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Help with Relief Valve Capacity Calculation

Help with Relief Valve Capacity Calculation

Help with Relief Valve Capacity Calculation

how would i calculate the capacity for a piece of equipment which has a MOP of 800# and it's MAWP is 110% of that - 880#. the relief valve is set for 850#.

from API 520, the total relieving pressure equals set pressure + 10% of set pressure + atmospheric, then obviously it would exceed the MAWP (850+85+~15=950 > 850). my question: is the relieving pressure now limited to the MAWP (850) & consequently, the overpressure term is ~1.7% instead of 10%?  or are you allowed to determine capacity at 110% of setpressure (i.e. 950#) even though the reliving pressure term is greater than the MAWP of that equipment?

RE: Help with Relief Valve Capacity Calculation


From I understand, it seems to me that you have made a mistake when interpreating your set pressure for your valve.

Yes you have to count 10% overpressure + Pa,but you are going to open your valve at 10% overpressure only, Pa is set to give pressure in PSIA, your MAWP is set (check on this) in PSIG, which is normally the case.

So you will have a valve opening at 850PSIG with a MAWP of 880 PSIG as well.

If this is not the case, Ops! you have a problem, your limit whne seting pressure reilef valves is the MAWP, you can't cross it.

Hope this will help.


RE: Help with Relief Valve Capacity Calculation

Kalhij, take a look at your pressure vessel code.  This is also explained in a lot of pressure relief valve manuals.  I'm going to use ASME Section 8, Div 1 as that is what I work with.  

A single PSV can not be set higher than the vessel's MAWP, in this case 880 psig although you can set it lower.  

The PSV must prevent the pressure in the vessel from rising more than 110% of MAWP or 968 psig.  As this is an upset condition, you are allowed to exceed the MAWP (under upset conditions when the PSV is relieving this is true, under 'normal' operations, you can not exceed the vessel's MAWP).  

If you have 2 or more safety valves, you are allowed to go to 116% of MAWP during an overpressure situation (the first safety valve must be set no higher than 100% of MAWP, the second or additional PSVs can not be set more than 105% of MAWP.

Fire is a somewhat special case.  If the overpressure is caused by fire, then the overpressure can reach 121% of MAWP.  

RE: Help with Relief Valve Capacity Calculation

thanks for your responses. perhaps my original post was confusing as both of you seem to think i said my RV set pressure was higher than my MAWP.  sorry for the confusion :>)

forget the terms i used before (MAWP,...) let me restate: the relief valve is set for 850#, the maximum oper. pressure of my device is 880#.

when calculating the capacity, the reliving pressure term equals: set pressure + 10% of set pressure + atmospheric. (let's forget the atmospheric part.)  my question is can the reliving pressure (i.e. 850+85=935#) be greater than 880# for this situation?  or should that relieving pressure be EQUAL to 880#??

hope this makes sense.

RE: Help with Relief Valve Capacity Calculation

Look at the 3rd paragraph in my post.  Yes, when you are relieving, the pressure is the vessel is allowed to exceed the MAWP of the vessel.  How much depends on what is causing the overpressure and whether you have one relief valve or several.

RE: Help with Relief Valve Capacity Calculation

thanks td2k & bill_bill,
after re-reading API 520 and having a simultaneous discussion with another tech source, i better understand this situation.  in API 520 sect 3.5 they give two examples in which both relieving pressures exceed the MAWP.

however, it now raises another question. the examples are for relief valve set pressures equal to and greater than the MAWP. but for both cases, the relieving pressure was the same!? it doesn't appear that the eqn for the relieving pressure term was strictly applied.
Relieving pressure (RP) = SetPress + 0.1*SetPress + P_atmos

the MAWP is 100#. in one case, the set press was 90#; for the other case the set press was 100#. in both cases the relieving press is 124.7# (psia).

have any idea why the 0.1*SetPress was not used for both cases?

RE: Help with Relief Valve Capacity Calculation

Was this a case where you had 2 PSVs on the same vessel and both were relieving?  

If this is the case (and I haven't pulled out API 520 to look at this), both PSVs will see the same inlet pressure during a relief and therefore that pressure should be used to determine their capacity.  

The 124.7 psia relieving pressure looks like it came from the 100 psig set pressure of the second valve + 10% + 14.7 atmospheric pressure.  The first relief valve, set at 90 psig, also has an inlet pressure, obviously, of 124.7 psia (effectively 22.2% overpressure).  The 10% is applied to the second valve as you need that overpressure, per API, to get full opening on the PSV (in reality, you might not need that much. Pilot operated PSVs can be fully open at as little as 3% overpressure but it depends on the manufacturer).  The 10% is a reasonable figure to use and is supported by API 520.

In this case, I suspect the first PSV has been set less than the vessel's MAWP as ASME section 8, Div 1 only allows you to set the second relief valve no more than 105% of MAWP.  With a 100 psig set pressure, this would suggest the MAWP is not less than 95.2 psig.

RE: Help with Relief Valve Capacity Calculation

this was not for multi-valve installation. the examples were both for single valve installations. they simply gave two separate cases in which the set pressure of the relief valve was different (one for 90#, the other for 100#).

the relieving pressure they calculated for both cases was 124.7 (psia).  i thought the relieving pressure = set pressure + overpressure.  and overpressure = 10% of set pressure.

is this not the case?

RE: Help with Relief Valve Capacity Calculation

Umm, normally yes.  I would expect the relieving pressure to be calculated as you said.

I've looked in API 520, 2000 version, in section 5 and can't see a calculation for 90 psig and 100 psig.  What version do you have?  It might be a typo but without knowing the specifics, it's hard to say.

RE: Help with Relief Valve Capacity Calculation

i have 7th ed. (1/2000). the section i'm referring to is 3.5, table 2.

RE: Help with Relief Valve Capacity Calculation

The example you are looking at is where you are protecting a vessel with a set pressure of 100 psig in both cases.  In one case, the PSV is set at 90 psig, in the other, 100 psig.  

Remember, the PSV is to protect the vessel, not the PSV thus the maximum relieving pressure is MAWP*1.1 + 14.7.  The 10% actually comes from the ASME code which again focuses on the vessel.  The pressure vessel code doesn't really care where you set your PSVs (as long as for a single one, you don't set it higher than the MAWP).  You could set it at 50 psig and use 120% as your accumulation which would give you a relieving pressure of 124.7 psia and meet Code requirements.

This difference (880 psig MAWP versus 850 psig set pressure) can arise during design when the MAWP of the vessel isn't initally known.  The process engineer will pick a design pressure and the vendor will manufacture the vessel.  Some companies require the manufacturer to go back, determine the weak component (shell, head, flanges, etc) and rate the entire vessel for that pressure.  Now, you could go back and change the PSV set pressure, update the impacted documentation, make sure the entire system that the PSV is protecting can handle the higher pressure but that takes time and money and may or may not be done.  Probably not for this small of a change, it's not like you are going to go to a smaller PSV and save enough money to justify the cost of the changes.

You could also do the calculation at 90*1.1 + 14.7.  You would have a lower relieving pressure, thus the capacity of the PSV would be lower.  If this was a new installation, you might either have to select the next orifice size or if this was an existing vessel, recommend to management to replace the PSV both of which cost money and isn't necessary to adequately protect the vessel.  Not recommended.

Read API 520.  All the sections in 3.5 refer to 110%, 116%, 121% OF THE VESSEL'S MAWP.

RE: Help with Relief Valve Capacity Calculation

TD2K, you wrote "The example you are looking at is where you are protecting a vessel with a set pressure of 100 psig in both cases.  In one case, the PSV is set at 90 psig, in the other, 100 psig."

i'm guessing you meant the MAWP was 100 psig in both cases - not the set pressure?

i guess my confusion lies in the calculation for a relief valve's capacity.  from RV manufacturer's catalogs, that equation is V (capacity) = (6.32*C*K*P*A)/([M*T*Z]^0.5)
the "P" in that equation is the inlet flowing pressure. where P = set pressure - inlet pressure loss + allowable overpressure + P_atmos.

so does this mean that allowable overpressure = 1.1*MAWP - set pressure??  if so, then i think i now better understand the equation.  my fault may lie in making that allowable pressure = 0.1* set pressure.

so in my original example:
RV set pressure = 850#
MOP (=MAWP?) of the equipment = 800#
therefore max accumulated press, MAP = 1.1*MAWP = 880#
allowable overpress = MAP - set pressure = 880 - 850 = 30#

assuming no inlet press losses, my resulting P in that capacity equation should then be:
P = 850 - 0 + 30 + 14.7

does this seem right?

RE: Help with Relief Valve Capacity Calculation

You are right, I should have written MAWP rather than set pressure, glad you caught that.

The relief valve manufacturer's use set pressure in their catalogs as you pointed out (good point, I never actually thought about that before).  I suspect the reason is that a PSV may be physically located on a vessel but be protecting the inlet piping and associated equipment, the pressure vessel and any associated equipment (eg. for a column the reboilers) and outlet piping and equipment.  All of these could have different 'MAWP', line losses when flowing to the PSV that might have to be taken into account and it's up to you to decide where to set the relief valve to achieve all of this.  Since the vendors likely don't want to get into a long discussion about this and perhaps set a case for someone arguing that 'this was your fault as we followed your example', they use set pressure (making the implicit assumption that the relief valve setting is at least not greater than any limiting MAWP).  At least, I could make a reasonable case for that arguement .  Whew, run-on sentence.

The allowable overpressure is always 1.1*MAWP (1.16 or 1.21 for multiple PSVS or fire respectively) for ASME pressure vessels.  As long as the PSV set pressure is less than MAWP, you can use 1.1*set pressure which will give you a conservative answer for sizing or reviewing the valve.  If you have a complicated system, pressure drops within the system (in additon to the inlet losses to the PSV) may have to be considered.  ASME Code allows you, with certain requirements, to protect multiple vessels with a single PSV as long as no vessel is exposed to pressures in excess of that allowed.  Since one of those vessels could be a long ways away, you might have to consider what pressure it would be at with the PSV relieving and is it adequately protected.  Piping is also part of this consideration but you may be able to allow for higher overpressures depending on the design code AND the company's practices.  I may be overcomplicating this for your initial question but I don't want to mislead you for a more complicated system.

Yes, the allowable overpressure for a pressure vessel is 1.1*MAWP - set pressure (not MOP).

MOP is not equal to MAWP.  MAWP is the limit of the vessel's mechanical design and may be equal to or higher than the design pressure.  MOP (maximum operating pressure) is the maximum pressure the vessel 'should' be operated at and is not a hard number.  It commonly is set based on some % of the MAWP or PSV set pressure to avoid accident opening of the PSV and to allow for normal flucuations in the process.  90% for conventional safety valves or 95% for pilot operated valves is common when selecting the MOP.  If the MAWP of the vessel is 880 psig, 90% of this is 792 psig and I suspect this is where the 800 psig MOP came from.  However, the MOP is not used in pressure relief valve calculations.  If the MOP was equal to the MAWP, you can NOT set the PSV at 880 psig.

RE: Help with Relief Valve Capacity Calculation

thanks TD2K.  this discussion has been very enlightening. guess i need to determine what the true MAWP is for our equipment....

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