Discharge head

Head. The tme meaning of the total developed pump head, H, is the amount of energy received by the unit of mass per unit of time (14). This concept is traceable to compressors and fans, where engineers operate with enthalpy, a close relation to the concept of total energy. However, because of the almost incompressible nature of Hquids, a simplification is possible to reduce enthalpy to a simpler form, a Bernoulli equation, as shown in equations 1—3, where g is the gravitational constant, SG is specific gravity, y is the density equivalent, is suction head, is discharge head, and H is the pump head, ie, the difference between H, and H. 

Total Dynamic Head The total dynamic head H of a pump is the total discharge head minus the total suction head h,. 

Before installation it is possible to estimate the total discharge head from the static discharge nead and the discharge friction head hj as follows ... 

Static Discharge Head The static discharge head is the vertical distance measured from the free surface of the hquid in the receiver to the pump centerhne, plus the absolute pressure at the hquid surface. Total static head hf, is the difference between discharge and suction static heads. 

The centrifugal pump is the type most widely used in the chemical industiy for transferring liquids of aU types—raw materials, materials in manufacture, and finished produc ts—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc. These pumps are available through avast range of sizes, in capacities from 0.5 mVh to 2 X 10 mVh (2 gal/min to 10 gaJ/min), and for discharge heads (pressures) from a few meters to approximately 48 MPa (7000 Ibf/iu"). The size and type best suited to a particular apphcation can be determined only by an engineering study of the problem. 

The discharge head is the vertical distance from the centerline of the pump (this would be the shaft on a horizontal pump) to the level in the discharge vessel. 

In this case, the pump must aspirate or lift the liquid up from the suction vessel into the pump and then push the liquid up into the discharge vessel. In this case the total head is the discharge head plus the suction lift. In all cases the total head is the work being performed by the pump. 

Install a small booster pump int(t the suction piping. The booster pump would have a reduced NPSHr for the system feeding it, and the discharge head of the booster pump would increase the Ha to the primary pump. 

Fiqiially, the pump will generate less flow as the discharge head or pressure requirements are increased. Obviously, both flow and head should be known before selecting a centrifugal pump. 

The pressure or discharge head is varied by adding and changing stages (impellers). 

The equation for determining the Ns is similar to equation for the Nss, except that it substitutes the NPSHr in the denominator with the pump s discharge head ... 

Where N = the speed of the pump/motor in revolutions per minute Q = the sc]uare root of the flow in gallons per minute at the Best Efficiency Point BKP. Eor double suction impellers, use I/2 BEP flow. H = the discharge head of the pump at the BHP. 

Series pumps theoretically offer twice the pressure at the same flow (Figure 8-29). The. second pump takes the discharge head of the first... 

Not enough discharge flow Excessive discharge Head Not enough NPSHa Worn or damaged impeller Inadequate foot valve size. Air aspiration or air pocket in the suction line. Plugged impeller or piping... 

Discharge head = 60 ft Discharge pressure head = 26 psig... 

This applies because the total head for a pump is total discharge head a( + ), minus ( —) the [suction head, a( + )J, or [suction lift, a( —)]. 

The discharge head of a pump is the head measured at the discharge nozzle (gauge or absolute), and is composed of the same basic factors previously summarized 1. static head 2. friction losses through pipe, fittings, contractions, expansions, entrances and exits 3. terminal system pressure. 

Note that the equivalent suction lift must be added to the total discharge head for the pump system to obtain the total system head. Keep in mind that the work the pump must accomplish is overcoming the suction losses (-f or —) plus the discharge losses, that is, + discharge loss (all) — (-f if head, or — iflift on suction losses, all). Thus, the suction lift becomes a ( — )( — ) or a ( + ) to obtain the total system head. Keep in mind that a vacuum condition on the suction of a pump never helps the pump, but in effect is a condition that the pump must work to overcome. 

Suction and discharge heads are determined the same as for centrifugal pumps. Total head and capacity are used in selecting the proper rotary pump from a manufacturer s data or curves. Since viscosity is quite important in the... 

Discharge heads on chilled water pumps and condensate pumps, if manufacturer is to furnish. 

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