In this article, we will discuss the Net Positive Suction Head Available calculation for Centrifugal pump.
What is Net Positive Suction Head Available?
Net Positive Suction Head (NPSHA) is defined as the absolute inlet total head above the head equivalent to the vapor pressure referred to the datum elevation. It is, therefore, the difference between the effective pressure at the pump’s datum elevation and the vapor pressure of the pumped liquid at the maximum pumping temperature, converted into meters of pumped liquid head.
NPSHA is specified for the rated flow rate.
Importance of NPSHA
NPSHA is very important in the pumping system to avoid Cavitations formation the pump. If NPSHA is decreased in a system will create sever cavitation effect in the pump and reduce pump life. (To read more about Cavitation click here)
Calculations of NPSHA for Centrifugal Pump
They are two types of suction condition, based on the pump centerline and pumping liquid level. They are
- Suction Head
- Suction Lift
The suction head and Suction lift are explained in the below picture.
For “suction head” condition
For “suction lift” condition
NPSHA – available NPSH (expressed in liquid column metres).
P1 – absolute pressure in the suction vessel (bar abs).
PV – absolute vapor pressure of the pumped liquid at the operating temperature in pump suction; in the case of cryogenic fluids, it is necessary to take into account the temperature increase in the suction pipe due to heat exchange with the environment (bar abs).
DPf1 – pressure drop in suction pipe (from the suction vessel up to the pump’s intake flange) calculated at the pump’s maximum rated flow (bar).
ρ – density of the pumped liquid at operating temperature (kg/m3).
(h1LL±hP) – vertical absolute distance between the minimum level in the suction vessel and the pump datum (m).
va – liquid velocity at pump intake flange (m/s).
Normally, the velocity head (va2/2g) is negligible and therefore not calculated.
The pressure in the above equation may be defined as gauge pressure: when using gauge pressure, P1 and Pv must be relative to the same atmospheric pressure. The use of a true local atmospheric pressure is very important in the case of high altitude locations.
Points to Remember for NPSH available Calculation
The following are some of the points to remember for NPSH available Calculation
- For services such as firefighting, BFW, or on-off services without control valve. NPSHA has to be calculated at 120% rated flow.
- When the pump sucks from vessels in which the liquid is at its boiling point and there are no temperature variations in the suction pipe (e.g. cooling or heating), the value for PV is the same as the operating pressure in the suction vessel.
- The NPSHA must be calculated by taking into account the rated (or higher) flow.
Methods to improve NPSHA
If the NPSHA is low in the pumping system. Then we required a special pump, to meet the NPSH margin. This will increase the cost significantly. To avoid this, the following are some of the methods to improve the NPSH available. They are
- Increase the operating pressure of the suction vessel, within the allowable pressure of the vessel.
- Increase the suction vessel elevation.
- Reduce the pressure drops on the suction piping by increasing its diameter and/or reducing its equivalent length.
- Provide cooling on the suction piping to decrease the vapor pressure.
- Chose different pump type (e.g. vertical barrel).
- For liquids at boiling point and ambient temperature or lower, provide insulation of the suction line to avoid the fluid heating up.
- Provide a low head pump (called “booster pump”) operating at low speed and with low NPSHR to increase the main pump suction pressure. The use of a common motor for the two pumps may be considered.
As we discussed at the beginning of this article, Net Positive Suction Head available (NPSHA) very important in the pumping system to avoid Cavitations in the Centrifugal pumps. Hence NPSHA in the pumping shall be maintained higher than the NPSH required for the pump to avoid cavitation.