Design calculations for high and low liquid levels in storage tanks

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This paper introduces the common design methods for high and low liquid levels in tank design. By mastering these design methods, both high and low liquid levels can be connected to a DCS for monitoring. When the liquid level reaches either the high or low threshold, an alarm is triggered to prompt appropriate actions. 1. High liquid level design 1) For tanks with fixed roofs, the designed storage high liquid level should be calculated using the following formula: h = H1 - (h1+h2+h3). Where: h – the designed storage high liquid level of the tank, in m ； H1 - Can wall height, m ； h1 - Height from the lower edge of the foam generator to the top of the tank wall, in meters ； h2 - Converted height in meters corresponding to the maximum liquid inflow rate into the storage tank over 10–15 minutes ； h3 – Safety margin, in meters; it can be set at 0.3 m (this value includes the thickness of the foam mixture layer as well as the expansion height of the liquid). 2) For floating roof tanks and internal floating roof tanks, the designed maximum liquid level should be calculated using the following formula: h = h4 – (h2 + h5), where h4 is the maximum designed height of the floating roof (at the bottom surface of the floating roof), in meters ； h5 – Safety margin, in meters; it can be set at 0.3 m (including the height required for liquid expansion and to accommodate the floating disk). 3) The designed storage height of the pressure tank should be calculated using the following formula: h = H2 – h2, where H2 is the height at which the liquid volume reaches 90% of the tank’s calculated capacity, in meters. 2. Low liquid level design: 1) It shall meet the requirement that cavitation does not occur in the pump within 10–15 minutes from the onset of the low liquid level alarm. 2) For floating roof storage tanks or internal floating roof tanks, the designed minimum storage level should be 0.2 m higher than the height at which the floating roof rests on the bottom. 3) It should not be lower than the highest point of the internal heater.