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Maximum Allowable Working Pressure vs Design Pressure

SonnieTP (Mechanical)
13 Jan 05 15:23
Does anyone has an experience on the HRSG design and configuration? The MAWP and the Desgn Press is the same on our Superheater and Drum section as well as the Reheater section of our HRSG. I ask the manufacturer why they did not place an allowance on both section and they quoted me the ASME Section.
No boiler shall be operated at a pressure higher than the maximum allowable working pressure except when the safety valve or safety relief valve are discharging, at which time the maximum allowable working pressure shall not be exceeded by more than 6%.  (quoted from ASME Code Sec.I, PG-21.1)"

The manufacturer placed a Safety relief valve on this area but the problem with this is, whenever the safety relief valve is opened to relieve the pressure it never closes back and sometimes it rupture the seal.?Are there any HRSG users out there that has the same design configuration as our? If so, what did you do to resolve the problem (aside from replacing the relief valve everytime they pop up?
codeeng (Petroleum)
13 Jan 05 17:14
Don't be confused between the MAWP and design pressure as sometimes they are/can be the same pressure. More importantly you must have a suitable margin between operating and design pressure.
bvi (Mechanical)
13 Jan 05 22:05
design pressure is what is specified, the equipment must be safely designed to contain that pressure.

MAWP is the maximum pressure the the equipment can contain, and still comply with code rules.  So, if MAWP=DP, fine, the equipment is OK.

However, if the MAWP is greater than the DP, then if you require the equipment manufacturer to calculate a MAWP, rather than permit them to assume that MAWP=DP, then you have equipment with a slightly higher maximum permitted pressure.

The relief valves are set relative to the MAWP because that is the maximum pressure per code the equipment would be permitted to contain.

There is no problem with MAWP=DP; you simply are not getting the advantage of a possible slightly higher pressure capacity of the equipment, if the vessel has a little "fat."
metengr (Materials)
14 Jan 05 8:09
This sounds more like a steam relieving capacity problem with the current safety relief(s) on your HRSG. Go back to the HRSG  OEM and have them review your steaming capacity requirements. You might need to install a different SRV or add some.
davefitz (Mechanical)
14 Jan 05 8:20
For drum type boilers, the "master stamping pressure " = design presure = MAWP , with the possible exception of the downcomers and lower waterwall inlet headers which need to add gravity head to the MSP, and ditto for economizers.

The design pressure at the main steam outlet header should be at least 3-5% below the actual expected normal full load operating pressure, to allow for margin /error in setting safety valves and other tolerances. A higher tolerance may be required if you have an HP bypas and a requirement not to lift releif valves during a full load rejection.

The set pressure for the 1st drum relief valve must be set at or below the MSP, and the relief valve at the main steam outlet is usually set lower than MSP by a value equal to the full load pressure drop thru the superheaters.
chicopee (Mechanical)
14 Jan 05 15:10
I have never heard of the MAWP being greater than the design pressure; however, the reverse can hold true when fired and unfired pressure vessels are derated. ASME/NB rated safety valves for boilers have their set pressures at MAWP's and their releiving capacities to the heat input; for air tanks (receivers) S.V.'s have their releiving capacities set to their compressor free air deliveries and set pressures to the MAWP of the tanks.  For boilers, the number of safety valves is based on not to excced 6% of the MAWP.
PAN (Mechanical)
17 Jan 05 5:17
MAWP can be greater than design pressure. I support the reply of bvi.
davefitz (Mechanical)
17 Jan 05 7:15
error report:
" ...should be 3-5% HIGHER than the actual full load operating pressure at teh STEAM DRUM..."
jte (Mechanical)
17 Jan 05 12:38

As BVI and PAN pointed out, it is quite common in my experience to have the possible MAWP be greater than the design pressure as specified by the process engineers. For example, the process engineer says they need 100 psi at 100°F and the fabricator runs the numbers and gets a required thickness of 0.52". The fabricator selects 0.5625" plate with no change to the corrosion allowance. At this point, unless the client specifies otherwise, the fabricator has two choices:

1) Do nothing. Apply the footnote for UG-99 which states that "The MAWP may be assumed to be the same as the design pressure when calculations are not made to determine the MAWP." The client is still happy, because their design requirements are met.

2) Do extra work. Go back, and recalculate the true MAWP based on the selected plate thickness. The client doesn't care since all he's checking is that his process design pressure is met.

Hmmm... which approach do you think most fabricators would take? The client who is smart enough to care has already specified that the MAWP be calc'd up front. The client who doesn't know won't recognize the extra effort anyway.

chicopee (Mechanical)
17 Jan 05 15:18
PAN,I suppose that there may be components or some feature  of a pressure vessel having design pressures greater than the MAWP, however, the stamped MAWP will be based on the  feature of the pessure vessel having the lowest design pressure vessel.  But since manufacturer's data sheets refer to calculated thichknesses based on MAWP, the term design pressure will not appear because both terms are synonomous.
JoeTank (Structural)
17 Jan 05 17:27
I think the point made by others here is that many times the manufacturer will simple set the MAWP shown on the data report and nameplate as the design pressure.  It's the easiest way to do it at the desk top level.

"I suppose that there may be components or some feature  of a pressure vessel having design pressures greater than the MAWP, however, the stamped MAWP will be based on the  feature of the pessure vessel having the lowest design pressure vessel"

Your statement is true.  Sometimes the limiting design feature of the vessel is the manufacturer's calculations.  Or, lack there of.

Steve Braune
Tank Industry Consultants

deanc (Specifier/Regulator)
18 Jan 05 9:03
Take a look at ASME SecVIII Div 1 Appendix 3 Definitions.

Also NBIC Appendix F.
codeeng (Petroleum)
19 Jan 05 17:12
It goes a lot further than this. If you really want the MAWP to be meaningful then it should be specified together with a limiting element, usually the shell thickness. This ensures that any rounding up of the shell thickness to accommodate standard plate thickness will be utilized for the extra pressure. If you ask a vessel manufacturer to establish the MAWP without limiting an element then all you'll get is a MAWP equal to the design pressure. This is because the manufacturer will limit, say, head thickness or nozzle reinforcement to the design pressure. I keep recommending to clients to address this in their specs but it never gets done.
JoeTank (Structural)
19 Jan 05 18:06
My experience on the limiting factors is somewhat different.  I have often observed that the nozzles are often the limiting factor, not the shell thickness.  Even though the shell thickness is often rounded-up to varying degrees, the extra thickness is typically allocated to the area repalcement calcs.  Therefore; no extra shell thickness is available for a higher MAWP.

Steve Braune
Tank Industry Consultants

jte (Mechanical)
19 Jan 05 18:27
Codeeng and Steve-

Yes, good point. I didn't address how to specify how a fabricator should design the vessel and establish the MAWP. My spec is something like "The MAWP shall be calculated based upon the acutal shell and head thickness chosen. The MAWP shall not be governed by nozzles, nozzle reinforcement, or flange rating."

Fairly often, though, I'm buying replacement vessels or exchangers for service where the process engineers have no desire to increase the pressure. In that case I modify the "max out the MAWP" approach to require that the max possible corrosion allowance be calculated with the MAWP set at design pressure. Similar general statement about nozzles etc not governing. I do not accept fractional corrosion allowances. All too often I'm dealing with equipment which has a nominal CA of 1/8" and we're in a FFS situation with it. Run a quick check of the calcs and find out that there is actually something like 0.17" of available CA which got rounded down to the next nearest 16th of an inch and reported as 1/8" (0.125") CA. At say 5 mils per year corrosion (so a nominal life of 25 years), that little round-down cost us 10 years of useful life of the equipment. Then you're in FFS / rerate city.


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