5 replies to this topic

[shantanuk100]

Hello All,

 

There is a lot of confusion prevalent regarding the terms Design Pressure (DP), MAWP, PSV Setpoints for pressure vessels, etc. so I'm sharing this document in which I've illustrated the differences between the terms, and their ascension in order of magnitude. 

Anyone with any suggestions, review, comments, or corrections kindly let me know.

Please check the revised version for some corrections that have been made as compared to the original file.

 

Thanks.

Attached Files

•  Guidelines on DP, OP, MOP, MAOP, MAWP, PSV Setpoints.pdf   230.75KB   323 downloads
•  Definitions of Pressure Vessel and PSV Design and Fabrication Values - Revised and Updated.pdf   212.83KB   253 downloads

Edited by shantanuk100, 22 November 2015 - 11:37 PM.

[afaruque]

Nice document. Keep up the good work.

[shan]

MAOP (Maximum Allowable Operation Pressure) is not defined in API 521.  Your introduction of the concept may cause more confusion.  Design pressure is the pressure, together with design temperature, used to determine the minimum permissible thickness or physical characteristic of each component, as determined by the design rules of the pressure design code.

[shantanuk100]

Dear Shan,

 

1. MAOP is not mentioned in API but I have seen numerous posts with people asking about what it is so I clarified it out.

The post is majorly focused on but not exclusively restricted to API 521 and is a general guideline for understanding the concept in order to avoid confusion. MAOP is ambiguous to many people, so here is a definition that explains what it is.

 

2. Regarding design pressure, what you say is exactly what I have mentioned about.

 

The Design Pressure and temperature are what are used to determine the minimum permissible thickness, wall thicknesses etc., as per the pressure design code but that is a consequence of the design pressure, not the basis for calculating it.

What you say is right but that is not the meaning of design pressure but rather a consequence of it.

What I have explained in the doc is about how the design pressure is found and what it actually means. 

 

I'm sure you would be aware of this but the design pressure is "The most severe case of process conditions (pressure/temp/flow etc.) that the vessel/piping is likely to face in its regular operation". This is the asme definition.

So we first decide the design pressure based on the most severe condition we are likely to encounter (Pressure / temp / corrosion) and then decide the thickness based on that design pressure.

 

Regards

Edited by shantanuk100, 23 November 2015 - 05:02 AM.

[Art Montemayor]

Shantanuk100:

 

Shan is correct in his brief comment.  There is more to the manner in which the various terms are defined in your document than meets the eye.

 

It is good that you have given attention and importance to the definition of the various terms used in pressure vessel design and fabrication as well as in pressure safety valve applications because this is an important topic for process and project engineers to master and dominate - whether involved in design or in process operations.  However, please refer to my copy of your document and the notations I have included in RED.

 

The main points that I have found in your document that, in my opinion, detract or mislead a young engineer from the actual definitions of each term are based on:

• Referring to pressure vessel codes or recommended practices, and not to natural gas pipeline codes or practices;
• The relationship of each term to process design where applicable as well as to fabrication mechanical design.

This last term is important to point out because it is an area remote and obscure to many young engineers who have not had the experience of fabricating vessels or have had the guidance of a mechanical engineer.  The first step towards the identification and specification of a pressure vessel lies in the realm of the process design engineer (usually an experienced Chemical Engineer by training).  The basic process design is converted into a detailed Datasheet by the combined input of the process design engineer and a project mechanical engineer.  This document is the cornerstone of the Request for Bids on the subject pressure vessel.  Once the successful bid is accepted and the purchase order issued, the final mechanical design is undertaken by the fabricator.  Until the final design is approved, the MAWP remains unknown.  It's value results as the value on the approved vessel fabrication drawings and its value usually depends on the fabricator's input calculations, approved welding procedures, and other factors.  Sometimes available steel plate thickness and economics determine this.  The owner or contract engineer's process design engineer has little to do with this, except through the project manager.  The point that I want to make here is that it is important to identify and know the limitations and circumstances under which these values are set and state this in their definition.  For example, you infer that the MAWP cannot be surpassed; yet this is frequently done in operation of PSVs.  This is not to approve of this or much less recommend exceeding the MAWP.  What is important is to recognize and understand how and why these values are set and their relationship to safe operation.

 

I welcome any and all further comments, experiences, critiques, or recommendations on this subject.

 

Attached Files

•  Definitions of Pressure Vessel and PSV Design and Fabrication Values.docx   22.93KB   126 downloads

[shantanuk100]

Dear Sir,

 

I have gone through the revised version you have included. Thank you for the feedback.

I have modified the document to remove any ambiguous terms or terms likely to cause confusion by interpretation and have retained your comments.

A few minor changes I've made.
 

1. I have separated the Process and fabrication based definitions of MAOP in order to remove any source of ambiguity.

 

2. Regarding the MAWP, sir,  I have already mentioned in the document earlier that we can exceed MAWP in specific cases as you had said. In this regard, I realise that my guideline for safety specifically asking not to exceed MAWP was causing the confusion so I have removed it. 

 

3. MAWP as we know can't be calculated by us but is to be obtained from the fabricator. My stating that the Mawp was usually around 25 % above OP was a general guideline, but since it is subject to rigid interpretation, I have removed it.

 

Please find the modified document attached below. 

Thank you for the effort and feedback.

 

Regards,

Shantanu

Attached Files

•  Definitions of Pressure Vessel and PSV Design and Fabrication Values.pdf   212.83KB   151 downloads

Edited by shantanuk100, 16 March 2016 - 06:42 AM.