Those of you that visit SAFTENG often know that I love the FM Global Data Sheets! These data sheets are NOW FREE (and have been for a couple of years). They serve as a GREAT AID in helping facilities understand their hazards and risks associated with all kinds of industries. In fact, although my practice is not endorsed by Global FM, I point clients to these data sheets when an employer will not purchase costly consensus standards, as often times FM Global has a “data sheet” that will address the fundamental issue the consensus standard addresses. Case in point… this week the Chemical Safety Board published a Safety Alert: Anhydrous Ammonia Safety Bulletin – Five Key Lessons to Prevent Hydraulic Shock (CSB). As with most CSB publications this alert is getting shared widely among the ammonia refrigeration industry and this is a GOOD THING; but did you know that FM Global’s Data Sheet 12-61, Mechanical Refrigeration had already brought to light the scenario that the CSB Alert did. Here is what FM Global’s Data Sheet 12-61, Mechanical Refrigeration Case Study #6 says…
3.2.6 Mechanical Refrigeration, Case Study 6
Liquid ammonia entered into evaporator coils and tubing of a refrigeration system during the defrost cycle. Injection of hot gas for defrosting impacted on the liquid ammonia that had accumulated, causing hydraulic shock waves which increased the pressure on system components. The increased pressure caused leaks to occur in the evaporator with subsequent contamination of products in the freezer space. The above scenario is typical of several significant ammonia contamination incidents that have occurred in recent years, where control devices were not provided in the system. These incidents occurred in high volume freezers at food processing facilities. System designs in each case did not provide for the draining of liquids from evaporator tubes and coils during the hot gas defrost cycle.
In this Data Sheet, there are six (6) “case studies” involving mechanical refrigeration, of which four (4) involve ammonia as the refrigerant.
Case Study 1 – Ammonia leakage from a valve packing concurrent with a broken drain connection resulted in the contamination of finished food products and raw food ingredients at a food processing facility.
Case Study 2 – Impact damage, lack of an action plan, contribute to large contamination loss of a food processing facility.
Case Study 3 – Liquid ammonia refrigerant escaped from a pipe broken by impact from a forklift.
Case Study 4 – Operator error allowed excessive ice buildup in an ice chiller thermal storage unit subsequently causing tube leakage and buckling of the walls and support members of the holding tank. (NOTE: this incident does not involve Ammonia as the refrigerant, but the learnings can be applied to NH3 refrigeration)
Case Study 5 – Water entered the system mixing with the refrigerant in a 750 ton (683 metric tons) hermetically sealed chiller at a textile mill, damaging the motor, compressor, and other parts. (NOTE: this incident does not involve Ammonia as the refrigerant, but the learnings can be applied to NH3 refrigeration)
Case Study 6 – Liquid ammonia entered into evaporator coils and tubing of a refrigeration system during the defrost cycle.
TIP: Before starting a PHA, share these DOCUMENTED events with the PHA teams so that you can set the tone for “just because it has never happened here – does not mean it will never happen here”. A couple of these case studies apply to Facility Siting, Human Factors, etc.
All in all, these FREE Data Sheets are a MUST HAVE in any safety professionals tool chest. If you can not afford to buy NFPA, API, ASME, ANSI standards and you want to learn about a topic from a VERY RELIABLE source, you need to register and get access by CLICKING HERE.
So if you are a fan of FM Global Data sheets the CSB Alert would have been old news and we could have been addressing our risks way back in 1993!
I should also point out that ASHRAE began a large-scale project, 1569-TRP, “CFD STUDY OF HYDRAULIC SHOCK IN TWO-PHASE ANHYDROUS AMMONIA” . The project was scheduled to begin in April 2014 and be completed by April 2015. Here is the “Background” statement for the project:
Over the past 30 years, a number of ammonia refrigerant spills have occurred as a result of a catastrophic failure of the piping associated with an ammonia evaporator or other low-temperature piping. Years ago, the reasons for these catastrophic failures were not understood. Over the past 20 years, careful analysis of these failures has revealed that large pressure surges generated inside the ammonia refrigeration system caused these failures. It was observed that even though the pressure surges were strong enough to break welds, nearby relief valves did not operate to protect the piping. These same relief valves performed adequately when tested after the event. The only explanation for this is that the pressure surges that occurred were of extremely short duration. It is now known that condensation induced hydraulic shocks were the most likely cause of these failures.
Loyko (1989) reports several failures in the piping of ammonia refrigeration plants: a 1 ¼ inch endplate blown off a coil header, a 4-inch weld cap blown off a wet suction return line, and a 16-inch end plate blown off a suction line. IIAR (1992) published Bulletin 116 which presented recommended piping practices to prevent the occurrence of some hydraulic shock events that have occurred. From Bulletin 116, the term “soft hot gas defrost system” originated where the hot gas control valves, timers and appropriate sequencing for closing and opening control valves and fan motors were described. Shelton and Jacobi (1997) reviewed the literature concerning the phenomenon of condensation induced hydraulic shocks. Hydraulic shock events not resulting in failure are reported by Glennon and Cole (1998) where events similar to “water hammer” were investigated. Also, both Loyko (1992) and Glennon and Cole (1998) recommend system piping designs
and operating conditions to minimize the occurrence of hydraulic shocks based on their analysis of the system operating conditions that led to the pressure surges. The literature cited above collectively makes two conclusions: 1.) failures occur in evaporators and two phase suction piping, and 2.) failures occur upon the initiation or the termination of hot gas defrost.
Martin et al. (2007) in ASHRAE Research Project 970-RP Condensation-Induced Hydraulic Shock Laboratory Study built a test rig around a 20’0” long 6” sch 80 pipe section that was fully instrumented to capture the hydraulic shock event. Slug formation, slug velocity, and shock pressure were a measure for 292 individual test runs over a period of eight months. Mass flow rates were varied so that thresholds for the occurrence of shocks were established for 6” sch 80 pipes at different initial void fractions. Condensation-induced shocks in steam-condensate systems have been researched by the U.S. Nuclear Regulatory Commission in Jones et al. (1979) and Izenson et al. (1988). Incidents of ammonia releases due to hydraulic shock are still occurring. Under the best scenario, a hydraulic shock that causes mechanical failure results in downtime for a plant. Large shocks result in downtime, plus health effects to plant personnel, and neighbor communities. Hydraulic shock events can have both a monetary and a health and safety effect.
You can read more about the project HERE.
IIAR also has a very well written Bulletin on the subject that ALL ammonia refrigeration processes SHOULD be following as their Design RAGAGEP:
Another GREAT IIAR resource is a Paper that was present at the 1992 IIAR Conference.