Improved Vacuum Relief Valve Developed by Department of Agriculture Officer

New Zealand Journal of Agriculture, Volume 63, Issue 5, 15 November 1941, Page 417

Improved Vacuum Relief Valve Developed by Department of Agriculture Officer

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W. G. WHITTLESTON,

Animal Research Station, Wallaceville,

MENTION was made in an article in the “Journal of Agriculture” for December, 1939, and January, 1941, of an investigation into the properties of relief valves and of work on the problem of relief valve design being carried out at Wallaceville. A full report on the results of our work on relief valves will appear in the “N.Z. .Journal of Science and Technology.” The present article describes an improved valve which has been developed at Wallaceville, ■ and which, after vigorous test, has proved itself to be superior to any valve yet exam-' ined. As a result of our studies on relief valves we suggest the following as a suitable standard of performance. When the valve is adjusted to blow off at a 15 inches vacuum with a flow / ' , • • of air (at room , temperature and pressure) through it equal to 0.10 cubic feet per second, the vacuum should not vary any more than one inch of( mercury between the limits

of 0.00 and 0.10 cubic feet per second air flow through the valve. A valve reaching this degree of sensitivity is desirable on a milking machine because, firstly, it will in no circumstances permit the vacuum to -rise to dangerous levels,.and, secondly, it will close completely immediately the vacuum falls by one inch of mercury, thus preventing unnecessary loss

of vacuum. However, while it is possible to obtain such sensitivity in a variety of ways, the valve must be robust, stable, and reliable. There are a few valves on the market which approach the above standard, but they are unstable. Further, the valve must be of reasonable cost and not- liable to become contaminated. As a result of earlier work on this subject, a diaphragm' type of valve was evolved which was 'very sensitive and fairly robust. It was, however, too costly to / produce. We were, therefore, compelled to turn to a different principle. It was evident from ' our studies on different types. that two. factors were, in the main, responsible for loss of sensitivity in a valve—(l) friction ,in the guide, and (2) pressure drop across the inlet ports. These have been reduced to negligible proportions in the new valve and stability has been maintained. When' reliability is wanted the weighted valve is ideal. Further, such valves may be con-

structed without guides, thus eliminating friction.

Overcoming Instability

But such valves are unstable. It has been noted that any restriction between the valve and the vacuum pipe tends to cause instability. A weighted valve opening directly into a l|-inch pipe may be quite stable at a 15 inches vacuum, but, with the increased air flow through it (the pump speed remaining constant) at a 10 inches vacuum, the valve may become hopelessly unstable. This problem has been solved by the use of an oil-filled dash-pot attached by a small spindle to the bottom of the weight. The small spindle passes easily through a guide, and has at its lower end a light plunger which moves in a shallow cylinder screwed on to the cross-piece carrying the guide. From its position, this guide, which, being light, can cause little friction, is always lubricated. Such an arrangement prevents the weight from swinging, and supplies sufficient damping to prevent “flutter” at the greatest air flows without loss of sensitivity. The dash-pot, once filled with ordinary rotary vacuum pump oil, will remain in action for very long periods, while the filling operation is extremely simple. The details of the valve are shown in the accompanying figure. It is evident that it is not convenient to alter the weight on such a valve when the blow-off vacuum is to be altered. This is overcome by the use of replaceable valve seats. The seat

may be altered by unscrewing the valve cover cap, removing the head and, after removing the holding screws, replacing the seat with one of smaller diameter for higher vacuum, and larger diameter for lower vacuum. Such an arrangement involves the use of a valve head capable of seating accurately in 'a number of different sized seats. Both hemispherical and conical valve heads have been found equally satisfactory in this respect. Small variations in vacuum could readily be. arranged for by, for instance, having a partly hollow weight into which lead shot could be poured, or by the use of rings which could be fitted without moving the main weight. However, once the valve is set it cannot readily be altered by unauthorised persons, and it is suggested that a fine gauze cover could be fitted to assist in preventing large particulars of dust, etc., from finding their way into the machine via the valve.

Outstanding Performance

Several such valves have been constructed at Wallaceville, and in . all cases the performance has been better than the suggested standard. One such valve is in use on the experimental milking machine, where it has given outstanding service.

It is evident that valves of this type

could be constructed in several ways. No attempt has been made to work out the almost unlimited arrangements which would be equally satisfactory. We have established the . principles of a valve which is capable of the performance required for optimum working conditions on a milking machine. The details of design may well be left to the manufacturers, limited only by certain considerations affecting the efficiency of the valve. The following points are important: — . (1) The valve seat should not be less than approximately 1 inch in diameter for use at a 10-inch vacuum. _ (2) There should be no obstruction between the valve and the machine. (3) There should be no. obstruction in the path of the incoming air.

Acknowledgments

The writer is indebted to the following for assistance in the work described:—Mr. H. G. Sawtell, who did most of the mechanical work involved in the investigation; Mr. S. Anderson, who operated the experimental milking machine on which tests were made and so assisted by giving practical criticism.