DUPLEX PUMPS like the one shown here have two steam and two water cylinders. Only one of each is shown, the other cylinders being directly behind these.

Duplex pumps have no "dead spots" because one or the other steam piston is always under force of steam (or compressed air). The two pistons are about 1/4 cycle out of synchronization with each other.

The steam enters the system through the top chamber on the left where the steam valve slides back and forth over the various intake and exhaust pipes. The black hole just under the steam valve is the exhaust vent for used steam.

The back pump controls the steam valve for the pump shown which controls, through the rocker arm in the middle, the steam valve for the piston in back.

On the water end of the pump, on the right here, the lower two valves are the intake valves and the upper valves are the discharge valves. A hydraulic accumulator is usually located just above the pump to smooth the flow and prevent water hammer.

As many as five sets of steam and water cylinders are linked together in various types of reciprocating steam pumps.

See also simplex pumps.

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