Hi everyone.

I'm looking for a little advice on the selection of a DP Transmitter on a MEG filtration line. The client have specified that it must be connected to the process, via capillary tubing and a remote diaphragm seal.

The instrument is to be Int. Safe and is to be capable of being brazed to the capillary tubing.

The process date across the filter is

Design pressure 255.4 barg
Design temp -20 to 50
operating pressure 90-220 barg
Dirty pressure drop bar VTA
operating temperature 21 degrees C
flow rate 4.7 m3/hr - 7m3/hr
fluid flowing MEG
fluid density 1113kg/m3
fluid viscocity 1.4-2.0
ambient temperature range -8 to 25 degrees C

Any help would be massively appreciated!

 

1) A datum is missing - what is the expected DP range?

It's a filter application, but is the expected DP across the filter 0.1 bar, 1 bar or 10bar? Is "dirty pressure drop bar VTA" supposed to mean something in this regard? It means nothing to me.

2) The pressure is getting up there, 220 barg is ~3250 psig.

I have Honeywell's DP and remote seal specs on my desk. The DP'w Maximum Allowable Working Pressure (MAWP) for most models (STD820, STD830 and STD870) is rated at 4,500 psi, 310.2 bar.

But the DP body/seal combination used for remote seals is only rated to 2500 psig (172 bar), well below the rated 220 barg operating process maximum. They might be able to do a 'special', but you'd have to ask. Other vendors are likely to have similar limits. So you might have to hunt for a body/seal combo that can take that pressure.

3) You should be aware that static line pressure contributes error to a DP measurement. Spec sheets state a static error component. Both Honeywell and Yokogawa have internal absolute pressure measurement on their DP's to compensate for static line pressures over 1,000 psig (68 barg). If you read the Rosemount spec carefully, you'll see it maxes out at 1,000 psig static line pressure.

Be aware of the static line pressure limit when searching for DP's with static line pressures over 68 barg.

4) Can you challenge the use of a capillary seal for ethylene glycol at room temperature? It's the stuff in automotive radiators. Sure, the line pressure is high, but why a remote seal?

I'm assuming that the brazing is to prevent leaks, so it's likely that threaded connections are not tolerated, but have you asked? If threaded connections are out, then all welded construction would be required for the seals and capillary.

An alternative might be an impulse tube/pipe welded to a football, which could flange mounted to the DP's flange. The process end of the tube/pipe could bebrazed to the process. A problem with brazing the 'seal' directly is the potential for the brazing heat to boil the fill fluid or even 'burn' the fill fluid and then claims of poor manufacturing of the xmtr assembly.