How to set the setting pressure of the spherical tank safety valve

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Introduction Safety valves for pressure vessels are essential safety devices to ensure their safe operation. The main functions of a safety valve are two: first, to ensure that the pressure vessel does not open under normal operating pressures, thereby maintaining the seal of the equipment ； Secondly, it is necessary to ensure that pressure vessels can open to release pressure in case of abnormal overpressure, thereby ensuring their safety and preventing them from failing due to overpressure or even exploding. Therefore, setting the opening pressure of the safety valve appropriately not only ensures the safe operation of the equipment but is also an important aspect of achieving an economical design for it, especially for large liquefied petroleum gas storage tanks that require significant investment. 1 Requirements related to standards and specifications: The construction of large liquefied petroleum gas storage tanks must meet the relevant requirements specified in the Technical Code for Safety of Special Equipment TSG21-2016, \"Regulations for Safety Inspection of Fixed Pressure Vessels\", issued by the General Administration of Quality Supervision, Inspection and Quarantine. (1) Safety valves are the most common type of overpressure relief device; the operating pressure of a safety valve is equal to its set pressure (also known as opening pressure). Furthermore, there is a certain deviation in the set pressure of the safety valve; this deviation should not exceed ±3% of the set pressure, or the larger of the two values, ±0.015 MPa. (2) Since the medium stored in liquefied petroleum gas spherical tanks is explosive, the standard GB/T12337-2014 \"Steel Spherical Storage Tanks\" stipulates that such tanks must undergo leak tests, and the most commonly used method for such tests is the airtightness test. The airtightness test pressure is equal to the design pressure. Safety accessories include safety valves; therefore, when conducting a gas-tightness test on liquefied petroleum gas cylinders, it is generally necessary to install the safety valves. (3) For liquefied petroleum gas spherical tanks, the designer shall specify the maximum allowable operating pressure in the construction drawings. The maximum allowable operating pressure is calculated based on the effective thickness of each pressure-bearing element in the spherical tank, taking into account all the loads applied to that element, and the minimum value is adopted. (4) According to the definition of the maximum allowable working pressure, and as shown in Figure 1, the difference between the effective thickness and the calculated thickness of each pressure-bearing element in the spherical tank is merely the rounding value of the thickness adopted during design. The calculated thickness of the spherical tank is the thickness required for each pressure-bearing element of the tank under the design pressure, and it is determined by taking into account all the loads that such elements are subjected to. Therefore, the difference between the maximum allowable operating pressure and the design pressure of the spherical tank depends solely on the thickness rounding value chosen by the designer. Images: Figure 1 shows the relationship between various thicknesses; Figure 2 illustrates the existing problems. For large liquefied petroleum gas spherical tanks, in order to minimize steel consumption and welding work, steel plates with odd thicknesses are often used, and the nominal thickness of the steel plates is reduced as much as possible by controlling the negative thickness deviation. Therefore, the thickness rounding amount generally does not exceed 0.7 mm, and sometimes it can even be virtually zero. As can be seen from the above, the magnitude of the thickness rounding value directly affects the difference between the maximum allowable operating pressure and the design pressure. Therefore, in larger liquefied gas storage tanks with better design efficiency, the maximum allowable operating pressure is often closer to, or even equal to, the design pressure. This will inevitably result in a situation where the opening pressure of the safety valve cannot be set above the design pressure and below the maximum allowable operating pressure, failing to meet all the relevant requirements specified in the standards and regulations cited above. 3 Solutions and Recommendations For the two problems mentioned above, there are two simplest and most straightforward solutions. First, by increasing the thickness of the rounded sections and using thicker spherical shell plates, the maximum allowable operating pressure of the spherical tank can be increased. Secondly, by reducing the design pressure of the spherical tank without thinning the thickness of the shell plates, the difference between the design pressure and the maximum allowable operating pressure is artificially increased. The third solution most commonly used in the industry at present is to set the setting pressure of the safety valve above the operating pressure and below the maximum allowable operating pressure. The maximum allowable setting pressure of the safety valve should be set as close as possible to the highest allowable operating pressure of the spherical tank, while the minimum allowable setting pressure of the safety valve should be set as high as possible above the operating pressure. At the same time, a stop valve is installed below the safety valve to ensure the smooth progress of the airtightness test. Another solution was proposed by combining years of experience in spherical tank construction and based on numerous tests. The basic ideas and specific approaches of this plan are shared as follows: (1) The setting pressure of the safety valve is still determined in accordance with the requirements of the third solution. (2) No shut-off valve is installed below the safety valve, or although one is installed, it is not closed during the airtightness test. (3) A set of safety valve backpressure devices is used to apply an appropriate backpressure on the outlet side of the safety valve, ensuring that it does not open during the airtightness test and thus allowing the airtightness test of the spherical tank to be completed successfully. 4 Conclusion The most prominent feature of the development in modern pressure vessel construction technology is the use of failure-mode-based design methods to ensure the functionality, safety, and cost-effectiveness of vessels throughout their entire life cycle. For large pressure vessels, the ability to better balance functionality, safety, and cost will better reflect the overall competence of the designers. This article presents some reflections based on the aforementioned principles of the “three characteristics” for pressure vessels, and seeks to propose some new approaches to solving related issues. Of course, in response to the issues raised in this article, other professionals have also proposed various alternative solutions. For example, the above-mentioned problems can be solved by methods such as performing secondary tuning on the safety valve. But like the four solutions mentioned in this article, all of them have more or less shortcomings. For example, some solutions have the drawback of insufficient cost-effectiveness, some sacrifice certain functionalities, while others fail to detect all sealing surfaces during airtightness tests or involve repeated disassembly and assembly.