Pump-Out CompressorThe most important component of any pump-out system is the pump-out compressor. The following options can be used for the pump-out function:
- Use the existing low pressure (booster) compressors and tie into the existing low pressure suction headers with the pump-out lines.
- Dedicate an existing compressor as the pump-out compressor.
- Install a new compressor as a pump-out compressor.
Facilities may wish to set up the pump-out system so that after maintenance operations, the compressor can be used to evacuate air to prevent it from accumulating in the refrigeration system. This air would be discharged to the atmosphere. If this type of system is used, a separate evacuation process should be used and strict administrative controls (such as locked valves, written procedures and training) should be implemented so that the atmospheric valve cannot be opened inadvertently, causing ammonia to be vented to the atmosphere during pump-out operations. Note that there are inherent risks associated with this operation and less-skilled maintenance personnel should not undertake it.
Pump-Out VesselA pump-out (suction) vessel should be located upstream of the pump-out compressor. This vessel should have the following features:
- The vessel should be large enough to hold any liquid ammonia and any lubrication oil that may collect during pump-out operations.
- The vessel should be equipped with a high-level alarm or a high-level compressor cutout to protect the pump-out compressor from liquid carry-over.
- Facilities may wish to equip the pump-out vessel with a heating system to vaporize liquid ammonia that may accumulate in the vessel. A heating system may be necessary if the pump-out vessel will be used to remove oil from the system. Heating options include internal hot gas coils, a steam jacket or electric heating bands.
- Facilities should consider installing a spring-loaded valve on the oil drain line from the vessel as an added safety measure.
- The pump-out vessel, by definition, is subject to repeated heating and cooling cycles. Thus, the pump-out vessel is at particularly high risk for failure due to corrosion.1 This vessel should be closely monitored for evidence of corrosion as part of the facility's mechanical integrity program.
Piping Manifold SystemThe piping manifold system may be either permanent (hard-piped) or temporary (hoses).
Hard-Piped Pump-Out Lines. One advantage of hard-piping lines to each piece of equipment in the system is that it eliminates the need for hoses and temporary tie-in points, thus eliminating potential leak points and the need to maintain a supply of hoses rated for ammonia service. In addition, using hard-piped lines increases the speed at which liquid ammonia can be removed from the system because facility operators do not have to locate and connect hoses. This is an important consideration if the pump-out system will be used in an emergency to remove liquid ammonia from equipment.
Hoses. Hoses may be used to connect equipment to the pump-out headers as needed. The advantages of the temporary hoses are reduced cost and decreased complexity. Because only a limited number of pump-out lines and valves are installed, the amount of labeling and training needed to operate the pump-out system also is minimized.
The following design considerations should be considered regardless of which option is chosen for the pump-out manifold:
- Thoroughly label the pump-out lines and valves and train operators on the use of the pump-out system.
- Carefully plan the design and operation of the pump-out system to eliminate the potential of trapping liquid ammonia between two closed valves in the pump-out manifold. Pressure gauges, vent lines and/or thermal relief valves may be needed to detect and prevent ammonia releases due to hydrostatic overpressure events.
- Design the pump-out system to minimize the potential for trapping liquid in low points in the pump-out manifold. Liquid traps or heating systems may be needed to prevent liquid accumulation in these low points.
- Ensure that the hoses used for temporary pump-out connections are rated and labeled for ammonia service and maintained per manufacturer's guidelines.
Cost of a Typical Pump-Out SystemIt is not possible to estimate the exact cost of a typical pump-out system due to the uniqueness associated with each ammonia refrigeration system. However, in preparation for this article, the author talked with representatives from several companies that have installed pump-out systems. The following estimates, while by no means providing a definitive answer, provide some guidance on the costs associated with pump-out systems:
- A pump-out system installed in a relatively small ammonia refrigeration system (ammonia charge of ~15,000 lb) cost ~$75,000.
- A somewhat larger ammonia refrigeration system (ammonia charge of ~125,000 lb) cost approximately $80,000 to $90,000 for a pump-out system.
- Another somewhat larger ammonia refrigeration system (ammonia charge of ~125,000 lb) has budgeted $100,000 for a complete pump-out system.
Part 3 of this column will look at a pump-out system is used at a frozen food plant. PCE