Intro4u2u

Many conventional sliding-stem control valves use threaded-in seat rings. Severe service conditions can cause damage to the seating surface of the seat ring(s) so that the valve does not shut off satisfactorily. In that event, replacement of the seat ring(s) will be necessary.
Before trying to remove the seat ring(s), check to see if the ring has been tack-welded to the valve body. If so, cut away the weld and apply penetrating  oil to the seat ring threads before trying to remove the ring. The following procedure for seat ring removal assumes that a seat ring puller, such as that shown in figure 5-4, is being used . If a puller is not available, a lathe or boring mill may be used to remove the ring(s).

1. Place the proper size seat lug bar across the seat rings so that the bar contacts the seat lugs as shown.

2. Insert drive wrench and place enough spacer rings over the wrench so that the hold-down clamp will rest about 1/4-inch above the body flange.

Slip hold-down clamp onto drive wrench and secure the clamp to the  body with two cap screws (or hex nuts for steel bodies) from the bonnet. Do not tighten cap screws or nuts.

3. Use turning bar to unscrew the seat ring. Struck seat rings may require additional force on the turning bar. Slip a 3- to 5-foot length of pipe over one end of the turning bar with a heavy hammer to break the ring loose. In addition, a large pipe wrench can be used on the drive wrench near the hold-down clamp.

4. After the seat ring is loose, alternately unscrew the flange bolts (or nuts) on the hold-down clamp and continue to unscrew seat ring.

5. Before installing new ring(s), thoroughly clean threads in the body port(s). Apply pipe compound to the threads of the new seat ring(s).

Note

Screw the ring(s) in to the body. Use the seat ring puller, lathe, or boring mill to tighten seat rings in the body. Remove all excess pipe compound after tightening. The seat ring can be spot welded in place to ensure that it does not loosen.

6. Reassemble the valve.