Operating, Design and Overdesign values 2

[Melimelo]

Hello,

When one wants to design any equipment, we define the OPERATING (which are the real ones) and the DESIGN (which are with 5% or 10% more to the operating ones) values of T and P.

I thought that :
OPERATING values = DESIGN values
OVERDESIGN values = DESIGN values + 5/10%

But I think it's false...My boss said to me:

OPERATING values = Real values found in calculations
DESIGN values = OPERATING values + 5/10%

What do you think?

Thanks a lot!
Méli )

 

[Melimelo]

Hello everybody,

Here is an interesting discussion I have found on the same subject (on another site:

http://www.cheresources.com/invision/index.php?showtopic=2544

):

It is not unusual for engineers to get confused with the terms "design pressure" and "Maximum Allowable Working Pressure" (MAWP). The two terms are not related mathematically; rather, they are related in a practical procedure that takes place during the actual fabrication of a pressure vessel.

Every Chemical Engineer - sooner or later - will have to deal with a pressure vessel fabrication or operation and it is sound and good advice that he/she should familiarize themselves with engineering terms employed and the logic of their application. In dealing with or specifying a pressure vessel, an engineer must resort to filling in or using a Vessel Specification Sheet - much like the one I have included in the attached Excel Workbook that gives you a host of Specification Sheets that you should be familiar with and employ in the course of your work.

The "Design" pressure is that pressure that the engineer decides is the value of the pressure at which the vessel will normally operate (or which it must withstand under operating conditions). This value must include any normal excess pressure that can occur during the vessel's operation. This is a discretionary value that depends on the background and experience of the design engineer. Sometimes the design value can be 10% over the pressure calculated (as in a simulation) or as much as 25% more. Good engineering judgment is employed in arriving at this design figure.

Once the Specification Sheet is received by the vessel fabricator, mechanical fabrication design takes place in which alloys, fabrication techniques, available materials, and other factors are taken into consideration to generate a fabrication drawing. Although the design pressure given is employed to generate the required vessel physical characteristics, some practical factors - such as available materials, fabrication efficiency factors, and alloys employed - will result in a vessel that not only meets the required design pressure, but often EXCEEDS it. This is a fortunate and conservative procedure because it ensures that the vessel will meet pressure safety expectations. The Maximum Allowable Working Pressure (MAWP) is a result of back-calculating the ultimate resulting fabricated vessel and is the prime factor in setting the pressure at which the corresponding vessel Safety Relief Devices will be activated. I consider the MAWP the most important pressure value attached to a vessel and one that should be clearly understood and stamped on the vessel for all to clearly read. The MAWP will change with time (as will the related design value) due to wear, corrosion, and vessel fatigue. This is why it is so important to religiously keep and maintain current and accurate data sheets and calculations on all pressure vessels as they are inspected and repaired through the years of service.

When you have a need to set a PSV on a vessel and you don't have its MAWP figure, you can employ the "design" pressure value - as long as it can be proven that the vessel is in as good a physical condition as the day it was fabricated. Note that I'm going to lengths to define the physical condition of the vessel. We often neglect to mention that we are ASSUMING that the physical condition of the vessel doesn't change from the day it was fabricated. This can be a dangerous assumption that doesn't necessarily apply. A vessel can undergo corrosion and wear as well as other chemical attacks through its use and lifetime. Physical and meticulous inspections and reports are essential to ensure that the vessel can be safely applied to a process --- especially to a high-pressure application. And I consider any pressure over 50 psig as HIGH PRESSURE. When a vessel explodes, it isn't the pressure that kills you; it's the amount of shrapnel and steel pieces that are blown about that do the damage. And even 50 psig can cause a considerable amount of serious damage if allowed to trigger a vessel failure.

I have never come across the term "Maximum Operating Working Pressure" and can only presume it means the same thing as MAWP. People are forever changing the writing of terms in order to suit their own likes and dislikes. MAWP was first described and is still employed by ASME in the USA and is the term I have always used to define what I have described in the above.

I hope this helps you understand the terms you have been confused with.

 

[djack77494]

Melimelo,
I have two comments about your most recent posting. First I have to disagree with your contention that the MAWP is based on the vessel being in a new/good as the day it was fabricated condition. Not true. We typically specify a corrosion allowance for a vessel, and the vessel can withstand the MAWP at the design temperature as long as it has not corroded in excess of the allowance. Thus a new vessel may be able to withstand quite a bit more pressure than a corroded vessel, but the corroded vessel can still withstand the full MAWP (as long as the corrosion is <= the allowance).

My second comment is that the MAWP is a true characteristic or property of the fabricated vessel. The design pressure is a minimum requirement established during the design phase. It is independent of the mechanical construction and pressure withstanding capability of the vessel and is really a non-issue once the vessel has been properly fabricated.