Questions about API598
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According to clause 5.9.1.3 of standard API598, for valves with adjustable stem seals, leakage through the stem seal during shell testing shall not result in rejection, and the manufacturer shall be able to prove that the stem seal can maintain a pressure at least equal to the valve’s rated pressure at 38°C without any visible leakage. Regarding the understanding of \"valves with adjustable stem seals,\" and in light of the contents of clause 5.9.1.4, it can be understood that the sealing at the stem area is adjustable; for example, the seal at the stem area may be an adjustable-type packing box. However, clause 6.3 of API 598 stipulates that shell tests should be conducted on valves that have already been installed, with both ends of the valve closed, the valve in a partially open position, and the packing gland tightened enough to maintain the test pressure – this process serves to test the packing box. I would like to ask: Rule 6.3 specifies that the stuffing box should be sealed at the same time as testing the housing, but if the valve stem seal falls under the conditions specified in 5.9.1.3, leakage in the stuffing box is very likely to occur. How should this be handled?
Well, what I’d like to ask is how to understand what it means when 6.3 and 5.9.1.3 are in conflict. For my packing area, it is only necessary to ensure that there is no leakage at the design pressure; however, the requirement for the shell is 1.5 times that value, and the shell testing includes a packing box test. There is a conflict between these two requirements. In cases where the design meets the special conditions specified in 5.9.1.3, the packing box can be designed to withstand a pressure of 1.5 times the design value without leakage, but in such cases it becomes impossible to maintain the pressure during the shell testing
Who says that packing only ensures no leakage at the design pressure? The shell strength test includes the filler position.
I understand what you mean. Stem sealing refers to the seal at the upper part, not the seal at the packing location
It has been understood that ordinary stop valves come equipped with an upper seal, so it is necessary to test this upper seal separately. When testing the upper seal, the packing must be loosened to avoid pressure being applied to it. To achieve proper sealing at the upper end, the valve stem needs to be fully opened; therefore, the packing is only subjected to 1.1 times the normal pressure during the process of sealing it. For valves without an upper seal, no upper seal is applied since one does not exist. During the testing of the valve body casing, since there is no upper seal, the packing is directly subjected to pressure ； When performing external leakage testing, the valve body is under pressure, and during the test the packing is subjected to pressure twice. To prevent leaks, after mounting the housing, operate the valve stem switch once, then tighten the packing area, and finally test for external leaks