Proving a relief valve is not neccessary

[SELM005]

Hi

I have got a 4 stage evaporator, the first stage of which is heated by LP steam. The steam pressume in the evaporator is 1.6 bar and the pressure upstream of the inlet control valve is 2.5 bar. The evaporator is pressure tested to 9 bar so I want to prove the relief valve is uneccessary so it can be taken off our Zurich inspection list. Any suggestions as to the calculation I need to prove this?

 

[edanyel]

Every PSV must have been designed for an specific and WORST CREDIBLE SCENARIO. If you just establish there's no way of achieve that risk condition then you don´t need a relief device.
Be aware, sometimes the scenario is external fire then no matter process coditions, you need a PSV.
Hope it helps
Rgds
Daniel

 

[jmiles]

Keep in mind as well that the it is the design pressure ont he evaporator that matters for the PSV, not your test pressure which is 1.3x your design pressure.

As well like Daniel said external fire can be a rough one to work with, but also make sure you look at deadhead pressure from and pump/compressor in the ssytem that is feeding the evaporator. or int his case since your likely comming off a steam boiler you need to look at what happens if you get blocked flow and that steam keeps building up. im not sure if you might be into double jeopardy there with the boiler and control valve. I guess that depends on your set up and where other relief systems are.

But in Short Daniel is correct, remove every credible case of over pressure and you wont need a PSV, but make sure you keep your documentation to show you looked at the different scenarios. As one of my colleagues said once, its better to be found incompetent than it is to be found negligent, so document everything.

 

[Homayun]

You should compare the Design Pressure of the your LP Steam System with the Design Pressure of your evaporator, NOT the Test pressure!

So If the LP Steam DP = Evaporator DP, then this means that control valve failure can probably not cause an overpressure in the evaporator. So protection against this scenario would not be needed.

 

[jmiles]

Homayun,

You are correct, you need to look at the design pressure of the evaporator, not the test pressure, BUT the design pressure of the LP steam system is almost irrelevant, what dictates the need for a PSV is the maximum pressure that the evaporator can see given a single failure in the system. and that pressure could be much higher than the design pressure of the LP Steam system, especially if you have to look at thigns like external fire.... or a failure in your steam boiler...

 

[SELM005]

Thanks everyone. I have managed to write the necessary justification

 

[psafety]

How is this worked around?

UG-125 GENERAL
(a) All pressure vessels within the Scope
of this Division, irrespective of size or
pressure, shall be provided with pressure
relief devices in accordance with the
requirements of UG-125 through UG- 137.

 

[skearse]

Depends on the system. UG-127(d) [part of A08] allows open flow paths or vents to be used as the sole pressure relieving device, or alternatively, the system may meet the requirements of UG-140 Overpressure Protection by Design (also part of the 2008 Addendum).

 

[psafety]

skearse

Depends on the system. UG-127(d) [part of A08] allows open flow paths or vents to be used as the sole pressure relieving device,
" Never saw this verbiage in ASME PV Code"

or alternatively, the system may meet the requirements of UG-140 Overpressure Protection by Design (also part of the 2008 Addendum).

"Then I'd assume he wouldn't need to be trying to prove what he is doing."

 

[skearse]

skearse

Depends on the system. UG-127(d) [part of A08] allows open flow paths or vents to be used as the sole pressure relieving device,
" Never saw this verbiage in ASME PV Code"

It's (still) there. Reading it right now. Check the A08...

or alternatively, the system may meet the requirements of UG-140 Overpressure Protection by Design (also part of the 2008 Addendum).

"Then I'd assume he wouldn't need to be trying to prove what he is doing."

UG-140 is the formalization of Code Case 2211 into the PV code. You don't get to just 'say' that you've got OP protection by design; there are a number of requirements you need to go through before you can claim it: process/system pressure is selflimiting, and is below the MAWP of the vessel; a full PHA must be completed to establish that there are no OP scenarios; etc., etc., etc.