Pumps and Hydraulics, Part 1 (of 2)

by N. Hawkins

d. The valves must be guided vertically by the valve-rod itself, the inside end of which must be kept in alignment by the usual form of tail-rod guide.

The vertical play at these parts should not exceed ¹⁄₈ of an inch.

e. The surface of valves must be machine faced and accurately fitted to a plane surface, and be steam tight when in contact with the seat of steam valve.

15. Steam Slide Valve Adjustment.

a. The lost motion at the valves and the setting of them must be determined by a solid hub on the rod, finished in the pump shop to standard dimensions, so that no adjustment is possible after the pump is once set up.

This hub may be forged on the rod and then lathe-finished to standard dimensions, or it may be made by turning down a rod of the size of the hub. It is believed that Tobin bronze can be safely forged after a little experience, if care is taken to maintain the proper heat.

It is recognized that the practice of making adjustable valve tappets located outside of the steam chest is a good thing in a large pump in constant service and operated by a skilled engineer, but for the infrequently used ordinary fire-pump, the utmost simplicity is desirable, and it is best not to tempt the ordinary man to readjust the valve gear.

The common form of lost motion adjustment consisting of nut and check nut at each end of the slide valve is not acceptable, as these nuts are liable to become loose and may be incorrectly reset by incompetent persons. A long rectangular nut in the center of the valve is also not acceptable, as it can be moved out of adjustment. A solid hub made as a part of the rod is required, as it absolutely avoids the possibility of the hub becoming loose, an accident possible with a separate hub attached to the rod.

The amount of lost motion should generally be such that admission takes place at about ⁵⁄₈ of the stroke of the piston, i.e., for 12-inch stroke R. H. valve will be about to open when L. H. piston has moved 7¹⁄₂ inches to 8 inches from the beginning of stroke. When piston is at end of stroke the ports should be full open.