Centrifugal Pump: Low NPSHA case
NPSH is Net Positive Suction Head. It is absolute inlet total head (in meter or feet) above the head equivalent to the vapor pressure referred to the NPSH datum plane. There are two terms regarding NPSH, which is NPSH available (NPSHA) and NPSH required (NPSHR). NPSHA is determined by the purchaser for the pumping system with the liquid at the rated flow and normal pumping temperature. Whereas, required NPSH (NPSHR) is NPSH that results in a 3 % loss of head (first-stage head in a multistage pump) determined by the vendor by testing with water and is the total absolute head above vapor pressure required at the pump inlet flange for safe operation at the pump operating conditions. To ensure that cavitation will not happen, NPSHA must always be greater than the NPSHR in any specified pump operating condition.
Sometimes mechanical engineer receives pump operation condition with very low NPSHA, it can be less than 0.5 m NPSHA for example. As project requirement normally require 1 m margin between NPSHA and NPSHR then it will make difficult to find the pump that can fulfill the requirement. There are some options that we can do regarding low NPSHA during design development:
1. Modifying pump suction condition
NPSHA is calculated based on the following formula:
NPSHA = (Hp + Hs − Hf) − Hvp
- Hp is absolute pressure head in the source in meters (feet)
- Hvp is liquid vapor pressure in meters (feet)
- Hs is static height calculated between source liquid level and pump datum in meters (feet)
- Hf is suction line losses in meters (feet)
It is noted from above basic formula that we can try the following in order to get higher NPSHA in our pumping system:
- Increase the pressure on the source of pumping system
- Increase static head which is liquid level in the suction source (vessel or tank)
- Decrease friction loss on the suction line by eliminate or reducing flow restriction, i.e., elbow, pipe length, valve etc.
- Decrease vapor pressure by reducing pump fluid temperature, it can be done by applying heat exchanger on the suction side.
Of course, not all above solution can be applied in every project. Especially when the project is revamping project which installing the pump on the existing system. The pump system source may be fixed and we do not want to touch or modify. But in the new plant or front-end engineering design, above solution can be feasible to be applied.
2. Pump type selection
The other solution is to select pump type that can increase NPSHA, so there will be sufficient margin between NPSHA and NPSHR at the inlet of pump impeller eye. The suitable pump type is vertical suspended pump VS-6 type. How we can get sufficient NPSH margin on this type of pump can be shown in the following Fig.1 as sample (dimension in mm).
Fig.1 NPSH on vertical pump VS-6
In this case we have NPSHA is only 0.9 m at the top of foundation. By selecting VS-6 pump type we have additional pump length which increase NPSHA to be 5 m at the 1st stage impeller inlet. In this sample with have 2.6 m NPSHR from the pump model selected. So, now we have NPSH margin is 2.4 m.