NPSH

This article is about Net Positive Suction Head. For the band commonly abbreviated to NPSH, see Natalie Portmans Shaved Head. For the fire hose coupling thread, see Glossary of firefighting equipment#N.

Hydraulic circuit

NPSH is an initialism for Net Positive Suction Head. In any cross-section of a generic hydraulic circuit, the NPSH parameter shows the difference between the actual pressure of a liquid in a pipeline and the liquid's vapor pressure at a given temperature.

NPSH is an important parameter to take into account when designing a circuit: whenever the liquid 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, as well as damage the system.

Centrifugal pumps are particularly vulnerable especially when pumping heated solution near the vapor pressure, 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. Careful design is required to pump high temperature liquids with a centrifugal pump when the liquid is near its boiling point.

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 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^[1]:

$NPSH = \frac{p_{0} - p_v}{\rho g} + \Delta z - h_L$

where h_L is the head loss between 0 and 1, p₀ is the pressure at the water surface, p_v is the vapour pressure (saturation pressure) for the fluid at the temperature T₁ at 1, Δz is the difference in height z₁ − z₀ (shown as H on the diagram) from the water surface to the location 1, and ρ is the fluid density, assumed constant, and g is gravitational acceleration.

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. Cavitation occurs at suction pressure levels below 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 simpler informal way to understand NPSH…^[2]

Fluid can be pushed down a pipe with a great deal of force. The only limit is the ability of the pipe to withstand the pressure. However, a liquid cannot be pulled up a pipe with much force because bubbles are created as the liquid evaporates into a gas. The greater the vacuum created, the larger the bubble, so no more liquid will flow into the pump. Rather than thinking in terms of the pump's ability to pull the fluid, the flow is limited by the ability of gravity and air pressure to push the fluid into the pump. The atmosphere pushes down on the fluid, and if the pump is below the tank, the weight of the fluid from gravity above the pump inlet also helps. Until the fluid reaches the pump, those are the only two forces providing the push. Friction loss and vapor pressure must also be considered. Friction loss limits the ability of gravity and air pressure to push the water toward the pump at high speed. Vapor pressure refers to the point at which bubbles form in the liquid. NPSH is a measure of how much spare pull you have before the bubbles form.

Some helpful information regarding atmospheric pressure; Atmospheric pressure is always naturally occurring and is always around us. At sea level, it equates to 101.325 kPa or approximately 14 Psi OR 10 metres of liquid pressure head. As we move higher up mountains, the air gets thinner and the atmospheric pressure reduces. This should be taken into account when designing pumping systems. The reason there is atmospheric pressure is simply due to earths gravity and its position in our solar system. It is a natural phenomenon and we are very lucky to have it as water wells and bores with shallow aquifers allow us to use this atmospheric pressure to our advantage.

We all know that pressure gauges exist on pumping systems and other machines to give us an indication of what performances are being achieved. We also use known pressures versus known performance in order to create a reference for system designs. An example would be an experienced pump technician or plumber knowing that a pressure of between 300 kpa and 500 kpa will provide adequate and comfortable pressure for household use.

A STANDARD PRESSURE GAUGE IS ACTUALLY READING INCORRECTLY. If it was reading correctly, it would show 100 kpa or 14 psi or 10 meters of liquid pressure head, before any system had been connected. Why? Because atmospheric pressure would make the gauge read this amount due to its presence. Manufacturers of gauges set them to read ZERO at sea level as a standard assuming designers will make allowances for the atmospheric pressure calculations themselves. Knowing this simple fact can make NPSH easier to understand.

If we now know that the gauge "should" really show 100 kpa or 10 meters of head pressure, then we can safely see that this gives us an instant advantage of 10 meters of head pressure at sea level. This means we can borrow against this and drop a maximum of 10 metres into or under the ground (or below sea level) reducing the gauge to zero and still get natural 'push' into our pump. Great for wells and bores with shallow aquifers within this depth! It is important to note that to get to exactly 10 meters may be difficult, but with the correct pipework and system design, it is possible to get very close.

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Once NPSH is fully understood, sizing and controlling pumps and pumping machines is a much simpler task.

NPSH is the liquid suction force at the intake of 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.

If we want to pump some amount of liquid, we have to ensure that this liquid can reach the center line of the suction point of the pump. NPSH represents the head (pressure and gravity head) of liquid in the suction line of the pump that will overcome the friction along the suction line.

NPSHR is the amount of liquid pressure required at 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 at intake port and the impeller is pushing the liquid out the discharge port, 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 low or near-vacuum pressure (negative NPSHA) to exist at the pump intake. This will cause the liquid to boil and cause cavitation, and the pump will not receive the liquid fast enough because it will be attempting to pump vapor. Cavitation will lower pump performance and damage pump internals.

At low temperatures the liquid can "hold together" (remain fluid) relatively easily, hence a lower NPSH requirement. However at higher temperatures, the higher vapor pressure starts the boiling process much quicker, hence a high NPSH requirement.

Water will boil at lower temperatures under lower pressures. Conversely its boiling point is higher at higher pressures.

Water boils at 100 degrees Celsius at sea level and an atmospheric pressure of 1 bar.

Vapor Pressure is the pressure of a gas in equilibrium with its liquid phase at a given temperature. If the vapor pressure at a given temperature is greater than the pressure of the atmosphere above the liquid, then the liquid will boil. (This is why water boils at a lower temperature high in the mountains).

At normal atmospheric pressure minus 5 psi (or -0.35 bar) water will boil at 89 degrees Celsius.

At normal atmospheric pressure minus 10 psi (or -0.7 bar) water will boil at 69 degrees Celsius.

At a positive pressure of +12 psi or +0.82 bar above atmospheric, water will boil at 118 degrees Celsius.

Liquid temperature greatly affects NPSH and must be taken into account when expensive installations are being designed.

A pump designed with a NPSHR suitable for cold water may cavitate when pumping hot water.

Some general NPSH Examples

(based on sea level).

Example 1: A tank with a liquid level 2 metres above the pump intake, plus the atmospheric pressure of 10 metres, minus a 2 metre friction loss into the 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), so the pump should lift from the well.

Now we will try the situation from example 2 above, but will pump 70 degrees Celsius (158F) water from a hot spring, creating negative NPSH.

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 600 mm 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. To work efficiently, this pump 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 600 mm minimum, totalling a total depth of 3 metres into the pit. (3.5 metres to be completely safe).

A minimum of 600 mm (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 damage may occur if a large pump has been sited incorrectly with an incorrect NPSHR value and this may result in a very expensive pump or installation repair.

NPSH problems may be able to be solved by changing the NPSHR or by re-siting the pump.

If an NPSHA is 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.

References

1. ^ Potter & Wiggert Mechanics of Fluids, 3rd Ed, p 612
2. ^ Simple NPSH & Cavitation Explanation http://www.marttechservices.com/pdf/ServiceBulletins/Service%20Bulletin_-_Net_Postive_Suction_Head_-_Cavitation.pdf