Fuel Gas Purge 1

[Falcon03]

Hello,

Does anyone have an experience with Fuel gas purge in instrument transmitters (pressure, level indication, etc.)? If so what are the advantages and disadvantages?

Thank you for your help.

 

[hacksaw]

Not uncommon where the instruments are pneumatic and the site isolated.

Fences and warning signs commonly posted when it is practiced.

 

[kbander]

Purge or flush is often used when you have a corrosive or fouling environment. For example, isobutane or propane flush is often used for HF Alkylation instruments, steam purges can be used in an FCCU when pressure taps could otherwise plug off, etc.

I wouldn't think that you would want to use a gas purge on a level tap though...


Regards,

Bob

 

[Falcon03]

Yes Bob I want to use the purge for our DP cell indication, since I beieve we have some liquid condensation inside the transmitter leges.

In order to avoid that, I was thinking to use fuel gas purge or better insulation. tks

 

[hacksaw]

My earlier comment was wrongly directed towards use of natural gas to operate pnematic devices. Appologies.

We has used purges on instrument connections frequently, but we were fortunate to have process where air, water, or nitrogen was compatable with the process.

We have used gas purges on instrument connections (specifically pressure taps and on d/p level connections to maintain dry-leg conditions).

The purges require regular attention and some means to indicate that a flow is maintained. Use of a hydrocarbon purge creates additional design/safety issues that I leave to you.

In one case we simple moved to larger impulse lines and with drain legs. The original design used 3/8" tubing. We could never completely drain the moisture (capillary effects). By going to 1" lines, the problem was solved.

We have also used air/N2 purges: rotameter with flow controllers worked best (expensive). This was used on level and filter d/p measurements. The flow controller can be eliminated, but may require regular inspection.

Process (gas) and buffer gas purges are commonly used in some rotating equipment in petro-chemical industry.

That said, you can also use remote seal trasmitters or use liquid filled impulse lines. they all require detail design considerations to make them successful,


Good luck,

 

[krsmani77]

If you have condensation problems, have you considered fuel gas itself may also condense? One of the solutions will be to provide drain pots (3" dia and 1 ft long) below the DP in continuation of the impulse lines and occasionally drain them(!!).Also try heat tracing and insulating the impulse lines. Are they taking off vertically from the measurement point? Best of luck.

 

[krsmani77]

One thing I forgot to mention: By measuring the purge gas line pressure itself(provided flow is low), one can know the level or the pressure at the point of injection ( provided the pressure drop is negligible). You can eliminate the the other instrument altogether(provided it is not a DP). That is two birds with one stone. I have seen such applications in measuring sump levels from the top.

 

[I2P]

Falcon,
The use of purge gas in transmitters either for level or pressure is quite common in the process industry,particularly where the service is such that it has a tendency to congeal,say for eg Vacuum gas oils,heavy petroleum fractions etc.but as i understand you want to use it to overcome the problem of liquid condensation.in that case it would be better to either provide a electric or steam heating to the impulse tubing,or provide drain pots at the low points. The routing of the impulse tubings may be altogether modified(by providing slope)so that condensation if any runs down back to the equipment and does not settle in the tubings.
using purge gas requires to know the hazards of the gas used,additional instruments like small rotameters etc. as has been pointed out the use of purge gas can eliminate the additional instrument be it a pressure transmitter otr level transmitter because the back pressure of the system on the purge gas can be used to sense the level as well as the pressure.

 

[Falcon03]

I would like to thank you all of you guys for your valuable information and help.