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PRESSURE CONTROL

principle requirements could employ a standard and therefore low cost reducing valve. The correct selection and deployment in the relevant part of the air system will achieve the lowest cost most energy efficientsystem.The penalty for poor regulationcharacteristics is that the outlet pressure will vary but in the bulk of compressed air applications, inlet pressures are fairly constant so this poses few problems.The penalty for poor flow character-istics is pressure drop which directly reflectsin energy costs. Every regulator suffers from some amount of pressure drop so for goodsystem design this is the more important property to examine.An important cost saving can beachieved by employing a reducing valve in conjunction with double acting cylinders where a reduced pressure can often be used advantageously on the non-working returnstroke and cost savings as high as 30% canbe achieved. This can be very important on multi-cylinder installations.A point common to all pressureregulators is that in order to work constantly and repeatability within their design limits they will require a supply pressure at least 1 bar higher than the required outlet pressure. They will work with a lower differential but performance can be impaired.

TYPES OF REGULATOR

In order to use compressed air most effec-tively and efficiently it is necessary to reduce the pressure to precisely the level required for its application.All pneumatic equipment has anoptimum operating pressure. Using it at ahigher pressure causes excessive wear, with no significant increase in output, whilst wasting the compressed air itself and the cost expended in generating it. If the compressed air is stored at this higher pressure and only used at exactly the lower level required for the application the storagevessel or receiver need only be topped upfrom some intermediate figure to the full capacity, which is more efficient. In order to achieve this optimum usage the compressor usually operates between two pressure levels, that is the receiver normally has a pressure switch set to give compressor cut- off at the required storage pressure (usually the highest achievable for filtration efficiency)and a lower level usually about 10 - 20%lower. This figure can be adjusted for the optimum when the receiver size, system flow demand and compressor output rating are considered. The outcome of this arrangement is that the compressor is not continually running, using up excess energy, producing more heat which produces more water, which must be removed (extra cost) to supply a system requirement at too high a pressure which causes excessive wear (extra cost) for no increase in output.A pressure reducing valve cantherefore generate cost savings greater than its purchase price in a short time period. Also it is mandatory in such applications as blow guns and cooling nozzles where the use of compressed air at high pressure is potentially hazardous.Pressure reducing valves or regula-tors have two principle characteristics whichmust be considered in establishing which toselect, their ability to keep the outlet pressure constant irrespective of the inlet pressure (called the regulation characteristic) and irrespective of the outlet flow (flow character- istic). Standard designs are manufactured which achieve certain levels of the ideal performance on each characteristic. A simple application with loose demands of the two Although Norgren produces a vast array of regulators they can be broadly broken into 4 types:-General Purpose Pilot OperatedPrecisionSpecial Purpose Most general purpose regulators areof the diaphragm type (figure 22). In general these are more sensitive than piston type regulators which tend to have better flow capacity for a given size. In the majority of compressed air systems response, ratherthan compactness for a given pipe size is themajor requirement, hence diaphragm type regulators are most common.Regulators can be relieving or non-relieving. The relieving feature allows for the system (outlet) pressure to adjust from a higher level to a lower one without actuating downstream equipment (this is done by having a vent hole through the diaphragm toatmosphere). Generally this relief hole is verysmall in relation to the regulator main ports so no more than a bleed flow can be achieved and this should not be considered a full relief or even safety relief device. Non-relieving versions do not havea connection from the downstream system to atmosphere and so can only be adjusted from a higher desired or achieved outlet pressure to a lower one by cycling down- stream equipment or using a 3/2 shut-off valve to expel excess air from the down-stream system.
Figure 22.GENERAL PURPOSE REGULATOR 11