2 replies to this topic

[Alejandro Guerrero]

Hi Friends,

This is my first post in the forum.

I have a question for experts:

When i have two block valves located at 30 meters with the pipe filled with
liquid and under sun radiation or fire (combustible liquid in pipe) i will
need a Thermal Expansion Relief Valve, this is clear. Thinking in a fire area of about 5000 ft2
as API indicate.

But, if the distance between the valves become short, for example 5 meters.
the sun may not heat the pipe so faster as the sunset appear, but the fire is a event
to define the Thermal Expansion Valve.

and if the distance between valves is 0,5 meters or 1 in ??

any know a criteria to define a minimun aceptable protection distance between 2 block valves
or i will need protect all situations when use a hydrocarbon fluid.

in my short experience in refinery i not see a lot of relief valves.

Regards

[latexman]

With an assumption of totally rigid pipe and fittings and complete hydraulic fill (no gas/vapor space), the length of pipe has no impact on the pressure attained. If the external conditions, sun or fire, are the same, a 1 meter run and a 1 km run of pipe (same diameter) will attain the same pressure. However, the volume of liquid relieved to maintain a fixed pressure is proportional to the length.

Can we come up with a rule of thumb that no thermal relief is needed if the distance between block valves is less than X meters? What if the toxicity of the chemical is like a poison? We'll need as many rules as toxicity classes.

What you'll find is pipe is somewhat flexible, there are pockets of space in pipes that are difficult to get completely hydraulically full, some fittings provide pockets of space, we intentionally tee pipes into pipes for vents and drains for maintenance reasons that form pockets, all control valves and most block valves leak through (it's just a matter of how much), all gaskets leak (again, it's just a matter of how much), we set up procedures so lines are not hydraulically full, operators operate valves and pumps so that lines are not hydraulically full, we monitor pressure transmitters in a line and momentarily open and close a block valve if it gets too high during certain conditions. A lot of factors and tricks of the trade to eliminate thermal relief PSVs.

Look out for lines of cool liquid that are normally "batched" into a warm room or area and there is more than one valve in the line that can be closed. Don't over look check valves! Sometimes, but not very often, you get one that holds real good! Ditto, for lines that are "shut down" for some reason.

There is one rule of thumb that has held up pretty good over the years. I tend to NOT put thermal reliefs on piping within the same unit or block (common operator group). If an issue develops, they will find a way to make it work so it won't fail the line or connections. I tend to ALWAYS put thermal reliefs on piping going from one unit or block to a different unit or block (different operator group), because each end has a valve and they might not communicate closely.

Hi

Has anyone tried to calculate the actual pressure rise using API 521 2007 ed.

Section 5.14.4

I can;t get the equations to converge for P2

regards

Ian