7 replies to this topic

[ayan_dg]

There is a pressure vessel whose MAWP is set at 100 barg (vessel is governed by ASME Div-1) , it has two identical relief valve each sized for 100% of the relief load and both are in service. One is set at 100 barg and the other at 103 barg. The maximum allowable accumulation is 106 barg. The inlet piping to the two valves are symmetrical.
When using the "3% Criteria" for sizing inlet line shloud I consider
i) Sum of rated capacity of the valves for the main header and individual rated capacity for the individual line.
ii) Rated capacity of only one valve considering the other valve is not present , as the lines are symmetrical the result will be same for the other one also.

[Erwin APRIANDI]

There is a pressure vessel whose MAWP is set at 100 barg (vessel is governed by ASME Div-1) , it has two identical relief valve each sized for 100% of the relief load and both are in service. One is set at 100 barg and the other at 103 barg. The maximum allowable accumulation is 106 barg. The inlet piping to the two valves are symmetrical.
When using the "3% Criteria" for sizing inlet line shloud I consider
i) Sum of rated capacity of the valves for the main header and individual rated capacity for the individual line.
ii) Rated capacity of only one valve considering the other valve is not present , as the lines are symmetrical the result will be same for the other one also.

Dear Ayan,

First clarification will be, why you have 2 x 100% PSV with different set point
and why both is in operation. Any specific criteria for this

Second, do both of the PSV connected to the vessel or there's only one connection
and there's an header splitting to two PSVs

For me, your second (ii) option is much more reasonable and appropriate, as I assumed
the second PSV is installed in case of the first one fail to open and the pressure inside the vessel reach to the second PSV set point. But this configuration seems strange, as ussually 2 x 200% will be used since PSV will undergoes annual inspection, so the stand by PSV is installed.
Therefore, during inspection of the first PSV, the vessel will still be protected by the second PSV so the set point mostly will be the same for both PSV

Edited by erwin.apriandi, 18 May 2011 - 03:45 AM.

[fallah]

Seems the vessel is a power boiler governed by AME Sec. I

Option i) is acceptable,provided that sum of the two PSVs rated capacities would be equal to total (100%) rated relief load (2x50%,30%+70%,...).Actually,2x100% operating PSVs wouldn't be logical.

Edited by fallah, 21 May 2011 - 05:08 AM.

[Lowflo]

Ayan,

There is no 3% rule in boiler code. Instead, the installation requirements are written prescriptively to ensure that PSVs operate in a stable manner. Thus, no need for inlet & outlet loss rules or calculation.

Read the installation rules and make sure your installation complies. They're are easy to understand. It's actually quite simple.

[fallah]

Ayan,

There is no 3% rule in boiler code. Instead, the installation requirements are written prescriptively to ensure that PSVs operate in a stable manner. Thus, no need for inlet & outlet loss rules or calculation.

Read the installation rules and make sure your installation complies. They're are easy to understand. It's actually quite simple.

Even though ASME Sec.I (in opposed to ASME Sec.VIII) isn't included a point indicates 3% rule for inlet line pressure loss,API 520 Part II has no exception to provide adequate confidence for ignoring 3% rule in power boiler application.

[Lowflo]

Ayan,

There is no 3% rule in boiler code. Instead, the installation requirements are written prescriptively to ensure that PSVs operate in a stable manner. Thus, no need for inlet & outlet loss rules or calculation.

Read the installation rules and make sure your installation complies. They're are easy to understand. It's actually quite simple.

Even though ASME Sec.I (in opposed to ASME Sec.VIII) isn't included a point indicates 3% rule for inlet line pressure loss,API 520 Part II has no exception to provide adequate confidence for ignoring 3% rule in power boiler application.

fallah......you're confusing ASME Sec I and Sec VIII. The OP's question is about ASME Sec-I boiler vessels. Sec-I boilers and Sec-VIII pressure vessels have fundamentally different rules and it's important that all relief designers understand these differences. When you're dealing with a Sec-I boiler PSV there are no sizing calcs, inlet/outlet loss calcs, relief scenario evaluations, and API 520 does not apply. Instead, the rules for Sec-I are prescriptive. You have to follow those rules exactly as written. If you do that, the task is simple and the design is safe. If you don't, the boiler inpector will not certify the installation and you can't legally operate the equipment.

[fallah]

fallah......you're confusing ASME Sec I and Sec VIII. The OP's question is about ASME Sec-I boiler vessels. Sec-I boilers and Sec-VIII pressure vessels have fundamentally different rules and it's important that all relief designers understand these differences. When you're dealing with a Sec-I boiler PSV there are no sizing calcs, inlet/outlet loss calcs, relief scenario evaluations, and API 520 does not apply. Instead, the rules for Sec-I are prescriptive. You have to follow those rules exactly as written. If you do that, the task is simple and the design is safe. If you don't, the boiler inpector will not certify the installation and you can't legally operate the equipment.

Lowflo...

I mentioned in my first post dated:21 May 2011 as "Seems the vessel is a power boiler governed by AME Sec. I"

In my second post dated:24 May 2011 as "Even though ASME Sec.I (in opposed to ASME Sec.VIII) isn't included a point indicates 3%......."

Everybody Looks to above statements would realize i understood not only field of OP's question but also differences of ASME Sec I with ASME Sec VIII especially about 3% rule...

Therefore in opposed to your statement i am not confusing....

But you are confusing about my statement regarding API 520 Part II.

As you may know,Part II of API 520 is about INSTALLATION (and 3% rule for inlet line loss..) not about :...sizing calcs, inlet/outlet loss calcs, relief scenario evaluations..." as you mentioned.

Of course,i haven't claimed API 520 Part II is applicable for PSV issues in ASME Sec I field but i submitted just a doubt it may be applicable due to not included any exception.

Edited by fallah, 28 May 2011 - 01:42 AM.

[chemsac2]

PSV inlet line loss is limited to 3% for PSVs designed for ASME section-VIII to be within blowdown pressure. Blowdown of typically 6-7%. Huddling chamber in PSV helps in achieving full lift within overpressure limit of 10%, but then one has to live with issue of blowdown during closing of PSV.

Thus, inlet line loss limit of 3% is result of need to limit overpressure to 10% for ASME section-VIII.

Since for ASME section I PSVs, overpressure is limited to 3%, I think some other mechanism is used to achieve full lift within 3% overpressure. Hence, I am not sure if 3% line loss criteria is applicable for ASME section I.

I would be glad to have views of che stalwarts on basis above.

Regards,

Sachin