PSV relief 6

[NiravK]

While designing relief valve, we consider 10% overpressure as a criteria, in general. What does it really mean?

1) Does this mean that the pressure in the system will remain at 110% value all the time during relief? (provided the cause of overpressure has not been mitigated).

2) Now consider that set pr.= max.allow.working pr.,
So, working pressure during relief will go upto 110% or 120%(Fire case). Does this accumulation harm the system components?

3) How does the mech.design pressure of system component will be decided? is it based on 'Set pressure', Max.allow.working pressure' OR 'accumulation'?

 

[quark]

There were some earlier discussions which state that you have to have 10% redundant flow capacity for PSVs(as a good design practice). In that case it need not be 110% always. Generally the relieving flow velocities will be generally higher.

As far as I am concerned, the design pressure is 1.5 times the MAWP.

Regards,

 

[TD2K]

Quark, the design pressure is equal to or less than the MAWP of the vessel. By 1.5, do you mean the hydrotest pressure (or 1.3 as allowed under the latest code version)?

Maybe we are talking about something different. To me as a process engineer, the design pressure is what I put on the data sheet initally, say 500 psig. When the vessel is actually designed and available actual head, shell and nozzle thicknesses selected, the vessel will likely be able to withstand somewhat more than my design pressure (unless the available plate for a component is exactly what I calculate I need, somewhat unlikely OR my design conditions are set by flange limits).


BAck to the OP. For pressure vessels in the US, you can not operate the vessel greater than its MAWP on a continuous basis. This is also the highest pressure you can set a single PSV protecting the vessel (if you have more than 1 PSV, the others can be set higher than MAWP but not more than 105% of MAWP). There is nothing stopping you from setting a PSV lower than MAWP however.

When looking at PSVs and allowable relieving pressures, you need to look at the ASME code (or other relevant codes for your area) for what you are trying to achieve. For the ASME code, for a single PSV other than fire, the code requires the PSV to prevent the vessel's pressure from rising more than 110% of MAWP. That means if the PSV is set at MAWP then it must have sufficient area to relieve the gas without the pressure exceeding 110%. Will the pressure actually reach 110%?

Consider that when you size a PSV you calculate the required area and then pick the next larger size PSV. If your relieving requirements were precisely correct, you could put the actual area back into the PSV equation and solve for the relieving pressure which would be lower than 110% of MAWP. However, picking the next largest orifice area (rather than having an orifice custom made for the precise area you calculated) provides a margin of safety in case your estimates of relieving flow rate, temperature, molecular weight, compressibility or other factors will have some errors in them.

Now, if you set a PSV lower than MAWP, you still only need to prevent the vessel from exceeding 110% of MAWP (for a single relief valve other than fire). The %accumulation for the valve itself could be higher than 110%, it's the vessel accumulation that is critical to ensure it won't be exceeded. Similar logic applies to multiple relief valves or fire cases.

If the vessel is properly designed for the conditions, 110%, 116% or 120% is unlikely to cause a vessel failure. First, the ASME code already has a substantial margin of safety built into it when you calculate the required thicknesses. Secondly, the vessel has been hydrotested to (typically in the US) 150% (or 130% today) of design pressure so you know it's seen and withstood higher pressures although not necessarily at the relieving temperatures.

 

[NiravK]

thnx both of u for reply.

i convinced by argument from TD2K..

Since vessel is already tested at 150% of it's design pressure, there should not be any harm to it or its piping components for comparatively smaller duratiion of exposure to pressures higher than MAWP.

any other idea ??

 

[hacksaw]

how long do you want your pressure vessel to last?

what is the risk to personnel (and equipment)should the vessel fail within the next twenty years?

What sevice conditions was the vessel designed for and is it being used in that service?

 

[quark]

TD2K,

Thanks for correction. Still I can't find out where I went wrong but I did make a mistake. Instead of 1.5 I should have said 1.1. It is a general practice that we consider 1.1 times MAWP as design pressure (I am not knowledgeable whether this is an international practice or not).

All your comments are well taken as usual.

Regards,

 

[Montemayor]

quark:

When discussing Design Pressure and Maximum Allowable Working Pressure (MAWP) with respect to pressure & vacuum relief devices, I have always identified the two as follows:

The key to sizing a pressure relief valve for pressure or vacuum relief is to make sure that the valve (with set point) chosen will flow the required amount of vapors at a tank pressure less than the MAWP of the tank. This insures that the tank's MAWP is never exceeded. The tank's design pressure is inherently equal to or less than the MAWP.

A pressure vessel's Design Pressure is simply the quasi-theoretical value calculated by the owner's process engineer or the fabricator's design engineer. This value serves as the target, or benchmark, by which the fabricator will proceed to actually fabricate the ultimate, final tank. Note that the Design Pressure is the target that the fabricator uses to ensure that his product will meet (& probably exceed) the specified pressure and vacuum conditions set forth by the purchaser/owner. In the real, practical fabrication of the pressure vessel the fabricator will employ those materials and components that - more often than not - exceed the specifications because certain components (such as the steel plates) are only available in standard thicknesses and grades. Another variable is the fabricator's incentive to employ his stored inventory of components and steel plate. In doing so, the fabricator again is often guided by his desire to employ his inventory rather than purchase additional materials. His incentive to rotate his inventory forces him to select applicable steel plate material that often surpasses the specifications - because of its existence and also because of standard sizing. As a net result, the newly-built pressure vessel's MAWP and MAWV are often in excess of the values specified for fabrication. These values may or may not be revealed to the Owner, depending on whether their calculations are specified as part of the agreed fabrication contract.

It is always conservative and, far more, safety-wise to employ the MAWP/MAWV values as the maximum criteria for setting the limits on storage tank operations. By doing so, the owner is forced to monitor the actual, physical conditions of his pressure vessels and take all wear, changes and modifications into consideration when attaching a maximum pressure and vacuum to a pressure vessel. This is in keeping with the spirit and intent of OSHA 1910 and all other safety regulations or guidelines.

I believe there has been some misconceptions betweeen the use or meaning of both labels. It is OK to use the Design Pressure as a substitute for the MAWP as long as the wear and corrosion of the vessel is such that the Design figure remains less than the MAWP. I always opt for working only with the more realistic and actual MAWP. This forces the owner to monitor his vessels (as the law's intent is intended) and always be aware of the real, actual conditions of his vessels and their physical limits at the time of the over/under pressurization. The Design Pressure is only an academic figure that converts itself into mere history the moment the vessel leaves the fabrication shop due to the fact that it really never represents anything other than what was specified - not normally what was really built and fabricated.

I hope the above helps in clearing up some ideas on the two terms and how/why they come into being.


Art Montemayor
Spring, TX

 

[quark]

Mr Montemayor

Good comments as usual and I do agree that with pressure relieving devices, I should concentrate on process primarily rather than mechanical engineering and it is a grave mistake if I wait till something happens. Thank you again TD2K and Montemayor.

Regards,

 

[hacksaw]

Best advice on the net as usual

 

[22082002]

Just a bit more information....
After finding out the release flows and fluid properties, at both upstream and downstream pressure levels, which a process engineer sends to PSV/Relief valve calculating engineer(i.e.the instru. engineer), that gentleman, I believe, adds additional 10% margin to size the orifice in the safety valve....

And then some people put margin for future expansions also...

So will the vessel really ever see MAWP????

Regards,
SAA