A Catechism of the Steam Engine

by C. E. John Bourne

429. Q.--Will you describe Harvey and West's pump valve?

A.--This valve is a compromise between the equilibrium valve, of the kind employed for admitting the steam to and from the cylinder in single acting engines, and the common spindle valve formerly used for that purpose; and to comprehend its action, it is necessary that the action of the equilibrium valve, which has been already represented fig. 34, should first be understood. This valve consists substantially of a cylinder open at both ends, and capable of sliding upon a stationary piston fixed upon a rod the length of the cylinder, which proceeds from the centre of the orifice the valve is intended to close. It is clear, that when the cylinder is pressed down until its edge rests upon the bottom of the box containing it, the orifice of the pipe must be closed, as the steam can neither escape past the edge of the cylinder nor between the cylinder and the piston; and it is equally clear, that as the pressure upon the cylinder is equal all around it, and the whole of the downward pressure is maintained by the stationary piston, the cylinder can be raised or lowered without any further exertion of force than is necessary to overcome the friction of the piston and of the rod by which the cylinder is raised. Instead of the rubbing surface of a piston, however, a conical valve face between the cylinder and piston is employed, which is tight only when the cylinder is in its lowest position; and there is a similar face between, the edge of the cylinder and the bottom of the box in which it is placed. The moving part of the valve, too, instead of being a perfect cylinder, is bulged outward in the middle, so as to permit the steam to escape past the stationary piston when the cylindrical part of the valve is raised. It is clear, that if such a valve were applied to a pump, no pressure of water within the pump would suffice to open it, neither would any pressure of water above the valve cause it to shut with violence; and if an equilibrium valve, therefore, be used as a pump valve at all, it must be opened and shut by mechanical means. In Harvey and West's valves, however, the equilibrium principle is only partially adopted; the lower face is considerably larger in diameter than the upper face, and the difference constitutes an annulus of pressure, which will cause the valve to open or shut with the same force as a spindle valve of the area of the annulus. To deaden the shock still more effectually, the lower face of the valve is made to strike upon end wood driven into an annular recess in the pump bucket; and valves thus constructed work with very little noise or tremor; but it is found in practice, that the use of Harvey and West's valve, or any contrivance of a similar kind, adds materially to the load upon the pump, especially in low lifts where the addition of a load, to the valve makes a material addition to the total resistance which the engine has to overcome. Instead of end wood driven into a recess for the valve to strike upon, a mixture of tin and lead cast in a recess is now frequently used, and is found to be preferable to the wood.

430. Q.--Is there any other kind of pump valve which is free from the shocks incidental to the working of common valves?

A.--In some cases canvass valves are used for pumps, with the effect of materially mitigating the shock; but they require frequent renewal, and are of inferior eligibility in their action to the slide valve, which might in many cases be applied to pumps without inconvenience.

431. Q.--Could not a form of pump be devised capable of working without valves at all?

A..--It appears probable, that by working a common reciprocating pump at a high speed, a continuous flow of water might be maintained through the pipes in such a way as to render the existence of any valves superfluous after once the action was begun, the momentum of the moving water acting in fact as valves. The centrifugal pump, however, threatens to supersede pumps of every other kind; and if the centrifugal pump be employed there will be no necessity for pump valves at all. There is less loss of effect by the centrifugal pump than by the common pump.

432. Q.--What is the best form of the centrifugal pump?

A.--There are two forms in which the centrifugal pump may be applied to mines;--that in which the arms diverge from the bottom, like the letter V; and that in which revolving arms are set in a tight case near the bottom of the mine, and are turned by a shaft from the surface. Such pumps both draw and force; and either by arranging them in a succession of lifts in the shaft of the mine, or otherwise, the water may be drawn without inconvenience from any depth. The introduction of the centrifugal pump would obviously extinguish the single acting engine, as rotative engines working at a high speed would be the most appropriate form of engine where the centrifugal pump was employed.

433. Q.--This would not be a heavy deprivation?

A.--The single acting engine is a remnant of engineering barbarism which must now be superseded by more compendious contrivances. The Cornish engines, though rudely manufactured, are very expensive in production, as a large engine does but little work; whereas by employing a smaller engine, moving with a high speed, the dimensions may be so far diminished that the most refined machinery may be obtained at less than the present cost.