Installation of Pressure and Temperature Transmitter

PWSlack has the right info on the temp. transmitter. Follow it.

There is no need for needle valves in this configuration. Simple ball valves will do; just make sure all wetted parts are compatible with process chemistry.

The presence of a needle valve between the RTD head and the tee demonstrates the RTD tip doesn't even make it to the tee, much less the main line. This is a bad installation of an RTD! Not only will there be unnecessary lag time introduced to the temperature measurement because of this, but there will also be an error in the direction of ambient air temperature.

Pressure transmitter is OK.

The RTD will never see any fresh process fluid. It will get its reading through thermal conduction of the process fluid and piping. This reading will always be lower than actual fluid temperature, due to heat loss of the stagnate fluid in the piping.

Get that temp probe in the main fluid stream or the readings will never be correct!

• Assembly of Pressure Transmitter with that procedure and using isolating valve is OK.

• But assembly of Temperature Transmitter is not OK, where your suggestion will lead to misreading of actual temperature of current fluid flowing. It will measure a temperature of a steady state portion of fluid, not representing the temperature of the whole fluid.

So you have to use a Thermowell using a NPTF half coupling (or flanged connection depends on the type of Thermowell) welded into 12" pipe which allowing a thermowell to be inserted at fluid inside the pipe, hence no need to use isolating valve.

Note. In case of pipeline less than 4", needed to install a thermowell, we have to make an enlargement to pipe size at portion of assembly of thermowell by welding a segment of pipe with larger diameter not less than 4" into the main line to allow a space inside the fluid for thermowell.

Just throwing this in the mix as no one else has - we need to consider the purpose of the RTD, as far as I've read, we're all assuming it is for measuring the temperature of the process solution flowing in the main line. This may not be correct.

Perhaps the RTD is not measuring the temp of the fluid in main line, but for some other reason. Perhaps it is to ensure accuracy of the pressure transmitter, or to ensure the PT is not operating above a max operating temperature.

I'd like to think, the engineer who designed the system is smart enough to know that the current configuration couldn't accurately measure the temp of the main process fluid.

Hope this helps.

Anthony

If the RTD is there to prevent the PT operating above its operating temperature then it will have a shutdown function. That function will be faster if, like many have suggested, the installation is altered so that the souce of heat or cold that could damage the transmitter, i.e. the main process line, is measured and shutdown quickly. Moving it to a thermowell in the main pipeline is still recommended.

Good point, though. The limits of temperatures and pressures of the process fluid are fundamental to most instrumentation and mechanical design considerations. It's strange how many instrumentation and mechanical specialists ignore these data in their postings and enquiries.

As mentioned already, the RTD installation is not correct if trying to measure process temperature. Depending on the process characteristics, the PT installation may also be incorrect. This PT installation is fine for a gas process, but for a liquid process the PT must be installed below the process piping to ensure an accurate reading..

Good point! I just wanted to add too that impulse lines should not be from the bottom of the pipe because of the possible presence of sediments or scale. This is straight from API RP 551. It is ONLY the PT installation that should be below the process piping in liquid service - self venting to avoid pockets of gas being trapped in the instrument.

Hi all,

The RTD and the pressure transmitter are used for the Flow compensation..The PT and RTD are installed just upstrem of the BWT ultrasonic flow sensor.

As i say, this is a 12" flare line. Is there any other alternative other than making a new tap on the pipeline for the thermowell

Cheers

Friend,

As i say, this is a 12" flare line. Is there any other alternative other than making a new tap on the pipeline for the thermowell <---- Is the flare line working right now?...I think yes, right.

How much is the pressure/temperature during normal operation of the plants in this flare line?

Are you flowing some toxic gas or dangerous gas in this line?

Is there any posibility of changes the needle valves by ball valves?

Where is the flow compensation done? (flow transmitter, DCS, Flow computer)

How much money is being losing if the flow measurement is not good or near to the reality?

Is it very important or neccesary/urgent get this temperature?, your boss or manager plant cannot wait to the next major repair in order to install the thermowell in this line.

Regards

JP

(post deleted by author)

Hello.

The installation is not acceptable.
What exists is a capture to install a pressure transmitter or indicator of pressure, with his respective valve of cut or process and drainage.
The sensors of temperature do not have valve of cut.
If it you wants to measure temperature a capture has to install only for this sensor, in a way such that this measuring the royal(real) value of the temperature that is happening(passing) for the pipeline of 12 ".

Excuse my English since it is very poor.

Edgar

The Pressure Transmitter is OK. But the Temperature will not measure the liquid in the pipe due to distance velocity lag.

Your comments in this thread are very much appreciated gentle men.

I would again make one point clear in this discussion that the service we are measuring is purely Gas. I believe I have mentioned the line as a flare line.

The line which connects the knock out drum and the Flare. So Its taken for granted that we measure the pressure and temperature of the Gas.

cheers