The introductions of OSHA's Process Safety Management standard and EPA's Risk Management Program have caused increased scrutiny of the ammonia refrigeration industry. Government regulators are conducting audits of ammonia refrigeration facilities and investigating ammonia releases. Notably, a significant number of these ammonia releases occur during plant tie-ins associated with maintenance work or system modifications when proper line opening procedures are not followed. Therefore, conditions may be right for companies to take a fresh look at a relatively old technology: pump-out systems. A properly designed and operated pump-out system may eliminate many of the releases that can occur during plant tie-ins.
Advantages of a Pump-Out SystemA dedicated pump-out system offers several advantages when performing plant tie-in procedures (see sidebar).
Enables Safer Operation of the Ammonia Refrigeration System. A dedicated pump-out system minimizes use of temporary hoses, temporary connections and temporary vessels. There are no hoses to drag around and no fittings to track down.
Pumping down a system to a very low pressure significantly decreases the amount of residual ammonia in the equipment, thus minimizing the amount that could be released during plant tie-in procedures. A dedicated pump-out system allows pressures to be lowered even below normal system operating suction pressure.
Also, facilities can develop simpler operating procedures with dedicated pump-out systems; this reduces the chance of a system operator making a mistake and releasing ammonia.
Provides Zero Emissions during Plant Tie-Ins. Pump-out systems should eliminate the need to vent residual ammonia into a bucket of water or to the atmosphere. When the pump-out system is properly used, the ammonia removed from a section prior to a plant tie-in would be contained elsewhere in the refrigeration system.
Without a dedicated pump-out system, extensive refrigeration system modifications may require the services of an outside vendor to store and/or dispose of large amounts of ammonia removed from the system. A pump-out system can simply transfer liquid ammonia from one vessel to another, eliminating the need to pump ammonia out of the system and subsequently dispose of it.
Minimizes Production Downtime and Costs. Plant tie-in procedures can be much quicker and simpler using a pump-out system, and the time saved can reduce labor costs and production downtime. This benefit would be especially useful for a facility that cannot schedule production shutdowns easily and that must conduct all service work or system modifications during evenings or weekends. Alternatively, the time saved frees facility personnel to perform other work that is important but less urgent.
A pump-out system enables one system operator to safely empty most equipment by turning a few valves. A second person would only be needed when opening the system to the atmosphere rather than during the entire plant tie-in procedure.
Improves Oil Collection Procedures. A pump-out system can be set up so that oil collects in a central vessel, thus eliminating the need for multiple oil pots located throughout the system. Eliminating these oil pots also eliminates the labor required to drain them, as well as the risk of ammonia releases associated with this task. Instead, oil would be drained from the central collection vessel on a less frequent basis.
Draining oil pots in production areas poses the risk of exposing employees to ammonia releases. Therefore, these pots typically are drained during off-times such as evenings and weekends. Having a central collection system allows oil to be drained more frequently from production-area components because the task is now performed in a closed system.
Enables Liquid Management. Pump-out systems enable system operators to turn a few valves to transfer relatively large amounts of liquid from one vessel to another without hoses. These transfers allow facilities to safely manage liquid in system vessels, for example, after system upsets that cause high levels in one or more vessels.
Improves Emergency Response Procedures. Mitigating an ammonia release is crucial to any ammonia emergency response procedure. (Smith, 2005) A pump-out system can be extremely beneficial in these situations, removing ammonia quickly and safely from equipment. For this reason, the emergency response procedures at some facilities emphasize the use of the pump-out system in an emergency because it enables them to remove relatively large quantities of ammonia from affected areas rapidly.
Improves Operator Satisfaction. One intangible but important aspect of a pump-out system is its effect on the system operator's job satisfaction. Numerous system operators who work at facilities with pump-out systems have indicated to the author that they would be very reluctant to move to a facility that did not have a pump-out system. Thus, a pump-out system may have hidden benefits in its ability to attract and retain system operators.
Part 2 of this column will look at typical features of a pump-out system and how one is used at a frozen food plant.
Sidebar: Plant Tie-In ProceduresAll equipment must be serviced or modified from time to time. IIAR Bulletin 107 describes the proper procedures that should be followed when making ammonia refrigeration plant tie-ins prior to equipment service or modifications:
- Reduce the system and equipment internal pressure to 0 psig before the tie-in process starts.
- Prepare the equipment required for the pump-out procedure. For example, verify that all personal protective equipment functions.
- Review the pump-out procedure and the tie-in procedure with personnel.
- Identify the valves, piping and connected equipment that will be shut down. Be sure to use appropriate lockout/tagout procedures and do not isolate any portion of the system piping or equipment that may trap liquid ammonia.
- Monitor temperatures and pressures during the pump-out process.
- Shut off the liquid feed to the portion of the system in which the tie-in is to be made.
- Continue operating any evaporators that facilitate refrigerant evaporation.
- Pump out until the pressure is below 0 psig, and (if appropriate), down to 15 to 20" HgVac. Let the pump-out compressor(s) continue to pump until they shut down on low pressure several times. A rise in pressure may indicate an incomplete pump-out.
- Let the system stand for several hours (overnight if possible). This allows any remaining liquid refrigerant to vaporize.
- Any signs of frost on uninsulated piping or valves may indicate the presence of liquid ammonia. If this condition persists after several pump-out attempts, check for leaking stop valves.
- After the pump-out process, the system pressure must be adjusted to near 0 psig before any cut-in is made. It is not advisable to have a deep vacuum when the system is cut into because air mixed with any residual oil and ammonia can form an explosive mixture. It is recommended that dry nitrogen be used to raise the pressure to just above 0 psig.