MAWP & DESIGN PRESSURE

http://cr4.globalspec.com/thread/28796/Working-Pressure-over-Design-Presure

http://cr4.globalspec.com/thread/21718#newcomments

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=mawp%20design%20pressure&order=asc

The official definitions from VIII-2

Design Pressure – The pressure used in the design of a vessel component together with the coincidentdesign metal temperature, for the purpose of determining the minimum permissible thickness or physical characteristics of the different zones of the vessel. Where applicable, static head and other static ordynamic loads shall be included in addition to the design pressure in the determination of the thickness of any specified zone of the vessel.

Maximum Allowable Working Pressure – The maximum gage pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature for thatpressure. This pressure is the least of the values for the internal or external pressure to be determined by the rules of this Division for any of the pressure boundary parts, considering static head thereon,using nominal thicknesses exclusive of allowances for corrosion and considering the effects of any combination of loadings specified in the User's Design Specification at the designated coincident temperature. It is the basis for the pressure setting of the pressure relieving devices protecting the vessel. The design pressure may be used in all cases in which calculations are not made to determine the value of the maximum allowable working pressure.

And I will add this:

The difference between design pressure and MAWP is not a simple relationship - they are related by the practicality of manufacturing. The design pressure is the pressure that the vessel will normally operate at PLUS a value that is added due to experience, nature of the vessel contents, any foreseeable pressure pulses, or any future increase in capacity. The (initial) calculations for the vessel are performed with this number.

However, during fabrication, it is often convenient/cheaper to increase the thickness of components because of material availability. If this is a one-off vessel with relatively small amounts of material it does not make sense to have a custom mill run of material which requires a minimum order. Take for example (I am just using random numbers - may or may not reflect reality) that the design pressure calculations show that 9/16" thick plate is required, but due to availability/standard products, this material is normally supplied in 5/8" plate - you would more than likely use the thicker material (as it would probably be cheaper anyway than a custom mill run). This increase in material, obviously, increases the strength of the vessel. The MAWP would result from "reverse engineering" this thicker material to obtain a new, higher MAWP over design pressure (factoring in ALL of the vessel components and corrosion allowance as well). If you don't have an MAWP you would set your pressure relief system to the design pressure.

So both MAWP and design pressure are applicable to all vessels. Sometimes, as a matter of "convenience" fabricators will simply say that MAWP = design pressure because they don't want to back calculate or be held accountable to the higher specified pressure. It can be stated in the purchase contract that a "real" MAWP be supplied as well, or that the limiting factors should not be vessel nozzles or other simple items - this is to avoid having a shell capable of more pressure, but because a small item such as a nozzle did not have the appropriate, relatively easy additional reinforcement added to not be the limiting factor - you lose out on the additional thickness of the shell.