Cavitation and NPSH Defined
NPSH is probably the most misunderstood aspects of pump hydraulics. It is very important to understand this concept, because NPSH problems are among the most common causes of pump failures, and are often mistakenly blamed for failures completely unrelated.
NPSH must be examined when using centrifugal pumps in order to predict the possibility of cavitation, a phenomenon which has both hydraulic and sometimes destructive mechanical effects on pumps.
Cavitation (See Figure above) is a phenomenon that occurs when vapor bubbles form and move along the vane of an impeller. (What causes the vapor bubbles to form in the first place is discussed shortly.) As these vapor bubbles move along the impeller vane, the pressure around the bubbles begins to increase. When a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble, the bubble collapses. The process is not an explode, but rather an implode. This collapsing bubble isn't alone, but is surrounded by hundreds of other bubbles collapsing at approximately the same point on each impeller vane.
The phenomenon of the formation and subsequent collapse of these vapor bubbles, known as cavitation, has several effects on a centrifugal pump. First, the collapsing bubbles make a distinctive noise, which has been described as a growling sound. This can be a nuisance in an extreme situation where a cavitating pump is operating where people are working. However, this physical symptom is usually the least area of concern with cavitation. Of far greater concern is the effect of cavitating on the hydraulic performance and the mechanical integrity of the pump.
The hydraulic effect of a cavitating pump is that the pump performance drops off of its expected performance curve, referred to as break away, producing a lower than expected head and flow. (See Figure below)
An even more serious effect of cavitation is the mechanical damage that can occur due to excessive vibration in the pump. This vibration is due to the uneven loading of the impeller as the mixture of vapor and liquid passes through it, and to the local shock wave that occurs as each bubble collapses.
The shock waves can physically damage the impeller, causing the removal of material from the surface of the impeller. The amount of material removed varies depending on the extent of the cavitation and the impeller material. If the impeller is made of a ferrous based material such as ductile iron, material is removed from the impeller due to a combination of corrosion of the ferrous material from the water being pumped and the erosive effect of the cavitation shock wave. If the impeller material is more corrosion resistant (ordinary bronze for instance), the damage which the cavitation causes is similar to a peening operation where a piece of relatively soft bronze is repeatedly struck with a small ball peen hammer. Materials like 316 stainless steel, with superior corrosion resistance and the ability to work harden under the peening action, have better ability to resist the metal loss associated with cavitation.
In any case, the removal of material , if it occurs at all, proceeds as long as the pump is cavitating. Pits can gradually be formed on the impeller vanes and, in the extreme, the removal of material can actually cause a hole to be eaten clear through an impeller vane. This removal of material from the impeller has the obvious effect of upsetting the dynamic balance of the rotating component. The result is similar to what happens if a car's tire isn't properly balanced, or if it loses one of the balance weights.
It is very important to remember that excessive vibration from cavitation can occur even without the material loss from the impeller described above. This is true because the vibration from cavitation is caused by the uneven loading of the impeller and the local shock wave as mentioned earlier, as well as by the removal of material.
The excessive vibration caused by cavitation often subsequently causes a failure of the pump's seal and/or bearings. This is the most likely failure mode of a cavitating pump, and the reason why NPSH and cavitation must be properly understood by the system designer and pump engineer.
Determining NPSHa using Pump Selection Software
The PUMP-FLO program along with all of the customized versions of our pump selection software provided by the pump manufacturers can calculate the Net Positive Suction Head available.
Net Positive Suction Head present at the pump suction is the head over and above the vapor pressure of the liquid being pumped. NPSHa is a function of the suction piping in the system and is independent of the type of pump in the system. It must be calculated by the engineer or pump user and supplied to the pump manufacturer as part of the pump specification.
NPSHa is calculated using the following formula:
NPSHa = PH – Hf - Hvp
P = The absolute pressure on the surface of the liquid in the suction vessel. (expressed in feet or meters of liquid)
H = The distance from the surface of the liquid in the supply vessel to the centerline of the pump impeller (expressed in feet or meters). The term is positive if the pump has a flooded suction, and negative if the pump has a suction lift.
Hf = The friction loss in the suction line including all piping, valves, fittings, and filters at the rated flow. This is expressed in feet or meters of liquid. Since the friction loss is a function of flow rate in the pipeline, insure that you use the design case flow rate when calculating the friction loss in the suction pipeline.
Hvp = Vapor pressure of the liquid at the pumping temperature (expressed in feet or meters of liquid.)
The pump selection program calculates the Net Positive Suction Head available is a two step process. First you must enter the fluid property of vapor pressure, then you can enter the system information.
To have the pump selection program calculate the NPSHa:
- Start the pump selection program (for PUMP-FLO select the Windows Start / Programs / FLO-SERIES / PUMP-FLO menu item.
- Choose the System / Fluid menu item to display the Fluid dialog box. In the fluid dialog box select the process fluid from the drop down list box and enter the fluid temperature. Then click OK.
- Choose the System / NPSHa menu item to display the NPSH Available dialog box.
- In the System Conditions for Calculation group box, enter the Tank Surface Pressure in gage units, along with the surface elevation of the liquid in the tank and the pump suction elevation.
- Specify the friction loss in the suction piping.
- Click on the Calculate NPSHa button. The calculated value is displayed in the NPSHa text edit box.
Once the NPSHa has been calculated, the pump selection software compares the selected pump's NPSHr to the NPSH available at the design flow rate to insure that the pump's requirements are met.
In step 2 above, if your process fluid is not listed in the drop down list box, you can manually enter the fluid properties. Additional fluid tables can be found on the Engineered Software Website. Here you will find a list of fluids that can be downloaded free, libraries of fluid data available on disk or individual fluid tables that can be purchased online.
The friction loss in the suction pipeline can be calculated using the PIPE-FLO available from the Engineered Software Website.
Volk, Michael. Pump Characteristics and Applications. Marcel Dekker, Inc. N.Y. 1996. p. 71-74