Air supply volume of the air seal device

Addition: Settings for gas seals – HG/T 20570.16—95 1 Function and composition of gas seal devices 1.0.1 Function 1.0.1.1 To prevent the material stored in tanks from being contaminated or deteriorated due to contact with external gases (air), as well as to avoid chemical and/or biological reactions with such external gases, it is often necessary to install a gas seal system. An air seal is used to maintain a certain pressure (positive pressure) inside the storage tank, preventing the contents of the tank from coming into contact with external air. 1.0.1.2 When the material stored in the tank is pumped out and/or the gas inside the tank condenses or contracts due to a drop in external temperature, this system automatically supplies additional gas to maintain an airtight seal, preventing external air from entering ； When material is fed into the tank and/or the pressure inside the tank rises due to increased external temperatures, causing the liquid in the tank to vaporize, the gas inside the tank can be automatically released into the atmosphere through a pressure relief valve. 1.0.1.3 Common gases used for gas sealing include nitrogen, fuel gas, natural gas, etc. The choice of gas for sealing should be determined based on the properties of the material being stored, the availability of such gas, and its economic viability. 1.0.2 Composition of the air seal device 1.0.2.1 The air seal device is shown in Figure 1.0.2–1. It consists of four components: a gas seal valve (also known as the main valve), a signal valve (also known as the control valve), a pressure reducing valve, and a needle valve. When the pressure inside the storage tank is below the set value, signal valve 2 opens, and gas seal valve 1 also opens accordingly. The high-pressure gas seal gas enters the storage tank after being depressurized by the gas seal valve 1, causing the pressure inside the tank to gradually return to the set value. Once this set value is reached, the signal valve 2 closes, and the gas seal valve 1 also closes accordingly. If the pressure inside the tank exceeds the set value, the suction valve of the pressure relief valve equipped with a flame arrester on the tank opens, allowing the gas inside the tank to be released and thus reducing the pressure to the set value. 1.0.2.2 To prevent overpressure or underpressure in the storage tank due to the failure of pressure relief valves and/or gas seal devices, a system that combines a liquid seal with a gas seal device can be used; the schematic diagram of its composition is shown in Figure 1.0.2–2. The function of the liquid seal equipped in the gas seal device is: (1) when the pressure relief valve fails, the liquid seal can serve to expel gases. That is, when the pressure inside the storage tank exceeds the set value, the gas in the tank can be released through the liquid seal. (2) When the gas seal device fails and the pressure inside the tank exceeds the set value, pressure can be released through the liquid seal, thereby reducing the load on the pressure relief valve. (3) When both the pressure relief valve and the air seal fail, and a negative pressure appears inside the tank, air can be drawn in through the liquid seal to prevent the tank from deforming or being damaged. 2 Selection and calculation of the gas seal device 2.0.1 Calculation of air supply volume 2.0.1.1 The air supply volume for the gas seal device in the storage tank should be greater than or equal to the sum of the air required to replenish the volume lost as a result of the pump drawing out the liquid stored in the tank, and the air needed to compensate for the condensation and contraction of the gas within the tank due to changes in external temperature. (1) The amount of air required to be pumped out to remove the liquid stored in the tank equals the pump’s maximum output capacity. (2) The amount of gas that needs to be added to compensate for the condensation and contraction of the gas inside the storage tank due to changes in temperature is specified in API Standard 2000, \"Venting of Aboveground and Belowground Storage Tanks,\" by the American Petroleum Institute. For storage tanks with a volume of 3180 m3 or more, this amount of gas depends on the surface area of the tank’s shell and roof; 0.6 m3 of gas per square meter of surface area is required to be added each hour ； For storage tanks with a volume of <3180 m3, 0.178 m3 of gas seal gas must be added per hour for each cubic meter of volume. The aforementioned gas volume allows the temperature of the gas inside the tank to change by 37.8°C per hour, which is on the safe side. Table 2.0.1 lists the gas supply requirements for common storage tanks due to changes in external temperature. The gas seal system for tanks subject to temperature changes requires a gas volume meter. Table 2.0.1: Tank volume, Gas volume. Tank volume, Gas volume, Tank volume, Gas volume – in m3 and m3/hr respectively. 10: 1.8; 800: 143; 7000: 1030. 15: 2.7; 1000: 178; 8000: 1140. 50: 9.0; 1500: 267; 10000: 1250. 80: 14.3; 2000: 356; 15000: 1630. 100: 17.8; 3000: 534; 20000: 2020. 150: 26.8; 4000: 684; 25000: 2300. 300: 53.5; 5000: 800; 30000: 2600. 500: 89. 6000: 920. Note: If the tank volume differs from those listed in the table, the required gas volume can be determined using interpolation. The gas volumes shown in the table are in standard cubic meters per hour. (3) Add the air volumes required in steps (1) and (2) to obtain the air volume required for the air seal device. 2.0.2 Calculation for selecting gas seal valves: Different manufacturers of gas seal valves use different recommended calculation formulas. The volume of gas and the appropriate valve model are determined by using the calculation formulas and size coefficients provided by the manufacturer of the selected product, in accordance with the required operating conditions. 2.0.3 For atmospheric pressure tanks equipped with breather valves, to prevent air from entering, the gas seal pressure value can generally be set at 0.0005–0.001 MPa of water column (gauge); this is an empirical value.

In other words, it is a conversion between the actual volume and the nominal volume; the amount of gas required for the gas seal is a fixed value, and even if the supply pressure changes, the actual amount of gas used for the seal remains unchanged.

Then, according to the calculations above, it should be a standard cube. Is there any difference between m3/hr and m3/h in the data?

It should be noted first that the American standard you cited is intended for low-pressure containers; if the pressure does not meet its requirements, you cannot use the data regarding the additional gas volume. The calculation isn’t difficult either: consider three aspects. One is the volume, in cubic units, of the medium pumped out per hour; this amount needs to be replenished with gas, which is expressed in standard cubic units ； Secondly, you need to consider the temperature changes inside the container: an increase in temperature leads to gases being released into the atmosphere, while a decrease in temperature results in a drop in gas pressure, which can be calculated using the ideal gas law ； Another thing to consider is a safety factor; that is, you add the two preceding values together and then multiply the result by a certain multiple.