Setting pressure of safety device 1

[zhlin]

Dear everyong,

If a pressure vessel or system is designed to 300 psi, is it allowed to set up the setting pressure of a safety device to 1.1 times design pressure?

Thanks in advance.

 

[Chance17]

No - the safety device actual activation can not exceed the MAWP.
But you may evaluate the relief capacity (flow) at 110% of set pressure.
The device may activate at any pressure lower than MAWP.

 

[zhlin]

Appreciate your quick response!

As I understand, a safety device will be designed to 1.1 setting pressure.

Did you mean "The device may activate at any pressure lower than MAWP. " is when it is set to a pressure lower than MAWP or there could be a situation it reacts below the setting pressure?

 

[JLSeagull]

The pressure relief valve opens at the set pressure but the pressure can build higher while relieving. The relief valve must have sufficient capacity to prevent the pressure rise to exceed 110% of the set pressure. Perhaps others can clarify if this is a misconception.

 

[zhlin]

Thanks. It really help me to understand the code.
Usually a pressure vessel will be operated under the design pressure and a safety device will be set to the design pressure.

But the setting pressure of the safety device will have tolarences. Does that mean a vessel possibley can't operate upto its design pressure if the setting pressure is on its min tolerance - less than the design pressure, right?

 

[pleckner]

You do not need to account for tolerances when you set your opening pressure. Tolerances are accepted by the codes as known deviations. As a matter of fact, the tolerances are limited by code. The small deviation in actual opening pressure that might be caused by these tolerances is more than accounted for in the mechanical design of the vessel and is accepted as such.

The set pressure, as given in previous answers, can be any value up to, but must never exceed, MAWP. It is common to not know the true MAWP during design so we would just set the relief device to open at the design pressure.

 

[zhlin]

Sorry, forgot sending "Thanks" to all of you.

 

[JAlton]

Perhaps these ASME Code references will be helpful.

The Safety-Relief Valve shall be set at or below MAWP, Ref. ASME Sec. VIII, Div. 1, Para. UG-134(a).

The allowable accumulation is 3 psi or 110% of MAWP, Ref. ASME Sec. VIII, Div. 1, Para. UG-125(c).

The Set Pressure Tolerance for Safety-Relief Valves is +/- 2 psi or 3% of Set Pressure, Ref. ASME Sec. VIII, Div. 1, Para. UG-126(d)

JAC

 

[dcasto]

yes you can if you use a relief valve that goes 100% open at set point, a pilot operated relief valve works well in this application.

 

[zhlin]

It is a rupture disk in this case.

 

[dcasto]

rupture disks are a different story and have their own standards. The manufacturer of the disk will tell you what the highest pressure that could be expected for the disk to fail, that value would be the 100% of the MAOP of the vessel. BUT, the disk could open 10% below the MAOP, depending on the manufacturers data.


One manufacturer sells disks with either a 0%, 5%, or 10% rating.

 

[JAlton]

IT IS NOT PERMISSIBLE TO SET A PRESSURE RELIEF VALVE ABOVE THE MAWP OF THE VESSEL UNLESS YOU HAVE MULTIPLE VALVES ON THE VESSELS. AT LEAST ONE PRV MUST BE SET AT OR BELOW MAWP. MAWP AND DESIGN PRESSURE ARE NOT THE SAME. IT DOES NOT MATTER WHAT TYPE OF VALVE OR THE VALVES OPERATING CHARACTERISTICS.

ASME SEC. VIII, DIV. 1, PARA. UG-134(a), When a single pressure relief device is used, the set pressure marked on the device shall not exceed the MAWP of the vessel.

JAC

 

[pleckner]

UG-134(b) tells a different story. The wording seems to allow a set pressure of up to 110% of MAWP where fire or some other source of external heat is the sole scenario.

"For pressure relief devices permitted in UG-125(c)(2) as protection against excessive pressure caused by exposure to fire or other sources of external heat, the device marked set pressure shall not exceed 110% of the maximum allowable working pressure of the vessel. If such a pressure relief device is used to meet the requirements of both UG-125(c) and UG-125(c)(2), the device marked set pressure shall not be over the maximum allowable working pressure."

And UG-125(c)(2) basically just says the PSV has to prevent the overpressure from rising more than 21% above MAWP.

"(2) When a pressure vessel can be exposed to fire or other unexpected sources of external heat, the pressure relief device(s) shall be capable of preventing the pressure from rising more than 21% above the maximum allowable working pressure. Supplemental pressure relief devices shall be installed to protect against this source of excessive pressure if the pressure relief devices used to satisfy the capacity requirements of UG-125(c) and UG-125(c)(1)have insufficient capacity to provide the required protection. See Nonmandatory Appendix M, para. M-13 for cases where the metal temperature due to fire or other sources of external heat can cause vessel failure prior to reaching the MAWP."

However, you aren't gaining anything because the relieving pressure is still nothing more than 121% of MAWP. So the sizing is the same even if the set pressure is at MAWP or 110% MAWP as UG-134(a) seems to allow for what I would call a rare case.

 

[JAlton]

UG-125 (c)(2) refers to additional valves beyond the protection required by UG-125 & 125 (c) (1). See Second Sentence of UG-125 (c) (2);

Supplemental pressure relief devices shall be installed to protect against this source of excessive pressure if the pressure relief devices used to satisfy the capacity requirements of UG-125(c) and UG-125(c)(1)have insufficient capacity to provide the required protection.

JAC

 

[pleckner]

Yes, I read all of UG-125(c)(2) and even pasted it into my post. And I've read it and re-read it and don't see where UG-134(a) only applies to supplemental valves. Once again a very poorly written section open to way too much interpretation. But as I wrote in my concluding remarks, you gain nothing by pushing the envelope so to me, it is really a moot point.

We can agree to disagree on what the exact wording means but the final result is the same.

 

[JAlton]

The answer to the original question remains, "No". UG-134(a) limits additional valves to 105% of MAWP provided at least one (1) PRV is set at or below MAWP.

UG-134(b) clarifies the situation. Vessels must have the required protection of a PRV at MAWP. If supplemental valves are used they are in addition to the PRV set at MAWP.

It is necessary to recall that the code words "shall", "should" & "may" have distinct meanings. "Shall" is a requirement, "Should" is a recommendation & "May" is permission. The permission to set "supplemental" valves above MAWP does not negate or supplant the requirement for a PRV set at or below MAWP.

You may disagree, but the is only one interpretation for the existing wording.

JAC

 

[pleckner]

There is no disagreemnt in the interpretation of those magic words, SHALL, SHOULD and MAY.

We both agree that a supplemental valve can be set no higher than 105% of MAWP. Then in what instance do you acknowlege the use of UG-134(b), the allowance for a PRV to be set at 110% of MAWP? I contend that UG-134(b) refers to a single PRV only, and for a fire scenario (or other external source of heat) only.

 

[pleckner]

This is an update to my last post. I was only partially correct.

After more digging and discussions I was reminded of the following in API 520, Part 1, 8th ed.:

5.4.3.4 Supplemental-device Installation

5.4.3.4.1 A supplemental-device installation provides relieving capacity for an additional hazard created by exposure to fire or other unexpected sources of external heat. The set pressure of a supplemental device for fire shall not exceed 110 % of the maximum allowable working pressure, MAWP.

5.4.3.4.2 Supplemental devices are used only in addition to devices sized for operating (non-fire) contingencies.

So, if you install a supplemntal PSV strictly for a fire scenario (or other external source of heat), then this supplemental PSV can be set at 110% of MAWP. But as I also said before, the total accumulation is still only 21% above MAWP so you really aren't gaining anything by setting this PSV at 110% of MAWP, relieving pressure will still be the same.

 

[786392]

I understand that any supplemental pressure relief is to minimize residual pressure stress exposure duration,

thus enhancing the vessel(containment's) withstanding chances for the considered credible scenario.

Best Regards
Qalander(Chem)