u11 cdu process variables

5/21/2018 U11 CDU Process Variables

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VIETNAM OIL AND GAS CORPORATION (PETROVIETNAM) November 2008DUNG QUAT REFINERY Rev. : 1OPERATING MANUAL Chapter : 3

UNIT CDU (011) Page : 30/38

Actions:

Action TagNo.

P&ID No. Service

P-1121A 124 Water pump stopped

P-1121B 124 Water pump stopped

011-FIC-025 124 Condensate flow controller set to manual andoutput 0%

This tripis manually reset by 011-UHSR-016

3.1.2.32. P-1122A/B Tempered water pumps protection (011-UX-024)

Purpose: When there is a low low level in the Tempered Water Drum D-1115, theTempered Water Pumps are stopped.

Initiators:

Initiator Tag No.TimeDelay

P&ID No. Cause

011-LXALL-0690.5seconds

126 Low low level in D-1115 (Water)

Actions:

Action TagNo.

P&ID No. Service

P-1122A 126 Tempered Water pump stopped

P-1122B 126 Tempered Water pump stopped

011-FIC-030 126 Tempered Water flow controller set tomanual and output 0%

This trip ismanually reset by 011-UHSR-024.

3.1.2.33. H-1101 Purge and Start-up sequence (011-UX-047 to 051)

For detail description of Heater purge, pilot gas, fuel gas, fuel oil and off gas ignitionsequences refer to the following drawing 8474L-011-A3501-0110-001-003, which has beenincluded in attachment 2.

3.2. OPERATING CONDITIONS

Refer to Process Flow Diagrams, which have been listed in 14.2 Process Flow Diagrams andMaterial Selection Diagrams, included in attachment 6.

3.3. PROCESS VARIABLES

This purpose of this chapter is to explain the main variables in Unit control.

The variables described are: CDU feed rate, fired heater outlet temperature, mainfractionator overhead temperature, pumparounds duty and stabilizer bottom temperature.

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3.3.1. CDU feed rate

The unit is fed by the crude charge pumps in unit 60. The crude oil flows through the cold

preheat train and enters into the desalter. In order to keep constant the desalter pressure,the crude oil flow at the crude charge pumps is set to the same flow as the measure at thefired heaters (011-FIC-069), by the controllers 011-FY-092 and 011-HY-002.

If the pressure in the desalter increases the inlet flow from crude charge pumps will bereduced.

3.3.2. Fired heater outlet temperature

This variable is very important, due to the fact that the column overflash is highly dependentof this temperature. If an increase in duty is required, the air flow is increased before the fuelrate is increased. On the other hand, if a decrease in heater duty is required, fuel flow rate isfirstly reduced and secondly air flow rate is reduced. This control scheme ensures that thefired heater is always operating with an air excess, to ensure a good combustion and avoid

risk of after burning in the stack.

3.3.3. Main fractionator overhead temperature

The overhead vapour temperature is controlled by the amount of heat removed from the toppumparound circuit. This temperature is important because the overhead product naphthaend point can be adjusted by changing the set point. If the temperature is higher than the setpoint, the top reflux duty will increase. As a result the overhead product end point andquantity will be reduced. On the other hand, if the temperature is lower than its set point, theoverhead product end point is lower than it should be and the control system will decreasethe top pumparound duty. So, if an increase in the overhead vapour product end point andquantity is desired, the top pumparound temperature set point will be increased.

3.3.4. Main fractionator pressure.The column operating pressure depends on the pressure controlled at the main fractionatoraccumulator drum D-1103. The operating pressure of a distillation column is related with therelative volatility of the components to be separated in the different streams. Relativevolatility is a measure of the differences in volatility between 2 components, and hence theirboiling points. It indicates how easy or difficult a particular separation will be. As pressureincreases the relative volatility approaches to one. This means that the vapour pressurecharacteristics of the components are more similar, and therefore they are more difficult tobe separated.

In addition, if the remaining variables keep constant, an increase in the column pressure willdecrease the overflash and therefore the internal column flow and the separation achieve willbe lower.

3.3.5. Pumparounds.

Pumparounds must be well controlled to obtain specific products. Pumparound duty controlis essential to keep internal liquid and vapour flow inside the column. The control is acting onthe recycled product flow to the column, and on the total pumparound duty (through dutycontroller).

Pumparounds are critical to maintain the optimum vapour/liquid rates in the fractionation ofthe column. Inadequate pumparound heat removal and poor pumparound distribution willresult in higher column pressure, poorer fractionation and an increase in the flash zonepressure. Excessive pumparound heat removal may result in difficulties in maintaining someend point and may cause internal flooding.



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3.3.6. Specification Adjustement.

The principal specifications to which the products from the fractionator can be adjusted are

the boiling point ranges and the flash point. In general, if the fractionation is good, the endboiling point of a side-cut equals the initial-point of the next heavier product plus a gap. Agap is the result of a good fractionation; however overlap is more normal and will increase asfractionation efficiency decrease. Product withdrawal flow rate are adjusted to provide therequired end points for the products and are generally set up from the top of the columnworking down.

As a general rule, increasing the drawoff rate will raise the end point of the side stream cutwhilst decreasing the drawoff rate will lower its end point. An indication of the end point is thedraw off temperature of the stream leaving the column. Increasing the product withdrawn,increases the draw off temperature and the product end point

The flash point of LGO, and HGO products can be modified by adjusting the flow of strippingsteamvia, FIC-017 and FIC-019 respectively (to increase the flashpoint the stripping steam

has to be increased). However, the stripping steam can only be increased to the point whereit starts to interfere with the end point of the next higher product. If a further increase in theflashpoint is required, other changes must be made, allowing an increase of the end point ofthe next higher product.

Kerosene flash point can be adjusted by the kerosene stripper reboiler E-1110 duty via 011-UIC-031 or, in case reboiler E-1110 is out of service, with steam flow via 011-FIC-015.

The specific gravity is directly related to initial and final product boiling point, so changing theproducts yields is required to achieve this specification.

The freeze point is related to chemical composition of the product. As final boiling pointincreases, the paraffin content in the product increases too, so the freeze point obtained willbe higher.

If residue entrainment occurs HGO can get black colour. This means that the washingsection (between HGO draw off tray and flash zone) doesnt have enough liquid flow. It isnecessary to increase the overflash or reducing the HGO draw off rate.

3.3.7. Stripping Steam in the Main fractionator column.

Increasing this flow rate, it is possible to decrease the content of lighter fractions still presentin the residue, increasing flash point and the initial boiling point of the residue. The strippingsteam flow,controlled by 011-FV-012 will affect temperature of the flash zone and amount ofoverflash.

3.3.8. Overflash

Overflash is that portion of the feed which is vaporized in addition to the overhead andsidestream products. The overflash condenses on the wash section plates and returns to theflash zone and bottoms stripping section. Its purpose is to prevent coke deposition in thewash section plates and preventing entrainment of residue up into HGO section, whichwould result in off spec HGO.

The CDU is designed for a minimum of 5% overflash.

3.3.9. Stabilizer temperature

The stabilizer bottom temperature has to be controlled in order to maintain the C4-

specification in this stream. This control is a cascade loop in which the stabilizer bottomtemperature is controlled by the high pressure steam flow through the desuperheater (DS-1101). If the bottom temperature decreases, the amount of high pressure steam will

increase, and therefore the reboiler duty, in order to raise the temperature again.



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The temperature in the top section at constant pressure defines the quality of the LPGproduct; this temperature is controlled on E-1122 (auto variable pitch controller). Settling thiscontrol at a lower temperature will decrease LPG C5 content and will increase the towerreflux. Increasing this temperature will increase quantity and C5 content in the LPG and willreduce the reflux rate.

3.4. INTER-UNIT CONTROL SCHEME

3.4.1. Atmospheric Residue from the CDU to the RFCC

3.4.1.1. Objective

The objective of this system is to control the level in the bottom of the Main Fractionatorcolumn and route as much of the outgoing atmospheric residue to the RFCC as is required

3.4.1.2. Description

This description should be read in conjunction with the following:

CDU P&IDs:

- 8474L-011-PID-0021-112

- 8474L-011-PID-0021-125

RFCC P&IDs:

- 8474L-015-PID-0021-301

Schematic 1: Atmospheric residue from CDU to RFCC

The level in the Main Fractionator, T-1101 is controlled by a typical Xxx40 three way split

range level controller 011-LIC-007 (direct acting). In the low range (0-33%) the signal from011-LIC-007 goes to low selector 011-LY-007B, which selects between 011-LIC-007 and thefeed requirement of the RFCC to control the flow to the RFCC. In the mid range (33-67%)011-LIC-007 resets the set point of 011-FQIC-026 and in the high range (67-100%) 011-LIC-007 resets the set point of 011-FQIC-027.

011-FQIC-026 and 011-FQIC-027 are parallel controllers used to regulate the flow of residueto storage. 011-FV-026 is a smaller valve than 011-FV-027 and will normally be sufficient tocontrol the amount of residue to storage whilst the RFCC is on-line. In the event that theRFCC is off-line then 011-FQIC-026 will be fully open and 011-FQIC-027 will control the flowto storage.

The RFCC feed is controlled by a split range level controller, which takes flow preferentiallyfrom the CDU by cascading onto 011-FQIC-029 via 011-LY-007B. If there is not enough

flow coming from the CDU then flow is taken from storage by acting on a flow controller FIC-402 in the line from storage.


u11 cdu process variables

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