Selection Of Manifold Integral or Traditional Type For DP Type Flow Transmitters

James Stewart's #2 post shows a Rosemount coplanar 5 valve manifold that connects to the two ports of a DP transmitter to the process piping/impulse tubing. It was the first shift away from a 4 decade industry standard since the 1940's, when pneumatic, then analog DP transmitters, and then smart DP transmitters used an industry 2 1/8" center-to-center flange mounting for a manifold valve.

The center-to-center dimension for the process ports (the holes in the center) were the same for all brands, all models from the pnuematic era on the left to the smart era on the right until the coplanar. The four threaded holes above and below the process port are for the bolts that hold the flanged manifold valve to the process heads.

A flanged 3 valve manifold. The flange bolts to the process heads shown above (the pneumatic transmitter has 'footballs', 1/4"-to-1/2" NPT threaded adapters, those would be removed for a flanged manifold valve)

The process piping connects to the NPT threaded connections on the side you can not see above:

My concern about coplanar venting gas in liquid line. Air or gas needs to be vented from the impulse lines in a liquid application.

The process head below shows a typical vent that is mounted on the side of the process head, above the center line, because air/gas rises above the liquid. The side vent is circled in red.

Here's a side vent. The hex head is a needle valve that opens or closes a channel to the vent hole, circles in red:

This red arrow in the graphic below shows the air/gas vent path when the side vent is opened:

That configuration shown above definitely works for the gas entrainment I've dealt with. I'm sure the coplanar works, but I'm always suspicious that gas can get trapped above the elevation of the vent, in the pressure chamber in the transmitter itself where the measurement is made. The coplanar side vent is shown below.

The fora have not been plagued with complaints of error low pressure error due to gas entrainment, but it's always made me wonder.

I had a couple drinks with an Emerson product guy a decade ago at an ISA show and he said that the coplanar design was really cost driven; it does not need the flange at right angles to the valve body that the traditional valved manifolds need. A coplanar can be machined from plate stock. Bar stool opinion or the truth? I don't know.