In the routine operation of the plant there should be no discharge to atmosphere. In well-maintained plants, it is not possible for a visitor to detect from the smell that ammonia is the refrigerant. Slight smells of ammonia sometimes occur because of leakage at valve packings, pump seals, venting from freshly drained oil, or from an air purger. A strong trend has developed for the use of capped valves, particularly those needing only occasional adjustment, in preference to the wheel type in order to avoid leakage at the packing. The quality of pump seals has improved over the past decade with the introduction of oil pressurized seals and the use of hermetic pumps to dramatically reduce leakage at pump seals.
A significant quantity of ammonia may be released when a safety relief valve opens, but the purpose of this relief of pressure is to prevent a serious rupture of a pipe or vessel. Nevertheless, some special measures for treating releases from safety valves may be warranted, and providing these measures may be manageable, because the precise location where the release will occur is known.
Three approaches to handling discharges from such point sources as relief valves are as follows:
• Direct discharge to atmosphere
• Absorption in water and subsequent disposal of the ammonia-water solution
The direct discharge of ammonia to atmosphere may at first appear to be a cop-out, but many refrigeration professionals on occasion discharge ammonia to atmosphere with a clear conscience, provided the concentrations that come into contact with plants, animals, and humans are maintained low. This position is justified by several factors21 such as that the release of ammonia into the atmosphere does not contribute to ozone depletion or global warming, and that ammonia is a naturally occurring compound in air and is a necessary ingredient for many biological processes.
One realization that became apparent in a study of concentrations in the vicinity of ammonia vapor releases was that, to reduce the ground-level concentrations, two measures should be attempted: discharge the ammonia at as high an elevation as possible and discharge the ammonia upward at a high velocity. Figure 13.11 shows ground-level concentrations for a release of 0.15 kg/s (20 lb/min) with a wind velocity of 4.5 m/s (10 mph). Curves B and C may be compared to the base case represented by Curve A, which represents concentrations when the stack height is 6 m (20 ft) above ground level and the diameter of the stack is 50 mm (2 in). The conditions shown for Curve A result in a maximum ground-level concentration of 115 ppm at a downwind distance of 40 m (130 ft). If the diameter of the stack pipe were reduced to 25 mm (1 in), as Curve B shows, the exit velocity is roughly four times that of the base case.
The peak concentration drops to 36 ppm at a downwind distance of 60 m (200 ft). The second approach, shown in Curve C, demonstrates that elevating the stack height to 12 m (40 ft) reduces the maximum concentration to 21 ppm at a downwind distance of 75 m (250 ft).
While the skillful treatment of direct discharges of ammonia into the atmosphere may give satisfactory results in the view of the plant operator, the discharge of a significant amount of ammonia usually must be reported for information purposes to environmental authorities. In the United States, a release of 45 kg (100 lb) or more in a 24-hour period must be reported. Furthermore, regulatory agencies sometimes do not permit even the careful direct discharge to atmosphere. In this situation, absorption of ammonia in water or flaring of ammonia might be required.