Jim Elsey helps you avoid common centrifugal pump mistakes
by Jim Elsey
June 18, 2018

The mechanical seal will need to be set after these other steps are completed. Most manufacturers do not install the coupling at the factory because it will just need to be removed for all of these aforementioned reasons.

Almost all pump problems occur on the suction side. There is a common and pervasive misunderstanding about how pumps work. Refer to above as a reference. Think of any pump system as three separate systems when trouble shooting issues in the field. The suction system, the pump itself and the system downstream of the pump. In my years of working on pumps and solving issues, 85 percent of pump issues occur on the suction side. When in doubt, it is a great place to start looking for the solution.

Always, always, always calculate the NPSHa. This is likely the most common and the most expensive mistake I witness in the field. People will erroneously think that because they have plenty of suction pressure or a flooded suction there is no reason to do these calculations. A few feet of friction or additional losses due to vapor pressure can wipe out that NPSH margin you thought you had. Insufficient NPSHa will result in cavitation in the pump impeller.

NPSHr has nothing to do with the system and is determined by the pump manufacturer. NPSHa has nothing to do with the pump and should be determined or calculated by the system owner or end user. I recently heard a phrase that the “pump becomes grumpy and grouchy” when there is an insufficient NPSH margin.

Understand cavitation. Cavitation is the formation of vapor bubbles in the fluid stream due to a drop below the vapor pressure of the fluid. The formation of the bubbles typically occurs just in front of the impeller eye since this is typically the lowest pressure in the system. The bubbles subsequently collapse downstream as they enter a region of higher pressure. The bubble collapse is what causes the damage to the pump impeller.

Cavitation causes damage. If the bubbles collapse in the middle of the fluid stream there is almost no damage. But when the bubbles collapse near or at the metal surface, they collapse asymmetrically and cause a small microjet. This collapse occurs on a nanoscale (1.0 x 10-9 or billionth). Local pressure forces involved can be higher than 10,000 pounds per square inch gauge (psig) (689 bar) or more, plus there is heat generated. This phenomenon can occur at frequencies up to 300 times per second and at speeds near the speed of sound. Note the speed of sound in air is approximately 768 miles per hour (mph) (1,236 kilometers per hour [k/h]) and varies somewhat with humidity levels. The speed of sound in water is 4.4 times faster at about 3,350 mph (5,391 k/h or 1,490 meters per second [m/s]). Because I started my career in the submarine world, I have to point out that the speed of sound is even faster in salt water.

Cavitation damage can occur at different locations on the impeller. “Classic” cavitation damage will occur approximately one-third of the distance downstream of the eye on the underside (low pressure side or the concave side) of the impeller vane. “Classic” because it is due to insufficient NPSHr. Cavitation damage may manifest at other locations on the impeller, but those instances usually are due to recirculation issues that are caused by operating the pump away from its design or BEP.

Cavitation is audible in the lower ranges. If you hear the cavitation noise (sounds like pumping gravel), it is likely cavitating. Just because you don’t hear the noise means nothing, since the majority of the noise range is outside the range of human hearing. Perhaps we should train dogs to help us detect cavitation? Cold water is typically the worst fluid for the consequential damage from cavitation.

Hydrocarbons have minimal effect from a damage aspect. Hydrocarbon correction factors exist and are based on empirical data. The rules for correction factors are covered in the Cameron Hydraulic Data book.

NPSHr is NPSH3. When a manufacturer states that the pump requires a certain amount of NPSHr at a given point, realize that the pump is already cavitating at that point with a 3 percent head drop because that is how NPSHr is measured. All the more reason to assure you have adequate margin.

Critical submergence is necessary to prevent vortexing. The vertical distance from the surface of the fluid to the pump inlet is the submergence level. The distance required to preclude air ingestion due to vortexing is the critical submergence level.

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