Pressure transmitters for steam application 7

[juliorosales]

Hi,

I'm working in an steam injection project. I need to measure differential pressure (choke valve DP is 1 to 270 psi) and absolute pressure (1100 to 1350 psig) at the steam injection line. I was looking for rosemount 3051 transmitters, but the application process temperature range is 570-585ºF, Rosemount PIT/PDIT are not suitable. Do you know pressure transmitters (PIT/PDIT) that is suitable for this application temperature range?

Thanks,

Julio Rosales.

 

[danw2]

Use a condensate pot filled with condensate at the end of an impulse line. Mount the transmitter below the condensate pot and the condensate will isolate the transmitter from the steam.

The graphic has been modified. The impulse line is not the reference leg, it is the measuring impulse line. The liquid head in the condensate pot and impulse line get 'zeroed out' at the transmitter.

 

[juliorosales]

Hi danw2,

So I don't need any special PIT. With the condensate pot I can isolate the instrument from the high tempeture.

Thank you for your answer.

Regards.

 

[roydm]

Yes, actually you can do it without a condensate pot for pressure but must have them for flow

 

[Compositepro]

The pot is only needed if you need to accurately read pressures in the inches of water range. At higher pressures the condensate in the sensing lines has an insignificant effect on the measurement. A simple pigtail loop in the sensing line will isolate the instrument from the steam temperature.

 

[roydm]

And if you mount the transmitter below the tapping point simple tubing is all that's required.
Refering to the lower diagram Danw2 posted I like to use 1/2" piping for the blowdown leg with 3/8 or 1/2" tubing to the transmitter. Arange the tubing with a few bends so it doesn't pull apart if you open the blowdown valve, the piping will grow in length quite a bit as it heats up.

 

[georgeverghese]

The arragements provided are good for measuring pressure only, but your original query requests for pressure and diff press measurement.

For DP measurement across the choke, you may have 2 choices

a) Place the dp cell below the measurement location and use diaphragm seals on both impulse lines leading to the dp cell

or

b) Place the dp cell above the measurement point and set up free draining impulse lines leading to the dp cell - slope should allow self drainage of steam condensate back into process lines.

Think there are dp cells that can give you a guage pressure readout also

The dp range of 1-270psi seems to be a wide range - check with the dp cell vendor if a dual range dp cell is required ( with auto switchover between lo and hi range).

 

[roydm]

Sorry George, you are wrong

For DP measurement across the choke, you may have 2 choices

a) Place the dp cell below the measurement location and use diaphragm seals on both impulse lines leading to the dp cell
The temperature of saturated steam at 1100 psig is around 550°F this would require D.C. 704 Silicone at least
or

b) Place the dp cell above the measurement point and set up free draining impulse lines leading to the dp cell - slope should allow self drainage of steam condensate back into process lines.
This would cook the transmitter, live steam would enter the transmitter and run back to be replaced by more live steam, the transmitter would approach the saturated steam temperature

DanW showed a couple of good examples that have been used for about 100 years

Think there are dp cells that can give you a guage pressure readout also

The dp range of 1-270psi seems to be a wide range - check with the dp cell vendor if a dual range dp cell is required ( with auto switchover between lo and hi range).

 

[georgeverghese]

Okay, didnt know there is a temp limit for the dp cell, so we could go for option (a) if we want to get a dp signal also

That means we can have the dp cell above the choke also, if required, with diaphragm seals in the impulse lines.

 

[roydm]

Yes you can use diaphragm seals in a case like this measuring a huge DP but for a typical flow application you will find you get errors or drift due to ambient temperature changes especially when the flow is low say 0.5" WC
In this case the DP is very high so you don't need anything as fancy as a condensate pot, just a simple tee or a pigtail syphon will do.
Where you have blowdown legs like in Danw2's 2nd example be careful to design the piping/tubing to allow for significant growth as the pipe expands with a sudden change in temperature, it can easily pull the tubing from fittings if it's rigid.
In reality you wouldn't use the blowdown legs too often because they will upset the transmitter signal for quite a while as the leg has to fill back up with condensate.

Diaphragm seals with capillaries are also expensive, so why use them when all you are measuring is water?

Condensate pots have another advantage in a vertical flow application, you can arrange them at the same elevation so at zero flow you get zero DP.