Interlock issues for liquid ammonia tank level
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The interlock system for the liquid ammonia storage tank provided by the design institute includes a mechanism that, when the liquid level in the tank is low, shuts off the discharge valve urgently and stops the discharge pump; this is integrated into the SIS system. Is this necessary? Is it because a low liquid level can also pose some safety risks? (PS: The discharge pump of the liquid ammonia storage tank is also the feed pump for the liquid ammonia vaporizer)
Follow the logic diagram provided by the design institute strictly, otherwise it will not pass the safety inspection
The interlock system for the liquid ammonia storage tank provided by the design institute includes a mechanism that, when the liquid level in the tank is low, shuts off the discharge valve urgently and stops the discharge pump; this is integrated into the SIS system. Is this necessary? Is it because a low liquid level can also pose some safety risks? Now, a interlock control for high and low liquid levels is designed, with shutdown at high levels, and a pump and pipeline protection mechanism at low levels to prevent vacuum formation.
The outlet of the tank is a liquid ammonia pump, which supplies ammonia to the ammonia vaporizer. If the liquid level in the ammonia tank is too low, causing the pump to fail to draw in any liquid, the gaseous ammonia in the vaporizer will flow back into the ammonia storage tank due to the pressure difference, resulting in overpressure in the storage tank.
Based on your manufacturing process, conduct a HAZOP analysis together with the process, equipment, and automation teams, and then make a decision
As mentioned above, HAZOP is used to analyze risk factors such as overpressure, as well as existing protective measures like safety valves. LOPA is then employed to determine whether these protective measures are sufficient, and whether a SIS is needed; additionally, it determines what the safety level of the SIS should be, whether it is SIL1, SIL2, or SIL3. Therefore, the design of interlocks is not decided arbitrarily; it is the result of safety analysis conducted by a team using specific methods.
The main concerns are pump evacuation, pump damage, and resulting leaks.