Problem No. 58: Should liquid ammonia be drained to the ammonia water tank or vent stack?
Liquid ammonia is one of the feeds to the urea plant. It typically comes from the ammonia plant at a pressure of about 20 bar. In the urea plant this liquid ammonia is further pumped up to synthesis pressure conditions, sometimes after passing through a buffer tank, a heater and/or a filter. According to the ammonia phase diagram, when liquid ammonia flashes to atmospheric pressure temperatures can drop to as low as -33°C under worst case conditions. In case one or more of the above mentioned items of equipment needs to be drained for maintenance, a decision needs to be made where to drain this liquid ammonia. In a urea plant the options are the ammonia water tank or the vent stack, but which is best option?