A New Automatic Valve That Regulates Flow
The valve shown in the accompanying illustration is a new British design of automatic valve to prevent excessive flows of water through a main. It will be observed that the pipe line is considerably contracted at the point of insertion of the valve, this contraction following Venturi Meter principles. The pressure drop therein is used to actuate an automatic trip gear. The valve is of the direct operated hydraulic type and the trip gear is made use of to actuate the cock controlling the pressure in the hydraulic cylinder. The use of the Venturi principle makes possible the accurate adjustment necessary to ensure closure at any desired rate of flow. When the valve is set to come into action at any predetermined rate of flow the piston in the main cylinder is at the top of its stroke. In one of the small cylinders seen in front of the main one is another piston and this is at the bottom of its stroke When the piston in the main cylinder is, as stated, at the top.
The space above the piston, in the small cylinder, is in direct communication through the cock previously mentioned with the smallest diameter of the Venturi tube while the space below the piston is in communication with the upstream side of the tube as seen in the front of the view. Thus there is a difference of pressure oh each side of the piston in the subsidiary cylinder.
Normally the weight hung at the end of the lever holds this piston at the bottom of its stroke. But when the difference of pressure on the top and bottom thereof due to the connection with the Venturi tube, reaches the point at which the gear is set to close, the weight falls, and in doing so reverses a four-way cock shown behind the lever and full pressure comes on top of the piston in the main operating cylinder. The ‘bottom is at the same time opened to exhaust. Hence the valve starts to close. This motion, however, must of course take place very slowly.
Commencing at a fairly rapid rate the tail rod of the operating piston which is inside the dome on the top of the cylinder has a pin which closes the valve on the pressure water admission pipe as it travels downwards. In consequence, the closing motion becomes slower and slower till final closure is reached.
To reset the valve, pressure is first equalized on both sides by means of a by-pass valve, the lever is pulled over to its original position, which is followed by the upward motion of the operating piston caused by the hydraulic pressure below it. The main valve is thus reopened and the piston in the subsidiary cylinder is forced to the bottom of its stroke, the weight on the end of the lever holding it in this position until the conditions of flow would call for the action already outlined.
By a mere alteration of the weight in accordance with a scale the valve can be set to operate at any rate of flow. In the case of a burst on the down stream side of the valve, in order to insure the closing thereof a Pitot tube is fitted to face the flow in the contracted area and is connected through the four-way cock (behind the lever) with the upper side of the operating cylinder. The dynamic force due to the velocity of the water induces a pitot head of sufficient magnitude to start the motion ofthe operating piston and cause it to close. As it closes the static pressure on the main begins to rise and the piston speed is accelerated.
The main valve is of the wedge door penstock type with gun metal jointing strips on both body and slide. The tail rod of the main piston has a pointer denoting on a scale on the dome the position of the valve; the cylinders are lined with gun metal, the connecting pipes are of copper, the cocks of gun metal and all holes drilled in the iron are copper lined.
MAJOR F. JOHNSTONE-TAYLOR.