Nearly every commercial and industrial facility in the world pumps water. Pumping cold, lubricating water is normally straightforward and only requires standard materials seals and lower-pressure components. However, as water is heated, its properties drastically change, and the pump specifications should change as well.
At 68 F, water has a specific gravity of 0.9982, a viscosity of 1 centipoise (cP) and a vapor pressure of about 0.3 pounds per square inch absolute (psia). At 176 F, water has a specific gravity of 0.9716, a viscosity of 0.355 cP and a vapor pressure of nearly 7 psia. Hot water is lighter, thinner and less lubricating and vaporizes to a greater degree.
Weight and viscosity continue to decrease as the temperature increases. Above the sea level atmospheric boiling temperature of 212 F, the pressure to maintain water as a liquid increases dramatically (see Figure 1).
The Effect on Pumps
The net positive suction head available (NPSHa) should be checked before pumping any liquid, especially hot liquids. Centrifugal pumps require that all incoming streams be in liquid form, so the liquid must not reach its vapor pressure. In most water systems with temperatures higher than 212 F, the water in a vessel or boiler is in a state of equilibrium—the pressure at the water’s surface is the same as the liquid vapor pressure. At the pump suction, the only energy keeping the water liquid is the vertical head measured from the liquid surface to the centerline of the pump impeller. Worse, some energy is lost because of friction in the suction piping.
The available energy is best expressed as NPSHa, which is the difference between the pressure at the pump suction nozzle and the liquid vapor pressure.
Pa = pressure at the water surface in psia
Pv = vapor pressure for the fluid at the given temperature in psia
He = difference in height from water surface to the pump suction centerline in feet
Hf = suction line friction loss in feet
SG = specific gravity of the fluid
Hot water systems tend to have low NPSHa, which puts them at risk for cavitation at the suction eye of the impeller. As the liquid accelerates through the suction pipe, pressure drops. Bubbles will form as the liquid drops toward its vapor pressure.
After the bubbles enter the impeller eye, the pressure begins to increase and the bubbles collapse. The collapsing bubbles produce tremendous localized pressures on the impeller’s surface and remove small amounts of metal. The tiny abrasions occur constantly as the liquid moves through the pump and, over time, will severely reduce the structural integrity of the metal. When pumping water, cavitation typically sounds like rocks hitting the impeller (see Image 1).
A 316 stainless steel (SS) impeller has greater resistance to short periods of minor cavitation and should be considered when the difference between the NPSHa and NPSHR is less than 2 feet. However, this impeller cannot make up for inadequate NPSHa.
Many pump users do not realize that pump manufacturers’ NPSH details are not guaranteed unless an NPSH test is purchased for a specific operating condition. An NPSH test can confirm the performance and is strongly recommended when NPSHa is less than 4 feet greater than the pump’s NPSHR.
After determining the NPSHa, the end user should consider the additional effects of hot water on pump performance.