A dry valve consist of an air or electrically operated shut off valve in the inlet line of the pump; usually mounted to the pump inlet port to simplify plumbing. Included in the valve is an orifice which permits sufficient fluid to pass to lubricate the pump parts when the valve is in the off (dry) mode. In some situations, a bypass flow control mounted to the pump outlet is used. The purpose of this is to re-circulate a portion of the outlet flow back to the pump inlet to prevent cavitation during high speed operation of the pump when the dry valve is open (wet) mode. This also reduces the flow rate to the compacting cylinders and thus compacting speed is not excessive preventing slamming and hammering of the compacting machinery. The compacting machinery is normally designed to operate at low pump speed which limits the rate of operation.
If, while the dry valve is closed (dry mode), pressure builds up in the discharge side of the system (above 20 PSI), the pump will be damaged. The dry valve in the off (dry) mode permits only a flow sufficient to lubricate the pump; while adequate to lubricate there is insufficient fluid to fill the pump displacement and the pump exhibits severe cavitation. No damage from this occurs unless there is pressure to cause the cavities (oil droplets) to collapse. System back pressure exceeding 20 PSI will provide enough energy to collapse the cavities (oil droplets) and damage the pump.
To prevent the possibility of backpressure buildup, a small valve may be added ahead of the system control valves to bypass a small flow, 2-3 GPM. This may be a flow control, bleed valve or similar device. See DVBP Port End Cover for P5000/5100 or P7500/7600 for Permco's bleed valve used in dry valve applications.
Permco offers (2) sizes of air operated dry valves which can be mounted to the pump inlet with a 1-1/2" or 2" SAE split flange porting. This valve is similar to other manufacturers products in general configuration. Because lubrication is limited while the dry valve is being used, Permco has available special thrust plates for the 3000/3100, 2500, 3700, 5000/5100, and 7500/7600 series pumps. These plates are necessary to assure adequate pump life. The dry valve should be inspected and the orifice size checked; Permco requires and orifice of 0.104 inches. If the orifice size is less than this, it should be drilled out with a .104 drill. Be sure to deburr and clean all chips from the valve.
THRUST PLATE PART # / PUMP SERIES
ZZ-0947-DV - 3000/3100 Series
W-0947-DV - 3700 Series
X-0947-DVS - 2500 Series, 5000/5100 Series
AZ-0947-DVS - 7500/7600 Series
Variations of installations will be found. In some instances, only a dry valve is used. In some cases, the operator must limit engine speed while compacting and must remember to close (dry mode) the dry valve at high engine speeds. A tachometer output with associated electronics may be incorporated which will automatically perform this function. Other Installations may incorporate the flow control, the flow control and back pressure valve, or just a back pressure valve. All of these arrangements may be found.