Pumping systems Cavitation

As the mechanical integrity of the pump system changes, the amplitude of vibration levels change. In some cases, in order to identify the source of vibration, pump speed may have to be varied, as these problems are frequency- or resonance-dependent. Pump impeller imbalance and cavitation are related to this category. Table 10-11 classifies different types of pump-related problems, their possible causes and corrective actions. 

This book is dedicated to pumps but we should mention that cavitation eould occur in other parts of the pumping system. Under the correct eireumstanees, valves and pipe elbows are also candidates to suffer damage from cavitation. 

Be aware that in some cases, you ll have to live with cavitation. Many pumps suffer cavitation for reasons of inadequate design, hor example, when operating only one pump in a parallel system, this pump tends to go into cavitation. Pumps that perform more than one dut through a valve manifold tend to suffer cavitation. Pumps that fill and drain tanks from the bottom tend to suffer cavitation. The last pump drawing on a suction header tends to cavitate. And of course vacuum pumps and pumps in a high suction lift arc candidates for cavitation. 

Provided that the system is initially clean and fitted with efficient air filters, metal edge-strainers of 0.005-inch spacing appear to be adequate, although clearances of vane pumps may be below 0.001-inch. It should be remembered that an excessive pressure drop, due to a clogged full-flow fine filter, could do more harm to pumps by cavitation than dirty oil. 

Example 4.1 It is desired to pump a wastewater to an elevation of 30 m above a sump. Friction losses and velocity at the discharge side of the pump system are estimated to be 20 m and 1.30 m/s, respectively. The operating drive is to be 1200 rpm. Suction friction loss is 1.03 m the diameter of the suction and discharge lines are 250 and 225 mm, respectively. The vertical distance from the sump pool level to the pump centerline is 2 m. (a) If the temperature is 20°C, has cavitation occurred (b) What are the inlet and outlet manometric heads (c) What are the inlet and outlet total dynamic heads From the values of the idh and odh, calculate TDH. 

NPSH. The net positive suction head is the most critical factor in a pumping system. A sufficient NPSH is essential, whether working with centrifugal, rotary, or reciprocating pumps. Marginal or inadequate NPSH will cause cavitation, which is the formation and rapid collapse of vapor bubbles in a fluid system. Collapsing bubbles place an extra load on pump parts and can remove a considerable amount of metal from impeller vanes. Cavitation often takes place before the symptoms become evident. Factors that indicate cavitation are increased noise, loss of discharge head, and reduced fluid flow. 

If a centrifugal pump is cavitating, several changes in the system design or operation may be necessary to increase the NPSHa above the NPSHr and stop the cavitation. One method for increasing the NPSHa is to increase the pressure at the suction of the pump. If a pump is taking suction from an... 

Prior to conducting a cavitation run, the water is circulated through the test loop a considerable length of time with the vacuum pump system in operation. In this way it is possible to minimize the air entrainment in the water which, as pointed out previously, appears to exert a considerable influence on the cavitation performance of the pump. For any given cavitation tests, the volume flow rate is held constant as the NPSH is incrementally decreased. As the vacuum on the system is increased it is sometimes necessary to reduce the incremental change in NPSH so that the so-called drop off point is obtained for any given pump test. 

For pumped systems where valve AP s decrease with increases in flow rate, it is desirable to check the flow regime at various flow rates. A valve may be noncavitating at flowsheet maximum but cavitating severely at 50—60 percent of flowsheet maximum. 

Now let s apply the hints and the formula to the following system figures and we can determine the NPSHa within one or two points. The important thing is that the NPSHa of the system is greater than the NPSHr of the pump. If the NPSHa should be inadequate, the pump is being starved, becomes unstable and cannot perform its duties. The inadequate NPSHa may lead to cavitation. 

As we ve said numerous times before in this chapter, the important thing is that the NPSHa of the system is above the NPSHr of the pump bv a sufficient amount to avoid stress and possible cavitation. If the NPSHa should be inadequate, there are ways to elevate it. Remember from the formula that five elements compose the NPSHa. Two of those elements, the Ha and the Hs, add energy to the fluid. And three elements, the Hvp, the Hf, and the Hi, subtract energ from the fluid. VVe must either increa.se the elements that add energ), or decrease the elements that subtract energy. To increase the NPSHa ... 

Inadequate NPSHa causes stress, vibration and maintenance on pumps because there is not enough energy in the iluid for the pump to perform its work. As you can see from the previous pages, the problems lie in system design and proper operating principles. When the NPSHa is below the NPSHr of the pump, the conditions are favorable for the pump to go into cavitation. Cavitation is tlie next chapter. 

It is important to darify that the pump does not eavitate, although people in the industry tend to say that the pump is eavitating. It is more correct to say that the pump is in eavitation or the pump is suffering eavitation. In realit - it is the system that cavitates the pump, beeause the system controls the pump. 

When cavitation occurs in a pump, its efficiency is reduced. It ean akso cause sudden surges in flow and pressure at the discharge nozzle. The calculation of the NPSITr (the pump s minimum required energy) and the NPSITa (the system s available energy), is based on an understanding of the lic]uid s absolute vapor pressure. 

This ty pe of cavitation is also called inadequate NPSHa cavitation. To prevent this ty pe of cavitation, the NPSHa in the system (the available energ) in the system), must be higher than the NPSHr of the pump (the pump s minimum energy requirement). 

As we ve di,seussed, system design is responsible for much of cavitation. Yet, the maintenanee mechanic is responsible for stopping and preventing eavitation. And certainly, it s the maintenance mechanic who has to deal with the results of cavitation, the constant changing of bearings, meehanical seals, damaged impellers, wear rings and other pump parts. 

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic becau.se they appear to suffer a lot of premature seal failure. Parallel pumps deserve double seals even if it s only a cold water system. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

The pump operates until the system temperature is normal and then the pump speeds up and cavitates. 

Do not confuse NPSH vdth suction head, as suction head refers to pressure above atmospheric [17]. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the border-line and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate wfith a full bite of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid. 

Cavitation of a centrifugal pump, or any pump, develops when there is insufficient NPSH for the liquid to flow into the inlet of the pump, allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or family of dimensional features related to the inlet and impeller eye area and entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating, and loss of suction flowt... 

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