There are-few situations where discharges of ammonia are planned. An exception might be where a plant is being decommissioned or reduced in size. Even when ammonia must be removed from a section of the plant being repaired or retrofitted, every effort should be made to recover the ammonia. Discharges from relief valves are not normal operation, although the specific function of the relief valve is to discharge ammonia to prevent a much more serious failure of a vessel. The designation of large releases that are discussed in this section primarily refers to those resulting from a rupture of a pipe, fitting, or vessel. The occurrence of such ruptures is almost always the result of an external impact such as a fork-lift truck striking a pipe or its support. The three major forms of release are: (a) as a vapor, (b) as an aerosol, and (c) as a liquid spill on the floor of the facility or earth outdoors. Several forms of these releases may occur simultaneously in a given incident.
One form of release, illustrated in Fig. 13.15, is when ammonia vapor escapes from a vessel containing liquid. In order to supply the vapor, liquid in the vessel must evaporate, and in doing so drops the temperature of the remaining liquid. As the temperature of the liquid drops, so does the pressure in the vessel, so the rate of discharge progressively decreases. For the idealization of no heat transfer to the vessel during discharge, calculations show that 20 percent of the liquid in a vessel initially at a temperature of 27°C (80°F) will vaporize to reduce the temperature to -33° C (-28° F), at which point the pressure inside the vessel is atmospheric, so flow stops. Actually, there will always be heat transfer to the vessel, so more than 20 percent of the liquid vaporizes and a small flow rate will continue to boil off.
A dramatic demonstration of an effective response to a vapor release from a liquid tank has been shown by the school of Hydro Care in Landskrona, Sweden, which is dedicated to the safe handling of ammonia. The technique used was to throw a canvas cover over the entire vessel. The cover provides thermal insulation such that the temperature of the liquid drops rapidly and the flow rate of ammonia vapor diminishes perceptively. Workers, using masks, then remove the canopy to repair the leak. If a long time is necessary, the vessel may have to be recovered.
Probably the most treacherous release of ammonia is in aerosol form, shown schematically in Fig. 13.16. Tiny droplets of liquid are dispersed thoughout the vapor, and the vaporization of this liquid develops a cold, dense combination which usually clings to the ground rather than rising quickly and away from people and vegetation.
Another form of release is one that might occur if a liquid line is ruptured and liquid ammonia spills onto the floor of a refrigerated space or machine room or onto the earth if the release is outdoors. The recommendation is that water normally not be applied to the liquid, Fig. 13.17. Immediately following the spill, the rate of vaporization will be high, but as the remaining liquid and the floor or ground on which it has spread becomes cold, the rate of vaporization diminishes rapidly. Ventilation of the enclosed space may be adequate to disperse the ammonia vapor until all has been evaporated. Experiments are going on currently with foams that could cover the liquid to retard the rate of vaporization even more.
The final topic associated with large releases is the concept of positive pressure ventilation. A machine room should be equipped with permanently installed fans of sufficient ventilating capacity as specified in Section 13.8. If a release occurs in a refrigerated space, however, installed ventilating capability will normally not be available. Large-capacity portable fans should be present on site for such emergencies. It may at first seem that one placement of the fan at an opening of the space is as good as another. The attempt to use the fan to exhaust air and ammonia fumes from the space, as shown in Fig. 13.18a, is likely not to be effective, however, because the bypass air from the exterior to the fan intake reduces the withdrawal rate of the fan from the space. Positive pressure ventilation, however, as shown in Fig. 13.18b, takes advantage of the jet issuing from the outlet of the fan to seal the opening.