Instruction Manual
D104197X012
NPS 8 CHP Valve
September 2016
11
4. Drive in the pin to lock the assembly.
5. If it is necessary to lap the seating surfaces, complete the lapping seats procedure before installing the seal ring. The
Trim Replacement procedure provides seal ring installation instructions and valve reassembly instructions.
Lapping Seats
Key numbers referenced in this procedure are shown in figure 4, except where indicated.
With metal‐seat constructions, lapping seating surfaces of the valve plug and seat ring (keys 3 and 6) can improve
shutoff. (Deep nicks should be machined out rather than ground out.) Use a good quality lapping compound of a
mixture of 280 to 600‐grit. Apply the compound to the bottom of the valve plug.
Note
The CHP valve uses spiral‐wound gaskets. These gaskets provide their seal by being crushed and therefore should never be reused.
This includes reusing a gasket after the lapping procedure has been performed.
An “old” gasket can be used to lap the seat, however the gasket must be replaced with a new gasket.
To preserve the effects of lapping, do not change either the position of the seat ring in the valve body cavity or the position of the
cage on the seat ring after lapping the seating surfaces. When the parts are removed for cleaning and replacement of the “old”
gaskets, return them to the original positions.
Use the following procedure to lap the seating surfaces.
1. Install the following parts according to the instructions presented in the trim replacement procedure: “old” seat
ring gasket (key 17), seat ring (key 6), cage (key 2) and “old” bonnet gasket (key 15) and three cage gaskets (key
16).
2. Install the valve plug and stem assembly (keys 3 and 4) into the cage.
3. Install the bonnet (key 14) over the valve stem, and secure the bonnet with four of the hex nuts (key 12).
4. Attach a handle, such as a piece of strap iron secured by stem locknuts, to the valve stem. Rotate the handle
alternately in each direction to lap the seats.
5. After lapping, disassemble as necessary (you may mark the position of the seat ring and cage with a soft tip
marker). Clean the seating surfaces, replace the gaskets, reassemble (taking care to return the seat ring and cage to
their original positions), and test for shutoff. Repeat the lapping procedure if necessary.
Trim Replacement
WARNING
Observe the warning at the start of the Maintenance section.
After all trim maintenance has been completed, reassemble the valve body by following the numbered steps below. Be
certain that all gasketed surfaces have been well cleaned. Key numbers referenced in this procedure are shown in
figure 4, except where indicated.
CAUTION
Inspect the seat ring, cage, bonnet, and body gasket surfaces. These surfaces must be in good condition, with all foreign
material removed. Small burrs less than approximately 0.076 mm (0.003 inches) in height (the thickness of a human hair)
