10 replies to this topic

[sheiko]

Hello,

In Europe, PED (Pressure Equipment Directive) states:

- Article 1 - Section 1: "This Directive applies to the design, manufacture and conformity assessment of pressure equipment and assemblies with a maximum allowable pressure PS greater than 0,5 bar."

- Article 1 - Section 2.3 : "Maximum allowable pressure PS means the maximum pressure for which the equipment is designed, as specified by the manufacturer.
It is defined at a location specified by the manufacturer. This must be the location of connection of protective and/or limiting devices or the top of equipment or if not appropriate any point specified. "

- Annexe I - Section 2.2.3.b: "the calculation pressures must not be less than the maximum allowable pressures and take into account static head and dynamic fluid pressures and the decomposition of unstable fluids. Where a vessel is separated into individual pressure-containing chambers, the partition wall must be designed on the basis of the highest possible chamber pressure relative to the lowest pressure possible in the adjoining chamber"

Also, according to API STD 521 - Section 3.23, the design pressure is equal to or less than the MAWP, while, the european PED seems to state that the design pressure is equal to or greater than the PS ...This seems odd to me.

I was wondering if anyone could help me understand this concept of PS (which is apparently not the MAWP)?
Could you please explain/clarify?


Thank you in advance.

[Andrei]

sheiko

In API 521, at page 30, Figure 4 - Pressure Levels, there is a diagram showing clearly the pressure you are refering at (100 on the scale) in relation with other pressures levels.

[Art Montemayor]

Sheiko / Andrei:

Regardless what you call it, "MAWP", "PS", or "Design Pressure", the issue that is emphasized and propagated in spirit through the API documents is one of SAFETY - in engineering design and in engineering operation.

What I mean by the above is the fact that hasn't been mentioned and brought out: WE, the engineers who design and the engineers who employ the vessels, machines, and other apparatus are responsible professionally, morally, and legally for the safety involved in using the same equipment. Therefore, please bear in mind that the term "Maximum Allowable Working Pressure" as used and mentioned in all the API and ASME standards, codes, and recommended practices, means: THE ACTUAL, EXISTING MAXIMUM AND SAFE PRESSURE THAT CAN BE IMPOSED ON THE SUBJECT EQUIPMENT AT THE TIME IT IS BEING EVALUATED. This means that we, the engineers, are responsible for knowing absolutely the existing condition of the subject equipment AT THE TIME IT IS EVALUATED FOR USE.

It does not mean that simply because a manufacturer labeled a vessel as having 1,000 psig as MAWP 5 years ago, that we can use that same MAWP value today without VERIFYING that it is applicable. The same applys to a "Design Pressure". You may "design" for it, but is it really the binding, and applicable maximum pressure that can SAFELY be applied? That is what ALL CODES, REGULALTIONS, AND LAWS ARE TRYING to get across to design and operating engineers.

It doesn't matter what we call it. The absolute, CURRENT, actual, maximum allowable pressure should be clearly defined and understood. Once that is a reality, then we can base our calculations on it, knowing that at that time the calculations are sound. With time, corrosion, temperature stress, metal fatigue, pressure shock, and other negative effects, the value of the maximum allowable pressure could easily diminish - whether we are aware of it or not. But regardless, WE ARE STILL RESPONSIBLE FOR ENSURING THAT THE SUBJECT EQUIPMENT IS SAFE FOR USE BY HUMANS.

The above fact(s) is just one or some of the reasons why operating companies should keep continuous maintenance and inspection records of all equipment that they operate.

I do not agree with the statement ""Maximum allowable pressure PS means the maximum pressure for which the equipment is designed, as specified by the manufacturer". What if corrosion has set in since the equipment was manufactured? What if the equipment was damaged during shipment? The maximum allowable pressure should be THE CONFIRMED AND IDENTIFIABLE EXISTING VALUE OF MAXIMUM PRESSURE THAT CAN BE APPLIED AT THE TIME IT IS BEING EVALUATED - and it is subject to being diminished as time, wear, age, corrosion, errosion, etc. all have an effect on the equipment.

[sheiko]

sheiko

In API 521, at page 30, Figure 4 - Pressure Levels, there is a diagram showing clearly the pressure you are refering at (100 on the scale) in relation with other pressures levels.

Andrei,

Just to fix ideas:

PS = Maximum allowable pressure (from PED)
MAWP = Maximum Allowable Working Pressure (from API)

I could not find the PS term in any of the API publications. Namely, i did not found figure 4 at page 30 in API STD 521 - 5th Ed. (2007), but figure 26 at page 31 in API RP 520 - 7th Ed. (2000). However, impossible for me to find any MAXIMUM ALLOWABLE PRESSURE (not working pressure) on this figure.

I make the above distinction because:
- According to API STD 521 - Section 3.23, the design pressure is equal to or LESS than the MAWP, while,
- PED seems to state that the design pressure is equal to or GREATER than PS ...

[Andrei]

sheiko,

You are right, the diagram I was refering at was removed from latest versions of API 521. The page number I was quoting was from 1997 edition.

[sheiko]

Sheiko / Andrei:

Regardless what you call it, "MAWP", "PS", or "Design Pressure", the issue that is emphasized and propagated in spirit through the API documents is one of SAFETY - in engineering design and in engineering operation.

What I mean by the above is the fact that hasn't been mentioned and brought out: WE, the engineers who design and the engineers who employ the vessels, machines, and other apparatus are responsible professionally, morally, and legally for the safety involved in using the same equipment. Therefore, please bear in mind that the term "Maximum Allowable Working Pressure" as used and mentioned in all the API and ASME standards, codes, and recommended practices, means: THE ACTUAL, EXISTING MAXIMUM AND SAFE PRESSURE THAT CAN BE IMPOSED ON THE SUBJECT EQUIPMENT AT THE TIME IT IS BEING EVALUATED. This means that we, the engineers, are responsible for knowing absolutely the existing condition of the subject equipment AT THE TIME IT IS EVALUATED FOR USE.

It does not mean that simply because a manufacturer labeled a vessel as having 1,000 psig as MAWP 5 years ago, that we can use that same MAWP value today without VERIFYING that it is applicable. The same applys to a "Design Pressure". You may "design" for it, but is it really the binding, and applicable maximum pressure that can SAFELY be applied? That is what ALL CODES, REGULALTIONS, AND LAWS ARE TRYING to get across to design and operating engineers.

It doesn't matter what we call it. The absolute, CURRENT, actual, maximum allowable pressure should be clearly defined and understood. Once that is a reality, then we can base our calculations on it, knowing that at that time the calculations are sound. With time, corrosion, temperature stress, metal fatigue, pressure shock, and other negative effects, the value of the maximum allowable pressure could easily diminish - whether we are aware of it or not. But regardless, WE ARE STILL RESPONSIBLE FOR ENSURING THAT THE SUBJECT EQUIPMENT IS SAFE FOR USE BY HUMANS.

The above fact(s) is just one or some of the reasons why operating companies should keep continuous maintenance and inspection records of all equipment that they operate.

I do not agree with the statement ""Maximum allowable pressure PS means the maximum pressure for which the equipment is designed, as specified by the manufacturer". What if corrosion has set in since the equipment was manufactured? What if the equipment was damaged during shipment? The maximum allowable pressure should be THE CONFIRMED AND IDENTIFIABLE EXISTING VALUE OF MAXIMUM PRESSURE THAT CAN BE APPLIED AT THE TIME IT IS BEING EVALUATED - and it is subject to being diminished as time, wear, age, corrosion, errosion, etc. all have an effect on the equipment.

I fully agree with Mr Montemayor.

However, the above definition of the PS term is from a directive, a legal text. This is why i would like to clearly identify it and understand it.

For info: http://ec.europa.eu/...d/index_en.html

So, is anybody familiar with this european directive in general and this PS term in particular?

[gvdlans]

I don't see your problem...

You state that according to PED the design pressure is equal to or greater than PS. However, you fail to give a direct quote from the PED directive where this is stated. I checked but couldn't find this statement in the PED.

You also state that PS is apparently not the same as MAWP but again you fail to give a direct quote.

As you writer yourself, PS is defined as "Maximum allowable pressure PS means the maximum pressure for which the equipment is designed, as specified by the manufacturer." Note that "manufacturer" in PED terms does not have to be the company that actually fabricates the equipment. In every project, it has to be decided who will take the PED role of "manufacturer". In the case of the design of a chemical plant, this can be the client (operating company), the EPCM contractor, a Project Management Consultant or the actual equipment manufacturer, or any other that takes the responsibility for compliance with PED.

Now this manufacturer has to define the PS, e.g. in the linelist for piping or in the equipment datasheets for equipment. Of course, the PS must be at least the design pressure, and must not be more than the MAWP. I would say that in general it is an advantage to use a PS equal to the design pressure since this may result in a lower PED class which means that there are less stringent requirements. Also, in the early stages of a project the MAWP is not known, and therefore the design pressure has to be used.

[sheiko]

gvdlans,

First, thank you for your input

If you re-read my first post, you will find the quote (Annexe I), and will probably notice that it is in disagreement with what you wrote: "Of course, the PS must be at least the design pressure..." (provided the so-called "calculation pressure" in the PED corresponds to the design pressure).

Also, it is my interpretation that PS and MAWP differ from each other, which needs to be clarified.

[gvdlans]

gvdlans,

First, thank you for your input

If you re-read my first post, you will find the quote (Annexe I), and will probably notice that it is in disagreement with what you wrote: "Of course, the PS must be at least the design pressure..." (provided the so-called "calculation pressure" in the PED corresponds to the design pressure).

Also, it is my interpretation that PS and MAWP differ from each other, which needs to be clarified.

So here is your assumption. You assumed calculation pressure is the design pressure and maximum allowable pressure is MAWP. I assume by carefully reading this annex that calculation pressure is similar to MAWP since it is the pressure that was used to calculate the resistance of the pressure equipment...

A larger quote from this Annex I is:

Resistance

Appropriate design calculations must be used to establish the resistance of the pressure equipment concerned.
In particular:

- the calculation pressures must not be less than the maximum allowable pressures and take into account static head and dynamic fluid pressures and the decomposition of unstable fluids. Where a vessel is separated into individual pressure-containing chambers, the partition wall must be designed on the basis of the highest possible chamber pressure relative to the lowest pressure possible in the adjoining chamber,

Note that the PED was written by lawyers, not by engineers

I also stated in my previous post that PS and MAWP can be different. Again:

Maximum allowable pressure PS means the maximum pressure for which the equipment is designed, as specified by the manufacturer.

Where manufacturer is the party that takes responsibility for compliance of the pressure equipment with PED. In many cases the PED manufacturer is NOT the actual fabricator of the equipment, but an engineering company or even the end user (the operating company).

[sheiko]

gvdlans,

Thank you so much for these clarifications.

You assume calculation pressure is MAWP. Then why not using directly the term MAWP?

Another question please: do you see any usefulness in introducing this PS term, when out of Europe they do not use it?

[gvdlans]

Again, PED was written by lawyers and politicians, not by engineers.

Main objective of PED was/is to remove trade barriers between members of the European Economic Area.

Also note that PED was not particularly written for the process industries. It is also applicable for equipment such as pressure cookers and fire extinguishers.

I do not see any usefulness in using whatever term, however PED is a law and therefore "we" have to comply with it. I have always used design pressure for PS and this worked well and is no big deal.