Thermal relief 1

[ab123456]

I have a pipe containing material at a temperature very close to its boiling point. On a hot sunny day the ambient temperature may exceed the boiling point of the material.

We will be fitting thermal relief valves however am unsure on how to size these. If I can quantify the solar heat gain i can calculate how much material boils off and then feed this information to the valve supplier for sizing.

How would I go about quantify the heat gain in a pipe due to solar gain?

The pipe in question is 3"NB unlagged carbon steel approx 20m long.

 

[Montemayor]

ab:

This subject has been dealt with and discussed thoroughly in many previous threads which can be accessed through the forum.

You don’t specifically state your material is a liquid. I assume it is.

A thermal relief valve is not “sized” for the application. You obtain a ½” or ¾” sized model and install it. Period. There is no documentation or calculations required by OSHA in the case of hazardous fluids. The reason the application is a no-brainer is simply because the amount of liquid expansion has been found to be empirically so small that it is immaterial to debate the actual flow rate. To obtain a relief –albeit, excessive in capacity – is considered practical and normal engineering practice. Although you can surely spend your time calculating the solar heat gain within your 20 meter-long, 3” pipe, I fail to see how that would affect the “sizing” of the thermal relief valve. What is more important is what you haven’t stated: what is the identity and nature of the liquid fluid you are relieving? How do you intend to physically carry out the relief? Does it pose a hazard to personnel and operations in the immediate area? Is it a pollution hazard or potentially toxic? Etc., etc.

 

[ab123456]

Montemayor,

Thanks for your reply I appreciate your assistance.

The line in question will be used to transfer acetaldehyde from storage tanks to various locations around site, the boiling point of the material is 21degc. The scenario i am concerned with is after a transfer valves are closed without first draining the line resulting in a full pipe closed at both ends.

The following day the ambient temperature results in a temperature in the pipe in excess of 21degc, the material starts to boil and as the line is closed at both ends the pressure increases and possibly exceeds design pressure.

Due to the phase change in this case I was considering this to more than a thermal expansion problem.

Do you think these concerns are justified or am i creating a problem where one doesnt really exist.

Thanks

 

[Montemayor]

ab:

Please don’t interpret my initial response as regarding your problem as “trivial”. It isn’t. Any potential for relief of a fluid (especially a hazardous one) is not a trivial occurrence and should directly addressed and safely resolved by engineers.

Your concerns for a potential overpressure are certainly justified and that’s why I’ve taken the time and effort to respond. As a chemical engineer you should recognize that if you have a “blocked-in” case of liquid acetaldehyde undergoing a temperature increase up to (and surpassing) its normal boiling point, you do not have a change of phase. You cannot vaporize acetaldehyde (or any liquid) while you have it totally blocked in as a liquid. That’s the reason for a tremendous hydraulic pressure increase within the trapped liquid while it tries to expand. While the liquid tries to expand, generating astronomical hydraulic pressure, it remains as a liquid. It will only vaporize (flash) once it is released to a lower pressure – presumably when the pipe bursts. And this is precisely what you are trying to mitigate.

The thermal relief device will efficiently relieve the excess pressure generated by the hot, expanding liquid – but at the expense (or trade-off) of generating a flash stream of acetaldehyde (albeit, small). If you cannot (or won’t) accept the introduction of a small flash stream of acetaldehyde into the atmosphere locally, then you can resort to the alternative method of relieving the expansion pressure: installing an expansion chamber. This latter alternative solution is more expensive, but it eliminates emissions.

I hope this helps you resolve this problem. Good luck.