NPSH. We know what it stands for—Net Positive Suction Head--but do we really know what it means? Or why it’s so important? More often than not, explanations of NPSH begin with confusing terminology and (even worse) confusing equations. That’s not a good starting point. The concept of NPSH is. Impeller severely damaged by cavitation
Very simply, NPSH refers to the amount of pressure at the suction of the pump. Often, you will see an A or R tacked on at the end. NPSHA or NPSHA refers to the amount of positive pressure available at the suction of the pump. NPSHR or NPSHR refers to the amount of pressure that is required at the suction of the pump to prevent cavitation.
Technically speaking, cavitation is what occurs when the pressure at the eye of the impeller drops below the vapor pressure of the fluid inside the pump. When this occurs, vapor pockets begin to form, traveling along the veins of the impeller. Once they reach the higher pressure inside the pump, these pockets will “implode” or collapse, causing extremely loud noise, and often causing severe damage to the impeller, shaft, and seal.
Practically speaking, cavitation is bad. It causes facilities in our area alone possibly hundreds of thousands of dollars a year. It can, if left unattended, literally destroy a pump. That is precisely why NPSH is such an important issue, albeit one that is rarely understood. It is critical in system design, and in pump selection.
What Determines Your Available NPSH?
The trick, of course, is making certain that you have enough NPSH available at all times, under all conditions, to meet the NPSH requirement of your pump. This is where the equation for NPSHA comes in:
NPSHA = 2.31(Pa – Pv) + (He-Hf)
Pa = Pressure in the receiver, psia
Pv- Vapor pressure of the liquid at its maximum temperature, psia
He- Elevation head ,ft
Hf = Friction losses in the suction piping at the required flow rate, ft.
Spgr = Specific Gravity
Fluctuations of temperature in a typical steam condensate system can result in cavitation if the system isn’t designed properly or the pump is incorrectly selected. Keep in mind that vapor pressure of a liquid decreases as its temperature falls. Positive head increases as the temperature falls. So, NPSHA will increase as the system temperature decreases. For example, at 212°F the NPSH available is 0 ft. But, by dropping the temperature to 200°F, the vapor pressure is reduced and the NPSH increases to 7.35.
Ways To Increase Your NPSHA
There are measures you can take to increase the NPSHA when a cavitation problem occurs:
(1) Lower the temperature of the condensate before it reaches the pump. This will decrease the vapor pressure. Condensate may be cooled by routing it through a cooler or flash tank. The downside of this is that it does waste some energy unless there is a recovery system in place to capture the lost heat from the steam.
(2) Pressurize the receiver. Just as system fill pressure can be increased in a closed system to increase NPSHA, a condensate receiver can also be pressurized. Pressurized condensate units with pumps designed to handle temperatures up to 250°F are available. Using a closed system such as this also avoids flash losses.
(3) Increase the static elevation by elevating the receiver that collects the condensate. However, if the condensate returns are low or below grade level, this is not an option.
(4) Put the condensate unit in a pit that is deep enough to increase the elevation head, thereby increasing NPSHA.
Decreasing The NPSHR
Sometimes increasing the NPSHA is simply not an option. Existing system designs may rule out all of the above options. In these cases, selecting a condensate pump with low NPSHR is the only way to avoid possible cavitation. For steam condensate systems with temperatures higher than 200°F, pumps are available with lower NPSH values than those of centrifugal pumps. Your JMP representative can assist you in selecting a pump that is designed for these particular circumstances.