NPSH - What it is and why understanding it is critical for pump selection and avoiding cavitation

Atmospheric Pressure

Until the early 17th century air was largely misunderstood. Evangelista Torricelli, an Italian scientist, was one of the first to discover that air, like water, has weight. He once said, “We live submerged at the bottom of an ocean of the element air.” The weight of this “ocean” of air exerts a force on the Earth’s surface called atmospheric pressure. Torricelli went on to develop the mercury barometer which now allowed for quantifiable measurement of this pressure. A mercury barometer (figure 1) uses a complete vacuum at the top of a glass tube to draw mercury up the tube. The weight of the column of mercury is equal to the weight of the air outside the tube (the atmospheric pressure). For this reason, atmospheric pressure is often measured in mmHg (millimeters of Mercury) or inHg (inches of Mercury), corresponding to the height of the mercury column. This atmospheric pressure controls the weather, enables you to breathe, and is the cornerstone of pump operation. 

Pump Operation

When asked how a pump operates, most people reply that it “sucks.” While not completely incorrect, it’s easy to see why so many pump operators still struggle with pump problems. Fluid flows from areas of high pressure to areas of low pressure. Pumps operate by creating low pressure at the inlet which allows the liquid to be pushed into the pump by atmospheric or head pressure (pressure due to the liquid’s surface being above the center line of the pump). Consider placing a pump at the top of the mercury barometer above: Even with a perfect vacuum at the pump inlet, atmospheric pressure limits how high the pump can lift the liquid. With liquids lighter than mercury, this lift height can increase, but there’s still a physical limit to pump operation based on pressure external to the pump. This limit is the key consideration for Net Positive Suction Head - NPSH. 

Net Positive Suction Head (NPSH)

NPSH can be defined as two parts:

NPSH Available (NPSHA): The absolute pressure at the suction port of the pump.

AND NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating.

NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. NPSHA MUST be greater than NPSHR for the pump system to operate without cavitation. Put another way, you must have more suction side pressure available than the pump requires. 

Vapor Pressure and Cavitation

To understand Cavitation, you must first understand vapor pressure. Vapor pressure is the pressure required to boil a liquid at a given temperature. Soft drink is a good example of a high vapor pressure liquid. Even at room temperature the carbon dioxide entrapped in the liquid is released. In a closed container, the soda is pressurised, keeping the vapor entrained. Water Soda Temperature affects vapor pressure as well. A chilled bottle of Coke has a lower vapor pressure than a warm bottle (as anyone who’s opened a warm bottle of cola has probably already figured out). Water, as another example, will not boil at room temperature since its vapor pressure is lower than the surrounding air pressure. But, raise the water’s temperature to 100°C and the vapors are released because at that increased temperature the vapor pressure is greater than the atmospheric pressure. Pump cavitation occurs when the pressure in the pump inlet drops below the vapor pressure of the liquid. Vapor bubbles form at the inlet of the pump and are moved to the discharge of the pump where they rapidly collapse, often taking small pieces of the pump with them. Cavitation is often characterized by:

? Loud noise often described as a grinding or “marbles” in the pump

? Loss of capacity (bubbles are now taking up space where liquid should be)

? Pitting damage to parts as material is removed by the collapsing bubbles

Noise is a nuisance and lower flows will slow your process, but pitting damage will ultimately decrease the life of the pump. The image below shows an impeller from a pump that has suffered cavitation (note the pitting and gouging along the blades, imagine the associated drop in performance). Often this is mistaken for corrosion, but unlike corrosion, the pitting is isolated within the pump (corrosion attacks the pump material throughout).

But back to NPSH - Calculating NPSHA

No engineer wants to be responsible for installing a noisy, slow, soon-to-be-damaged pump. It’s critical to get the NPSHR value from the pump manufacturer AND to ensure that your NPSHA pressure will be adequate to cover that requirement, but how do we calculate NPSHA?

The formula for calculating NPSHA: NPSHA = HA ± HZ - HF + HV - HVP

HA - The absolute pressure on the surface of the liquid in the supply tank

? Typically atmospheric pressure (vented supply tank), but can be different for closed tanks.

? Don’t forget that altitude affects atmospheric pressure (HA in Thredbo, NSW will be lower than in Sydney, NSW).

? Always positive (may be low, but even vacuum vessels are at a positive absolute pressure)

HZ - The vertical distance between the surface of the liquid in the supply tank and the centerline of the pump

? Can be positive when liquid level is above the centerline of the pump (called static head)

? Can be negative when liquid level is below the centerline of the pump (called suction lift)

? Always be sure to use the lowest liquid level allowed in the tank.

HF - Friction losses in the suction piping

? Piping and fittings act as a restriction, working against liquid as it flows towards the pump inlet.

HV - Velocity head at the pump suction port

? Often not included as it’s normally quite small.

HVP - Absolute vapor pressure of the liquid at the pumping temperature

? Must be subtracted in the end to make sure that the inlet pressure stays above the vapor pressure.

? Remember, as temperature goes up, so does the vapor pressure.

All too often, these calculations are faulted by a simple unit discrepancy. Most often, it’s easiest to work with metres of liquid. Adding the liquid name helps to be clear as well (metres of water, metres of petrol, metres of ammonia, etc.). Also, make sure to include the specific gravity of the liquid. As discussed above, a 10m column of mercury and a 10m column of water exert very different pressures at their base. 

Just the Beginning

Hopefully you now feel a little bit more knowledgeable regarding NPSH and especiall its importance when selecting a pump. A basic understanding can go a long way in identifying potential problems before they occur. Lifting liquids from underground tanks or rail cars, pulling thick liquids long distances or through hoses, handling high vapor pressure liquids such as LP gas or alcohol…these are just a few example cases of applications which pose the maximum risk of failure for the engineer who does not understand or account for NPSH.

There are plenty of NPSH calculators available on the web, and plenty of hydraulic engineers out there, try them and find one that works for you, but most importantly, remember, PUMPS DON'T SUCK! 

Arne Jensen - 2018

Acknowledgements:

Understand Net Positive Suction Head, Pumpschoolonline 2007

Total Head, N.P.S.H. & Other Calculation Examples Jacques Chaurette p. eng., www.lightmypump.com June 2003