Modification of pump curves by installing restrictive orifices at pump discharge
Ahmed Sarwat, M.Sc., CRL, VMA
Project Engineering Manager & Deputy Project Manager (Mega Projects, EPC, Engineering, Construction, Commissioning, Operation, LNG, Petrochemicals, Chemicals, Fertilizers, Natural Gas, Cryogenics, Carbon Capture (CCU)).
Sometimes, pumps with “steep” curves instead of “flat” ones are required in some applications (basically due to flow control needs). However, more often than would be desirable, curve steepness cannot be changed by modifying pump hydraulics and then the only remaining solution is to install an external orifice plate at pump discharge flange.
Hereinafter the orifice is considered as a part of the pump. A new combined “steep” H-Q curve is then obtained by subtracting head loss curve of the orifice plate from pump performance curve as shown below.
Just to remark that many users refuse this solution at least for the following reasons:
The inclusion of the orifice plate pressure losses into the pump performance curves, results in a diminution of the global system efficiency
The potential accidental loss of the orifice plate during transport, installation or maintenance leads the pump to follow the old “flat” curve and therefore to unexpected or even undesirable operation.
Nevertheless, it is sometimes the only one available solution, so we should be aware of this.
After that, manufacturer guarantees pump performance over the new curve resulting from combined action of the orifice plate and the pump itself.
If looking below at performance curves given by a determine John Doe manufacturer, it can be seen that only two of four curves are modified (H- Q and efficiency-Q) while the other two remain the same (power- Q and NPSH-Q).
The new Efficiency-Q curve is usually estimated by using the following equation:
- For a given flow rate Q1:
(Efn)1 = (Ef)1. [1-(ΔH)1/(H)1]
Being:
(Efn)1– new estimated pump efficiency (with orifice plate)
(Ef)1 – original pump efficiency
(ΔH)1= (H)1 - (Hn)1– head difference
(H)1 – original pump total head
(Hn)1 –new pump total head considering energy losses at orifice
Just to remark that many users refuse this solution mainly for the following reasons:
Diminution of the overall system efficiency.
Unexpected pump operation in case of loss of the orifice plate.