Design of LPG Vessels
LPG-Propane gas storage is in pressure vessels and it is almost always stored in its liquid form. These propane storage vessels can range from small camping canisters to BBQ gas bottles to larger gas cylinders and much larger LPG gas storage in Propane tanks - bullets. Gas storage depots may consist of large LPG-propane gas storage spheres or large buried tanks. The liquefied LPG-propane gas storage can also be underground, in custom-built or prepared storage caverns. Spherical or horizontal cylindrical type (bullet type) storage tanks are used to store LPG. For small capacity or underground installations, the horizontal cylindrical tanks are used, for sizes smaller than 100 m3 and spherical ones are used for higher capacities , for sizes greater than 500 m3. Many parameters should considered during vessel design. It shall meet the requirements of the ASME Boiler and Pressure Vessel Code, Section VIII, Division I or 2.
Design Pressure and Temperature
The design pressure of LPG vessels shall not be less than the vapor pressure of the stored product at the maximum product design temperature. The additional pressure resulting from the partial pressure of non-condensable gases in the vapor space and the hydrostatic head of the product at maximum fill shall be considered. Ordinarily, the latter considerations and the performance specifications of the relief valve require a differential between design pressure and maximum product vapor pressure that is adequate to allow blow down of the pressure relief valve (see API RP 520).
Both a minimum design temperature and a maximum design temperature shall be specified. In determining a maximum design temperature, consideration shall be given to factors such as ambient temperature, solar input, and product run down temperature. In determining a minimum design temperature, consideration shall be given to the factors noted in the preceding sentence as well as the auto-refrigeration temperature of the stored product when it flashes to atmospheric pressure. ASME Section VIII, Division I, has special rules for conditions where reduced temperature, as a result of auto-refrigeration or ambient temperature, is caused by coincident with a reduction in pressure. In such case it is required to evaluate the material (by impact testing if necessary) at the temperature of the product corresponding to a pressure that stresses the vessel shell to approximately 10% of the ultimate tensile strength of the shell material. When the vessel is repressurized, this must be done slowly to allow the temperature to increase as the pressure is increased.
LPG vessel design shall consider vacuum effects and be designed accordingly. Where an LPG vessel is not designed for full vacuum, some alternatives, in order of preference, are as follows:
a. Design for partial vacuum condition. This alternative is applicable when the vacuum conditions caused by ambient temperature conditions. The design pressure shall be equal to the minimum vapor pressure of the product at the minimum ambient temperature. In this situation, no additional protection against vacuum is needed.
b. Design for partial vacuum with a vacuum relief valve and a connection to a reliable supply of hydrocarbon gas. This alternative may compromise product quality.
c. Design for partial vacuum with a vacuum relief valve that admits air to the vessel. This alternative, under some conditions, may present a hazard from the presence of air in the LPG storage vessel, and this hazard shall be considered in the design.
Materials of Construction
All materials of construction shall meet the requirements of Section II of the ASME Boiler and Pressure Vessel Code. Materials with low melting point such as aluminum and brass, shall not be used for construction of LPG vessels.
The number of penetrations in any vessel shall be minimized, particularly those located below the working liquid level (i.e., below the vapor space). Flange connections shall be a minimum of ASME Class 150. All fittings shall be a minimum of NPS Y4.