When is NPSH testing of a rotodynamic pump desirable or indeed necessary?
Generally speaking, the industry specifies such testing when applications require the pumping of volatile liquids or when duty conditions require operation with sub atmospheric suction pressures or suction lifts.
Often overlooked however, are applications in the utility market sectors which could warrant verification of a pumps NPSH requirement (NPSH’R’) across its designed operating range. High capacity pumps utilised on critical utility applications in the: Power, water, fire protection and flood control markets are often complex hydraulic machines that are required to operate at varying flow rates, at which their NPSH’R’ can vary significantly.
It should be noted that the NPSH’R’ of a pump will vary significantly across the allowable or design operating range, particularly as specific speed and suction specific speed values of a hydraulic design increase. Higher specific speed and higher suction specific speed pumps, will often require a larger NPSH margin ratio, when compared to more radial type centrifugal pump designs, to ensure satisfactory performance. Understanding the NPSH performance, across the full operating range is thus quite critical. Additionally some specialised applications, such as that of fire water pumps, often have a requirement for NPSH testing at flow rates of up to 150% of best efficiency flow (e.g. NFPA 20).
Pumps as described above are generally tested, as the risk of failure is high, as are any associated costs.
NPSH Testing
NPSH testing of rotodynamic pumps can be quite challenging, owing to the need to vary the suction or inlet pressure element of the total differential pressure, while maintaining a constant differential pressure and flow rate. In short, if the suction pressure is reduced (lift thus increased) at a given flow rate, the discharge pressure must also be reduced proportionally.
NPSH’R’ (NPSH3)
The industry norm is to define the NPSH requirement (NPSH’R’) of a pump on the basis of NPSH3, whereby a value of NPSH ‘R’ is established on the basis of a gradual reduction in suction pressure until a 3% reduction in total discharge head of the pump is measured at a given flow rate. This is a rather crude process and at the point a 3% reduction in head is recorded, cavitation is well established, hence a pump will require more absolute pressure or available NPSH (NPSH’A’) at its inlet to perform satisfactory.
NPSH margin is a subject of its own and not something covered in this short paper. For guidance in respect of required margins above NPSH3, it is recommended that you consult ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin.
For contract specific applications, Pumpflow Ltd can provide advice and recommendations.
Pump Types and Configurations
Over-Hung and Between Bearings Pumps
The majority of process applications involve low to medium capacity, overhung and between bearing type pumps which can be tested on a traditional closed loop test facility with a retention tank. The
retention tank is used to control the suction pressure, using a vacuum pump if necessary to reduce pressure below atmospheric values. In instances, when the NPSH requirement of the pump is above atmospheric pressure, the retention tank may need to be pressurised, using a booster pump as necessary. In such instances the suction pressure is controlled by throttling the suction/tank outlet valve.
Ideally the suction or tank outlet valve is situated at least five (5) pipe diameters before the pump suction inlet and a flow straightening device is installed upstream of the valve, in order to stabilise the liquid flow, prior to it entering the pump.
Large vertical volute pump rigged for closed loop testing
Suspended & Electro Submersible Bowl Pumps
Testing of suspended or submersible bowl pumps creates additional challenges, particularly in the case of medium to high capacity pumps. Whilst performance testing is generally carried out in large open test tanks, varying the submergence for NPSH testing is a bit of a none starter as:
- Large volumes of liquid need to be dumped to drain to vary tank levels
- Minimum water levels are limited to roughly twice that of the inlet Bellmouth diameter of the pump, to prevent air ingression, thus preventing the lowering of absolute inlet pressure below that of atmospheric pressure.
- Unless the test tank is very deep, it isn’t possible to provide positive pressures of say two (2) atmospheres or more, often required for suspended bowl and electro-submersible pumps for offshore or deep well applications.
- Realistically only a single duty point NPSH’R’ value can be established.
There are a number of options:
Testing in a Pressure Controlled Caisson
Ideally Bowl type pumps should be tested suspended and sealed within a caisson, allowing inlet/suction pressure to be controlled and varied. A vacuum pump can be utilised to provide sub-atmospheric pressures and either the residual discharge pressure of the pump under test or a booster pump can be utilised to provide positive pressure in a closed loop system.
Long suspension length pumps can be assembled short length, leaving out some column/shaft assemblies and ideally contract specific vertically arranged thrust bearings and drive motors can be utilised for testing.
Electro-Submersible pump suspended in a pressurised caisson for performance & NPSH testing
Scaled Model Testing
A reduced scale model test is often performed by original equipment manufacturers in lieu of a larger scale contract specific pump. The test results of the model pump, including NPSH performance is then extrapolated to prove the performance of the full scale pump. This would generally apply when high capacity machines are to be tested, with flow rates above say 5m2/sec.
- This is an acceptable solutions so long its limitations are recognised:
- The model must be an accurate mathematical scale, without modification.
- Inlet velocity and flow conditions should be accurate to the full-scale design.
Needless to say, this can be an expensive labour intensive exercise, but much less expensive than the costs associated with the failure of critical equipment.
When Not to Test?
Clearly the complexity involved in NPSH testing can prove expensive, hence in clear cut cases with adequate suction pressure/NPSH available, across the pumps full operating range of the pump, it should not be necessary to test.
- As a guide for general applications involving stable fluids and relatively low energy, low specific and low suction specific speed pumps a NPSH margin ratio (NPSH’A’/NPSH’R’) greater than 1.25, for all operating conditions could be safe.
- When the manufacturer is offering products from a proven standard range with auditable test history.
The above factors will likely apply to many process and general industrial applications.
For further information on testing or the development of rotodynamic pumps please call +44 (0)161 667 6050 or email sales@pumpflow.net
