A frequent cause of pipe rupture is due to trapping liquid between two valves. If this liquid is cold when trapped, when it warms it can develop enormous pressures that may be high enough to rupture the pipe or valves. A section of pipe with two shutoff valves at either end, as shown in Fig. 13.2a, is an obvious example of the case where a pressure-relief valve must be installed. The discharge from this relief valve could be vented directly to atmosphere, but it is adequate and preferable to discharge to a section of the system that is itself protected by a relief valve.
A more subtle situation where liquid can be trapped is between a check valve and a shutoff or solenoid valve, as in Fig. 13.2b. This arrangement is common in liquid-recirculation systems with multiple pumps, where each pump is equipped with a check valve on the discharge side. If solenoid valves regulating liquid flow to the coils all happen to be closed, liquid will be trapped and during the no-flow situation may warm up and expand.
Figure 13.2c shows another combination capable of trapping liquid. Here an outlet pressure regulator downstream from a check valve can act as a shutoff valve. If the outlet pressure is high, the regulator will remain closed and the check valve will not permit liquid to pass back through it. Some commercial regulators are designed, however, to relieve against abnormally high pressures.
As Figs 13.2a through 13.2c show, the section of pipe in which liquid refrigerant might be trapped is equipped with a pressure-relief valve. Some designers specify an inlet pressure regulator in preference to a standard relief valve. The reason for this choice is that a pressure-relief valve opens based on a pressure difference. Thus, a pressure-relief valve rated for 1724 kPa gauge (250 psig) normally discharges to atmospheric pressure of 0 kPa gauge (0 psig) and thus opens with a pressure difference of 1724 kPa (250 psig). If the discharge of a relief valve shown in Fig. 13.2 vents to a pressure vessel operating at 1,034 kPa gauge (150 psig), it will actually require a pressure of 1724+1034=2758 kPa (400 psi) to open the valve. Inlet pressure regulators, on the other hand, open only against a spring pressure and are independent of the pressure at the valve outlet.
The ball valve of Fig. 13.2d is becoming popular because of its low pressure drop when completely open. If a ball valve in a liquid line is closed, it will trap liquid within itself. Most ball valves are fabricated in such a way that the seal is capable of venting to the upstream side of the valve when it is closed. To be certain, some installers drill a small hole in the ball to vent upstream.