Measuring pressure & temperature in circular pipes


Thread Starter


I am trying to work out how many straight lengths of (circular) pipe I need in order to take accurate pressure & temperature measurements. I have found lots of information and standards regarding pipe length requirements for flow meters, but nothing for simple static or total pressure & temperature measurements. Please can you help me understand how I would go about doing this? Do I have to adhere to the same guidelines as for venturi/orifice plates?
For truly accurate P & T static measurements, you would need either some flow conditioning into and perhaps out of the section or sufficient straight run (vis a vis flow meters) to assure theoretical parabolic flow profile, a maximum velocity (gases) of less than 30 Ft/sec, and proper instrument access (porting, location, etc.) as per ISA or other standards. If you don't follow these steps it is very easy to get dynamic head corruption of the signals. Depending on the instrument's accuracy this may or may or not be significant.

John Catch
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Depends on the accuracy being sought and the purpose of the measurment.

The flow testing labs have specific guide lines for pressure and temperature measurements. Routine pressure measurements in industrial processes are not as demanding and installation requirements are covered in most instrument handbooks.
For temperature, you get the temperature for point in the flow profile the tip of the thermowell ends up at, right?

Liptak has diagrams for placement of thermowells in his Temperature Measurement or Instrument Engineers Handbook.

E&H reproduced and enhanced illustrations of several thermowell pipe locations in their 2 page thermowell selection guide on pages 148 and 149 of their temperature catalog, here:
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Thanks everyone for quick and helpful responses. I think I need a compromise between long straight lengths to ensure developed flow and good measurements, but not too much to introduce pressure losses from friction or temperature losses through heat transfer. I will keep investigating best practice guidelines.
I knew I'd seen a reference to a pressure tap location spec somewhere but it took a day to recall where.

The valve capacity testing protocol in ANSI/ISA S75.02-1988 specifies pressure tap location and construction used for measuring upstream and downstream pressures during testing.

Liptak's Instrument Engineers' Handbook, Volume 2 (3rd ed) shows the pressure taps in section 4.14 (page 560) and has dimensions in table 4.14b for a section of pipe for testing valve capacity.

what you are missing as those are secondary issues, but to focus the conversation you need to define, the pressure and temperature ranges, fluid velocities, pipe sizes, and the level of accuracy you require.

It is one thing measure temperature to the nearest degree or two in non-traceable fashion, but for really accurate measurements you will discover just how difficult accurate measurements can be.

Lacking the details of what you are trying to measure, you can only get generalized responses that cannot really help you.