Design Pressure and temperature

[PIPING_DESIGNER]

I want to know if there are any formulas to calculate the design pressure and design temperature of a piping system from operating pressure and operating temperature.

 

[LittleInch]

No as you are not aware of the level of margin that the operator has applied to Design Pressure and Design Temp.

As a general ROT, the design pressure is commonly more than or equal to 10% higher than the Operating Pressure. Piping design pressure often (but not always) coincides with the MAWP of the flanges on that system at the design temperature. Pipeline design pressures tend to be much more bespoke.

Also as a general ROT, Max Design temp is often a min of 10C or higher than operating. Lower design temp is more variable and can be anything from 0C to much lower depending on the contents / location.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[gerhardl]

Found this comment:

'Ref. ASME Code Section VIII. the pressure levels at page 30 of 1997 edition. In general design pressure and design temperature are a function of maximum normal operating pressure/temperature plus a design margin (safety factor). This can be 10 to 20%.

Maximum normal operating pressure/temperature doesn't include human error, or process upsets or fire, for those kind of events there are protective devices (PSV's are some of them) that have to be considered during design pressure/ temperature selection.

All the respected engineering companies are providing a Design Pressure/Temperature Diagram of the plant that show all these values consistent on one document.'

As the post from LittleInch above: You cannot directly calculate as this would be an evaluation both of the construction as a whole with all relevant components, including safety devices and procedures, as well as consequences of any failure.

 

[davefitz]

Each vendor and each application has different rules or formulas. For Thermal Power plants that use boilers and steam turbines one may work backwards from the steam turbine stop valve inlet pressure during 5% overpressure operation plus any margin needed to allow the operation of a power actuated relief valve plus the presure drop in the steam transfer pipe plus any margin cuased by the machining tolerance in the steam turbine blade manufacturing process. In the old days, before numerically controlled lathes, that last margin could be as high as 5%. During a steam turbine trip , the boiler's pressure safety valves will lift and cause an overpressure above their lifting pressure by as much as 3% , so the design pressure may need to be set for meeting that condition.

The design temperature for transfer piping ( not heat transfer tubing) should be set with sufficient margin above the average predicted maximum operating temperature to account for errors in prediction of upstream heat transfer plus errors in the process temperature control and monitoring components. Over time, the oxidation of the temperature monitoring thermocouple junction can cause an error as high as 5F.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick