What causes medium leakage during pump shutdown cranking?
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One of the centrifugal pumps in our factory leaks fluid when the pump is shut down (the leakage is quite severe), but it operates normally when running and does not leak at all. What could be the reason for this? The medium being pumped is liquefied gas
This phenomenon occurs frequently with liquefied gas pumps; it may be due to the sealing O-ring not returning fully to its normal position after the pump is turned off, which leads to some leakage. This leakage causes freezing and creates gaps at the sealing points, but turning the pump back on again resolves the issue.
The sealing rubber rings for liquefied gas, both static and dynamic types, seem to be V-shaped PTFE rings, which perform better than O-rings. Liquefied gas absorbs heat as it vaporizes at normal pressure, and water vapor in the air forms a film that causes gaps at the sealed joints, leading to leaks; the situation may return to normal after operation begins. This situation occurs frequently
It’s a single-face mechanical seal, right? Liquefied gas is a medium that vaporizes very easily; the heat absorbed during vaporization causes the sealing surface to freeze up, leading to leaks. During operation, heat is generated due to the friction between the stationary and rotating rings, which prevents leakage. Since this situation could damage the seal when the pump starts, it is recommended to address it. This problem can be solved by introducing steam into the quick-cooling hole of the mechanical seal gland (or by blowing steam directly onto the seal gland), and then turning the shaft and starting the machine.
I have encountered this situation as well; it mainly occurs with double-suction pumps. There is no leakage when they are operating normally, but leakage appears when they stop running.
The normal phenomenon of thermal expansion and contraction, especially in liquefied gas pumps. When the pump is operating normally, heat is generated; the sealing gaskets and various components experience reduced gaps due to expansion. After the pump stops operating, these gaps increase as a result of cooling and contraction, and rapid cooling can lead to leakage of the fluid.
The spring of the mechanical seal did not rebound, causing a leakage in the seal; after turning the shaft several times, there was no longer any leakage.
Since the auxiliary sealing ring loses its sealing efficiency in cold conditions (as a result of thermal expansion and contraction), frictional heat during operation causes the auxiliary sealing ring to achieve the required pre-compression, resulting in better sealing performance. In our facility, the liquefied gas pumps are blown with steam before being started; nowadays, these pumps use dry gas seals or double-end face seals, which provide better sealing effects.