Net Positive Suction Head




NPSH is an acronym for Net Positive Suction Head. It shows the difference, in any cross-section of a generic hydraulic circuit, between the pressure and the liquid vapor pressure in that section. NPSH is an important parameter, to be taken into account when designing a circuit : whenever the liquid stagnation pressure drops below the vapor pressure, liquid boiling occurs, and the final effect will be cavitation: vapor bubbles may reduce or stop the liquid flow. Centrifugal pumps are particularly vulnerable, whereas positive displacement pumps are less affected by cavitation, as they are better able to pump two-phase flow (the mixture of gas and liquid), however, the resultant flow rate of the pump will be diminished because of the gas volumetrically displacing a disproportion of liquid. The violent collapse of the cavitation bubble creates a shock wave that can literally carve material from internal pump components (usually the leading edge of the impeller) and creates noise that is most often described as "pumping gravel". Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment. Considering the circuit shown in the picture, in 1-1 NPSH is : NPSH = P0 + H − Y − Vt (to be solved with coherent measuring units), where Y is the friction loss between 0-0 and 1-1, and Vt the liquid vapour pressure at the actual temperature in section 1-1. In pump operation, two aspects of this parameter are called respectively NPSHA or NPSH (a) Net Positive Suction Head (available) and NPSHR or NPSH(r) or NPSH-3 Net Positive Suction Head (required), where NPSH(a) is the suction pressure presented at the pump inlet port, and NPSH(r) is the suction pressure limit at which the pump's total differential head performance is reduced by 3% due to cavitation. It's important to note that cavitation occurs at suction pressure levels above the NPSH-3 level and pump damage can occur from cavitation even though the pump may continue to provide the expected hydraulic performance. A somewhat more simple method of understanding NPSH… NPSH is a very misunderstood and fairly difficult phenomenon to grasp and below is an attempt at the most simplest description along with some basic examples. Once NPSH is understood in its fullest, sizing and controlling pumps and pumping machines is a much simpler task. NPSH is simply the liquid suction force into a pump. In other words, the force of a liquid naturally “pushing” into a pump from gravity pressure plus liquid headpressure only - into a single pump intake. This means; NPSH = the net (left over) positive pressure of suction force into a pump intake after friction loss has occurred. Liquid head height or liquid head pressure + gravity pressure, minus friction loss, leaves a net head pressure of force into the pump. NPSHR is the amount of liquid pressure required into the intake port of a pre-designed and manufactured pump. This is known as NPSHR (Net Positive Suction Head Required). The pump manufacturer will usually clearly have a NPSH curve to assist you in the correct installation. NPSHA is the amount (A = available) to the pump intake after pipe friction losses and head pressures have been taken into account. The reason for this requirement? When the pump is receiving liquid into the intake port and the impeller is then pushing the liquid out at the discharge [disambiguation needed], they are effectively trying to tear each other apart because the pump is changing the liquid movement by a pressure increase at the impeller vanes, (general pump installations). Insufficient NPSHR will cause a very high negative vacuum pressure (negative NPSHA) that exists at the pump intake and the pump will not receive the liquid fast enough and in turn makes the liquid tear apart from the vapor pressure laws. This will create the liquid to boil and cause cavitation. Cavitation will lower pump performance and damage pump internals. At low temperatures the liquid can hold together relatively easily, hence a lower NPSH requirement. However at higher temperatures, the negative vacuum pressure starts the boiling process much earlier, hence a high NPSH requirement. Negative vacuum pressure & vapour pressure? Water will boil at lower temperatures whilst under a negative pressure or vacuum and the opposite for positive pressures. Water boils at 100 degrees Celsius at sea level and an atmospheric pressure of 1 bar. Vapor pressure examples: (vapour pressure = the temperature in which a liquid turns into a gas, which may change under negative and positive pressure environments). At a negative vacuum of -5 psi or -0.35 Bar water will boil at 89 degrees Celsius. At a negative vacuum of -10 psi or -0.7 Bar water will boil at 69 degrees Celsius. At a positive pressure of +12 psi or +0.82 Bar water will boil at 118 degrees Celsius. Liquid temperature greatly affects NPSH and must be taken into account when expensive installations are being sought. Therefore… if you are pumping with a NPSHR suitable for cold water, and you start pumping hot water, it may start to cavitate. Some general NPSH Examples: (based on sea level). Example 1: A tank with a liquid level of 2 metres above the pump intake, plus the atmospheric pressure of 10 metres, minus a 2 metre friction loss into pump (say for pipe & valve loss), minus the NPSHR curve (say 2.5 metres) of the pre-designed pump (see the manufacturers curve) = an NPSHA (available) of 7.5 metres. (not forgetting the flow duty). This equates to 3 times the NPSH required. This pump will operate well so long as all other parameters are correct. Remember that (+ or -) flow duty will change the reading on the pump manufacture NPSHR curve. The lower the flow, the lower the NPSHR, and vice versa. Lifting out of a well will also create negative NPSH; however remember that atmospheric pressure at sea level is 10 metres! This helps us, as it gives us a bonus boost or “push” into the pump intake. (Remember that you only have 10 metres of atmospheric pressure as a bonus and nothing more!). Example 2: A well or bore with an operating level of 5 metres below the intake, minus a 2 metre friction loss into pump (pipe loss), minus the NPSHR curve (say 2.4 metres) of the pre-designed pump = an NPSHA (available) of (negative) -9.4 metres. NOW we add the atmospheric pressure of 10 metres. We have a positive NPSHA of 0.6 metres. (minimum requirement is 0.6 metres above NPSHR). Now the pump should lift from a well. Now we will try the situation from example 2 above, but will pump 70 degrees Celsius (158F) water from a hot spring… Lifting out of a hot spring well will also create negative NPSH; however remember that atmospheric pressure at sea level is 10 metres, just like before. Example 3: A well or bore running at 70 degrees Celsius (158F) with an operating level of 5 metres below the intake, minus a 2 metre friction loss into pump (pipe loss), minus the NPSHR curve (say 2.4 metres) of the pre-designed pump, minus a temperature loss of 3 metres/10 feet = an NPSHA (available) of (negative) -12.4 metres. NOW we add the atmospheric pressure of 10 metres and we have a negative NPSHA of -2.4 metres remaining. Remembering that the minimum requirement is 600mm above the NPSHR therefore this pump will not be able to pump the 70 degree Celsius liquid and will cavitate and lose performance and cause damage. This pump now requires that it be buried into the ground in a pit next to the hot spring well to a depth of 2.4 metres plus the required 600mm minimum, totalling a total depth of 3 metres into the pit. (3.5 metres to be completely safe). A minimum of 600mm (0.06 bar) and a recommended 1.5 metre (0.15 bar) head pressure “higher” than the NPSHR pressure value required by the manufacturer is required to allow the pump to operate properly. Serious issues may happen and if a large pump has been sized incorrectly with regards to an incorrect NPSHR value which may result in a very expensive pump or installation repair. NPSH issues may be able to be repaired by installation changes or by changing the NPSHR. A very important and interesting thing to note is that if you have an NPSHA of say 10 Bar then the pump you are using will deliver exactly 10 bar more over the entire operational curve of a pump than its listed operational curve. Example: A pump with a max. pressure head of 8 bar (80 metres) will actually run at 18 bar if the NPSHA is 10 bar. i.e: 8 bar (pump curve) plus 10 bar NPSHA = 18 bar. This phenomenon is what manufacturers use when they design multistage pumps, (Pumps with more than one impeller). Each multi stacked impeller boosts the previous impeller to raise the pressure head. Some pumps can have up to 40 stages or more, in order to boost heads up to hundreds of metres.