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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
Krohne • krohne.com/safety 4
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 3
www.ChemicalProcessing.com
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products solutions services
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
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 5
www.ChemicalProcessing.com
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
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 6
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.
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 7
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.
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 8
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
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 9
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
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 10
www.ChemicalProcessing.com
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
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 11
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.
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 12
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
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 13
www.ChemicalProcessing.com
• 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.
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 14
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
www.ChemicalProcessing.com
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 15
Flow eHANDBOOK: Mull Potential Mistakes with Fluid Systems 16
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