Ammonia Pump-Out Systems, Part 3
January 1, 2007
In Parts 1 and 2, I discussed the advantages and features of a pump-out system. Now, I’ll demonstrate how those features can be put to use via a case history at a representative frozen food plant located somewhere in the United States. For illustration purposes, assume the 30-year-old plant has approximately 1,500 employees and contains a large ammonia refrigeration system (~225,000 lb ammonia) to cool and freeze the food products and cool the production areas at the facility. In this plant, a complete pump-out system was installed as part of the initial installation of the ammonia refrigeration system. It consists of a bank of pump-out valves, an oil regenerator, and a 50 hp pump-out compressor (figure 1).
With a system such as this, oil and ammonia can be pumped out from different parts of the refrigeration system, as described below. However, typically only one part of the system is pumped out at a time.
- The plant pump-out line goes throughout the facility and mezzanine areas and is used to pump-out space coolers, freezers and processing equipment in the production area.
- The DC pump-out line goes to the distribution center space coolers and freezers.
- The vessel/liquid pump-out line goes to the bottom of all ammonia vessels and the chillers in the compressor room.
- The intermediate/low stage pump-out line goes to all of the intermediate and low stage compressor separators.
- The high stage pump-out line goes to all the high stage compressor separators.
Figure 2 shows the pump-out manifold (top), which consists entirely of 1" piping, and the tie-in points located at each pump-out location (bottom), which are 0.5" piping. All of the pump-out lines are labeled Pump-Out with an arrow to indicate the direction of flow.
The oil and ammonia from the pump-out manifold collect in an oil regenerator. Hot gas flowing through coils inside the regenerator is used to evaporate ammonia from the oil. Once the ammonia is removed, the oil is drained from an oil drain valve located at the bottom of the regenerator via a spring-loaded valve.
The vapors from the oil regenerator can be routed to the pump-out compressor or to a low-pressure suction accumulator. Approximately 95 percent of the time, the vapors are routed to the low-pressure suction accumulator. The low-pressure system typically is preferred because it is easier to manage the liquid ammonia in the system if liquid is pumped out to the low-pressure suction accumulator. The pump-out compressor only is used when air is being removed from the system following maintenance operations or when the facility is pumping out from two systems simultaneously and thus the low-pressure system is already in use. Under normal operations, the pump-out manifold is typically vented to the low-pressure suction accumulator system even when the manifold is not in use, for two reasons. First, this prevents trapping liquid in the pump-out manifold. Second, with this method, liquid ammonia can be quickly removed from a piece of equipment in an emergency by opening one or two valves. A facility can install a pressure regulator in the pump-out manifold that will automatically vent the pump-out header to the intermediate pressure system if pressure builds up in the pump-out manifold.
The pump-out compressor can be set up to discharge to either the high-pressure discharge header or to atmosphere. Typically, the discharge line is routed to the high-pressure discharge header. The discharge valve to atmosphere is only opened if air will be removed from a piece of equipment during initial commissioning or following maintenance operations.
The facility should maintain a small supply of ammonia-rated hoses that are used to pump-out portions of the system where there are no hard-piped pump-out connections. Typically, these hoses are 100’ in length and contain valves on either end of the hose.
In Part 4, I will conclude by describing pump-out system uses.