Thermal Relief of ASME B31.3 piping

[StoneCold]

I have a simple question but I don't really see a code reference about it.
I have a flammable liquid full line. 2" Sch40 A106 if it matters.
I want to use a check valve around one of the block valves as my thermal relief.
The "discharge" side of the piping is vented back to the tank so pressure can not build up in that section of the line.
Is there any code restriction against doing that?
My other option would be a relief valve vented to the "discharge" side of the piping.

Thanks
StoneCold

 

[BigInch]

Check valve around a block valve. If you can do that, what use is the block valve?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[SNORGY]

I have seen this done on occasion with butane and LPG loading skids. Not saying I like it, but I have seen it done and the Authorities have accepted it.

 

[BigInch]

A thermal relief valve piped around the block valve.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[StoneCold]

How is a thermal relief valve going to be any different than a 40 psid check valve?
When the block valve is closed I can get a minute amount of flow in the reverse direction of normal flow with this side check valve.
I could use a thermal relief valve but the effect is the same as the check valve.

Comments?

StoneCold

 

[BigInch]

Obviously with a thermal relief valve you can set the pressure for what you need for thermal relief, not just a 40 psi differential.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[BigInch]

You may also want to consider that API RP 521 requires that backpressure be limited to 10% of the set pressure.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[StoneCold]

Big Inch
If I used the Thermal Relief Valve, I would definately fall under API RP 521.
But do I fall under that using the Check valve to create a flow path for trapped liquid?
I was originally going to let the relief valves vent on to the concrete containment. We are talking about a couple of mls of liquid.
But I like the idea of it being contained and going back into the system.
I have a another system where they vent into a small containment vessel but the liquid still evaporates and creates another small emmission.

What is everyone else doing for in plant process piping thermal relief?


Regards
Stonecold

 

[BigInch]

You're right, thermal reliefs are small volume & not exactly the same thing is it.
Mine are piped to oily water drain sys to sump.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[jnich]

This philosophy has been applied in larger crude oil systems I've worked on, but the check valve line that bypasses your process block valve also had smaller block valves installed upstream & downstream of the check valve. Normal process is to have the smaller block valves on bypass line closed. However, when closing your main process block valve you'd have to open both bypass line block valves to allow an open thermal relief path. Does that make sense?

 

[dcasto]

Top poster, you are perfectly correct in using the check valve.

I initiate the use of check valves in the late 80's. The line has purity benzene, where do you relieve purity benzene too? Later, we used then on all our double block and bleed valves. We even worked with Foster Valves to get the flange tapped to take the body check valve back into the pipe without having a tap on the line.

Eventually you will end up with a psv on the system because they will be no route for a check valve. But in the case of the benzene line, that psv was back at the begining near the process flare and vapor recovery systems were available.