Compatibility Of Slip-on & Weld Neck Flange

I see Spades has given a very good response.

The centre-line of the bolt holes (known as the PCD or Pitch Circle diameter) remains the same, irrespective of the style of the flange (WN slip-on etc..)

This is the common factor for the class of the flange (in this case 150#)

Same will apply for difference in style within 300#, 600# etc....

Weld on vs slip on is not the issue, the issue will be if there are different "faces" between the two. They both need to be flat faced or either raised face. The other consideration would be whether or not the flanges are 2 hole centered or it may not matter with instrument you are installing. Typically, when ordering flanged components/instruments/devices the vendor will ask raised face or flat faced.

Slip-On flanges on the meter should be compatible with Weld Neck flange on the pipe.

Because this is a flowmeter there may be a concern that there will a slight disturbance in flow pattern if ID of the flow meter is different than Id of Slip on flange.

For normal flow measurement this should not be a problem

With the wide variety of gasket materials and configurations, although not preferred, flat faced can almost always be bolted to a raised face flange simply by using a better gasket type for the application such as a Teflon envelope gasket instead of a thinner material.

If the flanges on the instrument are not aligned in reference to the holes, planes of the flange faces, etc. DO NOT BUY THE INSTRUMENT! That is a clear indication of sloppy design and/or fabrication. If those are sloppy what else is? If absolutely necessary a spool piece with flanges aligned to make up for the deficiencies can be fabricated and installed in the line.

Don't count on the vendors asking the right questions. Many vendor personnel don't know the difference between a slip on, a weld neck, socket weld, lap joint or a threaded flange so they will not ask. Don't wait for them to ask, TELL THEM!

If you really have a difficult situation or problem, ask the people who have worked with more flanges and piping than most engineers. Ask the pipe fitter/welder.

Good Luck, Old Salt