Flushing Ring

select the size of the flushing ring to match the process fluids properties.

if it is clean, like steam or sour water or solvents, then yes, you are correct, a 3/4" NPT flushing ring, swaged down to 1/2", and a 1/2" NPTdiaphragm seal will do the job. if it is not, then a 2" flushing ring, 2" flange connection, with a 2" diaphragm seal is a better choice.

the rating should match the piping line class, meaning the main process line. for example, if it is a 2" flange connection and if the line is rated for 600#, you have to select 600#. if it is 300# or less, you can select 300#.

The fact that you call this a FLUSHING ring tells me the product has sediments.

In that case will it not be better to have a 2" opening in the pipe than a 3/4" opening

to avoid plugging the tap?

The isolation valve (not the manifold valves) must meet the primary piping specification and should be increased to the size of the available flushing ("drip, calibration"). A 1 ½ inch ring should be matched with the diaphragm seal, either pancake or flanged. The seals are available in smaller sizes but larger is the proper direction for better response time and minimized temperature change effect. Vendors have programs to calculate the optimization of your seal choice based on plant and process conditions.

Seals are not always used only for chemical or particulate exclusion from the process lead lines, but on level applications the need for reference leg fill fluid maintenance (kept full with known, constant specific gravity) is removed (especially for the poor guy who climbs 30+ mtrs to check).

Pete