My last five articles covered the calculation of net positive suction head available (NPSHa). In the November 2018 issue, I summarized the NPSHa concept and said that firmly grasping the theory and completing the calculation can often be a tricky process. However, with an understanding of the basics and some practice, you can gain confidence and work through most applications. The five examples in the series were selected to cover almost every aspect you will encounter in the real world.
In most real-life NPSH issues, we are not the person that is conducting the initial NPSHa calculation for a system and initially selecting the pump. The more likely scenario is that we are stuck with an existing system problem, and the associated pump is cavitating toward a short and very expensive life ending. The guilty parties are gone or not talking.
Why Cavitation Is a Bad Thing
If there is insufficient NPSHa, the pump will cavitate. Cavitation causes pump damage and a reduction in performance. The pump damage manifests as mechanical seal and bearing damage. In the later stages, it can also destroy an impeller. All damage is expensive.
Most readers know that cavitation (classic) is the formation of vapor bubbles in the liquid. These bubbles form because the pressure on the liquid has dropped below the vapor pressure (NPSH required [NPSHr] exceeds NPSHa). This issue normally occurs near the eye of the impeller since this is the lowest pressure area in the suction system. The bubbles subsequently collapse when they reach an area of higher pressure at about one third to one half the distance along the underside of the impeller vane. The formation of the bubbles does little physical damage. Cavitation will affect the pump hydraulic performance. The collapse of the bubbles potentially creates serious damage to the impeller.
I will have an article explaining how cavitation causes damage in a future issue.
To preclude or mitigate cavitation, you must have more NPSHa than the pump requires.
NPSHr is also equal to NPSH3
How much NPSH margin you need to preclude cavitation varies with each application. The more margin, the better. Guidelines and rules of thumb are as plentiful and reliable as urban myths. I recommend you read American National Standards Institute/Hydraulic Institute (ANSI/HI) specification 9.6.1 to gain a better understanding. The liquid properties and the suction energy level are the differentiating factors.
How to Fix a Cavitating Pump
I am frequently asked this question, and I normally suggest a look at the NPSHa formula and its four components for the solution.
Using each of the four components from the formula, you can map potential solutions to solve the NPSHa problem.
NPSHa = ha – hvpa + hst – hf
ha = the absolute pressure. Absolute pressure as measured in feet of head of the liquid being pumped at the surface of the liquid. This will be barometric pressure if suction is from an open tank, or the absolute pressure existing in a closed tank such as a condenser hotwell or deaerator.
hvpa = the vapor pressure. The head in feet corresponding to the vapor pressure of the liquid at the temperature being pumped.
hst = the static head of the liquid over the pump centerline or impeller eye for a flooded suction in feet (positive value for flooded suction). Not all impeller centerlines correspond to the pump centerline.
hf = the total friction loss in feet of head for the suction side system.