Pumps and Hydraulics, Part 1 (of 2)

by N. Hawkins

It may be well to consider at some length the “Steam end” of the pump. This consists primarily of cylinders, together with their connections; these constitute the muscular system of the pump. The muscular or operating end is separate and distinct from the water end so far as construction is concerned but in operation the two are closely allied. It is therefore necessary as well as convenient to unite them in one equipment and thus enable the propelling mechanism to furnish a constant source of power.

So far as the elastic force of steam itself is concerned its history dates back to a period two hundred years B. C., when, as described by Hero, the force generated by steam was utilized for actuating certain devices constructed rather for curiosity than for any benefit which might be derived from their use. Very little advance was made in the construction of practical devices until the latter part of the eighteenth century when James Watt by his improvements placed the stationary engine on an operative basis and gave to the world what has proved to be the greatest invention of all time.

The first stationary steam engines were used for pumping water and were of the single acting type, in which the steam was admitted at one end of the cylinder, the opposite end being open to the atmosphere. The steam acting on the piston forced it to the limit of its stroke when the supply was cut off. The steam then condensed in the cylinder, forming a partial vacuum, and the force of the atmosphere upon the opposite side of the piston forced it back, causing it to complete its stroke before another supply of steam was admitted. This was a slow process, wasteful of steam and attended with many other inconveniences.

An improvement on this device was made in an engine built by Watt in 1774. This was a single acting engine but the condenser was separated from the cylinder. The valves were so arranged as to admit live steam into the upper end of the cylinder on the top of the piston and at the same time open the lower end of the cylinder to the condenser. The steam followed the piston in its downward stroke in which action it was aided by the partial vacuum formed in the condenser. At the completion of the downward stroke the valves were changed so as to close the ports to the steam supply and the condenser, and at the same time open a communication between the two ends of the cylinder equalizing the pressure above and below the piston. The weight of the pump rod on the beam or lever connection overbalanced the weight of the piston and caused it to complete the return stroke.

In 1782 the double acting steam engine was patented by Watt. This was a device in which the live steam acted on each side of the piston alternately, the opposite side of the cylinder being in communication with the condenser. The same patent covered the method of applying the principle of expansion of steam in the cylinder; a non-condensing engine was also described.

Note.—This invention was of great historical importance as it covered all the essential detail of modern practice in steam engine building and constituted the fundamental principle of all steam engines.

Improvements have been made in form and construction, necessitated by new adaptations which have been constantly developed. The requirements for higher speed, increased pressure which implies greater power, and the constant desire for greater economy in fuel have produced a variety of changes in detail but have not altered the fundamental idea.

When the steam after being utilized in the cylinder makes its exit directly to the open air, the engine is called single expansion for the reason that the action of the steam takes place in one cylinder during a single stroke, and what expansion takes place must be during one half of a revolution. When the steam from one cylinder instead of exhausting into the open air, is passed to a second cylinder, of larger area, and by expanding exerts a pressure on a second piston to aid in the completion of the revolution, the engine is called double expansion or compound, because the steam instead of completing its work in a single operation is afforded a double opportunity for expansion and an increased range of action. In the single cylinder the temperature of the walls is reduced in each revolution to correspond with that of the steam at the exhaust pressure.

This temperature must be restored by incoming steam at the beginning of a new stroke which means a reduction of power. With a double cylinder owing to the greater range of expansion, a higher temperature can be maintained in the first cylinder and a large amount of initial condensation is prevented. A still greater use of expansion may be obtained by the introduction of a condenser which allows the final exhaust to be carried below the atmospheric pressure to the extent of the vacuum formed. In stationary and marine practice triple and quadruple expansion engines are common. These are used in large units to give the greatest possible economy in fuel.