The temperature of the manual valve in front of the LPG safety valve is high, while the temperature of the manual valve behind it is low； The torch line, however, is frosted
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The temperature of the manual valve before the safety valve is high, while the temperature of the manual valve behind it is low； The torch line, however, was frosted and dripping ； Why? Let’s start with a real-world example: the safety valves on the top of a liquefied gas tank. Last night, it was found that the boundary valve of the flare line was frosted; the entire flare line corresponded to a line on the ground that was wet, with water dripping intermittently. Upon investigation, it was discovered that there were two safety valves, A and B, on the top of the liquefied gas tank (both of which were in use). The manual valve before safety valve A was warm to the touch, while the flange and valve body ahead of safety valve A were not warm; the manual valve connected to safety valve A was also not warm. The manual valve before safety valve B was not warm to the touch either. But I don’t understand why it’s hot here while frost forms at the torch line. Analysis result: It is determined that safety valve A has an internal leak ； Upon checking the historical records, it was found that no safety valve had been bypassed; it was concluded that there was a slight leak. The solution involved closing the upstream and downstream valves, isolating the system, and handing it over for maintenance. Were there any issues during this process? The temperature of the manual valve before the safety valve is high, while the temperature of the manual valve behind it is low ； The torch line, however, is frosted ； How to explain it
Firstly, frosting occurs on the torch line because the medium was originally in a liquefied state; upon entering the torch, the decrease in pressure causes the liquefied gas to vaporize and absorb heat, which leads to frosting on the torch line. As for the possible heat from the upstream valve, it might be an error caused by feeling it with your hand between the flare line and the valve. It is recommended to use a thermometer to compare the temperatures of the two hand valves.
This process is essentially one of liquefied gas gradually vaporizing as pressure decreases: the heat generated by the upstream valve is due to the high pressure at that time, during which the gas has not yet vaporized and remains in liquid form; the temperature carried by the liquid is its own temperature, and heat is released during this process. The temperature of the rear valve drops at the point when vaporization begins; however, the heat required for vaporization is only slightly more than the heat it possesses, resulting in a temperature decline ； Frosting on the torch line occurs because of a rapid drop in pressure, which causes a large amount of the medium to vaporize and absorb a significant amount of heat, resulting in a sharp drop in the temperature of the pipe walls and thus frost formation. Conclusion: Minor internal leakage in the safety valve! Removal and repair is the correct approach
I strongly agree with the analysis on the 3rd floor: the pressure is decreasing gradually, and this is due to the heat absorbed during vaporization
I agree with the third floor’s opinion: what leaks from the safety valve is liquid, so the temperature doesn’t drop. When the LPG liquid enters the pipelines, it vaporizes and absorbs heat, which causes frost to form in the pipelines. Use a wrench connected to the safety valve to listen for any sound of fluid flowing inside.
Thank you for your good advice; I’ll give it a try when I go to work tomorrow—use a wrench connected to the safety valve to listen for any sound of liquid flowing inside. It seems difficult to detect even a small amount of liquid leakage that way
The slight leakage from a safety valve functions as a throttling effect, caused by the rapid vaporization and expansion of the leaked liquefied gas. Similar to the cooling principle of household refrigerators.
Everyone, under normal operating conditions, the liquefied gas tank should not be full; what escapes through the safety valve should be gas, and there should not be a situation where a large amount of liquid vaporizes and absorbs heat.
Under high pressure, the expansion of gas also absorbs heat, leading to frosting in the pipelines
The temperature of the upstream valve is high because the pressure is high at this stage; the substance is still in liquid form and carries the temperature of the liquid itself, during the heat-release process. The temperature of the rear valve drops at the point when vaporization begins; however, the heat required for vaporization is slightly more than the heat it possesses, resulting in a cooling trend. A large amount of the medium vaporizes, absorbing a great deal of heat, which causes the tube wall temperature to drop sharply, resulting in frosting.
This post was last edited by lijianhuai on 2011-9-6 00:14. I agree with the view expressed in post 9 – under normal operating conditions, liquefied gas tanks should only be filled to about 85% capacity. In other words, what leaks out at the location of the high-pressure safety valve should be the gas phase inside the container. Although it contains PG components, it should not be a liquid. I agree with this view; under normal operating conditions, liquefied gas tanks should only be filled to about 85% capacity. In other words, what leaks out at the location of the high-pressure safety valve should be the gas phase inside the container. Although it contains component G, it should not be a liquid.