As a result, sealing becomes more difficult as the temperature of the liquid rises.ДЄs water is heated, the vapor pressure at higher temperatures will significantly increase system pressures. This is compounded by the increased internal working pressure, which forces the seal faces together. The lower viscosity at higher temperatures reduces the film thickness between the mechanical seal faces, leading to more friction and less lubrication. The resulting reading on a pressure gauge will also be reduced because the column of liquid weighs less and the resulting pressure is lower. P t = total developed pressure or differential head in psigДЇor a centrifugal pump, the lower density of hot water reduces the horsepower required for a given flow and head. P wp = pump design working pressure in psig The pump casing, bolts and flanges should be designed to withstand 150 percent of the pumpвЂ™s working pressure: The centerline-supported casing significantly reduces the pump alignment problems of foot-mounted casing designs. The pump casing can expand freely without being fixed vertically to the pump base.

This design feature allows the pump to thermally expand from its center of rotation, keeping the rotating components on the same axis with the pump casing. The American Petroleum Institute (API) Standard 610 recommends that pumps in this temperature range and above should be designed with centerline-supported casing. At temperatures above 300 F, pumps and piping systems start to expand dramatically. Most centrifugal pumps can handle water temperatures up to 250 F. Other PropertiesДЄfter determining the NPSHA, the end user should consider the additional effects of hot water on pump performance. An NPSH test can confirm the performance and is strongly recommended when NPSHA is less than 4 feet greater than the pumpвЂ™s NPSHR. Many pump users do not realize that pump manufacturersвЂ™ NPSH details are not guaranteed unless an NPSH test is purchased for a specific operating condition. However, this impeller cannot make up for inadequate NPSHA. ДЄ 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. A new bronze pump inducer (left) is subjected to severe suction cavitation (right). When pumping water, cavitation typically sounds like rocks hitting the impeller (see Image 1). The tiny abrasions occur constantly as the liquid moves through the pump and, over time, will severely reduce the structural integrity of the metal. The collapsing bubbles produce tremendous localized pressures on the impellerвЂ™s surface and remove small amounts of metal. Bubbles will form as the liquid drops toward its vapor pressure.ДЄfter the bubbles enter the impeller eye, the pressure begins to increase and the bubbles collapse. As the liquid accelerates through the suction pipe, pressure drops. Hot water systems tend to have low NPSHA, which puts them at risk for cavitation at the suction eye of the impeller. H e = difference in height from water surface to the pump suction centerline in feet P v = vapor pressure for the fluid at the given temperature in psia P a = pressure at the water surface in psia 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. Worse, some energy is lost because of friction in the suction piping. 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. 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 waterвЂ™s surface is the same as the liquid vapor pressure. Centrifugal pumps require that all incoming streams be in liquid form, so the liquid must not reach its vapor pressure. The net positive suction head available (NPSHA) should be checked before pumping any liquid, especially hot liquids.

Thermal expansion of liquids