In our discussion of Ammonia refrigerated warehouses, we should begin with an understanding of what we are dealing with. ANSI/ASHRE-15 classifies Ammonia as a Group 2 Refrigerant – prominently a toxicity hazard. Your main concern should be life safety. If you have a large system (on the order of 10,000 lb Ammonia) OSHA and EPA regulations apply which typically require Process Safety Management and Risk Management Programs. This will require a Hazard Review, mechanical integrity, emergency response and operator training. This will help prepare you for consideration of the fire hazard as well
Toxic levels of Ammonia are much less than the concentrations needed for flammable or explosive mixtures in air. However, one can not overlook the possibility of a flammable mixture, and once the safety of personnel and the public and the environment are assured, the next consideration should be protection of assets – the building and equipment and the stock.
Ammonia is somewhat more difficult to ignite and burns more slowly than most hydrocarbons. “Burns more slowly” is another way of saying the explosion is not quite as violent. The fire and explosion hazards are sometimes not taken as seriously as they should be. However Ammonia has flammability between 16% and 25% concentration by volume in air and will auto ignite at about 1200°F – easily attainable by sparking or shorting of electrical equipment.
It is valuable to know that higher than normal levels of ammonia are present – a tip-off of a leak. Detectors for Ammonia have to be fairly sensitive – typically in the 50 to 100 ppm range due to the low toxicity levels – and in refrigerated spaces they have to be reliable in cold areas. The selection of the proper detector is important. In refrigerated spaces, they should be located within 10 ft. of potential leak sources such as at chillers and evaporators and set to alarm at about 1,000 ppm or less. In mechanical equipment rooms general area coverage would be more appropriate. The detectors should also be calibrated frequently – about once a month to assure continuing reliability.
If storage is foodstuffs contamination is much more likely than a fire or explosion. With early detection, a ventilation system can be used to minimize the extent of contamination. In equipment areas, ventilation can reduce the possibility of an Ammonia concentration in the explosive range. Since Ammonia is highly soluble in water, spray or fog can be effective in reducing the Ammonia concentration. Control of ignition sources can reduce the probability of an explosion. Keep in mind that ventilation and absorption are the two basic features that can be employed to reduce the likelihood of an Ammonia explosion. This applies to the warehouse space as well as refrigeration equipment areas.
The discussion of Ammonia refrigerated warehouses can be divided between the refrigerated space and the space for the equipment that drives the refrigeration process. The former is first and foremost a warehouse storage area – the same as any other, except it is cold. The good sense and physical protection for safeguarding the warehouse space still apply, although somewhat complicated by the extreme environment. One thing that is found only in the refrigerated warehouse is the extension of the cooling system in to the space. This is typically limited to piping, chillers and evaporators. The piping is usually designed and installed to ANSI B31.5 “Refrigeration Piping.” In the warehouse space the most likely points of ammonia leaks or release due to mechanical failure is at the chillers or evaporators. This is where Ammonia detectors should be located. The detectors should initiate pre planned emergency personnel actions or in some cases start safety features automatically – things such as starting ventilation and isolating the Ammonia supply. Also, make sure you have a sound contingency plan for safeguarding perishables in the event the refrigeration system has to be shut down for repair.
The mechanical spaces associated with refrigeration systems should be treated like any other area handling materials with explosion potential. Here as in the warehouse, Ammonia detection is valuable but should be installed area-wide since the possibility of leaks can occur at numerous points in the areas. A bit more sophistication is in order as well. The detectors should automatically shut the Ammonia system, start emergency ventilation and de energize electrical equipment.
Your best bet is to consolidate the equipment in a well detached lightweight structure that will not expose the balance of the facility to losses in the event of a mishap. Next best would be a room with one common wall to the facility. An inside room should be against as many exterior facility walls as possible. A room in the interior of the facility or basements should definitely be avoided.
As long as you have at least one exterior wall, you have a means to vent explosion pressures away from the rest of the facility. Walls exposing the facility should be vapor tight, one hour fire rated, and explosion resistant (about 100 psi). Exterior walls should have tethered explosion relief panels (relief set for about 20 psf). Use of windows for venting should be avoided since the shattered glass shrapnel would be a personnel hazard. Relief venting area can be calculated by standard equations, which can be found in FM Global Data Sheet 7-13 “Mechanical Refrigeration,” Section 2.2.
Do not locate boilers or other flame producing equipment in the refrigeration room. Keep oils and combustible storage out of the room. The Ammonia accumulators and receivers should be outdoors. Electrical equipment in the room should be rated for Class I, Division 2 areas in conjunction with at least natural ventilation at the roof. If mechanical ventilation is used, power it independently of the rest of the room so shutting room power does not affect the fans.
If you already have a refrigeration system and the electric design does not meet Class I, Division 2 there are alternatives to upgrading. Continuous mechanical ventilation at the roof of at least 1 cfm/sq. ft. room area can be used, with a supervised failure alarm. The supervised Ammonia detection system should start emergency ventilation at 25% of the Lower Explosive Limit (LEL) and shut down all electric equipment in the room at 50% LEL. The emergency ventilation should be in addition to the normal continuous ventilation and be designed for the larger of 20,000 cfm or 10 cfm/room sq. ft.
Keep in mind that this is a general overview. There are may more specific design and programmatic features that can be employed and tailored to your specific system design and layout. Risk Logic, Inc. has the capability to evaluate your specific site to develop and recommend appropriate designs. We are available to assist you with your fire protection needs and look forward to that opportunity.
Risk Logic, Inc. can be of assistance in ensuring that all the above safeguards are being followed in an effort to avoid a major property loss..