Designing pumps for low Net Positive Suction Head (NPSH)
Amarinth designs high specification, low volume pumps on short leadtimes and has a wealth of knowledge selecting and designing pumps to operate in environments of a low Net Positive Suction Head (NPSH), such as on oil and gas platforms, Floating Production Storage & Offloading (FPSO) vessels and other applications with restricted space and headroom.
Selecting a pump is a balance of many factors, including the volumes and properties of the fluid to be pumped, total static lift, pipe size, pipe losses, the efficiency of the pumps and how frequently the pump will be run. Where space is at a premium or the cost of changing structures or pipework prohibitive, then engineers may also have to deal with a lack of suction head. Not taking the suction head into account fully can cause catastrophic cavitation to occur in the pump.
The catastrophic effects of cavitation
Cavitation occurs when the pump cannot get enough fluid flow into the impeller and the resulting reduction in pressure causes the liquid to vaporise and form bubbles. These bubbles can grow dramatically and choke an inlet, further reducing the flow of liquid and the performance of the pump. In addition, the bubbles can implode with tremendous force, literally tearing away at the metal surface. The resulting increase in stresses, vibration and noise can lead to premature component replacement and in some cases complete pump failure.
Designing for low NPSH
Amarinth always ensures that the NPSH available at the pump - NPSH(A) – which takes into account the suction static head, pipe friction losses, atmospheric pressure and the vapour pressure of the liquid exceeds that which is required by the pump to operate without cavitation occurring – what is known as NPSH(R). For a given NPSH(A), we generally aim to provide a pump with an NPSH(R) which is less than NPSH(A) by some 0.5m. Important factors that should be taken into account when designing for low NPSH include:
The physical location of the pumps and vessels
The pump should be as close to the tank or vessel as possible and the number of bends, valves and filters in the pipework minimised. Ensuring that the pipework is of the right size for the required flow rates and that the minimum level of liquid is sufficiently above the suction vessel outlet will reduce vortices and potential air entrapment at low levels.
Understand the lowest operating condition of NPSH(A)
There could be transitional conditions, for example the tank being emptied or an extreme condition of an instrumentation failure causing the tank to be at a “low-low” condition. Where the normal operating condition is relatively stable it may be possible to use this operating rather than designing for extremes of failure, which will result in significantly more headroom being required than may be available.
Due thought for safety margins
For example, API 610 recommends that that the pump manufacturer gives serious consideration to the difference between NPSH(A) and NPSH(R) when calculated using the vapour pressure of the lightest fraction in the oil being pumped. In offshore rigs or FPSO vessels the headroom is not available to engineer a solution if the calculations are only based on the lightest fraction. Calculating the vapour pressure based on the actual mix of fluids could halve the vapour pressure used in the NPSH calculation reducing the static head required by 10m.
Designing for pump efficiency
Pump designers usually design and select pumps to work at maximum efficiency and hence the lowest running costs. A number of parameters dictate the NPSH(R) for any given pump. However, pumps can be specifically designed for low NPSH, all-be-it at the cost of sacrificing some efficiency. This trade off may be worth considering in relation to additional civil and structural costs for the overall system, especially if the pump is not running continuously.
The impact of the Nss limit
Some specifications will not allow low energy pumps to operate outside of the Nss limit. Nss is a characteristic number for centrifugal pumps which relates Flowrate, Speed (RPM) and NPSH and is used to estimate the maximum practical speed at which the pump can run that avoids the possibility of cavitation problems. High Specific Energy Pumps with high suction specific speed are more prone to recirculation, leading to noise, vibration and, in some cases, severe erosion. In reality, the Nss value may be just a fraction over the limit dictated in the specification which means the risk to the pump is minimal and so long as there is no increased noise or vibration then it should operate without problem and so relaxing the Nss value could be considered.
Reducing the temperature of pumped fluid
NPSH(A) increases when the temperature of the fluid is lowered and so reducing the temperature of the pumped fluid can be an effective way to make gains. For example, if appropriate, injecting cold water into the fluid.
Full pump design service
Amarinth uses state-of-the-art CAD, CFD and FEA tools to design bespoke pumps, often in exotic super duplex alloys with double mechanical seals, that operate reliably in low NPSH environments.
Contact us to find out more about our low NPSH pumps and experience
Amarinth understands the complex nature of low volume high specification pumps. We also understand that you need a full suite of supporting documentation. Call us to find out more about our web documentation system that allows you to check all documents and receive copies of the contract documents 24 hours a day.
With its business agility, strong “can do” attitude and proven design expertise, Amarinth is the only choice for low NPSH pumps.