PUMP DUTY SELECTION
Most fluid flow and pumping design
programs will not automatically select an appropriate pump for
a particular system duty. If they did, the implication would
be that a dababase of possible pumps (capacity, efficiency and
NPSH curves) was available and the software would search through
this database locating suitable pumps.
We suggest that a pump selection
(duty point) be made in the following manner: Our fluid flow
design programs (Piping Systems FluidFlow and Fathom) allow a
"specification" to made at an I/O node:
- Specification - the required
value of pressure or flow
- I/O node - a position where
fluid is thought to enter or leave the pipe network (input/output)
Its not usually possible
to specify both flow and pressure at an I/O node. For instance,
if a desired inlet flow is specified at an I/O node then the
required pressure will be determined by the pressure drop in
the downstream pipework and fittings and static level changes.
And, alternatively, if the I/O pressure is specified, the software
will calculate the resulting flow into or out of the network.
This leads to an efficient way of
determining a required pump duty point.
Rather than try to model the
pump in the system immediately, simulate the pump with an estimated
pressure. Solve the model and view the calculated flow. Change
the pressure value until the desired flow is achieved. The set
pressure and the calculated flow values now constitute the required
duty point of the pump. An appropriate pump can be selected from
manufacturers catalogues, the performance curves entered
into the software and the pump then added to the model. (Alternatively,
you may have a known required flowrate to achieve - so specify
this and let the software calculate the pressure).
FluidFlow3 allows an 'Auto Pump'
component where you can set the desired flowrate and the software
then determines the necessary head. Both values then define the
pump duty point.