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Tuesday, September 29, 2009

Environmental Limitations

Every pumping application results in either positive or negative pressure as the throat of the pump stuffing box. A possitive pressure will force the liquid pumped through the packing to the atmosphere side of the pump. Higher pressure will result in greater leakage from the pump. This results in excessive tighening of the gland, which causes accelerated wear of the shaft or shaft sleeve and packing. For pressure at the stuffing box greater than 76 lb/in2 (5.1 bar), some mean of the throttling the pressure should be considered. A combination of hard and soft rings die formed to the exact stuffing box bore and sleeve dimension can be used. Harder rings at the inboard end of the box and at the lantern ring and gland break down the pressure and prevent the extrusion of the packing.

I the packing itself cannot be used to break down the pressure then a throttle bushing must be used. This is a typical arrangement for a vertical turbine where the stuffing box is the discharge pressure of the pump. Here the throttle bushing to used to bring the liquid to almost suction pressure, and most of the leakage through the bushing is bled back to the suction. When the suction pressure in the stuffing box. This prevents air leakage into the pump and excessive flow and wear at the bushing.

When a pump to fitted with a bypass line from the discharge, a valve can also be used to reduce the pressure at the stuffing box. This is another method for reducing pressure for the benefit of the installation.

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