Pumping Prescriptions
by Lev Nelik, Ph.D., P.E.
July 20, 2016

One of the claimed advantages of centrifugal pumps over positive displacement pumps is their ability to operate over a wide range of flow rates. Because a centrifugal pump operates at the intersection of the pump curve and the system curve, varying the system curve can easily change the operating point of the pump.

The convenience and simplicity of flow control by discharge valve throttling comes at a price because the pump is forced to run either to the left or to the right of its best efficiency point (BEP).

However, the real danger of operating the pump too far off-peak comes from suction side considerations: too far to the right, and you risk running out of net positive suction head available (NPSHA ), causing cavitation problems; too far to the left, and flow recirculation at the impeller eye will become evident through noise, vibration and damage. As a result, the flow must be limited on both sides of the BEP.

Flow controlFigure 1. Flow control of the centrifugal pump by the discharge valve (Graphics courtesy of the author)
operating rangeFigure 2. A pump operating range has limits.
Open and Pressurized tanksFigure 3a. Open tank Figure 3b. Pressurized tank Figure 3c. Tank under vacuum

Vapor Pressure

Consider the first limitation: high flow. A centrifugal pump stops pumping when liquid turns to vapor. This happens when the pressure somewhere inside the pump drops below the liquid’s vapor pressure.

Vapor pressure depends on temperature and a few other factors. Water turns to vapor at 212 F at atmospheric pressure when boiling in an open pot. In a closed pot, the water would reach higher pressure before it boils. Conversely, if the pressure were reduced (vacuum), water would boil at a lower temperature. Water will boil at room temperature if the absolute pressure is less than about 0.4 pounds per square inch absolute (psia).

Water has low vapor pressure, but other substances exhibit a very high value. Freon, for example, has a vapor pressure of about 90 psia, and ethane’s vapor pressure is about 700 psi at 80 F.

cavitation DevelopmentFigure 4. Development of cavitation

Knowing vapor pressure without relating it to a corresponding temperature is meaningless. Sometimes it is good to have a tabulation, or a graph, showing the relationship between the vapor pressure and temperature—the higher the temperature, the higher the vapor pressure.

A centrifugal pump is a “pressure generator,” produced by the centrifugal force of its rotating impeller. The pressure gets higher as flow progresses from the suction to discharge. This is why vaporization of liquid is most likely to happen in the inlet (suction) region, where the pressure is lowest.

In practice, it is difficult to know exactly when vaporization (cavitation) happens, so it is wise to keep some margin of available suction pressure over vapor pressure. The pressure (expressed in feet of water) is called discharge head at the pump exit side, or suction head on the inlet side. The difference is a pump-developed head, also called total dynamic head (TDH). These heads must include both static and dynamic components.

For water and other low-viscosity liquids, suction losses are usually low and often disregarded. For more viscous substances such as oils, these losses can be substantial and cause the pressure in front of the pump to drop below the vapor pressure, leading to cavitation. The inlet velocity must be minimized because the losses depend on velocity squared, which is essentially suction head dynamic energy.

Longer pipe runs, bends, turns and other restrictions add to inlet losses, leading to further pressure reduction in front of a pump. To avoid cavitation, what matters is not the suction pressure but how much higher the suction pressure is than the vapor pressure of the liquid being pumped. This is where the concept of NPSH comes in handy. NPSHA is simply the difference between this total suction head and vapor pressure, expressed as head, in feet.

Pump manufacturers conduct tests by gradually lowering suction pressure and observing when things get out of hand—the pump head would eventually begin to drop. For a while, as pressure decreases (i.e. NPSHA gets smaller), nothing obvious happens. A pump, operating at a set flow, keeps pumping and develops constant head. At some point, when the suction pressure (and corresponding NPSHA) reaches a certain value, pump head begins to drop. This typically happens rather suddenly.