Pressure measurement with diaphragm seal

[Indy]

Hi,
A contractor has offered a pressure gauge with a diaphragm seal. They plan to remotely mount this with impulse tubing. I have not seen this arrangement before usually I have seen a capillary tube to remotely mount an instrument with a diaphragm seal, has anyone seen this before?

Thanks

 

[mk3223]

It sounds okay, but what's the purpose to run the impulse tubing to a diaphragm seal gage.
It's a good question to the Contractor.

 

[danw2]

Capillary tubing is normally used to connect the diaprahm seal to the pressure instrument; gauge, transmitter, switch. This combination is assembled by evacuating the instrument side of the seal, the capillary, and the instrument with as hard a vacuum as can be pulled and then back-filled with the fill fluid.

The process side of the diaprahm is connected to the process with whatever that side of the diaphragm seal offers - direct sanitary fitting, flush flange, extended diapragm on flange, pancake, saddle, wedge, straight thread/o-ring fitting, other application-designed fittings, or the very common threaded connection using impulse tubing and fittings. An optional flushing connection can act as a bleed port for the process side installation.

 

[danw2]

I've seen a 'tree' with multiple instruments on a common diaphragm seal that needed some attention paid to its mechanical mounting, because it had too much mass to close couple to the process with a pipe nipple; impulse tubing provided the process connection.

 

[moltenmetal]

It's important to realize that a diaphragm seal assembly is also a thermometer, since it is liquid full and the liquid's volume changes with temperature. Keeping the volume of the thermometer low by using capillary tubing reduces the speed of response of the instrument but reduces the thermometric effect. Speed of response is not going to be an issue with a pressure gauge.

 

[Indy]

Thanks for the replies.

 

[TiCl4]

Is the impulse piping intended to be heat traced / insulated for exterior applications? That would be the only reason I could see to step up from capillary tubing to larger size piping - to allow for heat tracing/insulation to prevent freezing. Impulse piping will increase error from temperature swings, though.

 

[danw2]

>Impulse piping will increase error from temperature swings, though.

Capillary tubing between a seal and an instrument is a closed system and is subject to pressure change with temperature change - the thermometer effect.

But impulse piping is open to the process and the pressure seen by the instrument does not change with temperature, any more or less so than the process pressure itself changes with temperature. A wet leg on a vessel is open to the process at the top and the pressure is accurately transmitted through the liquid fill.

A vertical impulse tube includes the pressure 'error' of elevation head difference between the port/tap and the instrument.

 

[TiCl4]

Danw2, I had interpreted that the contractor was going to mount the diaphragm to the process, and instead used a larger pipe rather than capillary tubing between the diaphragm and the sensor. Thus, the impulse piping would be sealed in and would also have error from temperature swings. Now that I think about it, I'm not sure how the contractor would manage to replace the capillary tubing themselves.

I am not sure why they would buy a diaphragm seal pressure transmitter and not mount the diaphragm directly on the process. Perhaps it is to have a shut-off valve in order to allow the diaphragm to be replaced without shutting the process down?

Maybe I just read it wrong - clarification is required, I think. OP, is the contractor's proposed arrangement:

Process -> diaphragm -> impulse piping (instead of capillary tubing) -> sensor ?

or

Process -> impulse piping -> diaphragm -> capillary tubing -> sensor ?

or something else?

 

[Indy]

Hi
Proposal from contractor is
Process connection >>>> impulse tubing>>> diaphragm seal+gauge (combined unit.

Thanks.

 

[mk3223]

Indy, is there any purpose to have diaphragm seal gage?
If said, the process isn't chemical or corrosive fluid, there is no need to use diaphragm seal instrument. Just use the impulse tubing and regular instrument gage.
If for the chemical or corrosive process, the diaphragm seal is typically mounted directly to the process and run a capillary tubing to the instrument. The installation impulse tubing before the diaphragm seal, as Contractor suggested, can be a potential leak of the process, which doesn't make sense in term of design.

 

[williamhakespeare123321]

It is important to realize that a seal assembly with a diaphragm is also a thermometer, since it is full of liquid and the volume of the liquid varies with temperature. Keeping the thermometer volume low by using capillary tubing reduces the instrument's response rate, but reduces the thermometric effect. Answer velocity will not be a concern with a pressure gage.