Flow eHANDBOOK - Chemical Processing

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TABLE OF CONTENTSConsider Low-Flow Risk Assessment 5

Decreased operating rates caused by the pandemic can pose hazards

Don’t Slip Up With Slipstream Filtration 10

The optimum configuration depends on a couple of key factors

Tame Your Workhorse 13

Properly sizing your centrifugal pump and providing the right flow control can save energy

Additional Resources 16

AD INDEXEndress+Hauser • www.us.endress.com/chemical 2

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Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 3

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Many plants now run at reduced

rates because of COVID-19.

This can result in low flows that

present unique safety and operational

problems. So, here, we’ll look at risk assess-

ment for low flows and possible mitigative

approaches.

Plant design and equipment sizing usually

consider equipment turndown. However,

the tacit assumption is that low rate

operations will be short term. Thus, pro-

longed operation at low rates gets scant

attention.

In view of today’s situation, you should

perform a low-flow risk assessment. While

you can do this in many ways, adopting the

following steps generally fosters an effi-

cient evaluation:

• Assemble a small group of experienced

individuals from Operations, Maintenance,

Safety and Engineering.

• Establish protocols for conducting the

low-flow risk assessment.

• Divide the plant or sections of it into man-

ageable segments or nodes that contain

equipment — for example, pumps, com-

pressors, heat exchangers, valves, pipes

and control systems.

• Consider the impact of prolonged low-

flow operation on each component of the

nodes.

• Prioritize corrective measures.

As Figure 1 indicates, the adverse conse-

quences of low flow can include clogging/

plugging, under-deposit corrosion, pump or

compressor overheating, excessive vibra-

tion and loss of containment, poor heat

Consider Low-Flow Risk AssessmentDecreased operating rates caused by the pandemic can pose hazards

By GC Shah, Consultant



transfer and byproduct reactions. Of course,

clogging and plugging depend on the size

distribution of solids or particulates, which,

in turn, depends on the process and nature

of the fluid. As a rule of thumb, experts con-

sider that velocities below 2–3 ft/s increase

the chance of plugging.

It’s worth noting that low flow has some

positive aspects as well. Low flow to a

pump will require less net positive suction

head and, hence, would help avoid or mini-

mize the chance of cavitation when limited

suction head is available for the pump.

Low velocity will reduce the erosive impact

in services such as those involving sand

and slurry.

However, in risk assessment, we must focus

on adverse consequences and the means to

minimize safety/operational risk.

NODAL COMPONENTS Now, let’s look at some important

components. Of course, this list isn’t com-

prehensive but it does provide examples

of the types of key issues to consider and

potential mitigation measures.

Pumps: Plugging on the suction side and

pump internals might occur. Plugging can

further reduce flow to the pump, which, in

turn, might cause cavitation depending on

the liquid and its temperature and pressure

drop. If a plug on the suction side creates

vacuum, air could ingress, resulting in a

potential fire in systems handling flammable

fluids. Problems of cavitation, internal wear

and air ingress are less common with posi-

tive displacement pumps but can’t be ruled

out altogether.

Low velocity could cause solids to deposit

on pipe as well on pump internals, with a

potential for under-deposit corrosion. (See:

“Keep Under-Deposit Corrosion Under Con-

trol,” https://bit.ly/2ZtVq2V.)

Typically, pump rates below ≈15% of the

design best efficiency point could lead to

vibration or overheating. The minimum flow

point depends on the pump design and type

of fluid. Refer to pump curves and the ven-

dor’s input for safe minimum flow operation.

Excessive vibration could cause damage to

pump components including shaft, seals,

wear rings and bearings and, eventually,

could result in loss of containment.

SOME POTENTIAL CONSEQUENCESFigure 1. Low flow can lead to a variety of adverse impacts.

LOW FLOW

Heat transfer

Clogging, plugging

Corrosion

Overheating (pumps, compressor)

Vibration and loss of containment

Byproduct reactions



Consider:

• Operating a single pump if a service uses

multiple pumps. Pumps with variable

speed drives likely could accommodate

low flow operation by appropriate speed

reduction. If system pressure drop allows,

think about trimming impeller size but

realize this reduces pumping efficiency.

• Monitoring suction pipe, pump casing and

insulation if solids have high corrosion

potential.

• Adopting minimum flow recycle, also

called spill back flow.

• Monitoring vibration and using fire and

gas detectors and alarms to detect and

warn of leakage from seals/flanges.

Compressors: In a centrifugal compressor,

flow rates below the surge line rapidly result

in vibration, unstable operation and severe

damage to rotor, shaft, seals and bearings.

For flammable gas service, sub-surge flow

rates quickly could lead to loss of contain-

ment and potential fire. Obviously, unstable

operation also will adversely impact

upstream and downstream units.

Interestingly, low-flow operation in certain

ranges of flow tends to minimize potential

liquid entrainment from the demisters, suc-

tion drum or suction scrubber.

Consider:

• Using anti-surge system controls and

algorithms. You must provide sufficient

margin above the surge line so the oper-

ation stays away from surge in the event

of upsets, including rapid change in flow

rates to the compressor.

• Running one compressor, rather than two,

at low loads.

• Relying on unloaders or flow recycle for

low-flow control of positive displacement

compressors. Fixed-speed motors typi-

cally drive large compressors, so varying

speed usually isn’t an option.

Heat exchangers: Low-flow operation will

enhance fouling, scaling and sedimenta-

tion. Depending on metallurgy, this could

lead to under-deposit corrosion. In addi-

tion, heat transfer will decrease. In some

systems, this would entail more frequent

cleaning/maintenance. Heat-sensitive

materials could suffer higher rates of

decomposition.

Consider:

• Monitoring trend in pressure drop and

heat transfer coefficient.

• Using corrosion inhibitors.

Instrument systems: Low-flow operation

will impair the performance of several types

of sensors and transmitters.

Thermocouples and resistance temperature

detectors in thermowells will show slower

response, which will affect temperature

control loops adversely.



Many flow meters, such as Coriolis, turbine

and thermal ones, have large turndown

ratios that could accommodate low-flow

operation without significant loss of accu-

racy. However, others, such as orifice plate

meters with limited turndown (3 to 4), may

need resizing. At low flows, gear meters

could experience increased fouling.

Any pH probes in fouling/scaling service

would demand more aggressive or frequent

cleaning.

Although not a common occurrence, some

extractive sampling systems for gas chro-

matographs could require repositioning of

the sample tap in the pipe.

Consider:

• Re-ranging and re-calibrating sensors/

transmitters that low-flow operation

could affect.

• Monitoring the performance of critical

instruments, including those in safety

instrumented systems.

• Cleaning pH probes more frequently.

• Ensuring thermowells are installed in high

turbulence areas. For critical services, you

may need to adopt a voting arrangement

(e.g., 2oo3).

• Checking control valve openings. Typ-

ically, openings below 20% could pose

controllability problems. In such cases,

think about downstream throttling or

replacing with a smaller valve.

Piping: Liquid systems containing

suspended solids will show a higher like-

lihood of deposit formation and potential

under-deposit corrosion (depending on

metallurgy) in low-flow operations. Low

vapor flow also could accelerate de-en-

trainment of liquid streams in piping

systems. Stagnant liquid would facilitate

solids deposition on pipes and could lead

to under-deposit corrosion.

Consider:

• Providing appropriate treatment levels

of corrosion inhibitors/dispersants (if

applicable).

• Recycling flows (similar to spill back flows

on pumps) for pipe segments exposed to

low-flow regime.

• Implementing appropriate monitoring of

all potential locations where solid deposits

can form.

• Periodically checking pipe thickness and

insulation and updating service life based

on observed corrosion rates.

Distillation columns: Tower operating

range is set by weeping/dumping at the

low end and flooding at the upper end.

Because distillation involves a number of

subsystems, including pre-heater, column,

overhead condenser, reboiler, system

controls and other columns (if part of a

distillation train), operating/safety prob-

lems due to low flow could manifest in

multiple systems.



Possible issues that could arise include:

• Fouling or scaling systems could impair

heat transfer in the pre-heater; this could

cause unstable flow to the column.

• Low liquid/vapor traffic in the column

could lead to unstable column operation

— for instance, weeping, column dump-

ing, and potential vapor flow through the

downcomers.

• Low liquid traffic through the reboiler

could foster fouling and diminish boil-up,

which, in turn, will reduce vapor traf-

fic through the column and contribute

to enhanced weeping/dumping of the

column.

• Control systems could become unstable

because the control valve could fall out-

side the “good-operation-low limit,” i.e.,

below 20% open.

Partly because of multiple interactions,

distillation operations will necessitate sys-

tematic troubleshooting efforts: monitoring

operations, heat transfer rates, pressure

drops and product quality.

Consider:

• Avoiding structural changes to the

system, e.g., blanking off part of the

trays, changing the type of packing,

altering weir height, replacing the

preheater or the reboiler with a smaller

unit, etc., unless you expect low-flow

operation to endure for a prolonged

period (several years).

• Making changes that will allow reasonable

operation for the duration of low flows

— for example, reducing the pressure of

the column, increasing reflux/boil-up to

boost vapor/liquid traffic, using inhibitors

that could retard fouling, or maintain-

ing stripping steam or wash oil (refining

operations) to minimize poor stripping

and coke formation, respectively.

• Filtering out as many suspended solids

as practical before sending feed to

the distillation system, including the

pre-heater.

PROCEED PROPERLYLow-flow risk assessment should be thor-

ough and efficient. It should focus on

identifying safety/operational issues of the

equipment as well as upstream and down-

stream systems. Proper documentation and

follow-up, of course, are an integral part of

effective risk assessment.

GC SHAH, PE, CFSE, CSP, CFPS, is a Houston-based

consultant specializing in process safety, including

hazard analysis and fire protection services. Email him

at [email protected].



Contaminated hot oil had so

undermined the performance of a

vaporizer that it no longer could

provide the required duty. The contamina-

tion, which had occurred over time, had led

to fouling of the heat exchanger and fired

heater. Adding a filter for the hot oil clearly

was necessary. Configuring the filtration

system brought up an interesting question,

one worth exploring here.

Best practice for most hot oil systems is to

use a partial flow filter to keep the hot oil

clean. The filter treats a slipstream of hot

oil. This reduces the filter size (and cost)

while keeping the oil clean enough for reli-

able performance.

Figure 1 shows a simple hot oil system with

two possible slipstream systems: a more-flow

(MF) configuration (left) and a more-head

(MH) configuration (right). Figure 1 focuses

on the slipstream. The main circulating flow

to supply oil to the thermal load has its own

independent flow control system (not shown).

For the MF configuration, the slipstream

goes from the pump discharge to the pump

suction. As long as the circulating loop has a

higher pressure drop than the filter, the only

effect on the pump is a required higher flow

rate. The discharge pressure of the pump can

remain the same. In this system, the pressure

drop in the main circulating loop sets the max-

imum available pressure drop for the filter.

The hydraulics of the main loop potentially

limit filter sizing and life.

For the MH configuration, the slipstream

splits from the pump suction, goes through

Don’t Slip Up with Slipstream FiltrationThe optimum configuration depends on a couple of key factors

By Andrew Sloley, Contributing Editor



the filter and then returns to the main

flow. The pump capacity needed doesn’t

change but the pump discharge pres-

sure must increase by the pressure drop

through the filter.

Which system is better? To answer this, we

focus on the pump power, P. The hydraulic

power required is linearly proportional to

both the mass flow through the pump, M,

and the pressure rise over the pump, ∆Pr: P

α M ∆Pr.

For a new unit where you have full abil-

ity to select an ideal pump, the analysis

is straightforward.

To add detail to the example, let’s propose

a reasonable pressure drop of 12 psi across

the filter element to control filter operat-

ing costs and a pressure drop of 80 psi

across the main loop in the hot oil system.

The slipstream rate is 10% of the total flow

required to deliver the heat load.

Using the MF system, the pump discharge

pressure stays the same and the flow rate

rises by 10%. The pump power goes up by

10%.

With the MH system, the total pressure drop

increases to 92 psi from 80 psi. This is a 15%

boost in discharge head. The pump flow

rate stays the same but the pump power

rises by 15%.

Most hot oil systems follow this pattern.

The percent change in pressure drop

required for a reasonable filter element

life will exceed the percent change in flow.

This is because most hot oil systems have

relatively low slipstream rates and filter

applications generally need relatively high

pressure drops.

TWO CONFIGURATIONSFigure 1. The same size slipstream doesn’t mean the same size power demand.More-Flow Configuration

FC

Load

10% Filter

P1

More-Head Configuration

FC

Load10% Filter

P1



As the slipstream fraction goes up, the

advantage shifts from the MF to the MH

option in new systems. To go back to the

example, shifting to a 30% slipstream rate

increases the power demand of MF to

+30% while MH’s remains +15%.

For adding a slipstream filter to an exist-

ing system, the choice depends upon the

overall system hydraulics and configura-

tion. In some systems, neither option will

fit within existing equipment constraints.

Two other options, suitable particularly for

larger systems and existing systems with

constraints, are booster pumps and two-

stage pumps.

If the system is large enough, a booster

pump for the slipstream going to the filter

may make sense. It minimizes energy

consumption but incurs extra capital

expense for the additional equipment and

controls. Existing systems with signifi-

cant hydraulic constraints may require a

booster pump.

Some applications can benefit from install-

ing a pump that discharges flow at multiple

pressure levels. One example uses an API

pump with a modified shaft and back head

and a drilled-hole impeller second stage for

the slipstream flow. In many ways, this is

the ideal choice because it doesn’t require

extra flow and only the flow going through

the filter is at a higher discharge pressure.

However, industry is far less familiar with this

option; so, it has seen very limited adoption

despite its benefits.

You always should carefully check the

hydraulics of systems with slipstream

filters before arbitrarily selecting a flow

configuration. The best choice for an

application will vary with system hydrau-

lics and the size of the slipstream.

ANDREW SLOLEY is a contributing editor for Chemical

Processing. Email him at [email protected]

As the slipstream fraction goes up, the best choice changes.



The centrifugal pump is a workhorse

in the process industries. How-

ever, the way such pumps are used

frequently undermines their efficiency.

In particular, system designers typically

oversize pumps during the design stage

to ensure the pumps provide the needed

flow. In addition, the methods used for

flow control often are wasteful. In this

column, we address this pervasive pair

of problems.

Throttling valves. A valve on the dis-

charge side of the pump controls the

flow. This is the most common form of

centrifugal pump control. Partially clos-

ing the control valve (or balancing valve

in a static application) at the pump dis-

charge is a convenient but inefficient way

to reduce the flow of an oversized pump.

As the control valve closes, it reduces the

flow rate — but adds frictional resistance

the pump must overcome.

Flow resistance due to throttling wastes

a considerable amount of energy if the

valve constantly stays heavily throttled

(say, <60% open). The most common

improvements are:

• Trim the pump impeller. This is a low-cost

option and typically a good solution if the

required flow always is less than design,

the flow reduction isn’t too large, and the

required flow rate is fairly constant. (How-

ever, predicting resulting performance

demands care; see: “Centrifugal Pumps:

Avoid Surprises When Cutting Impellers,”

http://bit.ly/2DNd5rd.)

Tame Your WorkhorseProperly sizing your centrifugal pump and providing the right flow control can save energy

By Alan Rossiter and Glenn Cunningham



• Install a variable frequency drive (VFD).

VFDs save energy as the pump’s speed is

reduced. They are an ideal solution if the

pump’s capacity varies widely. However,

VFDs are comparatively expensive and can’t

always be justified. (For tips on applying

VFDs effectively, see: “Consider VFDs for

Centrifugal Pumps,” http://bit.ly/2Yao7yF.)

• Install a right-sized, high-efficiency pump.

If the pump is significantly oversized

for its maximum flow requirement or

if its hydraulic efficiency at the normal

operating point is low, this may be the

best option.

Recirculation. With recirculation, the

pump produces a constant flow greater

than the maximum process demand. Fluid

required by the process goes to down-

stream equipment while the excess fluid

flows, via a recirculation line with a con-

trol valve, directly back to the suction

tank. The constant recirculation of excess

fluid makes this the least efficient form of

flow control.

However, recirculation can serve an

important function by providing mini-

mum flow protection. If the flow through

a pump falls too low, pump damage can

result from rapid heating of the fluid

within the pump as well as induced pres-

sure pulses originating within pump

suction and discharge areas by recircula-

tion vortices. Stopping the recirculation

flow when it’s not needed to protect the

pump can achieve more-efficient minimum

flow protection.

In addition, you can arrange automatic

control valves for recirculation to open

only when the system flow is below the

pump’s minimum flow requirement. A

pressure sensor in the pump discharge

line can control the valves and open

them at a high pressure, or you can use a

spring-controlled mechanism set to open

automatically at a pre-defined pressure.

This arrangement greatly improves the

control system’s energy efficiency.

Parallel pumping with throttling valves.

Here, multiple pumps piped in parallel

discharge into a common header. This

arrangement usually relies on throt-

tling control valves to regulate flow in

different lines coming off of the main

supply header.

An opportunity to save energy often

exists in constant-speed parallel pumping

systems having more than three pumps.

Constant heavy throttling of control valves wastes a considerable amount of energy.



This is because the last pump started

generally doesn’t add a large amount of

additional flow to the system. The fourth or

fifth pump typically runs as an “insurance

policy,” in case one of the other pumps

fails. However, this insurance policy can

be expensive. Turning one pump off and

installing automatic controls capable of

starting an additional pump if the header

pressure falls below a pre-determined level

can ensure proper operation with fewer

pumps and lower energy costs.

For further details and examples, see:

Glenn T. Cunningham, “Rotating Equip-

ment: Centrifugal Pumps and Fans,”

Chapter 15 in “Energy Management and

Efficiency for the Process Industries,” Alan

P. Rossiter & Beth P. Jones, Wiley-AIChE,

2015, pp. 186-20.

ALAN ROSSITER is CP’s monthly Energy Columnist. Email

him at [email protected]. Guest contributor GLENN

CUNNINGHAM can be reached at GCunningham@

tntech.edu.

Understand Key Challenges in Process Safety Education

Process safety (PS) academic education has evolved slowly

over the past four decades. University chemical engineering

departments began to establish and deliver process safety

courses in the 1980s. However, the majority of chemical

engineering curricula still do not offer a standalone

course, and fewer make it a requirement.

Chemical Processing, in collaboration with the

Mary Kay O’Connor Process Safety Center, has

developed this Journal to help you better understand

process safety management and solve your challenges.

Predict and Prevent Well-Control EventsRethink Safety and Control Systems Design

UNDERSTAND KEY CHALLE NG ES IN PROCESS SAFETY EDUCATION

FEBRUARY 2021

MKO PROCESS SAFETY JOURNAL

Download the journal at www.chemicalprocessing.com/journals




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