There are several applications, particularly on suction lines, where pressure drops in the range of 2 to 4 lb/in.2 (psi) cannot be tolerated. Therefore, only valves that are capable of opening at very low pressure drops are suitable for this type of use. Some valves incorporate the floating disc principle and are ideally suited for such special applications. They are capable of opening full at pressure drops of 0.1 psi and below.
Larger-capacity valves are suitable for suction service when supplied with internal parts that are mechanically connected. With this arrangement, the piston is connected to the stem and plunger assembly, and when the coil is energized, the plunger assists in supporting the piston. As a result, the pressure drop through the valve is reduced to a bare minimum. Valves with the direct-connected assembly are designated usually with a prefix “D” to the type number.
When these valves are required for suction service, they are supplied with a spring support under the piston. The spring counterbalances the major portion of the piston’s weight, and therefore the valve will open with far less pressure drop than normal. Valves with the counterbalance spring are identified by the prefix “S” added to the type number.