We add a note to design pressure on our datasheets indicating that design pressure is at top of vessel.

Question: Per definition of design pressure in mandatory appendix 3, by default, the design pressure is referenced at top of vessel and design must add static head to design pressure to determine wall thickness of applicable zone of vessel. Do you agree that design pressure by default is referenced at top of vessel. Is my interpretation of code correct. Please advise.

Thanks

Hello Jagdev,

The pressure at the top of your vessel is the MAWP according to App. 3. This is your starting point for the determination of the design pressures for all components by adding the static head. Of course you can just design the whole vessel using one design pressure which covers all parts. It can (or has to if there is a static head) be higher than the MAWP.

However, because I know you are aware of these things I´m afraid your view on this matter is a little bit too complicate. You just don´t have a "design pressure by default" but a design pressure for each component.

Michael

Design pressure can not be higher than MAWP.

Design pressure specified in the datasheet is the design pressure at the top of the equipment and static head needs to be added for design of each component.

Most of the commercially available softwares consider appropriate static head based on the elevation of the component to be designed and the liquid levels.

In case of manual calculations, if design is not getting affected much, you can consider static head for the maximum liquid level and design all the components considering the same.

Sanjeev

Sanjeev

In case of column design of say 25 mtr. height.
and Operation level is 2 mtr.
what shall be Des pre to be used in calculations?

Client has aksed us to used De. pre + Dynamic Pressure

_________________
Niraj

Sanjeev,
I suppose we are talking about two different things. I like the last sentence of the definition of the MAWP:

You are calculating vessels for the maximum MAWP. However, we are using the design pressure for the design and try not to consider it as MAWP.
Michael

Michael,

I do not think we are discussing two different things. I also like MAWP definition as indicated by you in your post since it gives me flexibility not to calculate the MAWP and use DP in place of MAWP.

We need to first understand that MAWP (if calculated) shall always be higher than DP and this is the reason that code allow us to use a smaller value of MAWP ie DP (if MAWP calculation is not made) which is safer. Please refer to ASME Sec VIII Div 2 Part 2, section 2.2.2.1 (d).

Now let us try to understand the procedure of establishing operating pressure/ design pressure and MAWP

Operating Pressure/ Design Pressure: Operating pressure is established based on the process (for the worst anticipated condition) and design pressure is established considering some marging on the top of operating pressure. This Design pressure is specified on the datasheet when any one go out in the market to buy the equipment. We do not specify MAWP on datasheet.

MAWP: Thicknesses of various components/ equipment are established based on the design pressure (not the MAWP since it is not available in the datasheet) specified on the datasheet and rounded off to nearest higher available thickness in the market.
I will stop my design here if client do not want me to work out MAWP. If client want me to work out MAWP, based on my actual provided thickness, I will back calculate maximum allowable pressure for each component and consider the least calulated value as MAWP for the equipment. Now run the enire calculation again considering this least calculated value in place of design pressure so that this MAWP can be stamped on equipment name plate.
Since provided thickness are slightly more that the minimum required thinkness worked out based on design pressure, MAWP shall also be slightly more or at least equal to design pressure.

I think every one here agrees that client will provide you only design pressure and not the MAWP, which is worked out by the equipment designer.

As a client, I will definetely not accept a design if any supplier offer me an equipment with a lower MAWP than the DP specified in my datasheet.
I am paying for an equipment with my minimum requirements and will not accept if it does not meet those minimum requirements.

Now last question is how does MAWP helps and why not we design only with DP considering MAWP same as DP: As an owner, MAWP provide me a flexibility with my process and allow me to consider a higher pressure than my design pressure (with a limit upto equipment MAWP). This is very common in many de-bottle necking projects and many other places where design pressure needs to be increased for various process reasons.

Regards

Sanjeev,
If we aren´t speaking about two different things, we are at least watching the same problem from different points of view. Your design pressure comes from your client, and my design pressure is according the Code. According to UG-21:

You are completely right. However, the main thing is that the MAWP as stamped on the nameplate is verified by the calculation.
Michael

Michael,

ASME Sec VIII Div 1, Section UG 21, Design Pressure (DP): Fully agree with code requirements but coincident pressure and temperatures are not decided by equipment fabricator or designer. This data comes from User based on their process requirement.

ASME Sec VIII Div 1, Appendix 3, Definition of Terms, Design Pressure (DP): The pressure used in design of a vessel component together with coincident design metal temperature, for the purpose of determining minimum permissible thickness. (Please note thickness is determined based on DP and not the MAWP)

ASME Sec VIII Div 1, Appendix 3, Definition of Terms, Maximum Allowable Working Pressure (MAWP): Maximum gauge pressure at the top of the completed vessel in normal operating position…………………..This pressure is the least of the values of internal or external pressure to be determined by the rule of this division for any of the pressure boundary parts, including the static head thereon, using nominal thickness exclusive of allowance for corrosion and considering……….(Please note MAWP is calculated using nominal thickness and to arrive to a minimum required/ nominal thickness we have to first start with a design pressure)

ASME Sec VIII Div 2, Part 2, Section 2.2.2.1 (d) Specified Design Pressure: I would consider this specified design pressure same as design pressure defined in ASME Sec VIII Div 1. Please note this is covered under responsibility of user and not the manufacturer. I think in my last post also I should have used user in place of client.

I agree with your statement that MAWP as stamped on name plate is to be verified with calculation however please note MAWP calculation is based on using nominal thickness that means a backward calculation after you have selected your nominal thickness.

I also agree that we are looking at the same problem from different angles. You are considering MAWP as your design pressure and I am considering design pressure which is established based on user’s process requirements and specified in the datasheet and/ or user design specification.

I think this discussion is going too lengthy so finally to cut it short my last question is:

Can you stamp a vessel with MAWP (DP in your definition) lower than the specified design pressure in UDS and/ or datasheet??

Regards