Understanding Suction Cavitation & Avoiding Ugly Pump Rental Repair BillsNPSH is the abbreviation for Net Positive Suction Head, the condition most often associated with suction cavitation. This can be a confusing concept for people who use pumps so the following is an explanation in plain English.

atmospheric pressure, often referred to as vapor pressure (14.7 psi, 34 feet head or 1 bar). If pressure is less than 14.7 psi, 34 feet or 1 bar, water will change to vapor or boil at a lower temperature.

If the resistance on the suction side of a pump is subtracted from the prevailing atmospheric pressure, the result is Net Positive Suction Head available or NPSHa. As stated earlier, each centrifugal pump requires a certain amount of positive pressure to keep water in a liquid state: Net Positive Suction Head required, or NPSHr. When water hits the eye of the impeller and NPSHa is greater than NPSHr, then water stays in a liquid state. If NPSHa is less than NPSHr, water will turn to vapor, or boil, and bubbles will form.

FACT: At sea level, water molecules change from liquid to vapor at 212° F or 100° C. The common term is that water “boils.” In fact, what actually happens is that the movement of the liquid water molecules, in response to the heat, is enough to push against the prevailing

FACT: Every centrifugal pump impeller requires a certain amount of positive pressure against the eye of the impeller to keep the water in a liquid state. During pumping, a vortex forms at the eye of the impeller when water is pumped. The atmospheric pressure at the eye of the vortex is lower than prevailing atmospheric pressure which helps keep water flowing into the eye. The resistance of moving the water to the eye of the impeller (gravity and/or friction) reduces the available atmospheric pressure when the water reaches the eye of the impeller.

VAPOR PRESSURE

NPSHr AND NPSHa

fig. 1 Unable to hold their shape due to the surrounding pressure, the bubbles implode with the final implosion causing a water jet of tremendous force to strike the leading edges of the impeller vane.

fig. 2 An example of an impeller that suffered suction cavitation. Note the pitting at the leading edges of the impeller vanes.

sound of the imploding vapor bubbles). This is suction cavitation. The damage caused includes pitting to the leading edge of the impeller and the front wear plate. The pump loses efficiency and productivity. An example of an impeller that suffered suction cavitation appears below. Note the pitting at the leading edges of the impeller vanes (fig. 2).

FACT: As the vapor (“bubbles”) leaves the eye, the vanes of the impeller strike the vapor and pressurize it, and now NPSHa is greater than NPSHr. The result is that the vapor (“bubbles”) are unable to hold their shape due to the surrounding pressure. The bubbles implode. The final implosion (fig. 1) causes a water jet of tremendous force to strike the leading edges of the impeller vane. The resulting sound is like the pump is “pumping rocks” (the

SUCTION CAVITATION

Deductions from NPSHa Atmospheric pressure at sea level is 14.7 psi which converts to 35 feet of positive head or 1 bar (metric). However, when water reaches the eye of the impeller in a suction lift centrifugal pump, as many as three reductions in pressure may have occurred:

1.Elevation above sea level. Atmospheric pressure is lower at higher elevations so instead of starting with 14.7 psi (or 35 feet head as at sea level), the prevailing pressure is lower which reduces the amount of NPSHa.

2.Temperature of product pumped. As mentioned, water at ambient temperatures will convert to vapor at 212° F (100°C). However, if the water is at an elevated temperature, the amount of heat required to overcome vapor pressure is reduced. Therefore, NPSHa is reduced.

3.Total Dynamic Suction Head or TDSH (gravity and friction resistance on the suction side of the pump). Prevailing pressure is reduced by the vortex created at the eye of the impeller. As mentioned, the amount of reduction is determined by the Total Dynamic Suction Head (gravity and friction resistance). This amount is subtracted from the prevailing atmospheric pressure resulting in NPSHa at the eye of impeller.

Solutions to suction cavitation

NPSHr curves

Suction cavitation occurs when NPSHa (available) is less than NPSHr (required). The solution is to reduce the amount of resistance on the suction side of the pump. Follow the logic stream:

» Reducing resistance (gravity and/or friction) on the suction side of the pump will increase the amount of NPSHa (available) which will now be more than the NPSHr (required) of the impeller which will keep water in a liquid state at the eye of the impeller which will eliminate suction cavitation, or more simply stated: NPSHa > NPSHr.

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Add 1.6 feet of NPSHa for all field applications

Every centrifugal pump has an NPSHr curve that quantifies the amount of positive pressure required by the impeller and therefore must be available at the eye of the impeller to keep water in a liquid state. Consult the NPSHr curve to understand the NPSH required for the centrifugal pump model that you’re using. In the example below, approximately 6-7 feet of positive pressure are required for this pump model.

34 feet- 29 feet of TDSH

5 feet of NPSHa which is less than the 6-7 feet of NPSHr of the impeller

34 feet- 15 feet of TDSH

19 feet of NPSHa which is greater than the 6-7 feet of NPSHr of the impeller

Example #2: A pump is operating with 29 feet of Total Dynamic Suction Head. Subtracting this from the available 34 feet leave just 5 feet of NPSHa.

Example #1: A pump is operating at sea level and pumping ambient temperature water. The Total Dynamic Suction Head (gravity and friction resistance) is 15 feet. Subtracting 15 from 34 leaves 19 feet of NPSHa. A pump operating at 1,000 gallons per minute (gpm) would require about 6 feet of positive head.

Result: No suction cavitation will occur (NPSHa is greater than NPSHr)

Result: Suction cavitation will occur (NPSHa is less than NPSHr)

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Charts exist for elevation and temperature deductions, and deductions would be made according to the conditions of the pump application. To further illustrate the concept of Total Dynamic Suction Head, consider the following examples.

Remedies

Summary

To avoid suction cavitation, reduce Total Dynamic Suction Head however possible. This could include:

1. Reducing the gravity resistance by lowering the pump closer to the product being pumped.

2. Reducing the friction resistance by increasing the diameter of the suction hose and thereby reducing velocity and resulting friction.

Suction cavitation can be managed and eliminated, and potential ugly pump rental repair bills can be avoided, by understanding NPSHr and calculating NPSHa in each application. For help or more information, contact your local Xylem Rental Solutions pump expert.