OPUS

Specialty Item Pressure Reducing Valves

Image provided by Watts Water Technologies

The primary purpose of a pressure reducing valve (PRV) is to maintain a constant and reduced pressure in a piping system which is supplied from a higher pressure source. Establishing adequate building line pressure at the beginning of a system, and reducing pressure later as needed, is easier and more effective than attempting to start at low pressure and increase later.

In a steam system, PRVs should be installed either right side up or upside down to prevent condensate from collecting on the valve seat. This can prevent water hammer and deterioration of the valve seat.

Types of PRVs
There are two types of pressure reducing valves. One consists of a single pressure reducing valve, and the other type consists of two valves known as the two-stage, pressure reducing station.

Single PRV Types
Primarily, a single pressure reducing valve is of two types: the spring/lever action type and the external pilot line control type. A description of each is as follows:

Spring/Lever Action Type PRV
There are two types of spring/lever action type, pressure reducing valves. They are both operated by a simple diaphragm attached to the valve stem. One type uses a valve stem controlled by a spring loaded diaphragm. The other type has a weight and lever controlled valve stem. The spring/lever action type offsets the difference between the high pressure and reduced pressure sides. The reduced pressure side controls the amount that the valve opens. When the reduced pressure side is at the required high pressure, this high pressure acts on the diaphragm to drive the valve closed. When the reduced pressure side drops in pressure, the balance is lost and the valve opens. These actions occur within the valve body.

The pressure settings of spring/lever action valves are adjusted by a set screw on the spring, a shifting of the weight on the spring, or by a shifting of the weight on the lever. Spring/lever action type, pressure reducing valves should be used for normal conditions where severe changes in pressure are not experienced on either side of the valve.

External Pilot Line Type PRV
The external pilot line type, pressure reducing valve has a pilot, and is similar to the simple configuration of the spring/lever action type. The external pilot line type also has a diaphragm and spring connected to the valve stem. However, the external pilot line type differs from the spring/lever action type because it is controlled by a pilot line. The pilot line runs externally from the downward side of the reduced pressure line, to the pilot connection on the valve. The pilot is connected to the PRV diaphragm to maintain the PRV in a closed position. The spring offsets the pressure differential between the high  and reduced pressure sides. The reduced pressure side drives the valve closed when it is at the desired pressure. If the reduced pressure side drops, the balance between sides is lost, and the valve opens. This valve configuration should be used for severe conditions when the pressure changes greatly at a rapid rate.

Two Stage, Pressure Reducing Station
It is not always possible to reduce pressure from a high initial supply pressure to a required low pressure by using a single pressure reducing valve. A double installation of two single pressure reducing valves is necessary. It is known as a two stage, pressure reducing type station. The first valve reduces the initial high pressure to an intermediate pressure and the second valve reduces the intermediate pressure to the desired low pressure. The two-stage, pressure reducing station has the advantage of reducing velocity noises, reducing wire drawing of valve seats, and permitting more reasonable diaphragm sizes. In a steam system, PRVs should be installed either right side up or upside down, as required by the manufacturer, to prevent condensate from collecting on the valve seat (refer to manufacturer’s literature). Such installation can prevent water hammer and deterioration of the valve seat.



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