Spherical or horizontal cylindrical type (bullet type) storage tanks are used to store LPG. The horizontal cylindrical types are usually used for small capacity or underground installations and Spherical ones are used for higher capacities. The design of high-pressure LPG storage tanks is critical. Many parameters need to be considered during design. This article will provide basic information for the same.
Selection of LPG Storage Tank Types
A tank-type will usually be selected considering the cost or the size for transportation. The spherical type is usually employed for sizes greater than 500 m3. The horizontal cylindrical type is usually used for sizes smaller than 100 m3. Both types will be applicable for volumes ranging from 100 to 500 m3. The type of this capacity range will be decided by the total weight. Where the tank is installed underground, the horizontal type shall be selected, even if the vessel capacity exceeds 100 m3.
LPG Storage Capacity
Definition of Capacity
- Nominal capacity- All this capacity can be used, defined as below in Fig. 1. This capacity is usually used as a tank name.
- Geometrical capacity- Volume inside a vessel which is called “a water volume” in NFPA.
- Storage capacity- The volume from the tank bottom to the maximum design level. This volume varies depending on the operating temperature.
- Net-working capacity- Volume between HLL and LLL or HHLL and LLLL
(1) Maximum liquid level (maximum Storage Capacity)
Many countries specify a maximum liquid level (max. storage capacity) in their regulations. In countries that have no such regulations, NFPA shall be applied. NFPA-58 and 59 specify details of the maximum liquid level including liquid volume correction factors and equations concerning capacity and temperature (Refer to NFPA 58 Para. 4-4 and Appendix-F)
Few regulations specify that a vapor space of 10% shall be secured under the severest conditions, thus resulting in the following equation.
V = W/0.9d
Where V = tank geometrical volume (m3); W = Storage capacity (kg) and d = Density at the maximum design temperature (kg/m3)
NFPA specifies the coefficient of the above equation, i.e. 0.9 as follows.
- 9 to 0.95 at 100° F
- 98 to 0.99 at the maximum storage temperature.
This maximum liquid level fluctuates according to operating temperatures as below Example ;
The following figures are the results of example calculations according to the physical properties of Pure Propane per NFPA.
From the above, it is not possible to set a fixed level for the highest limit point. Therefore the highest limit of level should be compensated with the storage temperature or a differential pressure type level indicator shall be used.
(2) Minimum levels
Refer to Fig. 2
H2; 150 mm or 10 minutes from the maximum filling volume
H3; A height of Deadstock area. The height shall be calculated by the reasonable dead stock volume.
The recommended height for the spherical tank is shown below.
- D: Diameter of the sphere
- H: Height of level
- V: Sphere volume
- Vb: Sectional Volume of the height H
- H4: 300 mm/ minimum 100 mm
Note 1; High and low level (HLL and LLL) alarm shall be set at the maximum and the minimum operation respectively. If high-high and low-low level (HHL and LLL) for an emergency shutdown or an automatic diversion system are provided, set points shall be selected at lower than the maximum and higher than the minimum design, but not inside of the maximum and the minimum operation.
Sphere Maximum Capacity
The maximum sphere capacity is limited due to the wall thickness. The wall thickness is limited by the manufacturing and the stress relief requirement.
Operating and Design Conditions
(1) Operating temperature: Operating temperatures are not so important for the design of tanks; they are merely used to design pumps connected to tanks. The maximum operating temperature and minimum operating temperature as pump design bases shall be determined separately. The operating temperature of a tank shall be determined based on the following conditions.
- The temperature of rundown from process units
- Ambient air temperature (annual mean or annual highest mean temperature)
- The temperature of products when they are received from a tanker.
(2) Operating Pressure: An operating pressure shall be an equilibrium pressure at operating temperature. Where the mole fraction of contents of the liquid in the tank fluctuates, the most severe case in normal operation shall be considered.
(1) Design Temperature: A design temperature shall be determined based on the assumed highest temperature, with consideration given to input heat generated by solar radiation. Generally, design temperatures are specified per country based on the ambient air conditions of the district where the plant facilities are to be constructed. Major oil companies may have their own design standard of temperature selection. Where the country’s regulations or the client’s design standards do not specify design temperatures, NFPA shall be applied. Design temperature determination standards are closely connected with design pressures.
Major oil companies, in some cases, have specified the lowest design temperature as a design standard; they employ the equilibrium temperature of a tank internal at atmospheric pressure as the lowest design temperature. Low-temperature service materials, therefore, shall be used for tanks storing propane or lighter fluids.
(2) Design Pressure: The equilibrium pressure of a tank internal at the design temperature shall be used as the tank design pressure. Where the country’s regulations or the client’s design standards do not specify a design temperature, NFPA shall be applied as per the table below. Some major oil companies specify a higher temperature e.g. 65° C to be a mechanical design temperature, in their standards. In this case, however, they do not employ the equilibrium pressure of the internal at the specified temperature as design pressure, but the design pressure will be specified separately or the minimum design pressure specified in NFPA is otherwise used.
Note 1: Refer to NFPA 58, Para. 8-2.2
The NFPA specifies the equilibrium pressure at a design temperature of 41, 46 and 54° C, respectively, to be a design pressure, for each type of vessel as given below.
- Vessels up to 4.5 m3 incl. in-capacity (54°C)
- Vessels over 4.5 m3 in capacity (46°C)
- Underground vessels (41°C)
LPG Storage Tank Nozzles
(1) Tank nozzle information to be provided by basic engineering. The following items of nozzle information shall be provided by the basic design group.
- Size, number, and location of inlet and outlet nozzles. Note: The pump suction nozzle shall be inserted by 300 mm from the tank bottom.
- Size, number, and location of the sampling nozzle(s) and water draw-off nozzle, if required
- Size and number of the spare nozzle(s), if required
- Size and number of vent and drain nozzle. Minimum one vent and drain nozzle shall be provided.
- Size and number of nozzles for safety relief valves. Minimum one spare PSV shall be provided.
(2) Nozzle for instrumentation
Nozzle information for instrumentation will be provided by others.
(3) Nozzles to be decided by the detailed engineering group
Instrumentation on LPG Storage Tanks
Generally, two-level instruments will be installed to permit mutual calibration to be carried out, because LPG tanks cannot open without the tank shut down. One level instrument may be permitted if it is possible to remove and calibrate it by installing an isolation valve such as a radar type. To use the LPG tank capacity as effectively as possible, it is necessary to compensate the level with temperature instrument or use differential pressure type level instrument
Generally, a temperature indicator shall be installed at the bottom crown
Generally, two pressure gauges should be provided at the sphere top and bottom. One pressure instrument should be provided and indicated in the control room. Two pressure relief valves, each have a 100% capacity shall be provided. This configuration allows the PRV maintenance without a sphere shutdown.
A Water draws offline shall be installed on each LPG tank. Two isolation valves shall be provided on the water draw offline: a distance of more than one meter shall be provided between the valves to prevent freezing the valves as figures below. As an alternative system, a water draw off pot is provided, and the vent line from the water pot is returned to the flare line or the LPG tank.
Insulation and Painting: For aboveground tanks, in some cases, cold-insulation or fire protection may be provided, according to the client’s request. In such a case, it is possible to reduce the safety valve relieving capacity.
Tank Heaters or Coolers: A tank heater or cooler shall not be installed in the tank. However, an external heater may be required in the coldest areas, i.e. North East of China or Siberia, to avoid a vacuum in the tank.