A penstock is a single-faced valve consisting of an open frame and a door. This form of valve is normally located in tanks or channels as a means of controlling flow into a pipe. There are different types of penstock available, main are:
Penstocks for operating against pressure, i.e. pressure forcing the door onto the frame.
Penstocks for operating against off-seating pressure, i.e. pressure forcing the door away from the frame.
Penstocks designed to accommodate both seating and off-seating pressures.
Penstock frames may be circular or rectangular. In the latter case a preferred proportion of width to depth is 2:3 for vertical form, and 4 :3 for horizontal form. Frames and gates are commonly made of cast iron, although plastic materials (usually reinforced steel) are also used for gates operating in aluminium. stainless steel or epoxy-coated frames for corrosive applications. Frames may be for channel or wall-mounted application. Sealing faces in suitable materials are embedded into both the frame and door surfaces.
Handwheel operation of the gate is normal, using a rising stem supported by a suitable headstock or bracket. The advantage of a rising stem is that the screw thread at the bottom of the stem is not usually immersed and is readily accessible for lubrication. A non-rising stem eliminates the need for a headstock and merely rotates through a nut in the penstock door. The threaded portion at the bottom is then usually immersed in the product being handled.
Seating pressure can be accommodated by the use of side wedges only. Off-seating pressure requires the use of bottom wedges or top and bottom wedges. Penstocks subject to sealing pressure normally seal tighter than penstocks for off-sealing pressure duties. Both types can be made virtually drop-tight with correct installation, distortion of the gate frame at the time of installa tion being the determining factor as far as leakage is concerned.