MechTech June 2016 Final

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Pump systems, pipes, valves and seals

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Mechanical Technology — June 2016

he world is split into two camps

when it comes to improving the

energy efficiency of pumps. The

component-based approach is

being driven in Europe through legislation

and setting minimum efficiency levels

for pump and motor manufacturers. The

systems approach has been championed

by the USA ever since the US Department

of Energy piloted a successful energy

savings project in the mid 90s in China.

Although it started off as an electric

motor optimisation project, it was very

quickly discovered that the major sav-

ings’ opportunities came from looking

at the pumping system, rather than just

concentrating on pumps and motors.

The main difference between the

component and systems approach comes

down to how wide you intend to set your

system boundary when evaluating a

pumping system. Take the typical pump

system of a pump taking fluid from a

reservoir and pumping it to a discharge

tank a suitable distance away and at a

higher elevation – as shown in Figure 1.

The component approach

Let us start with the box surrounding only

The systems approach to pumping

Figure 1: The main difference between the component and systems approach comes down to how wide you

intend to set your system boundary when evaluating a pumping system.

In this issue we welcome new columnist, Harry Rosen, from TAS Online and 2KG Training.

Rosen is currently one of the two international pump experts for the United Nations

Industrial Development Organisation (UNIDO). In this, his first column, he outlines the

main differences between the component and systems approaches to pump efficiency

analysis and optimisation.

the pump and motor. Power is measured

from the MCC, flow rate from the flow

meter situated just downstream of the

pump, and head from the difference

in pressure between the suction and

discharge pressure gauges. The pump

efficiency is calculated to be 75.4% and,

by comparing this to that on the pump

curve from the manufacturer, we find

that it is close to the maximum of 79%

efficiency for this pump. On a component

level the pump is operating efficiently and

does not warrant any further attention.

The system approach

Now let us expand the system bound-

ary to incorporate the flow control valve

(FCV). This opens to allow bypass flow

back to the suction side when demand is

low. It is thus not the flow rate through

the pump that is important in our exam-

ple, but the flow to fill the second tank,

or to supply a downstream process. The

energy consumed for pumping any liquid

back to the suction tank is wasted energy.

If we expand the system boundary

once again to incorporate the pressure

control valve (PRV), we get the true

picture of the system demand in terms

of flow and pressure. The pressure down-

stream of the PRV is what the system

actually requires, and the pressure loss

through the control valve must also be

treated as wasted energy.

By using the flow rate at F2 and the

pressure after the PRV in our calcula-

tions, we can determine the overall

system efficiency, which could be dra-

matically less than our original calcu-

lation of pump efficiency. Our system

level opportunity would be to remove the

throttling valve, close the bypass line and

find another way to meet the required

system demand – by installing a VSD,

trimming the impeller or changing the

control methodology, for example.

If we assume that 20% of the flow rate

is being returned to the suction tank, and

the pressure drop across the throttle valve

is around 30% of the upstream pres-

sure, then the overall system efficiency

can be calculated to be around 42%.

Suddenly there is a major energy savings

opportunity. This is the benefit of look-

ing at the system rather than individual

components.

A case study: The bypass flow at

a sugar mill in the Philippines

The system:

Sugar mills provide great op-

portunities for reducing pumping energy

costs. There are numerous pumps used in

all aspects of the process, as well as for

cooling of process heat. In addition, many

mills have cogeneration plants with boil-

ers running on bagasse, the high calorific

dry pulpy residue left after the extraction

of juice from sugar cane. These plants

require additional pumping systems for

boiler feed water and cooling pumps for

condensing steam back to water.

The system investigated included

four hot-well pumps (three operating

normally) that pump hot return water

from the refinery to a set of spray pans – a

low cost alternative to traditional forced-

convection cooling towers. The water is

cooled down through natural convection

by approx 10 °C and then pumped by

another set of pumps back to the plant

Pumping systems 101: