Piston valve is a Seatless & Glandless valve. Piston valve valve up to the size 6" do not have any stuffing box. The salient feature of piston valve is that of the piston operation. The piston slides between the two packing rings made from specially imported material and do not rotate, hence there is minimum ware. The only warring part is the packing ring and that to after an extensive use. The two packing rings are separated by a ported lantern bush. The bottom ring rests on a cast machined groove of the body and upper packing ring is being held and remained pressed by a bonnet on top. The bonnet rests on the top ring, when tightened evenly by the nut of the stud presses the lantern bush which in turn presses the bottom ring. This is done by keeping the valve in closed position. Thus the mechanism is aligned compressed and ready for operation. Here the bottom ring of the valve serves the purpose of the seat while the upper ring serves the purpose of a gland. The advantage of piston valve is when any leakage is observed from the hole of the bonnet, specially provided for this purpose, it can be stopped on the line. This is done by tightening the nut of the bonnet evenly, keeping the valve in closed position. The maximum thickness of the ring is utilized and when the valve cannot be further tightened to stop the leakage, It is only required to replace the sealing rings. This can be done on the line itself. The pressure drop in piston valve is considered as high as 18%.
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