4 replies to this topic

[ogpprocessing]

Dear CHE Experts,

 

In some project P&ID legeneds I have seen a requirement to locate temperature measurement downstream of flow element?
 

 

Why is temperature measurement located downstream of flow element?

 

How much distance should be considered as minimum? Is this dependent to flow meter type?

 

 

[Shivshankar]

ogprocessing,

 

Whenever you come across such questions, try to think reverse what will happen if you put temperature element upstream ? 

 

I don't know what type of temperature element and flow element you have, but consider as Thermowell. (Upload P&ID)

 

Temperature probes should be located downstream of flow element so the well can be in flow stream for accuracy which not disturb velocity profile upstream of the flow element (as you don't want to cut your flow at upstream).

 

For your next question go through books or standards related to flow measurement.

 

http://www.flowcontr...ng-requirements

 

Regards

Shivshankar

Edited by Shivshankar, 06 April 2013 - 08:51 AM.

[Jome128]

Dear ogprocessing

 

I agree with Shivshankar's point. Temperature measurement device usually with thermowell shall be install 3~5 pipe diameters away from elbow, flow meters or other device. But, I think it is ok to install temperature device upstream flow device away from 3~5 pipe diameters. Please correct me if I am wrong.

 

Thanks.

[markymaark]

We'd have to know the purpose of both the Temperature indicator and flow element.  Otherwise we're just giving our best guesses.

[JMW]

The only downside to locating the thermowell downstream is that the response time of the flow meter is usually much faster than that of the temperature measurement.

This means that if temperature delayed response is critical it is better to install upstream but make sure to be outside the meter required upstream flow conditioning. This is most likely to be critical where a step change might be expected in flow and/or temperature.

Hence if the temperature sensor responds slower, the delay is partially offset by the flow delay i.e. the time taken by that portion of the fluid to flow to the meter.

However, as ever, it is always best to follow the industry/international standards or manufacturer's recommendations about the location of all instruments where appropriate.