VALVE COMPONENT REPAIRS
8.4 Seat Ring Replacement
This section focuses on the replacement of welded-in and threaded seat rings. Repair of seat rings is addressed in Section 8.3.
Seat rings are found in most gate, swing check, and tilting disc check valves in which the seat is 90˚ to the bonnet bore. Seat rings are found in about half of the globe valves, angle valves, lift checks, and Y-type valves, with the balance being manufactured with integral seats.
Welded seat rings are installed with a partial penetration seal weld joining the seat ring to the valve body. In gate-type configurations, the weld is located in the flow stream area of the valve as shown in Figure 8-4a. In globe-type valves, the seal weld is found either above or below the seat at the discretion of the manufacturer (see Figure 8-4b).
These welds are typically 1/8 inch to 3/8 inch (3.2 to 9.6 mm) deep, providing a seal to the leak path under the seat ring. This weld is not considered a structural weld.
Seal Weld
Figure 8-4a
Typical Gate Valve Seat Ring Seal Weld [27]
Seal Weld
Figure 8-4b
Typical Seal Weld Locations for Globe-Type Seat Rings [28]
Threaded seat rings are generally found in control and safety valves of all pressure classes and in Class 150 and Class 300 pound gate, globe, and check valves. Threaded seat rings are designed to stay in by preloading like a bolt thread or by welding. These can be either tack welds or a full diameter seal weld located on the upstream side of the seat ring.
There are two basic seat ring configurations for gate valves. The most common configu-ration has the seat ring pocket that is machined parallel to the flow bore as shown in Figure 8-4a. If the seat is at an angle from the bonnet bore centerline to accommodate a wedge, the angle is machined into the seating end of the ring prior to applying
hardfacing. In the second seat ring configuration, the seat ring pocket is machined into the valve body at an angle to the flow bore (see Figure 8-4c). This design allows for a uniform length seat ring, which makes in situ machining more difficult but makes alignment of the new ring much easier. All threaded-type gate valve seat rings are also made in this configuration.
Seat Ring Angle (Typically 5˚)
Gate Valve Body
Seat Ring Pocket
Figure 8-4c
Seat Ring Angle Generated by the Angle in the Seat Pocket [14]
Globe valve seats rings, welded-in and threaded (see Figure 8-4d), are installed parallel to the bonnet bore. This is true for horizontal, angle, Y, and, Y-angle designs.
Lower Stem
Packing Stuffing Box
Packing Top Closure
(Bonnet)
Gasket Guide Bushing
Inner Valve
Seat Ring
Body
Guide Bushing Gasket
Bottom Closure
Drain Plug
Figure 8-4d
Example of Globe Valve with a Threaded Seat Ring (Courtesy of Anchor/Darlington Valve Company)
In situ seat ring replacement is quite common in most globe-type valve applications.
Only recently, with the advances of in situ machining and welding equipment, have some of the valve manufacturers and service companies been able to replace welded-in gate valve seat rings in a cost-effective manner. Several field service companies can replace seat rings in most gate-type valves 8 inches to 10 inches (203 mm to 254 mm) nominal size and larger. Smaller gate valves should be removed from the line and
Replacing globe valve seat rings (perpendicular to the bonnet bore axis) is easier than replacing gate-type valve seat rings (parallel to the bonnet bore axis), due to the accessi-bility of the seat straight down the bonnet bore. Welding, machining, and lapping equipment is available for all sizes and pressure ratings. Because of the straight-in approach to the seat, welding and machining can be performed with either machine or manual repair processes.
The replacement of threaded seat rings is much more common, even in situations where the valve is welded in-line. Simple replacement due to a damaged ring can be performed with grinders and special spanner wrenches. The repair challenge for valves with
threaded-in seat rings is when the body threads have been damaged or eroded away. Like that of valves with welded-in seat rings, the globe-type configuration is quite repairable with the portable boring bars currently available. In situ repair of gate valve body threads is limited to very special “one-off” applications above 12-inches (305-mm) diameter.
This section simply addresses the replacement of welded-in and threaded seat rings.
After the seat ring is replaced, the mating disc or poppet must be fitted to the new seat ring to achieve the desired seal. In many cases, particularly with the tilting disc and swing check valves, the fitting operation can be just as difficult as the seat ring repair.
8.4.1 Repair Assessment and Strategy
In cases where the hardfacing material is thin, has multiple indications, or is of a
hardenable type, seat ring replacement is generally the proper approach. This approach avoids the potential welding problems associated with hardfacing materials. The re-placement ring is already hardfaced and, therefore, the only welding required is that of the ring to the valve body, utilizing a compatible filler material.
A thorough evaluation should be performed prior to committing to a seat ring replace-ment. Minimal tolerances are often employed, making material removal or distortion from welding in the seat ring critical to obtaining a leak-free seat. This operation re-quires significant resources and management support to ensure that mockup training is performed and that the necessary equipment is available.
When evaluating the option of seat ring replacement versus valve replacement, the following questions should be asked:
• Can the flaw be removed by grinding without impacting the sealing area, thus avoiding a seat ring repair or replacement?
• Does the crack or flaw penetrate into the valve body or threaded area such that a welded repair of this pressure boundary material will be required? If so, are the proper welding procedures qualified and materials available to do a base metal repair?
• Are qualified and experienced welding, machining, and valve repair personnel available to properly support the repair?
• Are sufficient rigging points and space available to move equipment in and out of the valve?
• Are replacement seat rings available?
• If needed, are replacement wedges, discs, or poppets available?
• If body threads are damaged, can the valve design be changed to accomodate a drop-in seat ring that is seal-welded?
8.4.2 Repair Prerequisites
8.4.2.1 Generic Prerequisites
The following should be considered prior to initiating a repair:
• Record the manufacturer name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate.
• Obtain drawings and material specifications from the manufacturer.
• Obtain and review the maintenance and repair history to see if there have been any changes to the original design.
• Procure a replacement pressure seal ring and an oversize ring if the valve has a pressure seal bonnet.
• Procure replacement seat rings and have the supplier ready to finish machining them after taking final measurements in the field. If the plant has machining capabil-ity, the rings can be shipped to the job site for final machining.
• Procure a replacement wedge or disc if required.
• Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspec-tion “hold points.” All measurements and findings should be recorded for a final report.
• Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line.
• Install a protective liner over the pressure seal bore area.
• Determine if PWHT equipment and personnel are available for replacing seat rings in valve bodies fabricated from P-Nos. 4 or 5 materials.
8.4.2.2 Gate Valve Specific Prerequisites
• Establish match marks on both sides of the wedge with respect to the matching seat prior to removing the wedge from the valve body. This will ensure that the wedge is re-installed in the same direction.
• Perform a blue check and record the seating location on the wedge as shown in Figure 8-3b. The blue check will provide the valve repair technician with the infor-mation necessary to determine what type of repair or adjustments will be required for the wedge to seat properly upon re-installation.
• Measure and record the wedge dimensions as shown in Figure 8-3c.
• Measure and record the minimum distances between the valve seats at the locations shown as shown in Figure 8-3d.
• Utilizing a low stress punch, establish two reference points behind each seat ring as shown in Figure 8-3d. and record the reference dimensions from the seat ring face to the reference mark. These will be used to re-establish the seat face location when fitting the new seat rings.
• Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line.
8.4.2.3 Swing Check Valve Specific Prerequisites
• Remove the disc assembly and record any damage. Particular attention should be given to the seat, disc, hinge pins, and hinge pin bushings.
• Perform a blue check and record the seating location of the disc. The blue check will provide the valve repair technician with the information necessary to determine what type of repair or adjustments will be required for the disc to seat properly upon re-installation.
• Measure and record the seat width as shown in Figure 8-3f.
• Utilizing a low stress punch, establish three reference points on the seat ring 120˚
apart as shown in Figure 8-3f and record the reference dimensions. These will be used later to re-establish final seat face location when fitting the new seat ring.
• Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line.
8.4.2.4 Tilting Disc Check Valve Specific Prerequisites
• Remove the disc assembly and record any damage. Particular attention should be given to the seat, hinge pins, and hinge pin bushings.
• Perform a blue check and record the seating location on the disc as shown in Figure 8-3g. Measure the poppet seat angle and verify with the manufacturer’s drawing.
The blue check will provide the valve repair technician with the information neces-sary to determine what type of repair or adjustments will be required for the disc to seat properly upon re-installation.
• Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line.
8.4.2.5 Horizontal Globe, Angle Globe, and Lift Check Valve Specific Prerequisites
• Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides.
• Perform a blue check and record the seating location on the poppet as shown in Figure 8-3i. This should be a thin line of contact.
• Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3j.
• Measure and record the ID and OD of the poppet seat, and the OD of the poppet wear ring.
• Measure the poppet seat angle and verify with the manufacturer’s drawings.
• Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line.
8.4.2.6 Y-Type Globe and Lift Check Valve Specific Prerequisites
• Remove the poppet (plug) and record any damage. Particular attention should be given to the seat ring, poppet seat, body guides, and poppet guides.
• Perform a blue check and record the seating location of the poppet. This should be a thin line of contact.
• Measure and record the inside diameter of the seat ring, the outside diameter of the seat angle, the distance from the top of the seat to the bonnet flange surface, and the body bore diameter as shown in Figure 8-3k.
• Measure and record the ID and OD of the poppet seat, and the OD of the poppet wear ring.
• Measure the poppet seat angle and verify with the manufacturer’s drawings.
• Locate and establish the seal weld centerline. Compare with the manufacturer’s drawings and establish the cut line.
8.4.3 Seat Ring Removal
Seat ring removal is performed by several methods. When selecting a removal process, issues such as accessibility, time/schedule constraints, radiation levels, and air quality should be considered. The methods most commonly used include:
• Manually grinding out the seal weld
• Grinding out the seal weld with automatic systems
Rotational Drive
Mounting Fixture
Weld Torch
Figure 8-4e
Split View of Valve Body (In-Line) Exhibiting the Seal Weld Fixture [27]
• Machining out the complete seat ring
• Removing the threaded-in seat ring by grinding out the tack or seal weld and un-screwing it with a special spanner wrench
Grinding is a time-consuming process that also presents radiation exposure and air quality concerns. Several companies have developed manipulators that get the mainte-nance personnel out of the valve (thereby reducing the overall radiation exposure) and provide some time savings over the manual method. Grinding can also be used when equipment or qualified machinists are not available.
Machining out the seal weld or complete seat ring is quick, clean, and precise when performed by a qualified machinist with well-designed equipment. Globe valve seat rings can be removed with a variety of commercially available boring bars, as well as some special bars developed by repair organizations.
Machining out gate and check valve seat rings is performed very effectively with some of the special machines developed by service companies specializing in field machining and welding. Machines designed and manufactured for gate valve seat ring repair were commercially available by mid-1996.
The sequence for removing a seat ring is the same for both gate and globe valves. Extreme
machining, the machinist should look for a crack in the machined groove indicating that the seal weld has been penetrated. If the machined groove reaches a depth of 3/8 inch (9.5 mm) and a crack is not visible, a determination should be made about going deeper or adjusting the cut location. It is extremely important that the seat ring pocket depth is not increased by cutting into the base material because this alters the seat ring reference.
After severing and removing the ring, the valve body should be prepped as shown in Figure 8-4f to install the new ring.
Valve Body
Flow Bore
Finished Prep
Figure 8-4f
Valve Body Preparation for Replacement Seat Ring [11]
During service, the crevice between the seat ring pocket and the seat ring (or the seat ring threads) can fill with corrosion particulate and other materials resulting in a stuck seat ring. Methods that can be used to remove the seat ring include:
• Welding lugs on the ID bore and pulling and rotating the ring
• Run weld beads along the ID of seat ring to shrink the ID
• Heat valve body around seat ring to expand the pocket