutoff.

To assure good shutoff, the valve must have a soft seat.

b) Deaereator Level Control Valve:

The purpose of this valve is to maintain a level in the deaereator, an open

style of feedwater heater. It controls the amount of condensate flow into

the deaereating vessel. The service conditions of this valve vary directly

with the plant load. During start-up, the pumping load is small, the valve

inlet pressure is high and the outlet pressure is low, because the deaereator

pressure is not built up yet. In this case, there is a need for cavitation

prevention and the flow capacity required is very low.

As the plant load increases, the need for high flows and the condensate

pump can’t maintain the same pressure head at these higher flows. The result

is lower inlet pressure to the valve. Concurrently, the line pressure to the

deaereator is building, putting backpressure on the valve. These higher flows

with lower pressure drops create a need for higher capacity of the valve but

less resistance in the trim.

The requirements of this valve are:

n High rangeability

n Cavitation protection at low flows

n Low resistance at increasing flows

n Tight shut-off is not essential because this valve is open at all times the

plant is up and running.

In a characterized stack all discs are not

the same, but rather are chosen to provide

precise variable flow versus pressure drop

over the full range of the valve.

w 8 w 9

II. Feedwater System

This is the portion of the plant where feedwater is taken from the deaereator by the boiler feedpumps and sent through the

high pressure heaters, the economizer, and finally into the boiler. The fluid is brought to full outlet pressure of the boiler

and its temperature raised by heat recovery for efficiency of the system.

a) Boiler Feedpump Recirculation Valve:

See attached, “Boiler Feedpump Recirculation Valve.”

b) Boiler Feedwater Regulator:

Depending on the A/E, utility and boiler manufacturer, the feedwater flow will be controlled either by a variable

speed feedpump or a high capacity control valve.

The boiler pumps may be motor driven, which are generally constant speed and therefore constant outlet pressure, or

steam driven with variable output. (A fluid coupling on a constant speed motor driven pump can be utilized to get

variable output.)

In any case, a control valve for feedwater regulation to the boiler takes the fluid from the pump outlet and regulates the outlet

flow rate to the boiler demand.

The service of this valve is similar to the DEA level control valve, except at a significantly higher pressure.

The fluid is taken from the DEA into the boiler feedpump and the pressure is raised to boiler operating pressure (most cases

are over 3,000 psi). This is the inlet pressure to the feedwater regulator. At start-up and low loads, the pumping load is small

and the pump outlet pressure is high and the drum pressure is not built up yet. In this case, there is a need for cavitation

prevention and the flow capacity is very low. As the plant load increases, drum pressure increases and flow rate increases.

The pump cannot maintain the same pressure head at these higher flows. The result is lower inlet pressure to the valve and

high back pressure on the valve. These higher flows with lower pressure drops create a need for high capacity of the valve

with less resistance in the trim. Many plants utilize a start-up valve and a main valve for this service. The start-up valve

would have trim to cope with the low flow and cavitation condition, and the main valve take over the flow increased

and differential pressure decreased. The CCI DRAG® valve can be built with characterized trim to cover the full range of

operation conditions in one valve.

Requirements of this valve:

n High rangeability

n Cavitation protection at low flows

n Low resistance at maximum flow

n This valve should have at least Class IV shut off.

w 8 w 9

Feedwater Regulation

Bailey D10 and DRAG®

w 10 w 11

Typical Feedwater System with Bailey

w 10 w 11

Typical Feedwater System with DRAG®

w 12 w 13

Boiler Feedpump Recirculation Valve

In a power plant, the boiler water is circulated in a closed loop by a feedpump driven either by electric motor or steam

turbine. The pump takes the water from the deaereator or high pressure heaters and boosts the pressure to the system

requirements (3800–5500 PSI in a supercritical universal pressure boiler). These pumps require a certain minimum

amount of flow to avoid overheating and cavitation problems. In order to protect the pump when the boiler feed flow

requirement is less than the minimum permissible flow through the feedpump, a recirculation system is used to return a

portion of the high pressure flow back to the condenser (or sometimes to the deaereator) and thence to the pump. (see

diagram for a system schematic.)

Ideally, the recirculation system would meter the flow in response to the pumps’ requirements. This indicates the need for a

modulating control valve which normally would be closed, but if power to the valve’s actuator should fail, the valve would

need to open. Also, if the system is off, the valve is fully open. When the pump is turned on, the valve is flowing 100%

and closes as system flow increases. The valve would need to handle water with thermodynamic conditions at the inlet of

3800–5000 PSIG to 10 PSIG. Thus, during modulation of full flow conditions, the valve must handle high pressure drop

and substantial flashing, yet manage to minimize erosion, cavitation and noise problems.

While minimum flow protection would normally be required only during station startup and shutdowns, in actual practice,

such systems might be used for extended periods of peak power or in the case of nuclear plants, forced operation at reduced

percent power to qualify operating permits. It is apparent that minimum flow systems must be capable of continuous

operation—their purpose is to protect $100,000-plus feedpumps from severe damage. Since spare feedpump capacity is

not generally used, damage to a feedpump can have serious consequences in terms of restricting full power capability for

extended periods of time. Each feedpump is always installed with an individual recirc. system.

When shut, the pressure could build up to as high as 5500 PSIG and the valve can be expected to hold “drop-tight” against

leakage. Should the valve leak, even the tiniest amount of flashing seat and water mixture would cut the seating surface like

a rough wire and in a short while, the shutoff function of the valve would be lost. (See diagram). Excessive leakage cuts into

the efficiency of the plant, both through direct energy loss and because the boiler cannot be fed the full rated flow. In the

extreme case, a shutdown of an entire power plant may be necessitated because of a single leaking valve.

Thus, the statement of the problem is this: there is a need for a boiler feedwater pump recirculation valve (BFPRV) that

will:

1) Break down water pressure from ~ 4000 PSI to atmospheric or vacuum pressure:

n Without trim erosion due to flashing service conditions.

n Without cavitation

n Without mechanical vibrations and noise.

2) Remain leak-tight for long periods of time. (one year or more).

3) Modulate automatically and open in case of failure of power to the valve operator.

This application is probably the most severe application of a control valve in the power plant.

Trim Materials for Recirc. Valves

PLUG: 400 series stainless heat treated for hardness

SEAT RING: 300 series stainless

Some customers specify Stellite #6 for seat surface. Corrosion problems may occur in feedwater systems. Typically, boiler

feedwater is treated with ammonia at the deaereator to eliminate excess oxygen. Ammonia, combined with hydrazine

create amines which attack the cobaltchrome material in Stellite#6. The net result is corrosion, and subsequent errosion

of Stellite #6.

w 12 w 13

Boiler Feedpump Recirculation Valve

A CCI DRAG® valve of globe configuration

is shown located near pump discharge

with flow to deaerator. An angle CCI

DRAG® valve is also available, should piping

configuration dictate. Systems discharging to

the condenser are normally angle valves.

Actuator control schematic shown is for 3–15 psi modulation

signal, with increasing signal tending to close valve. The

actuator is fitted with a spring for fail open on loss of air

signal. A snap-acting relay is provided to ensure the valve is

seated at maximum signal. The snap-acting relay is set as

shown so the valve modulates between 10% and 100% open.

Schematic of typical once-thru,

universal pressure,

w 14 w 15

How Feedpump Recirculating Valves Withstand Severe Duty

As appears in Power, Aug. 1987Of all power plant services, that of the boiler-feedpump recirculation valve is perhaps the most severe. Differential pressure across this valve may be as high as 4000 psi and, to make matters worse, pump manufacturers today are specifying higher minimum flows (up to 25%) to protect feedpumps against damaging temperatures and high radial shaft loads when boilers are operating at minimum load or on hot standby.

Curtis Sterud, Control Components Inc., Rancho Santa Margarita, Calif., explains the essential characteristics of recirculation-valve design for minimum downtime: (1) The valve must produce the large pressure reduction (from pump-discharge to deaereator pressure) without generating excessive velocities and flashing that would create destructive cavitation, and (2) It must have a tight seal when shut off to prevent rapid seat and plug failure through leakage.

Pressure reduction (and the resultant velocity increase) is accomplished through stages: from a few stages in multiple-plug valves, up to 20 stages in radial-disk valves. At rated flow there is a margin of pressure difference between valve-discharge pressure and liquid vapor pressure, which prevents cavitation. This is usually achieved through piping loses between the valve and the deaereator, though a choke-flow restriction may be included in the piping or the deaereator shell.

But at reduced flows, downstream velocity controls are ineffective and cavitation can occur within the recirculation valve, leading to rapid valve-seat failure and the inability of the valve to shut off tightly. Valve-seat failure is intimately related to velocities within the valve and is not restricted to the effect of velocity at the valve seat itself.

In a radial-disk recirculation valve (below) 28 or more radial disks dissipate pressure through friction in a series of tortuous paths (inset). Since these paths are fixed in each disk in the stack, individual disk flow is constant and velocity is limited to about 100 ft/sec at any degree of flow modulation through the valve.

Another important aspect of velocity control is the downward flow through the annular space between the plug and the inside of the stack of disks. This downward velocity can be quite high—up to 600 ft/sec. It must be minimized and deflected away from the seating surface. Several design refinements are needed to achieve this:

n Stacked disks must be oriented so that the radial flows from their exhaust ports are not lined up vertically, but alternated. The resulting annular flow around the plug interrupts the downward annular flow.

n A continuous weir around the inner edge of each disc ensures homogeneous flow from the stack and serves to control total velocity. Flow from discs below the bottom of the plug interrupts the downward annular flow and deflects it away from the seating surface (3rd figure).

Seat designs and materials have varied widely over the years. Sterud considers that most of them have been inadequate and favors a design that uses a relatively soft 300-series stainless steel seat and a hardened 400-series stainless steel plug. A 3° difference between the machined faces of the valve’s conical seat and plug (last figure) produces a line contact between plug and seat.

This seat design is used with a closing force of about 1000 lb./linear inch of seat/plug mating. This far exceeds the yield strength of the softer seat material and is almost 50% higher than the seat load used in most recirculation-valve designs.

To eliminate the possibility of uneven downward flow around the plug during very low-flow operation, the actuator is provided with snap-action relay that closes the valve completely below about 10% of rated flow.

w 14 w 15

III. Main Steam System

Babcock & Wilcox Drum Boiler Water/Steam

The main steam system covers the portion of the plant that takes the steam from the boiler, sends it through the superheaters, and into the high-pressure turbine. The steam exiting the high-pressure turbine is sent through a reheater, then fed into the low-pressure turbine. Finally, after all potential energy is extracted from the steam, it is dumped into the condenser to start the whole process over again.

Large generating units were designed generally for base-loaded operation. However, with increased emphasis on planned cycling operation of fossil-fired boilers, there are new demands on the control of boilers during start-up and low load operation.

Conventional drum boilers can be operated with wide variation in load, including complete shutdown and re-start, without sacrificing heat rate or cyclic life. Modes of operation include variable drum pressure, constant throttle pressure and dual pressure.

With “variable drum pressure,” the turbine throttle valves are nearly wide open and the throttle pressure varies with drum pressure. This operation is relatively slow in response to load demand.

With “constant drum pressure,” the turbine throttle valves are nearly wide open and the throttle pressure varies with drum pressure. This operation is relatively slow in response to load demand.

With “constant drum pressure,” the turbine load is changed by modulating the turbine throttle valves. The low load efficiency is achieved by sequenced turbine control valves and partial ARC throttling at the expense of a large change in impulse chamber temperature.

“Dual pressure” operation involves wide variable throttle pressure, with the pressure controlled by wide range superheater division valves. The drum pressure is controlled at a high pressure above 2000 psi. For this type of control, there is little change in turbine metal temperatures, or in drum saturation temperature over the load range. Load response will be at least comparable to that for “constant throttle pressure” operation.

The “dual pressure” mode of operation is a system incorporated in some B& W drum boilers. B&W incorporated CCI DRAG® valves in five locations of this system. These valves are:

n BW100—Feedwater Regulatorn BW501—Secondary Superheater Stop & Bypass Valven BW502—Primary Superheater Bypass Valven BW518—Main Steam Attemperatorn BW519—Reheat Outlet Steam Attemperatorn BW510—Turbine Bypass

The superheater division valves (BW500 and BW501) are used below about 70% load to maintain drum pressure, yet provide reduced throttle pressure to the turbine. The BW502 valve permits firing proportional to steam flow during an unloaded transient, and limited over-firing at low load. The 502 from the drum with the steam exiting the superheater and reheater to hold temperature at the turbine without the concern for water into the turbine, which might result from water attemporation. The BW100 feedwater regulator is a highrangeability valve for this service.

w 16 w 17

Typical Start-Up After Overnight Shutdown

The 500 and 501 valves have been closed to “bottle” up the boiler overnight. The boiler pressure will have decayed somewhat, so initial firing will be to bring drum pressure and temperature up. The 502 valve is used to bypass the steam to the condenser. When steam temperature is established, the 501 valve is opened to admit steam through the superheater and initially through the 510 valve to the condenser. This if for warming flow. The 510 valve is closed, and turbine throttle valves are opened to 70%. The turbine is rolled (turbine throttle pressurized at about 200 to 300 psi. The 501 valve opens to increase the turbine throttle pressure which is turbine load. With the low flows involved, the steam attemporation at superheater outlet (518 valve) and reheater outlet (519 valve) controls the turbine temperature without the danger of water into the turbine at less than 15% load. As load goes above 70%, the BW500 valve is opened to 100% open and the turbine throttle valves control load of the turbine from there to full load.

It should be obvious that the five valves we provide for this system are crucial to the successful operation of the system.

The 502 valve starts with low temperature water at drum pressure ( ~ 2000 psi and 300°F). The flow rapidly changes in temperature as the leg of water is displaced by 2000 psi saturated steam which is approximately 650°F; this is a significant thermal transient. The valve should be over the plug, gasket seal, with linear disk stack for this service.

The 501 valves starts with high inlet pressure (approximately 2400 psi) and very low outlet pressure (0 to 100 psi). The valve must control flow to load the turbine and then control flow as turbine load (pressure) is raised. This requires a characterized disk stack similar to the BW100 feedwater regulator valve. The customer would like the system to have an inherent linear characteristics, i.e., valve stroke linear, with load increase. This requires that we plot the Cv required for each position of the valve for load required.

The 518 and 519 valve flow conditions are about the same, i.e. ~ 2000 psi saturated steam ~ 650°F inlet and 0 to 300 psi outlet pressure. The trim can be linear, and under plug flow.

There are other severe service applications which are common to both drum and once-through units. These are attemperator spray valves, soot blower control valves (for plants which use steam for soot blowing), and auxiliary steam valves for steam from main steam to boiler feedpump turbine.

w 16 w 17

Sootblower Header Control Valve

A good regulating (modulating) valve is required to control the pressure in

the sootblower header. The valve must have good rangeability because the

flow varies considerably during the sootblower cycle. As the sootblowers

open and close, the header control valve must respond quickly to avoid

pressure surges which would pop the safeties on the line.

A Class V shutoff is required because any leakage through the header

control valve would increase header pressure and pop the safeties when the

sootblowers are closed.

Another consideration is thermal transients. A valve closed for a period of

time will cool somewhat. When opened, the trim heats up much faster than

the body.

This CCI valve for sootblower header control has been designed to meet

the above requirements. The disk stack is characterized with 14-turn and

eight-turn expanding disks with Pressure Equalizing Ring (PER) grooves for

minimum fluid velocities and high rangeability. The flow is over the plug

to protect the seat from trash damage. The plug is unbalanced with high

actuator load for good shutoff.

w 18 w 19

Sootblower Header Control Valve

w 18 w 19

Attemperator Spray Control Valve

These valves control the amount of water required to control the steam temperature exiting the superheaters. (Primary, secondary and reheat).

The water source for the attemperation after the primary or secondary superheater is from the main feedpump generally after the economizer section of the boiler. The pressure drop across the valve is low and conventional valves have usually been used. However, there is a very wide rangeability requirement. Single drop-type valves throttling at the seat tend to wire open and leak.

The water source for the reheat attemperation may be the same as for the superheat or it may be from some intermediate stage of the pump. In either case, the pressure drop across the valve is significantly high and velocity control trim is required.

We recommend the same valve for both areas. The disk stack is characterized with 14 turn and 8 turn expanding disks with (PER) for minimum fluid velocity with reheat spray and wide rangeability in both applications. The flow is over the plug to protect the seat surfaces from trash damage. The plug is unbalanced with a high actuator load for good shutoff.

w 20 w 21

Bailey Single-Port Control Valve

ANSI Class 1500 & 2500, Class A1 Valves

w 20 w 21

IV. Heater Drain System

There are two sets of heater systems in a normal power plant. The low- pressure heaters heat the condensate coming from the condensate pump so it is near saturation when it gets to the deaereator. The other set, called high- pressure heater, heats the feedwater coming from the boiler feedwater pump so that it is near saturation when it enters the boiler. Both systems work with same way, with the exception of the heating fluid. In the low-pressure heaters, exhaust steam from the LP turbine is used, while the high-pressure heaters use extraction team from the reheat section. See typical schematic, below.

The level of condensate in the heaters must be controlled for best system efficiency, so the drain system is fairly elaborate. There are emergency heater drain valves which bypass the fluid to the condensor. There are heater drain valves between each other. Each heater is at a lower pressure than the preceeding heater. The fluid in the first heater is saturated water as the fluid flows through the drain valve to the next heater, the fluid flashes, the flashed steam passes over the tubes containing the condensate, and the heat of the steam is absorbed by the tubes warming the condensate. At the same time the steam temp is reduced to saturated water. This saturated water is let down to the next heater and the same process occurs.

The problem is that the condensate in the bottom of the heater is at saturation. When the condensate is drained and loses just a small amount of pressure, it flashes, and erosion damage to the control valve and associated piping is common. The important thing in choosing control valves in this application is to use a characterized disk stack to range low flow away from the seat, with sufficient turns to keep velocity as low as possible. Angle valves, or globe valves with oversized ends and a flow distributer integral with the seat ring to limit velocity will also combat erosion. In addition, the body material should be A217C5, or A182F5.

Globe DRAG Heater Drain Valve with Flow Distributer Integral with the Seat Ring

w 22 w 23

Monticello Steam Electric Station—Unit 3

Texas Utilities Services, Inc.

Dallas Power & Light Service Company

Texas Electric Service Company

Texas Power & Light Company

Monticello Steam Electric Station—Unit No. 3

Mt. Pleasant, Texas

B&W Contract No. UP-124

w 22 w 23

B&W Once-Through Startup System Using Bailey 201 & 207 Valves

All boilers require a minimum fluid flow through the furnace wall tubes at

even a minimum firing rate to protect the furnace tubes from overheating.

Protection is provided by circulation of a minimum amount of feedwater

and the use of a startup bypass system.

w 24 w 25

Startup Bypass System in Once-Through Critical Pressure Units

Many variations exist when comparing the startup systems provided by each of the boiler manufacturers. Functionally, they all have a common purpose as elaborated below. Notice in Figures 1, 2 and 3, the physical differences illustrated for each of the once-through boiler manufacturers. Combustion Engineering, Inc., (Figure 3 on Page 52) provides circulation pumps to recirculate fluid through the furnace pumps and convection walls and, in this way, protect the tubes from overheating. (Figure 3 on Page 52) Babcock & Wilcox Company and Foster Wheeler Corporation (Figure 2 on Page 52) require a minimum pumping rate be established to provide this same protection.

Because of these individual differences, control systems vary on each of these units insofar as the actual coordination of the valves in each startup system. However, again analyzing the job that has to be performed, the functional objectives of all systems are the same, namely:

1) As noted above, provide protection to keep furnace tubes from overheating by maintaining a minimum flow of fluid through the furnace. Care must be taken to keep the pressure of the fluid in the furnace circuit at a pressure well above saturation, thus preventing any flashing from occurring in the furnace circuit.

2) All systems provide some means of circulating water through the system for both a cold and hot water cleanup through the use of a polishing system.

3) All systems provide for an orderly sequence to startup and initially load the unit as follows:

a) By rejecting flow to the flash tank or separator during startup, provisions are made for hot cleanup operation and initial build up of enthalpy.

b) To assist in building up the heat in the boiler, in a minimum time, both the water and steam in the flash tank are put back into heat recovery in the deaereator and/or feedwater heaters during startup and low-load operation.

c) When the enthalpy level in the flash tank or separator reaches some minimum desired level, steam can be admitted to the superheater and main steam lines for warming purposes.

d) By bypassing steam to the condenser through a turbine bypass valve and/or the turbine above seat drains while regulating the heat input, better matching of steam temperature to turbine metal temperature is achieved prior to rolling the turbine. This improvement is relative to that obtained with a drum type boiler where temperature is obtained as a function of the firing requirements for the pressure required.

The turbine bypass valves’ function to provide initial steam to heat steam lines and roll the turbine. Through these valves, it is possible to establish steam flow through the superheater to turbine roll and synchronize the turbine with minimum upsets. (B&W = 210, FW = U, CE = SD)

w 24 w 25

B&W Units for Base Loaded Operation

B&W Units for Base Loaded Operation

B&W once-through units were originally designed with Bailey valves in the

startup system. (Figure 1 on Page 52)

Bailey valves was owned by B&W. B&W aquired CCI in 1971 to utilize the

advantages of the DRAG® valve in the severe service applications in the

steam generation units. At the same time, the manufacturing of the Bailey

valve was moved to CCI in California.

So, CCI supports the startup system valves of old or new B&W units,

whether Bailey or CCI drag valves are used.

The following diagrams show typical B&W units with Bailey valves in an

older plant, and CCI DRAG® valves in a plant after 1974.

e) Following turbine synchronization, turbine load is increased using flash tank or separator steam available. When the available flash tank or separator steam is depleted, additional load is obtained by opening the in-line stop valves, thus, admitting furnace outlet fluid directly into the superheater. In the case of a Combustion Engineering or Riley boiler, pumping rate must be increased at the same time. Once the stop valves are fully opened, the flash tank or separator are taken out of service.

Control systems must be properly programmed to recognize the following facets during this period:

1) Furnace circuit pressure must always be maintained.

2) Throttle pressure is increasing during this period; thus, the amount of stored fluid and heat must be increased. Load and steam temperatures are likewise increasing, which also demands additional heat and fluid storage.

3) Saturated steam from the flash tank or separator to the super heater is being replaced with steam directly from the evaporating section of the boiler. This means the enthalpy leaving the furnace section must be maintained at an enthalpy level approximately that of steam leaving the flash tank. By properly programming the opening of the inline stop valves (B&W = 200, FW = Y, CE = BT)and changes in pumping and firing rate, outlet steam temperatures can be maintained during this transfer to straight through operation.

Thus during startup and low load operation prior to the turbine load exceeding the minimum feedwater flow, the control system must utilize the bypass system valves as an extension of pressure control and feedwater flow control. During this period, the heat input must be properly controlled to provide the required steam conditions at the turbine, recognizing that some heat is being lost through the bypass system until it is taken out of service.

w 26 w 27

B&W Once-Through Unit(Older Style Unit)

This design requires a startup flow of 25% to 30% of rated capacity.

For startup, these units circulate the fluid thru the boiler, 202 & 207 valves, the

flash tank and to the condenser, until the fluid temperature is proper for steam

at about 600 psi in the flash tank. Then steam flow is allowed through the 205

valve to the secondary superheater and 210 valve for warming. Then, when the

temperature is OK, the 210 is closed and the turbine is loaded to minimum

load. This could be 800 psi throttle pressure and about a 25% load. Pressure

upstream of the primary superheater would be about 3500 psi for supercritical

units and about 2800 psi for sub-critical units. The turbine throttle valves are

set at this minimum load setting. Then the 201 valve is opened as the 202 and

207 are closed to pressurize the downstream secondary superheater.

During the startup, primary superheater pressure control is maintained by the

202 valve. When the pressure drop across the 200 (201) valve is about 300 psi,

the 200 valve is opened 100%. Note that when secondary superheater pressure

is higher than the flash tank, the 205 valve closes (its a check valve). Load is

now increased to 100% by opening the turbine throttle valves. At this time,

the controls are set to open the 202 valve if the primary superheater pressure

exceeds a certain limit.

The load can be varied by turbine throttle valves. However, the load must be

changed very slowly because when the throttle changes to change load, there is a

change in steam temperature across the turbine. Turbines are not to be thermally

cycled. Therefore this type of unit is base loaded, that is, operated at constant load.

B&W introduced cycling of their once through units by incorporating a “401”

valve. This valve replaces the 200 & 201 valves.

w 26 w 27Schematic of the Startup of a Base-Loaded B&W Once-Through UnitOlder unit with Bailey E40 & D10 valves in the startup system

w 28 w 29

After 1974, CCI DRAG valves replaced the

Bailey Valves in the Startup System

Schematic of a base loaded B&W once-through unit using DRAG valves in the startup system

w 28 w 29

Detailed description of a typical startup sequence of a base loaded B&W unit

Cold water cleanup mode, no firing in this mode.

1. Approximately 25% of full load flow is established.

2. The 200, 201, and 202 valves are closed.

3. The 207 is set to maintain 600 PSI at the inlet to the primary superheater.

4. The 241 (flash tank level control valve) is open, dumping all flow to the condenser and to the condensate polishing

system. The flash tank will be flooded during this mode to allow the 241 to pass total startup flow.

5. The 242 valve is kept closed until the flash tank level starts to fall below the flash tank centerline, it will then open

until the flash tank level hits a predetermined high level set point.

6. Circulation is maintained in this manner until the cation conductivity entering the economizer and at the 207 valve

inlet is below 1 µΩ.

w 30 w 31

Detailed Description of a Typical Startup Sequence of a Base-Loaded B&W Unit

Initial Firing Mode

1. Firing is initiated in the boiler.

2. All flash tank drain flow will be transferred from the condenser to the deaereator. The 241 valve is held closed and 230 (deaereator

water pegging control valve) is held open until the deaereator is pegged at its full-load operating pressure (approx. 140 PSI). After

the deaereator is pegged the 230 valve will limit flow to the deaereator to maintain its pressure at set point.

3. The 241 will control flash-tank level about its normal level after the deaereator is pegged.

4. When the fluid temperature at the primary superheater inlet exceeds 300 F, the primary superheater outlet pressure setpoint

will be ramped automatically from 600 to 3650 PSI at the primary superheater outlet.

5. As a temperature leaving the primary superheater increases the 207 operates to maintain a programmed primary superheater

outlet temperature.

6. At the temperature of 300 F, the 207 valve opens to a minimum position. As the temperature leaving the primary superheater

increases, the 207 operates to maintain a programmed primary superheater outlet temperature.

7. The 220 (H.P. heater steam control valve) and the 240 (flash tank overpressure control valve) will open to limit the flash

tank pressure at its set point of 500 PSI.

8. During this period, the secondary superheater will be boiling out to remove all water.

9. The flash-tank pressure increases as firing is continued.

w 30 w 31

Detailed Description of a Typical Startup Sequence of a Base-Loaded B&W Unit

Initial Turbine Roll Mode

1. At a flash tank pressure of 300 PSI, 205 (low pressure superheater nonreturn valve), will open.

2. The 210 (turbine bypass valve), is opened to pass approximately 2% of full load through the superheater to warm the

main steam lines. The flash tank pressure will continue to increase as firing is continued to its set point of 500 PSI.

3. At the flash tank pressure of 500 psi, the turbine can be rolled, approximately 2% of full load flow is required to roll the

turbine. The 210 valve should be kept open to pass an additional 2% flow to the condenser.

4. The firing rate should be adjusted until the gas temperature is approx. 50ºF above the desired temperature to the

turbine.

5. After the turbine steam requirements have been met, the 220 (flash tank steam to H.P. heater) valve will be opened

to limit flash tank pressure to 500 psi.

6. The 241 valve is still maintaining flash tank level, the 230 valve is maintaining deareater pressure (approximately

140 PSI).

7. When the capacity of the 220 valve is exhausted, the steam entering the turbine should be increased to 1000 PSI. This

will increase the flash tank set point to 1000 PSI.

8. The 220 and 240 valves are automatically set to hold the flash tank at its set point of 1000 PSI

9. At a flash tank pressure of 1000 PSI, the turbine can be synchronized and loaded. The unit load is ramped to

approximately 7% load.

10. The 210 valve is closed after the turbine is synchronized.

w 32 w 33

Detailed Description of a Typical Startup Sequence of a Base-Loaded B&W Unit

Transfer to Once-Through Operation

1. When the load on the turbine reaches approx. 7%, the 201 (pressure reducing valve) will begin to open. This will allow

steam to flow directly to the secondary superheater, rather than to the flash tank.

2. Pressurization of the secondary superheater occurs as the 201 is opened. (The turbine throttle valves are set to maintain

approximately 7% to 25% load as the secondary superheater is pressurized. When the secondary superheater pressure

exceeds the flash tank pressure, the 205 valve will close.

3. The 201 valve will continue to be opened at a predetermined rate to allow the turbine load to increase to approx. 25% of full load.

4. As the 201 opens, the 207 will close to control the primary superheater outlet pressure at its set point.

5. The flash tank drain flow to the deaereator will decrease as the 201 valve is opened. The deaereator pressure will decay as

the flash tank drains increase. When the deaereator pressure decreases below 25 PSI, the 231 valve will open to hold the

deaereator pressure at 25 PSI with flash tank steam.

6. The flow to the secondary superheater is through the 201 valve until its capacity is exhausted, which is typically around 25% of

full load. The 200 (high-pressure stop valve) will then be opened to achieve full pressurization of the secondary superheater

7. As the 200 and 201 valves are opened, the 202 valve will close to hold primary superheater outlet pressure at 3650 PSI.

8. As the flow to the flash tank decreases, the heaters and deaereator will be pressurized by steam from turbine extraction points.

9. As the load on the turbine reaches 25% the 202 and 207 valves will close and their opening set point will be

4250 PSI. These valves will now act as relief valves during a unit trip.

10. The 260 valve (flash tank warming line non-return valve; not shown, bypasses the 231 valve) will be

opened to pass a small amount of

steam from the deaereator back to the

flash tank. This is required in order to

keep the flash tank warm in case the

202 or 207 valves open for

overpressure relief.

11. During this time the 241 valve

operates to maintain flash tank level.

w 32 w 33

B&W Units for Sliding Pressure Operation

In a turbine generator, the electrical power output is dependant on the pressure entering the turbine. First, the boiler is fired and brought up to a constant discharge pressure. The turbine is equipped with several valves, known as turbine throttle valves, which regulate the turbine inlet pressure. As load decreases, the valves may move closed to reduce turbine inlet pressure. All valves may move closed equal amounts in unison (full arc admission) or they may close sequentially (partially arc admission). This is known a constant pressure operation.

Constant pressure has two adverse effects when large load changes occur. First, the turbine will experience temperature fluctuations, which will create fatigue and reduce its life. Second, the net thermal efficiency or heat rate of the turbine drops at lower loads.

Sliding pressure operation is designed to eliminate these problems.

Figure 1 shows the constant pressure system. With this system the turbine throttle valves control the inlet pressure to the turbine proportional to plant load.

Figure 2 shows a sliding pressure system. Here, a control valve (401) is installed upstream of the secondary superheater. Although the turbine throttle valves are still in the system, they are held wide open and plant load (turbine pressure) is varied by the sliding pressure control valves. The temperature change due to throttling at the 401 valves is adjusted at the secondary superheater. Temperature at the turbine is constant at all loads.

After startup and transfer to the once-through operation, the load will be raised to 100%.

With constant pressure systems, the transfer is at approximately 25% load. Pressure at the turbine throttle valves is brought up to approximately 3500 psi and load on turbine raised via the turbine throttle valves.

With sliding pressure control there are options of 70% or 100% sliding pressure control.

With 70% sliding pressure control, a larger 201 valve is installed. The turbine throttle valves are set at 70% load and the turbine throttle pressure is controlled by the 201 valve up to that load. The 200 valve is opened and then the load is raised on constant pressure control by the turbine throttle valves to 100%.

With 100% sliding pressure control, the 201 and 200 valves are replaced by valves which combine the functions of the 201 and 200 valves. These valves are called 401 valves. Systems with this design operate by setting the turbine throttle valves wide open and controlling the turbine throttle pressure with these 401 valves throughout the load range.

w 34 w 35

Sliding Pressure Control

The sliding pressure unit has a few modifications designed to satisfy three basic requirements of operation. These requirements are:

1. Capability to be reliably started up and shut down to make them available for two-shift operations.

2. Extended unit load turndown while operating in the once-through mode. To maximize the capability to reduct unit load during

off-peak demand periods without placing the boiler bypass system in service; thus maintaining reasonable heat rates at reduced

loads.

3. Capability of variable pressure operations the once-through mode to optimize operation of the unit for load cycling. To

extend the range of operation of the unit in the once through mode it is necessary to reduce the boiler minimum feedwater

flow requirements for furnace protection.

Modifications

1. In constant pressure units, the first pass of the furnace consists of four parallel riser circuits (sidewalls and front and rear

walls). In sliding pressure units this consists of two passes in series; pass 1 is through sidewall risers and pass 2 is through

front and rear wall risers. In addition a bypass valve around pass 1 is installed (263 valve), to limit the flow through

the sidewalls to 125% of original design. This bypass is to limit the additional pressure drop created by the dual pass

arrangement of the lower furnace. A second bypass valve (264 valve) around pass 2 is added to further reduce this pressure

drop; this allows a minimum feedwater flow rate of approximately 10% of full load flow.

2. For capability of the boiler/turbine temperature matching during startup and for accurate mainstream and hot

reheat control while operating on the bypass system, steam attemperators are added for the secondary superheater and

reheater. The second superheater steam attemperator requires the addition of valves 218 and 205C. The latter is used

to maintain enough

differential pressure

between the flashtank

and the throttle to

support the

attemporation

function. The flash

tank steam is used

for superheat steam

attemperation, since

this function is

required only while on

the bypass system. The

reheat steam attemperation requires

a 219 valve which takes steam from

downstream of the 401 valve, since this reheat

steam attemperation is needed while on the bypass system

and also during once-through operation at lowload generator.

3. The 202 valve is eliminated from the cycle and the 207 valve is used for all

functions formerly done by the 202 and 207 valves.

4. The key valve(s) for sliding pressure is the 401 valve. This valve is the turbine pressure

control valve. The valve must control load from as low as 10% at 2500 PSI pressure

drop to 100% at approx. 50 PSI pressure drop with a linear installed stroke vs. load

characteristic.

w 34 w 35

401 Valve

Disk Stack

w 36 w 37

Schematic of Foster Wheeler Base Load Design Super Critical Unit with Flash Tank

This is Foster Wheeler’s version of a startup

system utilizing a flash tank. The main

difference is that flow to the flash tank in

startup is through two valves in series; this is

the “W” valve and the “P” valve.

(Figure 2 on Page 52)

w 36 w 37

Once-Through Systems by Foster Wheeler

Once-through steam generators by Foster Wheeler incorporate start up

bypass systems to maintain a minimum cooling flow through the furnace

circuits when starting up. Other provisions are also built into the system

to satisfy turbine throttle steam requirements and to give maximum heat

recovery during starting. There are two designs; one using an external flash

tank, and one using integral separators.

1. EXTERNAL FLASH TANK SYSTEM

A bypass system utilizing an external flash tank system for rolling,

synchronizing and initial loading of the turbine is shown first. At some load,

usually that which matches 1000 psi throttle pressure, the steam flow to the

turbine is switched from the external flash tank circuit to the main line flow

path. In this system, shown in the schematic (Figure 2 on Page 52), throttle

steam to the turbine is initially furnished through valve N with division

valve V closed. When throttle pressure is to be ramped from the 1000 psi

level to full pressure for loading to 25% of full load, division valve V is

slowly opened and valve N is closed. Valve P closes to generate the pressure

ramp.

SYSTEM DRAWBACKS

When the external flashtank system is used with a cycling unit, it is difficult

to provide optimum steam conditions to the turbine (temperature and

pressure) during loading and ramping to obtain minimum starting time

without degradation of turbine cycling life. This characteristic is especially

true when the pressure ramp achieves full pressure at 25% load.

In general, the flash tank system, to achieve proper fluid enthalpy at the

boiler division valve, starts the ramp at a higher than optimum load and,

as a result, turbine control valves close slightly during ramping to properly

follow the ramp program (pressure versus load). In addition to this effect,

throttle steam temperature to the turbine may decrease (dip) or exhibit

reversing trends during ramping as a result of changing from the external

flash tank loop to the main flow path. The cumulative effect of the

foregoing for a hot start is to cause a decrease in turbine first stage shell

temperature. The first stage outlet inner shell temperature is measured and

used as an indication of adjacent shaft surface temperature. For repeated

hot starting, fatigue damage causing surface cracks on the shaft must be

avoided. To keep cycling life expenditure for the turbine at a chosen level

when starting in this manner, either the time for hot starting must be

greatly increased or the number of hot starts at minimum time must be

limited. For cycling service, this restriction on operation is unacceptable.

w 38 w 39

Applications for CCI Valves in Flash Tank Systems

‘W’ VALVE

Pressure reduction. This valve is usually sized for approximately 25%

boiler flow at 300 psi differential pressure with an equal percentage

characterized disk stack.

‘P’ VALVE

Superheater bypass valve. This valve discharges to the flash tank. During

start up the valve is used in series with “W” valve to control boiler pressure

as temperature is raised. When the unit is on line the “P” valve is closed

and functions as a pressure relief valve for the boiler. The valve must have

good shutoff. The pressurized seat valve, like the 207 valve in the B&W once

through system (Figure 4A), should be used here. The disk stack can be MS

2000 16 turn type with linear characteristics.

RECIRCULATION VALVES FOR CONDENSATE,

BOOSTER AND MAIN FEEDWATER PUMPS

These valves are the same as on the B&W system.

‘D’ VALVE

Flash tank level. This valve is the same as the B&W 241 valve.

‘E’ VALVE

Flash tank level (along with “D” valve). This valve is the same as the B&W

221 valve.

‘C’ VALVE

Flash tank to H.P. heater. This valve is the same as the B&W 220 valve.

‘B’ VALVE

Flash tank to deaereator. This valve is the same as the B&W 231 valve.

‘A’ VALVE

Flash tank to overpressure control valve. This valve is the same as the B&W

240 valve.

w 38 w 39

Schematic of Foster Wheeler Cycling Super Critical Unit with Integral Separator

w 40 w 41

2. FOSTER WHEELER INTEGRAL SEPARATOR SYSTEM

To overcome the disadvantages of the external flash tank system, the Integral Separator Start Up System was developed, which incorporated main line separators in the high-pressure circuitry. The schematic on page 39 (see also Figure 5 on Page 53) shows this system.

To avoid thick-walled pressure vessels that would limit starting and loading rates, multiple separators are employed at the primary superheater inlet. For the case where the furnace circuits operate at full pressure, a pressure reducing station (W valves) is installed upstream of the separators to provide variable pressure operation of the superheaters.

Steam and water at lower pressure downstream of the pressure-reducing station enters the separators. Separated steam flows to the primary and finishing superheater and then to the turbine. A spray station between the primary and finishing superheater controls final steam temperature during startup. Drain flow leaving the separators is collected in a drain manifold and routed through breakdown valves P to heat-recovery subloops and/or the condenser.

By adding controls to adjust the firing rate to hold separator pressure to set point, this startup operation becomes similar to that used for drum-type boilers. The minimum startup flow required is recycled through the heat recovery subloops back to the steam generator and can be considered similar to the recirculated downcomer flow of the drum unit.

By maintaining circulating flow always in the main line flow path for both start up and on-line operation, a simplicity of operation results. There is no matching of bypass flow enthalpy to saturated steam enthalpy before starting the pressure ramp, and as a result, the system inherently avoids a throttle temperature decrease or erratic temperature swings during pressure ramping. In addition, the pressure ramp can match the turbine characteristic without the need to adjust turbine control valves to a more closed position.

By virtue of these inherent characteristics of the Integral Separator System, the turbine first-stage shell temperature during ramping is maintained at steady or increasing values. For hot starting, this mode of operation permits more rapid starts while maintaining full turbine cyclic life. For application to cycling (two shift) units, the system permits rapid warm and hot starting for the 7500 cumulative cycles required.

The simplicity and repeatability of operation of the Integral Separator System makes it a more acceptable system for the operators. Repeatability of conditions for starting the ramp is easily achieved.

The Integral Separator System achieves full throttle pressure at 25% to 70% load depending on the capacity of the pressure reducing valves (W valves).

For the turbine, sliding pressure operation to the 70% load plateau gives a more uniform thermal gradient for the turbine while changing load.

At a 70% turbine valve opening, turbine and steam generator hot start times match at approximately 90 to 100 minutes from light off to full load. The basis for the calculations is that turbine life expenditure is not to exceed .01% per cycle, or permissible cycles are 10,000. The comparable time for 25% turbine valve opening is 140 to 150 minutes. Further increases in capacity beyond 70% yield only marginal reductions in hot start times.

Once-Through Systems by Foster Wheeler

w 40 w 41

Applications for CCI valves in integral separator system

‘W’ VALVE

Pressure reducing station. This valve is usually sized for approximately

70% boiler flow at 300 psi differential pressure with an equal percentage

characterized disk stack.

‘P’ VALVE

Superheater bypass valve. During startup, this valve is used in series with

the “W” valve to control boiler pressure and level in the separators. The

“P” valve discharges to the heaters, deaereators, and condenser through the

“E,” “F,” and “D” valves.

RECIRCULATION VALVES FOR CONDENSATE,

BOOSTER AND MAIN FEEDWATER PUMPS

These valves are the same as on the B&W system.

‘D’ VALVE

Separator to condenser. This valve is the same as the B&W 241 valve.

‘E’ VALVE

Separator to heater. This valve is the same as the B&W 220 valve.

‘F’ VALVE

Separator to deaereator. This valve is the same as the B&W 231 valve.

‘MX’ VALVE

Auxiliary steam to BFPT (attemperator at low flow). This valve is used to

inject saturated steam into the steam exiting the platen superheater during

system low flow operation.

‘PR’ VALVE

Auxiliary steam to BFPT. This valve takes steam from the platen superheater

outlet during system low flow operation.

w 42 w 43

Schematic of Combustion Engineering Super-Critical Unit, Base-Loaded Design

The combustion engineering combined-circulation supercritical boiler incorporates a startup system which has many of the same features and benefits as the B&W design. The main unique feature which differentiates the CE unit from the B&W unit is the integral recirculation system in the boiler, which separates waterwall protection from flow requirements. Integral recirculation allows for lower minimum flow of approximately 10% of full boiler load flow which not only minimizes heat rejection during startup but allows the transfer from the bypass system to once-through operation to take place without a sudden drop in steam temperature. For the CE unit, the transfer from recirculation to once-through operation occurs without operator intervention. Increasing waterwall pressure drop, which is due to increased flow, causes a reversal of pressure across the check valve in the recirculation line. Once the check valve is closed, the operator has the option of leaving the recirculation pumps in service or shutting them down. The concept of separating waterwall protection from plant cycle requirementsis also used in CE’s sliding pressure units. (Figure 3 on Page 52, Figure 6 on Page 53)

w 42 w 43

Combustion Engineering,Once-Through Unit Base Loaded

w 44 w 45

BT Valve

w 44 w 45

Typical Hot Restart Curves C–E Combined Circulation Units

w 46 w 47

Combustion Engineering Supercritical Once-Through Type Unit

Startup system modification to allow frequent load changes from maximum down to low load with minimum effect on the turbine thermal stress

w 46 w 47Description of CE Once-Through Unit and a Modification forSliding Pressure to 70% Load

The CE unit incorporates a “Combined Circulation” system.

In a drum unit, circulation of water within the unit provides a cooling flow in the furnace tubes to prevent overheating. In a once-through steam generator, a turbine bypass system is used during startup and low load operation to handle the minimum flow required for furnace wall cooling. This flow is usually 30% of max flow.

BE Boiler extraction valve. Startup flow and waterwall pressure control valve.

BTB Boiler throttle bypass. Control waterwall pressure at transfer from startup system to once through operation. Also pressure control when opening BT valves.

BT Boiler throttle valves.

SA Steam admission. Passes steam from the separator tank to the turbine while on the startup system. It is a check valve when on once through operation.

SP Spillover valve. Controls separator pressure when on the startup system.

WD Water drain valve. Controls level in the separator.

IS Superheater spray control valve.

IR Reheater spray control valve

SD Steam drain valve. Bypass turbine when warming lines and depress at shutdown.

FWB Feedwater control valve. Controls feed-water flow from 5% to 25% unit flow.

For large supercritical units, 30% turbine bypass is a technical and economic handicap. Therefore the “Combined Circulation” design utilizes furnace wall recirculation rather than a turbine bypass system; a recirculation line takes the fluid from the furnace wall outlet and discharges it into the inlet of the furnace wall system. A circulating pump at the inlet of the furnace wall system maintains the required minimum furnace wall velocities, at lower loads automatically, by recirculaton superimposed on the once-through flow.

The unit through-load (flow to the turbine), as maintained by the boiler feedpump, increases in direct proportion to unit load. The recirculated flow, as maintained by the circulating pump, supplements the through-flow over the lower load range in a manner which protects the furnace walls by raising the total flow through the walls to a safe level, regardless of feedwater flow. At approximately 10% load, the pressure drop through the furnace wall system equals the head produced by the circulation pump and the stop-check in the recirculation line automatically closes. The recirculation then ceases to produce recirculation of furnace wall flow but continues to contribute its positive head to the total unit through-flow, in this manner acting as a booster to the boiler feedpump.

SUBJECT: Startup valve modification to allow frequent turbine load changes without affecting the turbine life.

BACKGROUND: The unit shown is a Combustion Engineering–designed once-through critical pressure 565 MW system.The startup system incorporates Sulzervalves. The schematic is shown below:

w 48 w 49The startup for the CE unit, and the startup valves interface is shown for a base-loaded plant:

w 48 w 49

The startup shown is for a base loaded unit. The Sulzer valves and actuators are designed to be operated within the conditions shown.

For instance, the first BT valve is shown to start to open at 18% load. The pressure drop across this valve at 18% load is 1700 psi. The BT actuators are sized for that pressure drop. The valve should not (and probably cannot) be opened at less than 18% load because of the greater pressure drop. The valve, plus packing friction, is the actuator load. Thus, actuator load is proportional to system pressure drop.

For lowload operation at less than 30%, the unit may operate with the turbine throttle valves at 30% and the BT and BTB valves in control between 10% and 30% load. The unit could also operate at less than 10% by transfer from once- through to the BE valve and separator startup system.

To reduce load down to the low load is a reverse of the startup. The turbine throttle valves are used to change the load down to 30%, the BT valves are brought down to control, and then the BTB valves are as well. The load reduction via the turbine throttle valves must be done slow enough so as to minimize thermal stresses on the turbine. There is a fluid temperature reduction with throttling. Once on the BT valves, the pressure throttling is across the BT valves and the temperature to the turbine remains constant because of the superheaters after the BTs. Changing load while on the BT valves is “sliding” pressure. This means that varying the load in the 30% to 100% load range on the turbine throttle valves is slow, but once on the BTs, the load can vary relatively fast as far as the turbine is concerned.

The Sulzer BE, BTB, and BT valve are extremely heavy-duty valves. They were designed for the startup and shutdown of the unit as discussed so far. However, extended time at low load using these valves and actuators was not in the original scope.

So the question is, “What should be done to address extended low load or sliding pressure operation of the unit?”

If sliding pressure (extended service time) is done, the BT valve trim should be changed from ‘linear’ to ‘equal-percentage’ flow characteristic. Also the actuator size should be increased to enable full pressure range operation on the BT valves.

The equal-percentage characteristic is required so there is a smooth change in flow as each successive BT valve is opened or closed. Especially important is when the first BT is opened. At that time the BTB is controlling waterwall pressure. The BTB trim has about 10 times less plug area than the BT plug area. When the BT opens, the BTB valve must close to maintain waterwall pressure as the flow is increased through the BT valve. With the significant difference in plug size, the equal percentage trim in the BT would allow smooth increase of flow while minimizing waterwall pressure swing. The equal-percentage trim requires a larger seat ring bore in the BT valve in order to maintain the same maximum capacity (Cv). The BT valves’ combined maximum capacity must be at least the same as before. The resistances between the pump and turbine (called “parasitic power”) must not increase. The forgoing is addressing extended operation on the BT and BTB valves down to about 10% load on once-through operation.

The unit could transfer to the startup system for low load down to approximately 7%. However, transfer from once-through to the startup system introduces problems of feedwater control and feedwater chemistry. The frequent cycling of the unit for sliding pressure or for low load operation is best done on once-through operation.

With BT valves modified with equal-percentage trim, increased seat ring size for capacity, and larger actuators, the unit can be operated on “sliding pressure” to a higher load than 30%. Shown below is a 70% system showing the startup and once-through ranges of the Sulzer valves:

w 50 w 51

The system could be configured with only the BT valves. However the BTB

capacity addition at 100% load is of benefit for minimizing parasitic power.

w 50 w 51

The operation of the bypass system can be broken down into two basic areas

of control: the low load and pressure portion of the pumping and firing

controls, and the flash tank sub-loop controls.

The pumping and firing rate controls include the control of the boiler

feedpump, the firing rate and control of critical control valves.

These critical valves for the three major once-through boiler manufacturers

are shown below:

The flash tank subloop controls include the following valves in the three

boiler designs:

B&W, FW and CE, Nomenclature of Startup Valves Listed According to Function

w 52 w 53

Figures 1 Through 3 Base-Loaded Configurations

Figure 1

Babcox & Wilcox

Once Thru Unit

Figure 2

Foster Wheeler

Once Thru Unit

w 52 w 53

Figures 3 Through 4 Base-Loaded Configurations

Figure 3

Combustion Engineering

Once Thru Unit

Figure 2

Riley Stroker

Once Thru Unit

w 54 w 55

Figures 4 Through 6 Sliding Pressure Configurations

Figure 4

Babcox & Wilcox

Once-Through Unit

Figure 2

Foster Wheeler

Once-Through Unit

Figure 3

Combustion Engineering

Once-Through Unit

w 54 w 55

Typical Layout for HP

and LP Bypass Valves

Power Plants which Utilize

Turbine Bypass Systems

w 56 w 57

Stein Boiler

w 56 w 57

Sales and servicelocations worldwide.

CCI World Headquarters—CaliforniaTelephone: (949) 858-1877Fax: (949) 858-187822591 Avenida EmpresaRancho Santa Margarita,California 92688 USA

CCI Switzerland formerly Sulzer ThermtecTelephone: 41 52 262 11 66Fax: 41 52 262 01 65Hegifeldstrasse 10, P.O. Box 65CH-8404 Winterthur, Switzerland

CCI KoreaTelephone: 82 31 985 9430Fax: 82 31 985 0552/326-17, Pungmu-DongKimpo CityKyunggi-Do 415-070South Korea

CCI JapanTelephone: 81 726 41 7197Fax: 81 726 41 7198194-2, ShukunoshoIbaraki-City, Osaka 567-0051Japan

Contact us at:info@ccivalve.com

Visit us online at:www.ccivalve.com

Throughout the world, companies rely on CCI to solve their severe service control valve problems. CCI has provided custom solutions for these, and other industry applications for more than 30 years.

DRAG is a registered trademark of CCI.©2000 CCI 372 n 11/00 300