Reducing risks associated with hot gas defrosting & avoiding liquid hammer effect

At the Gustav Lorentzen 2014 (GL2014), Yitai Ma and Haoxiang Gu from the Key Laboratory of Efficient Utilization of Low and Medium Grade Energy in China presented the paper titled “Research on risk of hot gas defrosting and liquid hammer in ammonia refrigeration,” which discusses how to optimise ammonia freezers in order to ensure workers’ safety. In this study, the authors also provided insights from the “Guide Book of Specific Controlling Technology,” which provides ammonia refrigerant related businesses with guidelines on safe ammonia usage.

Hot gas in full ammonia unit can cause dangerous liquid hammer phenomenon

Ma and Gu’s GL 2014 paper investigated the principle of hot gas defrosting and how to prevent the liquid hammer phenomenon, referring to two ammonia leak accidents that occurred in China in 2013.

The authors argued that the reasons for these accidents were:

• Wrong operation of hot gas defrosting
• Defect caused by welding in the end of the low pressure pipe
• Leak of ammonia caused by the cap falling off

When the hot gas defrosting was under operation, the pipe was found fluctuating violently, ending with the rupture of the cap and causing the ammonia leak.

“Hot gas defrosting is an efficient and energy-saving solution,” say authors

The purpose of hot gas defrosting is to prevent that the condensation liquid flows into the compressor. The panelists contended that despite the accidents in China, among the numerous ways to defrost, hot gas defrosting (artificial or automatic) remains an efficient and energy saving solution if conducted appropriately.

In hot gas defrost, which has similar characteristics to a heat pump, the ammonia vapour at the high pressure side is sent into the evaporator and defrosted with the latent heat of condensation. When the process of defrosting ends, the system should start working again once the pressure and temperature are decreased. It is important that the pressure of the gas remain low in order to guarantee its manipulation, therefore isolated controlling valves are needed. Hot water or an electrical heater can be used during the defrosting process.

Liquid explosion and liquid hammer during hot gas defrosting

The hydraulic hammer effect, which appears in the 70 - 80°C temperature range, occurs when steam and condensation water combine. This can happen when water pumps start working or valves are opened so the water flows towards a lower pressure, a flow that reaches high speeds in a short time. When this high-speed water meets with an obstacle, such as a valve or a cap, it may cause fractures and significant leaks (hydraulic hammer effect).

A similar effect can occur in hot gas defrosting if condensation liquid is left in the discharging pipe by accident. As the occurrence of liquid hammer is quick and unstable and cannot be tested with general parameters, specific measures must be taken quickly after defrosting to avoid liquid from evaporating rapidly and forming liquid slug.

To prevent liquid hammer from occurring due to the sudden opening of valves, it is critical to control the throttling process. The authors cited Danfoss, which develops a kind of steam exhaust electric valve that relies on the pressure differential before and behind the valve and controls the opening in two steps.