Drum level measurement with DP trx

[stzatzop73]

On site we have differential pressure transmitters used to measure drum level, at pressures of 110 bar, 25 bar and 4 bar respectively. the sensing legs of the DP trx have demineralized water. In the winter, the trace heating is not enough so when there is no generation the water lines get frost and the signal has problems. If we put glycol in the sensing legs, then probably we will need to make again the zero of the trx? is there any problem since glycol is more heavy than water?

 

[Weegie]

Many refinery instrument maintenance crews in cold climates use ethylene glycol as a freeze protection/barrier fluid in impulse lines for frost protection & keeping the process fluid out of the transmitter/manifold assembly. As you most likely have a 'high' & 'low' pressure filled impulse lines(different lengths/elevations) to your transmitter, it would be advisable to 'zero' your transmitter.

 

[glowing]

Yeah, I would just get sealed leg capillaries and be done with it. Tell them the minimum temp you get to. I have installed this for -50C aps using Rosemount.

If your heart is set on wet legs (uggg) glycol gets pretty thick in the cold. you might need to go from a 60/40 glycol (assuming that is what you are using) to a 20/80 or a methanol gut mix.

open the pocket book and get capilaries...you wont regret it.

 

[lacajun]

glowing, I've seen capillaries heat traced and insulated in cold climes to prevent or minimize the temperature effects on the fill fluid. Have you seen or heard about that?

 

[Weegie]

Apologies there. I did mean an ethylene glycol mix.
There also used to be a commercial barrier fluid for purchase for these type of applications too but, I can't remember the brand name.
I've never seen capillaries being heat traced. Your choice of capillary fill fuid would be chosen to suit your specific climatic conditions.

 

[Zinskie]

From a maintenance standpoint, filled impulse lines have both advantages and disadvantages compared to capillary transmitters:

The sensing lines can be filled right at the transmitter, and can be isolated at the transmitter to periodically check operation and calibration whereas to check capillary transmitters you have to go to the process taps; on tanks these nozzles often are not readily accessible without scaffolding or other means.

On the other hand if the lines are not refilled regularly or if someone unwittingly opens the equalizing valve between high and low taps, the fill fluid level will not remain consistent and the transmitter's output will not be accurate.

Personally I am partial to "wet legs" over capillaries in all instances save those most prone to clogging such as very heavy oils, VTB, and catalyst slurries to name a few.

 

[lacajun]

I prefer wet legs over capillaries, too. I've seen variations in readings with temperature swings with capillaries. They electrically traced and insulated the capillaries to prevent this. I don't know if it worked or not but it drove up the cost of the installation.