Another critical time when there is danger of liquid hammer is upon termination of defrost. Near the end of the hot-gas defrost the pressure in the coil builds up to that of the pressure-regulating valve relieving the refrigerant condensate. If the return to refrigeration operation consists of no more than the sudden opening of the solenoid valve between the coil and suction line and reopening of the liquid supply, the high-pressure vapor in the coil rushes into the suction line. In a liquid-recirculation system, there will surely be some liquid in the liquid/vapor suction line. The high-velocity gas can drive some of this liquid to the end of the suction line, as shown in Fig. 13.5, or to the first elbow where an enormous impact is generated.
To prevent this situation11, a bleed valve can be installed to bypass the pressure-regulating valve on the refrigerant condensate line. When defrost is terminated, close the solenoid valve in the defrost gas line, and open the solenoid in the bleed line. This allows the pressure in the coil to drop slowly, and when the large valve between the coil and suction line opens there is little pressure to vent from the coil.
Here is a word of caution. The bleed valve is small and may become blocked with foreign matter. If this happens, the valve might open as required, but the pressure equalization does not occur. A periodic check is advisable to be sure that the coil pressure subsides at the desired rate.
A pattern that emerges from inspections of the incidents of rupture occurring in a coil or in the suction line at the initiation or termination, respectively, of defrost is that (1) the metal in the region of rupture is at a very low temperature, and (2) the rupture usually occurs at a weld. The metal at these low temperatures may be exhibiting brittleness, and high-quality welds are mandatory, especially in these sections of the system.