relationship between design and operating conditions

[jonesey]

Anybody know if there is a guideline/standard on what the minimum differences can be between design and operating conditions ? We have a case where our process dept. wants to increase the gas outlet temperature from a fired heater from it's normal operating temperature of 326degC up to 450degC, where 450degC is the specified design temperature of the associated piping system. I think i can remember something in a standard somewhere about that design conditions should always be higher than operating conditions (20-30degC i think) and it was not advisable for operating temperatures to be the same as design conditions. Any help/advice would be appreciated.

 

[NozzleTwister]

jonesey,

Each company or client has their own guidelines. Typiclly I see a bump of 25 or 50 deg. F. (most often 50 deg. F.)over normal operating for the design temp. to cover any fluctuations from the norm. This is just for systems that won't see large excursions due to upsets.

Many systems can and will see temperature excursions much greater than the 50 deg. F. guidelines. This is common with steam systems, heater systems, heat exchanger piping, lines subject to steam out, regeneration or other systems with predictable and possible temp. deviations. Your Process Engineer should be the best source for this.

The operating and design conditions for a system should be realistic. Your conditions will most likely be lower than the equipment design conditions and the limits of you pipe spec.

To design a piping system for temperatures in excess of what it will see under any circumstances will require additional flexibility, additional pipe materials, and additional support requirements and is just pouring money down the drain.

Good luck,

NozzleTwister
Houston, Texas

 

[jte]

Jonsey-

As NozzleTwister pointed out, it is pretty much up to the operating company to set the design condition to operating condition margins.

I recently went through a similar rerate. Wound up having to replace several large expansion joints. Not cheap, but worth it from a process perspective. Be aware that if your existing piping/ducting and its supports are carbon steel that you are transitioning into a temperature range where creep will begin to play a role. For example, creep governs VIII-1 allowable stresses for SA-36 starting at 700°F (~370°C) while SA516-70 starts at 750°F (~400°F) [ref: ASME II-D]. Keep this in mind when you are studying the rerate.

jt

 

[codeeng]

Process engineers often apply piping design conditions to the equipment however your margin over operating temp. appears to be excessive. Typical values as mentioned above generally suffice. You should discuss the cost impact of using the higher temp. with your Process group.

 

[jsummerfield]

For an extra wrinkle, as many engineers set the design conditions for vessels and piping systems based upon the tables for carbon steel, many instruments with stainless steel flanges require a piping break as the stainless limits are lower. For example, a stainless steel thermowell may require ASME class 900 RTJ connection in an otherwise ASME class 600 line.

John

 

[Shofa]

Theoretically, new system can work safely under the design conditions. However, risk will be there due to any increase in the boundary conditions coming from fluctuations from the norm.

For existing system, you have to rerate it before applying the design conditions.