Relief valves are designed to open automatic-ally when the pressure in the line or the unit becomes too high. They are commonly installed in steam, water, and oil lines, and on various units of machinery where pressure must not exceed a certain limit. Relief valves prevent the building-up of an excessive pressure which may be caused by such conditions as the sudden closing of an outlet valve or the failure of a reducing valve. There are many different types of relief valves, but most of them consist of a valve body contain-ing a disk or ball. Under normal pressure condi-tions, the compression of a coil spring holds the disk or ball on its seat. When the pressure in a valve exceeds the resistance of the spring, the disk or ball lifts off its seat and the pressure is reduced until it falls below the pressure for which the valve is set.

Relief valves should be set at the lifting pressure specified by the manufacturer. The ten-sion on the valve spring can be adjusted by means of an adjusting nut. The nut should be locked when the desired setting is attained. Since the set-ting for a specific valve will depend on the design of the valve and its use, the instructions in the applicable manufacturers technical manual should be followed when any relief valve is being set.

Continual lifting (popping) of a relief valve indicates either excessive pressure or malfunc-tioning of the valve. Either condition should be corrected immediately. A relief valve which is not operating properly should be removed, disassembled, cleaned, and inspected. The disk, or ball, and the seat should be checked for pitting and excessive wear. The spring should be carefully inspected for possible defects. When a relief valve is removed for any reason, the spring tension must be reset.

Relief valves must never be locked in the closed position, except in an emergency. When emergency measures are taken, the valves must be repaired or replaced as soon as possible after the emergency.

REDUCING VALVES

Reducing valves are used to provide a steady discharge pressure lower than the supply pressure. They are used on gland seal lines, galley steam lines, heating system lines, and on many other reduced-pressure lines. A reducing valve can be set for any desired discharge pressure, within the limits of the design of the valve. After the valve is set, the reduced pressure will be maintained regardless of changes in the supply pressure, as long as the supply pressure is at least as high as the desired delivery pressure.

Two types of reducing valves are in common use, the spring-loaded reducing valve and the pneumatic pressure-controlled reducing valve. Reducing valves of the pneumatic type are of two designsthose which regulate low temperature fluids such as water or oil, and those which regulate high temperature fluids such as steam or hot water.