foil flubs with flow - chemical processing

Flow eHANDBOOK

Foil Flubs With

Flow

TABLE OF CONTENTSDeftly Move Liquids Out of Danger 5

Consider a number of factors when designing an emergency transfer system

Identify Orifice Plate Issues 9

Various culprits can compromise flow measurements

Follow These 6 Tips for Sight Glass Selection 13

Knowing the forces detrimental to the glass can prevent a system shutdown or catastrophic

failure

Additional Resources 19

The SITRANS FS230 clamp-on ultrasonic flowmeter with haz-

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hazardous application challenges. Available approvals include

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The flowmeter has an accuracy of 0.5 to 1.0% of flow rate for

all liquids and pipe sizes and repeatability of 0.25% according to

ISO 11631. All sensors come with the industry standard of 100-Hz

data update rate that detects the smallest variations in flow 100 times every second.

A SITRANS FS230 flow system consists of the SITRANS FST030 digital transmitter paired

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pipe, such as liquid and gas, hydrocarbons, chemical, water and wastewater, HVAC, power,

food and beverage, and pulp and paper, pharmaceutical and mining. For more information,

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PRODUCT FOCUSULTRASONIC FLOWMETER HANDLES HAZARDOUS CHALLENGES

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Flow eHANDBOOK: Foil Flubs With Flow 2

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AD INDEXKrohne • us.krohne.com 8

L.J. Star • www.ljstar.com 4

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Check out Chemical Processing ’s

eHandbook Series! These eHandbooks are an excellent resource on various topics, solution applications and specific industries providing information to help solve your challenges and plant problems.

• Flow January 23, April 9, October 8

• Powder & Solids February 13, May 14, August 13, November 12

• Process Control March 12• Training March 26 • Refining & Petrochemical

April 16 • Pollution Control

April 23• Steam Systems May 7 • Digitalization June 11 • Pressure Measurement

June 18

• Distillation July 9• Level July 16 • Heat Transfer August 6 • Mixing August 20 • Water/Wastewater

September 10 • Process Safety September 17 • Motors & Drives

September 24• Flow October 8• Reliability October 22 • Energy Efficiency

November 5 • Hazardous Dust

December 3

2020 eHandbook schedule:

Sharpen YOUR STEAM

SYSTEMS BEST PRACTICES

Steam Systems eHANDBOOK

IMPROVE YOUR REFINERY

Refining & Petrochemical eHANDBOOK

Foster These

Flow Best Practices

Flow eHANDBOOK



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De-inventory safety standby systems

may sit for years before use. In fact,

unless they are intended for unit

shutdowns as well, the objective is never

to activate them. However, when called

upon, the systems should handle extreme

circumstances reliably and with minimum

operator intervention.

Liquid de-inventory systems fall into four

categories: pressurized; displacement;

gravity drain; and pump-out. In pressur-

ized systems, a high-pressure reservoir of

gas forces liquid from the system into a

reservoir. In displacement systems, a liquid

displaces the process flow and forces it to a

destination. In gravity drain systems, liquid

can empty into a reservoir at a lower eleva-

tion. In pump-out systems, a pump moves

the liquid to a safe reservoir outside the

unit. Pressurized and displacement systems

are relatively rare. Plants commonly opt

for gravity drain and pump-out systems to

transfer liquid inventory.

Gravity drain systems work reliably as

long as the process elevation, drain res-

ervoir elevation and connecting piping

are adequate. Often, though, e.g., when

a vessel needing emptying is at or near

grade, there’s no reasonable way to make

the elevations work.

For pump-out systems, no widely accepted

design rules exist. Different philosophies and

regulatory regimes can lead to disparate

pump-out system choices. So, let’s briefly

look at some areas needing decisions.

Deftly Move Liquids Out of DangerConsider a number of factors when designing an emergency transfer system

By Andrew Sloley, Contributing Editor



Determining the net positive suction head

available (NPSHA) to use for a pump-out

system requires answers to questions about

three major factors:

1. Will compositions be changing dra-

matically from normal operation? One

example would be all the liquid from the

trays in a tower dropping into the tower

bottoms and needing pumping out. On

large towers with steep composition

profiles, this can result in vaporizing

mixtures in the tower boot. The head

required (NPSHR) to prevent vaporiza-

tion in the pump suction may be less

than that for the usual assumption of

bubble-point liquid.

2. Will system pressure be at normal con-

ditions? Do the pump-out contingencies

include loss-of-containment? In this

case, the pump must operate while the

system is depressurizing. Again, the

pumped liquid may contain vapor.

3. Must the pump drain the system to very

low liquid levels to remove as much

inventory as possible? Here, the NPSHA

should reflect the low liquid level.

There are no easy solutions for pumps that

will have vapor in the feed during pump-out.

The best course of action is to reduce NPSHR

and use a slower-speed pump. A 1,800-rpm

pump can tolerate more abuse for the same

conditions than a 3,600-rpm pump. Don’t

rule out even lower speeds. In one case, I

specified 900 rpm for a pump-out system.

Pump-outs often occur when operators are

fully engaged in dealing with other issues.

So, a base assumption is that once the

pump-out system is turned on, the operator

doesn’t have to think about it again.

A pump-out

system usually

makes most sense.



Low NPSHR pumps often suffer problems

with suction recirculation. They should

have a recirculation line to alleviate this.

To make the system as reliable as possi-

ble, use an orifice in the recirculation line

rather than rely on an active control valve

for low-flow protection. This mandates

increasing the pump capacity.

The flow recirculation loop shields against

suction recirculation and gives some pro-

tection against temperature rise during

blocked flow.

If the downstream flow is blocked, the work

going into the pump will pass into the recir-

culating liquid. That liquid may recirculate

inside the pump or in an external loop. In

either case, unless there’s heat removal in

the loop, the temperature of the liquid will

increase. An external loop has larger inven-

tory, so temperature will rise more slowly

even without external cooling.

Unless the recirculation loop has some

method of heat removal, the recirculation

will slow — but not eliminate — the effect

of fluid heating. If the loop includes a

cooler or the recirculation goes upstream

of the pump to a location where it can

cool, then the recirculation loop will help

lower pump heating.

Adding recirculation to the pump suction

increases pump reliability. The recircu-

lation loop helps reduce problems from

suction inlet recirculation and, at a mini-

mum, cuts the impact of fluid heating at

zero net flow.

Opting for a slow-speed pump and a

robust design with a recirculation loop

generally offers significant advantages

— including, importantly, minimizing the

need for attention by operators when they

might be very busy.



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Orifice plates abound at many

plants because they provide a

simple and inexpensive way to

measure flow rate. An orifice plate induces

a pressure drop in a fluid that should accu-

rately correlate to its flow rate. If the orifice

plate meets established standards for its

geometry and is correctly installed in the

process line, then, as long as we know the

properties of the fluid, we can get flow

measurements to within 1% accuracy.

However, in practice, many factors can

lead to flow rate errors of up to 15% or

more. These include mis-location of the

instrument, improper installation, damaged

plates, blockages and unexpected flow

regimes (laminar versus turbulent or two-

phase versus single-phase).

One of my very first troubleshooting assign-

ments was attempting to determine fuel

consumption and efficiency for a number

of large thermal cracking furnaces. The

complex fuel gas system at the plant had

evolved over decades. Fuel compositions

varied over time and in location — and

composition wasn’t continuously measured.

Additionally, branches of the system used

different flow meter types; some branches

lacked any flow meters and, thus, required

mass balance calculations based on data

from meters installed elsewhere. Adding

to the challenge, meter maintenance was

sporadic and poorly documented. Appar-

ent errors for fuel consumption on some

heaters exceeded 30%. In other cases, two

meters in series on the same pipe gave

readings that differed by more than 15%.

Identify Orifice Plate IssuesVarious culprits can compromise flow measurements

By Andrew Sloley, Contributing Editor



Every time an eager new-hire engineer

was between assignments in the plant,

that person was tasked with trying to sort

out the fuel system. Long-established

history had shown the assignment to be

thankless and progress only occurred in

small steps; my experience bore that out.

Nevertheless, I did learn some import-

ant lessons. Today, we’ll focus on those

related to orifice plates.

First, most orifice plate errors, except for

installing one that’s too small, tend to give

flow rates lower than actual.

Backwards installation of an orifice plate

is a classic error. For a reasonable beta

ratio (~0.5) and in turbulent flow, expect

an error of 12–15% too low a flow rate.

Orifice plates have a specific installation

direction that should appear on the tab

on the orifice plate along with dimension

information. If you can reach the orifice

plate, this is easy to check. Accessing the

plate may be a problem, though. Accu-

rate measurement may require a length

of straight pipe upstream equivalent to

up to 90 upstream pipe diameters for

sufficient flow conditioning (see: “Think

Straight About Orifice Plates,” http://

bit.ly/2U2Qksx). This length of straight

run often only occurs in pipe-racks,

making getting to the orifice plate diffi-

cult and time consuming. Additionally, in

rare cases, the plate stamping is on the

wrong side, which easily can lead to back-

wards installation.

Orifice plates should be flat but can

undergo buckling or bending during

manufacture or transport. Badly buck-

led plates are easy to spot, so rarely get

installed. Somewhat buckled ones often

are installed, though. If the plate is buck-

led toward the upstream direction, the

flow rate will err high, while one buckled

Somewhat buckled

orifice plates

often are installed.



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toward the downstream direction will err

low. When installed in bolted flanges, the

buckling usually decreases due to the

flange forces on the plate. However, don’t

count on this. Errors due to buckled and

bent plates usually are 7–8% or less.

Plates have a specific edge geometry on

the orifice. Pay particular attention to

the shape of the edge and measure the

dimensions carefully to make sure the

plate is acceptable. If the sharp edge isn’t

manufactured correctly or is damaged,

the discharge coefficient of the orifice

changes. This tends to result in low flow

errors in the 4–6% range. If enough wear

occurs, then the entire orifice size changes

and errors can get much larger.

Deposits may accumulate on the plate

surface or edge. Deposits also may build

on the pipe on either side of the plate.

Errors from these problems can reach

nearly any value. A surface deposit only

on the plate likely will lead to an error of

4% or less. Deposits on pipe that inter-

rupt flow to or from the orifice can cause

errors exceeding 15%, which can be high

or low depending upon where the depos-

its are.

Problems with orifice plates often prevent

plants from closing material balances within

acceptable error ranges. However, they

aren’t necessarily the only causes. As in all

troubleshooting, start by assessing available

data and only use correct information.



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Sight glass applications require vary-

ing levels of consideration during

the design phase. In all applications,

sight glasses will be subjected to forces

involving pressure, temperature, thermal

shock, caustics, abrasion or impact. The

design approach to each application must

take these conditions into account. Table 1

compares several types of site glasses

and their ability to withstand these vari-

ous conditions.

The risks are real. When a sight glass fails, it

can be extremely dangerous. When a sight

Follow These 6 Tips for Sight Glass Selection Knowing the forces detrimental to the glass can prevent a system shutdown or catastrophic failure

By John Giordano, L.J. Star

COMPARISON OF SIGHT GLASSES FOR CRITICAL APPLICATIONSTable 1. Determining the right site glass for a critical application will depend on their ability to with-stand various conditions.

Temperature Application

Thermal Shock

Resistance

Corrosion Resistance

Abrasion Resistance

Pressure Capability

Impact Resistance

Glass Disc Soda Lime

Up to 300°F Poor Poor Poor Moderate Poor

Fused Sight Glass Soda Lime

Up to 300°F Moderate Poor Poor Good Good

Glass Disc Borosilicate

Up to 500°F Good Good Good Good Good

Fused Sight Glass Borsilicate

Up to 500°F Good Good Good Excellent Excellent

Quartz Disc Above 500°F Excellent Excellent Excellent Good Moderate



glass fails catastrophically, it can cause

severe operator injury and even death.

Furthermore, a catastrophic sight glass fail-

ure can create costly downtime. In a system

made primarily of metal, the weak spots

generally are sealing joints and glass. Typ-

ically, the failure of a sight glass on a piece

of equipment or within a piping system will

halt the whole process until the equipment

can be repaired or replaced. Moreover, this

failure may lead to scrapping the process

media. In a pharmaceutical process, the

product loss could cost millions of dollars.

Extreme forces, whether internal or exter-

nal, can have a detrimental impact on the

glass components’ visibility and strength.

Even minor cracks, scratches or abrasions

can be a source of weakness within the

glass and most likely will lead to failure.

Sight glasses are highly engineered prod-

ucts (Figure 1). These tips on how to select

a sight glass will help you to meet your

critical application needs. Six conditions

— temperature, thermal shock, corrosion,

abrasion, pressure and impact — and how

to design for them, are addressed.

TEMPERATURE The temperature within a process system

will have an effect on the sight glass.

One must consider all possible extremes

within which the sight glass must be able

to operate. Depending on the tempera-

ture range, certain glass types will perform

better than others. At temperatures less

than 300°F, standard soda lime glass may

be used unless the application is for phar-

maceutical processing, requires resistance

to corrosive chemicals or may be subjected

to thermal shock.

For applications that involve tempera-

tures up to 500°F, borosilicate glass may

be used. At temperatures greater than

500°F, such as in high-temperature steam

applications, quartz or sapphire glass is

recommended. Figure 2 shows the gen-

eral temperature ranges for common

optic materials.

SIGHT GLASSESFigure 1. Sight glasses are highly engineered products designed to withstand harsh con-ditions.



ABRASION Glass abrasion — physical

wearing down of surface

material — may occur with

fluids that contain granu-

lar particles in suspension

or with particles carried in

process gases. This erosion

of the glass may limit visi-

bility and affect its strength.

When designing for an

abrasive environment, it is

critical to prepare a routine

maintenance schedule to

evaluate the glass materials.

Glass material can be

inspected either visually or

using ultrasonic equipment,

which is a nondestructive

way to analyze the wall

thickness and determine

whether abrasives have

reduced the glass mate-

rial’s thickness. It also is

helpful in these conditions

to mount a shield on the

process side of the window

to extend the useful life of

a sight glass.

PRESSURE Pressure may be specified

as working, design, test or

burst.

• Working pressure is

the maximum pressure

allowable within an

operating pressurized

environment.

• Design pressure is the

maximum pressure that

the system has been

designed to withhold,

including a safety factor

typically specified by

American Society of

Mechanical Engineers

(ASME).

• Test pressure is the value

typically specified by an

end user to go above and

beyond the vessel design

pressure to ensure that

the components will not

only meet the design

criteria but also incorpo-

rate a level of safety that

exceeds it.

• Burst pressure is the

amount of pressure at

which a component will

fail. Typically, this test is

performed only in highly

safety-critical environ-

ments such as nuclear

facilities. Achieving burst

pressure is a costly test

as it requires the manu-

facturer to destroy the

component.

The glass materials

selected, the unsupported

diameter and the glass

thickness all play a role

COMMON OPTIC MATERIALSFigure 2. Quartz has the largest general temperature range for operations requiring sight glass.



in determining a sight

glass assembly’s pressure

capabilities.

The two types of sight

glasses are a conventional

glass disc and a glass

disc fused to a metal ring

during manufacturing.

Conventional glass typi-

cally fails when subjected

to significant tension. With

fused sight glass windows,

the metal ring’s compres-

sive force exceeds the

tensional force (i.e., pres-

sure) and, as a result, the

sight glass will not fail. The

metal ring squeezes the

glass and holds it in radial

compression.

Fused sight glass win-

dows offer high pressure

ratings and high safety

margins. The strongest

fused sight glasses are

made from duplex stain-

less steel and borosilicate

glass; this combination

creates the highest com-

pression. Figure 3 shows

the operating pressure

and temperature of fused

borosilicate sight glass

compared to fused soda

lime sight glass at differ-

ent temperatures.

IMPACT Some applications involve

objects that impact the

sight glass. An exam-

ple is a food mixer in

which hard chunks of

matter may strike the

glass. Another example

is a wrench dropped by a

worker that hits the sight

glass. While these events

seldom are enough to

cause immediate failure,

they can create scratches

or gouges that may pro-

vide a point for tensional

force to concentrate. It’s

always recommended that

scratched sight glasses

be replaced immediately.

Fused sight glasses offer

the greatest protection

from these situations.

THERMAL SHOCK Thermal shock can cause

cracking as a result of

rapid temperature change.

Some glass types are par-

ticularly vulnerable to this

form of failure due to their

low toughness, low ther-

mal conductivity and high

thermal expansion coef-

ficients. One situation in

which thermal shock may

PRESSURE/TEMPERATURE COMPARISONFigure 3. This chart compares the operating pressure of fused Borosilicate sight glass and fused Soda Lime sight glass at differ-ent temperatures. Source: “Compression vs. Fusion in Sight Glass Construction” by Karl Schuller, Herberts Industrieglas GmbH. Used with permission.



occur is during washdown, when cold water

comes into contact with a sight glass on a

heated vessel. Thermal shock also can occur

from within the vessel. This can take place

during startup when hot or cold media are

introduced or during clean-in-place/steril-

ize-in-place (CIP/SIP) operations.

During these situations, media are intro-

duced at a temperature very different

from that of the sight glass. Initial contact

can cause a rapid temperature change

in the glass, resulting in failure. Another

thermal shock hazard can occur during

autoclaving.

If thermal shock is a potential risk within

the process system, then, at a minimum,

borosilicate glass should be specified.

Borosilicate glass has a considerably lower

thermal coefficient of expansion than

soda lime glass, making borosilicate glass

more tolerant of sudden temperature

changes. Fused quartz has even greater

capability for more extreme temperature

environments.

The following calculation is used in deter-

mining the thermal shock parameter or the

resistance of a given material to thermal

shock.

kσT(1 – ν) RT = ________ αE

where: k is thermal conductivity, σT is

maximal tension the material can resist, α

is the thermal expansion coefficient, E is

the Young’s modulus and ν is the Poisson

ratio.

CORROSION Laboratory-grade glass is a formulation

of minerals and chemicals that is inert to

almost all materials except for hydrofluoric

acid, hot phosphoric acid and hot alka-

lis. Certain process media are caustic or

acidic and can etch the glass. The result

is a cloudy view with weakened integrity

that requires the sight glass to be replaced.

Hydrofluoric acid has the most serious

effect, where even a few parts per million

will result in an attack on the glass.

Careful consideration of the chemicals

present within a cleaning process is nec-

essary to ensure that the glass material

will not be impacted. For further details

regarding the physical characteristics of

borosilicate glass, ASTM E438 “Standard

Specification for Glasses in Laboratory

Apparatus” is available as a reference

material. The useful life of a sight glass in

these cases may be extended with shields

mounted on the process side of the glass.

Made of mica, fluorinated ethylene propyl-

ene (FEP) or Kel-F material, these shields

are not as transparent as glass, so there is

a tradeoff in visibility.

Corrosion also is a factor with the metal

used in a sight glass window. Most

system designers know which type



of stainless steel must be used to

handle their caustic or acidic pro-

cess medium, and they will specify

this steel to their sight glass supplier.

In some cases, a sight glass may be

mounted in such a way that the metal

ring doesn’t come in contact with the

process fluid, and therefore lower

cost steel may be used (Figure 4).

With a bolt-on sight glass mounted

on a vessel, only glass and Teflon

are exposed to the process medium,

thus, instead of expensive Hastelloy,

lower cost carbon steel may be used

in the sight glass ring (Figure 5).

JOHN GIORDANO, is national sales manager,

food & beverage, L.J. Star. He can be reached at

[email protected]

BOLT-ON SIGHT GLASS CUTAWAYFigure 5. In this cutaway view of a bolt-on sight glass mounted on a vessel, only glass and Teflon are exposed to the process medium. Instead of expensive Hastelloy, lower cost carbon steel may be used in the sight glass ring.

BOLT-ON SIGHT GLASSFigure 4. A bolt-on sight glass enables the metal ring to be mounted so it doesn’t come in contact with the process fluid.



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