8 replies to this topic

[dhns]

Dear all,

could anybody explian about "Difference between design pressure and maximum working pressure"?
for frocess vessels.

Regards,
Dhansekaran.R

[djack77494]

could anybody explian about "Difference between design pressure and maximum working pressure"?
for frocess vessels.

Excellent question, Dhansekaran. Early on in the design of a facility, the engineer is working on the design of the equipment that will accomplish the project goals, and especially will meet the process needs. During this phase, he will specify the design pressure for any vessels in the project. This is the a pressure selected such that it can contain the process fluids under all normal operating conditions. Usually the definition of "all normal operating conditions" extends to include startups, shutdowns, and many other "off-design" conditions.

This information passes to a vessel designer. He/she selects available plate that will suffice for constructiong a vessel meeting the engineer's requirements. Because the plate is available in discreet sized, the designer often selects "next size thicker", which is then used to construct the vessel. The end result of using plates, welds, heads, etc. that actually exceed the engineer's requirements, is that the vessel itself has a little "extra" that is "thrown in". If one took the actual vessel as constructed and calculated the pressure it could withstand, then one would have the vessel's maximum allowable working pressure (MAWP), which always meets or exceeds the design pressure.
Regards,
Doug

[HallDN123]

Just to add on top of the previous post, corrosion will reduce a vessel's MAWP, which may drop below the design pressue and require re-rating of the vessel.

[Art Montemayor]

HallDN123:

Yours is a great added value to Doug's specific, detailed, and informative response to a basic and important question.

When considering the importance and application of both terms, it becomes a vital issue as to WHEN to apply each of these terms and UNDER WHAT UNDERSTANDING. There is a clear and important reason as to why ASME and API always refer to the set pressure on a PSV as the MAWP - and NOT the design pressure. ASME assumes that the design engineer always takes the actual and real physical state of the vessel into consideration. True, the use of the design pressure is often the more conservative one; however if, as you well point out, no importance is given to the actual physical state of the vessel walls with respect to the original state, a grave mistake can be made in certain instances. That is why it is so vitally important for a plant to impose and rigorously maintain a pressure vessel and storage tank inspection and documentation control system year-round. ALL pressure vessels and storage tanks should be thoroughly inspected and calibrated on a routine basis AND documented. All PSVs related to these vessels should be confirmed as being applicable to the SAFE set pressure. If not found in the original physical state, the PSVs should be replaced or re-set and the safe set pressure of the vessel documented in plant records.

We often neglect to take the continuous corrosion effect on a vessel into consideration when we do process calculations and make daily decisions in our process plants. NO process vessel or storage tank can be reasonably expected to remain in the original, fabricated MAWP calculated for it when it was fabricated. Corrosion, wear, and stress will have negative effects throughout its entire life and this must be reflected in the settings that we apply to pressure relief devices on these same vessels. Storage tanks are not the seemingly benign, safe type of vessels that we all would be led to believe. They, too, require routine inspection and re-rating and they can cause grave harm and damage if routine inspections are not applied through their useful life.

Please excuse my extending the explanation to the basic question. I believe that the reasons behind these engineering terms have a definite value and important explanations. I would hate for any engineering student to take these definitions as mere clerical importance or semantics. As in all engineering, there is a DEFINITE IMPORTANT REASON for their existance and understanding.

[katmar]

I have no quibble with the definitions and recommendations given by the posters above, but there is a situation that I come across regularly that always leads to confusion and arguments over the definitions. This is when tall vessels, for example distillation columns (towers), are to be rated as pressure vessels.

If a distillation column will operate at 200 kPa(g) and it is 20 m tall then when it undergoes its in-situ hydraulic pressure tests (or if it floods in operation) we could have a situation where the top of the column is at 200 kPa(g) while the base "sees" 400 kPa(g). This is a significant difference and the lower part of the column must be designed for this higher pressure.

Thus you can have the situation where a vessel is stamped with a design pressure (or MAWP) of 200 kPa(g) but the designer must in fact have used a higher pressure in the actual calulations of the plate thickness at the bottom of the column. I do not know the codes well enough to know whether it is covered by a particular regulation, but it is something that needs to be taken into account.

[Qalander (Chem)]

Dear Katmar,Realy good added dimension at times;this might get overlooked. Height adds column head aspects in a more pronounced manner.

Best Regards to all above posters as this all had great relevence.
Qalander

[djack77494]

I know this can be somewhat confusing, but keep in mind that we are using a single number, be it the design pressure or the MAWP, to represent the entire vessel. Added to this is the fact that the vessel will exist in various states of repair (disrepair?) over its useful lifetime. Thus we must understand the importance and use of this single number.

I have seen the use of multiple MAWP definitions. This is disturbing since it leads to confusion and possibly avoidable errors. Personally, I like to think of MAWP to be the pressure that the vessel can safely withstand in a hot and corroded condition. Additionally, this pressure is one that may develop at the top of a horizontal vessel or the top tangent line of a vertical vessel. As noted above, the bottom of a tall vertical vessel may experience a considerably higher pressure. This of course must be considered in the vessel's design, and an experienced vessel designer would "automatically" allow for this fact in his design.

Expanding on the entire concept of vessel specification a bit, I would hope that all are aware that a vessel specification by a process engineer would include dimensions or capacity, a design pressure, a design temperature, a material of construction and corrosion allowance, and a minimum design metal temperature (MDMT) along with a coincidental pressure. Add a design external pressure (vacuum) if applicable. I would hope that someone would introduce a new topic dealing with TEST pressures so we can see how that parameter relates to these others.
Doug

[mscahmed]

Dear Katmar,

Regarding to your question, It’s a debits question fro a lot of Engineer in the oil and gas industery .

I make things easy for you , that I have a process and operations experiences in different processing plants.

• Let take a figure 100 Bar is the normal operating pressure for vessel X.
• The setting for pressure safety valve will be 125:135 bar range.
• The hydro test pressure will be 150 bar (This pressure is the Max allowable working pressure” if you go higher the vessel materials will fail" also this pressure will generate the max. stress for the vessel materials, for this reason not recommend to reach to this pressure more than one time in the good life time of the vessel unless any repair or maintenance happen.

I hope to declare this issue for you


Regards

Ahmed Farghaly.MSc,MBA.

[kkala]

...could anybody explain about "Difference between design pressure and maximum working pressure"?
for process vessels. Regards, Dhansekaran.R

The question does not mention "maximum allowable working pressure" (MAWP), but "maximum working pressure", which could mean (a) maximum operating pressure or (β) MAWP, if "allowable" is neglected by apparent error. Above posts are based on assumption (β).

Edited by kkala, 26 January 2011 - 03:50 PM.