Determination of the test pressure for the shell side of a shell-and-tube heat exchanger

This is applicable to fixed-tube-sheet heat exchangers, and is mainly used for determining the hydrostatic test pressure for the tube ends

Because if the design pressure of the shell side is low, the test pressure for the shell side will naturally be lower than that for the tube side. If there is no dew on the pipe fittings during the shell side pressure test, it cannot be guaranteed that there will be no dew on them during the pipe side pressure test. The reason is simple: this situation often occurs during pressure testing of pressure vessels. When the pressure reaches 10 atmospheres per square centimeter, the welded joints do not leak; but when the pressure rises to 11 atmospheres per square centimeter, the welded joints suddenly start to leak. Therefore, in such cases, when designing the shell side, it is necessary to take into account that the test pressure of the shell side must be greater than or equal to that of the tube side.

It is not possible to inspect the pipe joints during the pressure test of the tube side; if the pressure on the tube side is higher than that on the shell side, it cannot be guaranteed that the joints are fine even if they show no problems during the shell side test.

In recent years, an overseas engineering company, when specifying the requirements for the heat exchangers in a project it was designing for our country, proposed that \"for heat exchangers with a large difference in design pressures between the tube side and the shell side, the design pressure on the side with the lower pressure should be increased so that the pressure used for its hydrostatic test equals the design pressure of the higher-pressure side (the so-called 2/3 principle)\". Since then, some domestic engineering companies have also included it in their own engineering regulations. The 2/3 principle originates from clause D2.1 of Appendix D (Reference – Recommended Practices) of API 660-2003, the Standard for Shell and Tube Heat Exchangers. Article D2.1 states: “The heat exchange tubes in the high-pressure unit fail.” If the heat exchanger is subjected to a large operating pressure difference between the shell side and the tube side, the impact of tube failure should be considered. Note: For detailed information, refer to references (4) and (5). ” Reference (4) is API RP 521-1997, \"Guidelines for Pressure Relief and Pressure Reduction Systems\", and reference (5) is \"Petroleum Society – Design and Safe Operation of Shell and Tube Heat Exchangers in the Face of Heat Transfer Tube Failures\". At present, reference (4) has been found, while reference (5) has not yet been found. In Reference (4), item 3.18 pertains to failures in heat exchange equipment; clause 3.18.2 of it states: \"It is very rare for all the tubes to rupture and a large amount of high-pressure material to flow to the lower-pressure side of the heat exchanger, but accidents are possible.\" Smaller leaks may cause little overpressure during heat exchanger operation, as the standard hydrostatic test pressure is 150% of the equipment’s design pressure. Equipment failure, in other words, when the low-pressure side (including the upstream and downstream systems) is designed using a pressure that is at least 2/3 of the high-pressure side’s design pressure, it is impossible for a loss of head from the low-pressure side to atmospheric pressure to be caused by pipe rupture. When there is a practical difference between the design pressure and the operating pressure on the high-pressure side of the heat exchanger, it is possible to use the maximum possible system pressure in place of the high-pressure side design pressure based on individual operating conditions. When the actual test pressure on the low-pressure side is below 150% of the design pressure (referring to the 2/3 rule), this lower pressure should be used to determine whether overpressure protection is required. When the low-pressure side of the heat exchanger (including the upstream and downstream systems) is designed in accordance with the aforementioned 2/3 principle, pressure relief due to tube rupture is not required. For newly installed systems, increasing the design pressure on the low-pressure side can reduce risks. When it is considered that this margin is approaching, the upstream and downstream piping and equipment systems must be estimated. ” It should be noted that the standard hydrostatic test pressure is 150% of the equipment’s design pressure, as specified in the ASME codes prior to 1999. The so-called 2/3 principle states that 150% of the design pressure on the low-pressure side should be greater than or equal to the design pressure on the high-pressure side; in other words, the design pressure on the low-pressure side should be greater than or equal to 100/150 (2/3) of the design pressure on the high-pressure side. The current ASME code provisions have changed 150% to 130%, and the 2/3 principle should accordingly be changed to a 100/130 (10/13) principle. According to China’s **standard GB150, the hydrostatic test pressure is 125% of the equipment’s design pressure. Following the so-called 2/3 principle mentioned above, the design pressure on the low-pressure side should be greater than or equal to 100/125 (4/5) of the design pressure on the high-pressure side; therefore, this 2/3 principle should also be adjusted to the 100/125 (4/5) principle. Therefore, if the heat exchanger is required to be designed in accordance with ASME codes, the design pressure on the low-pressure side should be greater than or equal to 100/130 (10/13) of the design pressure on the high-pressure side ； If the heat exchanger is required to be designed in accordance with the Chinese standard GB150, the design pressure on the low-pressure side should be greater than or equal to 100/125 (4/5) of the design pressure on the high-pressure side. (For convenience, the 2/3 principle will still be used in the following description.) Based on the provisions of section 3.18.2 of API RP 521-1997, the following implications can be drawn: (1) When the design pressure of the lower-pressure side of the heat exchanger (including the upstream and downstream systems) is set to at least 2/3 of the design pressure of the higher-pressure side, there is no need to incorporate additional pressure-relief measures for overpressure protection due to tube rupture on the lower-pressure side of the heat exchanger (including the upstream and downstream systems). ⑵ When the design pressure of the lower-pressure side of the heat exchanger (including the upstream and downstream systems) is set to less than 2/3 of the design pressure of the high-pressure side, pressure relief measures with additional overpressure protection in case of tube rupture must be considered for the lower-pressure side of the heat exchanger (including the upstream and downstream systems). ⑶ When pressure relief measures with overpressure protection have been installed on the lower-pressure side of the heat exchanger (including the upstream and downstream systems), the design pressure for that side (including the upstream and downstream systems) does not need to be at least 2/3 of the design pressure of the higher-pressure side. Based on the above implications, a key issue in the chemical processing units we have designed at present is whether pressure relief measures with overpressure protection have been installed on the lower-pressure side of the heat exchangers (including the upstream and downstream systems). If no pressure relief measures with overpressure protection are provided on the lower pressure side of the heat exchanger (including the upstream and downstream systems), it is necessary to use a design pressure for that side (including the upstream and downstream systems) that is at least 2/3 of the design pressure of the higher pressure side ； If pressure relief measures with overpressure protection are already in place on the lower pressure side of the heat exchanger (including the upstream and downstream systems), it is unnecessary to use a design pressure for that side (including the upstream and downstream systems) that is at least 2/3 of the design pressure of the higher pressure side ； If the equipment and pipelines in the systems upstream and downstream of the lower-pressure side of the heat exchanger already have pressure relief measures for overpressure protection, but are not designed with a pressure level of at least 2/3 of that of the high-pressure side, then using only 2/3 of the high-pressure side’s design pressure for the low-pressure side of the heat exchanger serves no purpose other than increasing the cost of the heat exchanger. Based on the above analysis, since various material flow systems in chemical plants use design pressures determined by their own operating conditions, and overpressure protection measures have been installed in each of these systems, there is no need to increase the design pressure on the lower pressure side of the heat exchangers. In 2008, the Technical Specifications for Shell and Tube Heat Exchangers developed by Sinopec’s Engineering Department removed the clauses related to the “2/3 principle”. In July 2009, I received a letter from a designer at the foreign engineering company mentioned above, in which it was stated that the company’s current approach regarding the aforementioned \"2/3 principle\" is to generally ignore it; only when the client insists strongly and can provide sufficient reasons will the company act according to the client’s request.

If they are more or less similar, it’s fine to proceed; if the difference is too large, could an ammonia leak test be conducted?