Page 71 - FoodFocusThailand No.194 May 2022

SOURCE OF ENGINEER
Piping System Design

for Energy Saving in Pump

Piping systems are parts of important engineering systems in the food industry. The systems involve the
transmission of various kinds of fluids, either in open forms (from one point to others, no return) or for circulation
purposes (supply-return). Energy is required to transmit fluid to the destination at the preferred flow rate. In
most cases, the piping systems are over-design, which gives some margin for flow rate control. Reduction of
flow rate with proper methods to suit process requirements could significantly lower energy costs.

During the factory design stage, piping system designers will In systems that require variable flow rates, such as machine cooling
gather end-user requirements for each piping system, such as type systems that vary the flow rate of cooling water with the heat load to
of fluid, temperature, pressure, and flow rate at each destination. The maintain machine temperature, control can be done through control
requirements become the design goal. Locations of the source, for valves. When the control valves are throttled, the flow rate and pump
example, machine room and/or the storage tanks, are then considered. power consumption are reduced, as per the equation above. However,
The piping system is then designed to connect the source to the another way to further reduce the power demand is by implementing a
destinations. Designers will choose the pipe material to suit the fluid variable speed drive – VSD. By reducing the speed of the pumping motor,
and conform to related industrial codes, then lay out the pipelines on the pump curve shrunk according to Affinity Law, as shown in the dashed
the architectural drawing of the factory, identify the critical path, and line in Fig. 2. This results in a new operating point, point C, which has
size each pipe section according to the required flow rate. The required the same flow rate as point B, but with a lower pressure drop. Many case
pressure is then computed in order to select suitable a pump(s). The studies in the past have confirmed over 50% energy saving through the
design could be revised multiple times during the course of a project installation of VSD in systems that operate in reduced flow for a prolonged
until the factory is up and running. period. Return on investment usually takes a few months. A precaution
Fluid flow in pipe against the resistance always suffers energy in using VSD is in the system with high lift; pump speed reduction may
loss, which results in pressure drop. Energy loss in flow is caused lower the performance curve so much that the fluid cannot reach the
by the combination of major loss and minor loss—the major loss destination.
results from friction between the fluid and the pipe surface. The
corresponding amount of pressure drop depends on types of fluids,
types, sizes, and lengths of pipes and varies with the square of velocity More Information Service Info C012
(or flow rate), according to the Darcy-Weisbach equation. This can
be drawn as a graph as an example in Fig. 1 (log-log scale, pressure
drop represented with height of fluid – head). Minor loss causes by
changes in velocity during flow through fittings, elbows, valves,
accessories, and equipment (for example, heat exchangers). During
the design process, the designer does not know the exact quantity
of fittings, so 25-50% is added to the pipe length to account for a
minor loss. The pressure drop is then combined with elevation (no
elevation in closed-loop circulation). Adding the required exit pressure
and some margin of safety, the required pump pressure is obtained.
The required pressure and flow rate can be plotted as a necessary
operating point, point A in Fig. 2.
A parabola passing through the operating point is called a
system curve. The pump’s performance curve is a characteristic of
each specific pump. The designer will choose the pump which its
characteristic curve passes through the required operating point
and has high efficiency at that point.
When the piping system is in operation, pressure drop in the piping
system is usually lower than the design estimation due to the fact that
designers always over-size the pipes and components, which results
in a higher flow rate than required. Pump manufacturers could trim
the pump impeller to move the operating point to the design value.
The plant operator can also throttle the valve(s) to adjust the flow
rate. Valve throttling causes increasing minor loss, which makes the
system curve steeper. This moves the operating point to point B. As
the power consumption of the pump varies with flow rate, valve
throttling can reduce pump power consumption according to the
following equation:
E =
where E is electrical power consumption (watts)
Q is flow rate (m /s)
3
is pressure added to the system
(discharge – suction pressures) (Pa)
and is overall efficiency of pump and motor (30% - 80%)

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