2000 Hook-up Book

Condensate Pumping

In nearly all steam-using plants, condensate must be pumped from the location where it is formed back to the boilerhouse, or in those cases where gravity drainage to the boilerhouse is practical, the condensate must be lifted into a boiler feed tank or deaerator. Even where deaera tors are at low level, they usually operate at a pressure a few psi above atmospheric and again, a pump is needed to lift condensate from atmospheric pressure to deaerator tank pressure. Electric Condensate Return Pumps When using electric pumps to lift the condensate, packaged units comprising a receiver tank (usu ally vented to atmosphere) and one or more motorized pumps are commonly used. It is impor tant with these units to make sure that the maximum condensate temperature specified by the manufacturer is not exceeded, and the pump has sufficient capacity to handle the load. Condensate temperature usually presents no problem with returns from low pressure heating sys tems. There, the condensate is often below 212°F as it passes through the traps, and a little fur ther subcooling in the gravity return lines and in the pump receiver itself means that there is little difficulty in meeting the max imum temperature limitation. See Fig. II-74 (page 119). On high pressure systems, the gravity return lines often con tain condensate at just above 212°F, together with some flash steam. The cooling effect of the piping is limited to condensing a little of the flash steam, with the remainder passing through the vent at the pump receiver. The water must remain in the receiver for an appreciable time if it is to cool sufficiently, or the pump dis charge may have to be throttled down to reduce the pump’s capac ity if cavitation is to be avoided. See Fig. II-75 (page 119).

The PUMP NPSH in any given application can readily be estimated from: NPSH = hsv = 144 (Pa - Pvp) + hs - hf W Where: Pa = Absolute pressure in receiver supplying pump, hs = Total suction head in feet. (Positive for a head above the pump or negative for a lift to the pump) in psi (that is at atmos pheric pressure in the case of a vented receiver).

SYSTEM DESIGN

head above the pump inlet is already fixed by the pump manu facturer, it is only necessary to ensure that the pump set has suf ficient capacity at the water temperature expected at the pump. Pump manufacturers usu ally have a set of capacity curves for the pump when handling water at different temperatures and these should be consulted. Where steam systems oper ate at higher pressures than those used in LP space heating systems, as in process work, con densate temperatures are often 212°F, or more where positive pressures exist in return lines. Electric pumps are then used only if their capacity is downrated by partial closure of a valve at the outlet; by using a receiver mount ed well above the pump to ensure sufficient NPSH; or by subcooling the condensate through a heat exchanger of some type. All these difficulties are avoided by the use of non-electric con densate pumps, such as the Pressure Powered Pump ™ . The Pressure Powered Pump ™ is essentially an alternating receiver which can be pressurized, using steam, air or other gas. The gas pressure displaces the conden sate (which can be at any temperature up to and including boiling point) through a check hf = Friction loss in suction piping. W = Density of water in pounds per cubic foot at the appropriate temperature. Pressure Powered Condensate Pump

Pvp= Absolute pressure of

condensate at the liquid temperature, in psi.

The absolute pressure at the inlet to the pump is usually the atmospheric pressure in the receiver, plus the static head from the water surface to the pump inlet, minus the friction loss through pipes, valves and fittings between the receiver and the pump. If this absolute pressure exceeds the vapor pressure of water at the temperature at which it enters the pump, then a Net Positive Suction Head exists. Providing this NPSH is above the value specified by the pump man ufacturer, the water does not begin to boil as it enters the pump suction, and cavitation is avoided. If the water entering the pump is at high temperature, its vapor pressure is increased and a greater hydrostatic head over the pump suction is needed to ensure that the necessary NPSH is obtained. If the water does begin to boil in the pump suction, the bubbles of steam are carried with the water to a high pressure zone in the pump. The bubbles then implode with hammer-like blows, eroding the pump and eventually destroying it. The phenomenon is called cavitation and is readily recognized by its typical rattle-like noise, which usually diminishes as a valve at the pump outlet is closed down. However, since in most cases pumps are supplied cou pled to receivers and the static

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