5 troubleshooting strategies for optimizing oversized pumping systems

8/21/2019 5 Troubleshooting Strategies for Optimizing Oversized Pumping Systems

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There are several potential ways to

address an oversized pump and system

condition, including the following:

1. Install a different pump.

2. Modify the control strategy to in-

clude a flow recirculation line.

3. Trim the impeller.

4. Install a variable frequency drive

(VFD).

5. Reduce pump speed.

1

Install a Different PumpInstalling a different pump will typically

be the most expensive option and may

only be feasible if considerable energy

savings or reliability savings are avail-

able from the current installed condi-

tion. Many pump upgrades will require a

new baseplate and piping modifications

on the suction and discharge. Given

this fact in tandem with the upfront

cost of the pump itself, this would likely

be a last resort if none of the other

possible solutions presented hereafterare deemed to be viable. If a different

pump is chosen, then other reliability

improvements should also be evaluated

with the pump setup, such as fixing

any piping issues, baseplate issues, or

pump upgrades that may not have been

available when the original pump was

installed.

2 Add a FlowRecirculation Line

Adding a flow recirculation line on the

14 | May 2015 Flow Control Magazine

OPTIMIZING

OVERSIZEDPUMPING SYSTEMS What to do when your pump is too big for your system

By Randy Riddell, CMRP

Figure 1. Impeller reduction and recirculation line operating points

Various studies have shown that many process pumps

are oversized. Optimizing oversized pumping systems

can save large amounts of energy, as well as make an

impact on pump reliability by reducing pump vibration and

extending bearing, seal and impeller life. Oversized pumps

will operate to the left of the best efficiency point (BEP) on

the pump curve, creating significant internal recirculation,

low-flow cavitation, and high shaft loads.

C O U R T E S Y

S C A

YSTEM DESIGN | Pumping Systems


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pump will improve the pump efficiency

as the pump operation moves to the

right on the curve, but the total energy

consumption will increase with a flow

bypass control strategy. This can be a

viable option if the pump is small with

low-flow conditions without significantconsequences for wasted energy. The

new operating point for adding a flow

recirculation line can be seen in Figure 1.

3 Trim ImpellerOne of the most common actions for an

oversized pump is to trim the impeller.

Trimming an impeller reduces energy

consumption due to excessive head

from an oversized pump; however it

does little for over capacity of the pump

sizing, as shown in Figure 1. Trimmingan impeller can, in some cases, lead to

lower pump efficiency.

While trimming the impeller has

drastic effects on the pump, determin-

ing how much to trim the impeller is typ-

ically dictated by the system and/or con-

trol valve operation. The impeller should

be trimmed to get the control valve in

the system to operate in a reliable posi-

tion, which is typically 40 to

60 percent open. If the con-

trol valve differential pressure(ΔP) is in a more closed posi-

tion (e.g., 25 percent open),

then the impeller should be

trimmed to obtain a more

reliable control valve position

(55 percent open). The pres-

sure drop can be estimated

by the control valve equation

by getting the Cv for each

valve position.

Figure 1 shows a pump

operating at 1,000 GPM at65 ft. with a control valve

typically running 25 per-

cent open. The calculated

pressure drop difference to get the

control valve at 60 percent open was

15 ft. The impeller was trimmed from

~13.25x12.5” to ~12.25x10.62” di-

ameter to accomplish a 15 ft. head re-

duction.

4 Install Variable

Frequency DriveVariable frequency drive (VFD) applica-

tions on pump systems can be a good

solution to remove wasted energy by

removing a control valve. By removing

waste head from the control valve pres-

sure drop, the pump can slow down.

This reduces the pump brake horse-

power (BHP) needed due to the lower

head requirement.

Most of the time when a VFD is in-

stalled, the original motor is left in place

and motor efficiency losses are small

and neglected. During a conversion to

a VFD, the original motor may go from

90 percent loaded with 0.87 power fac-tor and 94.5 percent efficiency to 50

percent loaded with energy reduction,

which may lower the motor power factor

to 0.6 and lower the efficiency to 93

percent. On the other hand, the pump

efficiency could increase 5 to 10 per-

cent, depending on the orig-

inal operating point.

VFDs are only viable forhigh-friction head systems,

not high-static head sys-

tems. A VFD is typically only

justified when 50 percent of

the total head is due to dy-

namic head in the system.

VFDs are also not gener-

ally feasible if the process

system feeds a lot of differ-

ent control valve systems.

In these types of systems,

header pressure control isrequired for the VFD con-

trol to meet all of the sys-

tem pressures required to

manage all feed sources. This typically

results in a fairly high system pressure,

just slightly lower than the fixed-speed

design, as the highest pressure source

becomes the driving factor in the pres-

sure setpoint. With multiple sources, it

is important to carefully evaluate each

control valve operation in the system.

As such, it may be difficult to justify aVFD on these type of control systems.

Figure 2. 1200 RPM pump curve original operating point

C O U R T E S

Y

S C A

Reducing pump speed may be thebest option for optimizing an over-

sized pump if the process condi-

tions and pump hydraulics will sup-

port it. Lowering pump speed will

improve the hydraulic cavitation

characteristics of the pump by

lowering net positive suction head

required (NPHSr), improving bear-ing life, and saving energy.

“

”



5 Reduce Pump SpeedReducing pump speed may be the best

option for optimizing an oversized pump

if the process conditions and pump

hydraulics will support it. Lowering

pump speed will improve the hydraulic

cavitation characteristics of the pump

by lowering net positive suction head

required (NPHSr), improving bearing

life, and saving energy.

Consider the pump curve in Figure2, which was operating at 1200 RPM. It

originally had a 300-horsepower motor.

Suction energy adjustments to NPSH

resulted in an NRSPr of 28 ft. The net

positive suction head available (NPSHa)

in the system was 15 to 19 ft. Insuf-

ficient NPSH caused pump cavitation,

which elevated typical vibration levels

as high as .4 in/s. The control valves in

the system operated 25 to 35 percent

open, which presented the opportunity

to reduce pump head.

This pump operating at a slower

speed, 900 RPM, shown in Figure 3,

presented a good alternative to the

original oversized condition. Since the

pump was slowing down and couldonly slightly reduce the head, a larger

impeller had to be installed. This pump

would need a 20.75” impeller operating

at 6,000 GPM at 85 ft. Due to shifting

the operating point, the pump efficiency

improved to 78 percent. Due to system

head reduction, this pump would only

require 165 BHP, which also meant a

smaller 200-horsepower motor. This

would be a critical factor, as moving to

a smaller motor horsepower would allow

the 900 RPM frame to fit on the exist-

ing 300-horsepower baseplate, whichmade this an even more attractive so-

lution.

When suction energy and NPSH

margin were calculated, the NPSHr for

this pump at a lower speed was 12

ft. This would drastically reduce the

pump’s cavitation potential. As a result,

vibration was reduced to 0.11 in/s. This

was not a huge surprise as suction en-

ergy is a function of the square of the

speed. The pump system control valves

were now operating in the 50 to 70 per-cent open range.

Not every pump system will work

out where all the factors align to allow

a modification along the line of what is

described above; however, looking at all

options will help produce the best pos-

sible result. No matter what your budget,

there is usually some measure of incre-

mental improvement that can be em-

ployed to improve the efficiency and reli-

ability of an oversized pump system. FC

www.sca.com

YSTEM DESIGN | Pumping Systems

Find related content @ flowcontrolnetwork.com…Search on:FLOWSTREAM

Cavitation | Energy Efficiency | Impeller | NPSH | Pump Curve | Reliability

16 | May 2015 Flow Control Magazine

Randy Riddell ,

CMRP, CLS is the

Reliability Manager

for SCA at the Barton

Mill in Alabama. He

has over 25 years

of industrial experi-

ence with a career

focus on equipment

reliability. Mr. Riddell has a BSME

from Mississippi State University and

is a Certified Maintenance & ReliabilityProfessional from the Society of

Maintenance & Reliability Professionals.

He can be reached at Randy.Riddell@

sca.com or 256 370-8105.

Randy Riddell

Figure 3. Pump curve for same pump operating at 900 RPM C O U R T E S

Y

S C A

No matter what your budget, there is usually

some measure of incremental improvement that

can be employed to improve the efficiency and

reliability of an oversized pump system.“

”

5 troubleshooting strategies for optimizing oversized pumping systems

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