## Size Of Steam

To calculate the size of the steam end required for a specific application, the basic principle of steam pump operation should be considered. A simple schematic of a steam pump is shown in Figure 24.

In order for the pump to move, the force exerted on the steam piston must exceed the force on the liquid piston that is opposing it. The force on the steam piston is the product of the net steam pressure and the steam piston area. The net steam pressure is the steam inlet pressure minus the exhaust pressure. The force acting on the liquid piston is the product of the net liquid pressure and the liquid piston area. The net liquid pressure is the pump discharge pressure minus the suction pressure or plus the suction lift. This may be expressed algebraically as follows:

where Ps = net steam pressure, lb/in2 (bar) As = steam piston area, in2 (mm2) Pl = net liquid pressure, lb/in2 (bar) Al = liquid piston area, in2 (mm2)

Because the pistons are circular, the squares of their diameters are directly proportional to their areas, and the above formula can be rewritten as where ds = steam piston diameter, in (mm) dl = liquid piston diameter, in (mm)

In practice, it is necessary for the force on the steam piston to exceed the force opposing it on the liquid piston by a considerable amount. This is because of mechanical losses, which include stuffing box friction, friction between piston rings and cylinder of both liquid and steam ends, and the operation of the valve gear. These losses are determined by testing and are accounted for in size calculations by introduction of a mechanical efficiency figure. Mechanical efficiencies are expressed as a percentage, with 100% being a perfect balance of forces acting on the steam and liquid pistons as expressed in the previous formula. Because the efficiencies of two identical pumps may vary with stuffing box and piston ring packing tightness, the efficiencies published by manufacturers tend to be conservative.

With the mechanical efficiency factor inserted, the formula of forces becomes

STEAM STEAM

IN EXHAUST

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