VP of Engineering, Thompson Pump
Dale Conway is vice president of engineering for Thompson Pump. In this position, he oversees all engineering departments and technical aspects of the pump business, such as manufacturing engineering, quality assurance and research & development. Mr. Conway earned a bachelor’s degree in Mechanical Engineering from the University of Central Florida. He is a trained ISO-9001 internal auditor and successfully implemented an ISO-9001-compliant quality management system at Thompson Pump. He has attended and taught many pump- and pump cavitation-related seminars throughout the country and has authored several technical papers. Mr. Conway can be reached at 800 767-7310 or firstname.lastname@example.org.
Q: Pump cavitation is typically classified into two general categories – inertial and noninertial cavitation. What is the difference between inertial cavitation and noninertial cavitation?
A: Cavitation in general terms is used to describe the behavior of voids or bubbles in liquid. Any time a flowing liquid falls below its vapor pressure, vapor bubbles can form. If the flowing liquid is then subjected to pressures above the vapor pressure, these bubbles can implode causing damage, which is called cavitation. Pump cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation, and noninertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Noninertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field.
Q: What are the typical causes of pump cavitation? What are the typical end results of cavitation in pumping systems?
A: The cause of cavitation in pumps is usually due to insufficient NPSH (Net Positive Suction Head) energy on the suction side of the pump. NPSH is the energy required to push the liquid into the pump. This can be caused by:
- Having the pump at too high of a distance above the fluid source
- Having too small of a diameter of suction pipe
- Having too long of a distance of suction pipe
- Having too many fittings on the suction pipe
- Handling a liquid with a low vapor pressure
- Running the pump too fast
The end result of cavitation is the collapse of the vapor bubbles inside the pump, which can cause several problems. The first problem is a reduction in the pumping capacity of the pump. If the pump is unable to keep up with the incoming flow, then an overflow situation may occur. Cavitation also causes damage to the pump. The collapsing vapor bubbles can cause excessive vibration, which can cause rotating parts, such as the impeller, to contact non-rotating parts, such as the wear plates or wear rings, causing damage. Excessive vibration may also cause premature failure to mechanical seals and bearings. Cavitation can also damage the wetted components themselves from contact with the imploding vapor bubbles. In these instances, the energy that is released when the vapor bubbles implode causes pieces of the metal to break off and collide with other moving parts. The damage typically occurs to the impeller and can severely reduce the operating life of the pump.
Q: What are some common warning signs that may signal the end-user that they are experiencing pump cavitation?
A: If the pump is cavitating, it will typically vibrate, deliver less flow and make a noise that sounds like marbles going through the pump. The sound may start out at a low level and increase in intensity over time as material is chipped away and the surface of the parts becomes rougher. This is due to the additional energy required by the drag (friction) on the fluid from contacting the rough internal surfaces of the pump.
Cavitation is often confused with another phenomenon called air entrainment. Air entrainment occurs when air is allowed to enter the pump on the suction side and expands as it enters the impeller eye. This can often reduce the flow of the pump and cause vibration from disrupting the laminar flow stream through the pump. Air entrainment can cause similar damage to bearings and seals. Unlike cavitation, however, this problem can be easily remedied by simply identifying air leaks and fixing them.
An interesting point about cavitation and air entrainment is that some experienced pump users have actually injected small amounts of air into pumps that were cavitating to attempt to stop cavitation. By injecting air into a pump that is cavitating, the air bubbles cushion the impact of the imploding vapor bubbles and reduce the NPSHr of the pump, thus lessening the cavitation. This technique, however, should only be used by skilled pump technicians, as too much air can cause priming problems and, further, adding air typically reduces the pump’s capacity, which could cause an overflow condition.
Q: Why is cavitation so prevalent in and around the pumping system as compared to other segments of the process line? What other segments of the process line are particularly susceptible to cavitating conditions?
A: Cavitation frequently occurs in pumps because of the varying pressures in pumps. Centrifugal pumps operate from the principle of creating a low pressure at the eye (center) of the impeller, and atmospheric pressure forces the fluid to the eye to fill the void. As the fluid approaches the eye of the impeller, the pressure drops, and if the pressure drops below the vapor pressure of the particular liquid, it will boil and cause vapor bubbles to form. As the fluid leaves the impeller eye, it is now exposed to higher pressures (due to the rotation of the impeller inside the casing), which can rise above the vapor pressure of the liquid, causing the vapor bubbles to implode.
Cavitation can also occur in valves where the pressure drops suddenly and there is a chance for the fluid to drop below its vapor pressure. This can often occur in throttling type valves, such as gate valves or ball valves. If the pressure differential from one side of the valve to the other becomes too great, the fluid can vaporize across the valve and implode on the downstream side of the valve. The way to avoid cavitation in valves is to size them properly for the proper velocities. Valves are typically sized for velocities less than 15 feet per second to avoid the possibility of cavitation.
Q: What are some common best practices end-users can employ to prevent pump cavitation?
The collapse of vapor bubbles inside a pump can cause severe cavitation damage on the impeller, resulting in negative process conditions such as vibration, decreased flow, and noise.
A: Always calculate the NPSHa (Net Positive Suction Head available) from the system, and compare it with the NPSHr (Net Positive Suction Head required) by the pump. The NPSHa should always be one to two feet above the NPSHr of the pump to prevent cavitation.
The NPSHr is a function of the pump design and cannot be changed. The NPSHa is a function of the system parameters and can be changed. Included in the NPSHa is the atmospheric pressure, vapor pressure of the liquid being pumped, static height from the water level to the pump, and friction losses. The atmospheric pressure is related to the altitude. At higher altitudes, the atmospheric pressure is less and subsequently there is not as much energy available to push the liquid into the pump. The vapor pressure varies by the type of liquid and the temperature of the liquid. If the liquid is allowed to cool before the pump, it can often be pumped easier. Regarding the static height from the fluid level to the pump, it is often possible to move the pump closer to the fluid to increase the NPSHa. To reduce the friction losses, larger diameter pipes can often be employed to increase the NPSHa and thus prevent pump cavitation.
If it is not possible to increase the NPSHa as described above, then the pump user should search for a larger pump or pump that runs at a lower speed with lower NPSHr.
Q: From a technology perspective, what systems can end-users employ to help them more effectively diagnose and mitigate pump cavitation?
A: The most effective solution is to listen to the pump and to evaluate the flow. Flow can best be determined using flowmeters, and there are several types commercially available, depending on the type of fluid being moved. Listening to the pump can be accomplished by the naked ear by trained personnel or by using suitable noise level meters. More sophisticated vibration measuring equipment can also be employed to detect cavitation. These portable devices can connect to the pump bearing housings to detect movement (displacement) in the pumping system.
Q: In your experience, what are some of the most troublesome application occurrences of pump cavitation? How were these cavitation issues resolved?
A: Among the most common applications that are susceptible to cavitation are applications that have high-suction lifts with little-to-no discharge heads, as is the case with bypassing sewage from manholes. In these applications, the duty point does not fall on the typical performance curve because there is insufficient discharge pressure. In these applications, it is called operating “too far to the right of the curve.” The way to fix this is to put artificial pressure on the discharge of the pump. This can be accomplished by using smaller-diameter discharge hose or placing a throttling valve in the discharge line.
Other examples are pumping heated liquids that are already close to their boiling points. In these instances, the fluid cannot be lifted and must be provided a suitable distance above the height of the pump.