Specify max P/T for process measurement instrumentation 2

[carol2005]

Here is an opinion on how to specify the max pressure and temperature for process measurement instrumentation.
In order to prevent the pressure/temperature rating of the instrumentation becoming a weak point of the system, for the max pressure or temperature specified on the datasheet, process upset scenarios (except fire) should be take into account in additional to the normal operating range. The suggestion was that,
1.For process measurement instrumentation on vessels, the max P/T data should be specified as being consistent with the vessel rating, i.e. to use the vessel design pressure or PSV set pressure whichever lower as the max pressure, and vessel design temperature as the max temperature for the instrumentation;
2. For process measurement instrumentation on piping, the max P/T data should be specified as consistent with the piping specification. such as ANSI 600# class with correspondence temperature range.

I agree this will ensure the inherent safety design/selection of the instrumentation. My concern is, the design/selection of transmitters usually gives 1.5 times of safety factor for over range protection. If the max P/T data is specified as the same as vessel or piping rating, could that result in over-rated instrumentation or increased project cost?

How would do you specify the max pressure and temperature for process measurement instrumentation?

 

[CJKruger]

carol2005, on vessels I prefer it if the instrumentation design P+T match the vessel design P+T. This is why I hate the practice of designing vessels for a minimum design temperature of 500 F.

On piping I prefer to match the pipe spec, and not the max allowed by the flange rating.

 

[JLSeagull]

Caution, Generalizations Follow:

Differentiate certain mechanical devices such as pressure gauges from more complex devices such as transmitters; and online instruments from inline instruments. One generalization does not fit instrumentation in general.

Each instrument specification data sheets should reflect the design conditions including the maximum pressure and temperature.

If working with Class 600 process systems, we normally don’t sweat the common transmitter ranges for pressure. This generalization should work for most pressure transmitters that are measuring ranges above 50 psig, perhaps not quite as good if measuring draft ranges such as 0-1 inch water column. I normally buy Rosemount 3051S series. If you are dealing with some OEM type instruments you better check all the details to verify that the pressure generalization works for your instruments. You can select a range that is well below the maximum pressure rating of the process. Nothing bad happens if you specify a range of 0-50 psig and the maximum process pressure is 1000 psig. If the instruments are applied to a Class 600 carbon steel piping specification with a design pressure based upon the steel material group, you must check the flange ratings for any stainless steel matching flanges for flanged instruments. Carbon steel material groups are rated for higher pressure than stainless steels or exotic flange materials at the same temperature.

We treat the installation detail as necessary to accommodate extremes in temperature – both high and low.

Full vessel design for the pressure and pressure differential pressure transmitters is about free. Not so with gauges. Select gauges that read the maximum pressure around 2/3 of the gauge scale. OK, nothing really bad happens at 0.75 or 100 percent. At 125 percent of scale the gear for the pointer goes past the drive gear and perhaps the bourdon tube begins to straighten in a manner that does not return to the original shape. Call the gauge broken at this point but still not ruptured. The rupture pressure is lots higher than the scale for a new gauge without any pitting etc.

Inline instruments such as valves, most flow meters etc. are pressure vessels and must meet the same criteria as required for the pipe and equipment.

 

[JLSeagull]

The pressure must be reduced to apply some measurement instruments, including most process analyzers. If an analyzer is published at 70 psig maximum, you can measure the fluid that operates at 3000 psig. However, you must provide the pressure reduction, overpressure protection, etc. while also assuring that the fluid still represents that which you want to measure. Compared to pressure instruments, the analyzer topic requires it's own thread.