4 replies to this topic

[victorvikram]

Dear All,

I have a simple question to ask. The question is about design temperature (DT). In some DT calculation, the formula used is:

DT = MOT + 10degC

I have seen that DT = MOT + 15degC or even 20degC. What is the basis to this formula? Appreciate your help.

[ankur2061]

victorvikram,

This is not at all a formula. It is a guideline which a company (engineering or operating) might have adopted for the purpose of standardization of the engineering and design practices. This may be different for different companies.

I have used DT = MOT + 20°C for a company as well as DT = MOT + 25°C for another company.

Norsok standard P-001, "Process Design" has a section devoted to Design Temperature. Norsok Standards although created for off-shore oil and gas have found acceptance in many other areas of process design. The link for this standard is available at:

http://www.standard....ue&preview=true


Hope this helps.

Regards,
Ankur.

[Dacs]

I've used MOT + 28°C on my Middle East projects.

This design margin is usually based (and will vary) on company experience and there's no hard and fast rule on this.

However, I need to point out that you have to consider Maximum Operating Temperature, which is not necessarily (but usually) the same as the normal operating temperature.

[Technical Bard]

The design margin used for any parameter should be set based on how much uncertainty you believe you have in the determined value and how much variability there could be.

In the "old days" when we couldn't run hundreds of simulations of every permutation and combination of inputs, margins had to be higher because we really didn't know as much. Many companies manuals have retained these "large margins", even though the design effort is usually much more rigorous than it was 30-50 years ago.

Determining the Maximum Operating condition depends on assessing what you realistically believe you will see during startup, normal operation, upsets, and shutdowns (both controlled and emergency). You might do this with steady state simulations, dynamic simulations, past experience with real plants or some relatively simple calculations. The more work you do on assessing this and whether you the engineer think you've covered all of the design conditions means you could reduce the margins.

For the specific topic here, +28ºC is obviously an old number, because it is equal to +50ºF, and in many cases is very conservative if you have done the amount of engineering normally done in many countries today. +10ºC would have to mean you have a great level of confidence in your maximum operating temperature, and that you have imagined a lot of operating scenarios and assessed them relatively rigorously - or you have a lot of operating data for that process indicating that the temperatures don't move around a lot, even during upsets and transients. I would be nervous in most cases to use a margin of only 10ºC without (well-built) dynamic simulations (which takes very trained people to do well).

One key thing to think about is whether there is a fundamental physical or thermodynamic limit that will prevent you from getting significantly higher temperatures. For example, in high pressure saturated steam services, where no superheat is possible/likely, the design temperature does not need to be significantly higher than the saturation temperature at the design pressure - and since the saturation temperature of steam doesn't change much at very high pressures (> 80 bar), your design temperature margin might be very small, but still safe.

[S.AHMAD]

1. Technical bard has good points to consider.
2. For operating temperature below 200^oC, the allowable stress for c.s remains constant below this temperature. The only concern is for higher temperature.