Technical Safety BC released an investigation of a split thermostatic expansion valve. As a result of scheduled maintenance on a liquid ammonia line, ammonia was trapped within an isolated piping line on the downstream side on the thermal expansion valve under the diaphragm. During this maintenance the trapped ammonia boiled off within the control valve diaphragm increasing the pressure. This increased pressure caused the diaphragm to split apart on the weld.
The most likely the cause of the diaphragm failure is overpressure due to trapped liquid ammonia on the downstream side of the thermal expansion valve that applied an excessive force to the inside of the diaphragm.
