Basic procedures
Valve repair can be performed in the field, with the valve still installed in the piping system, or in the shop. In-situ valve grinding and repair is often more effective because it eliminates the need to remove the valve from the system. The only time shop maintenance is more effective is when a valve needs to be overhauled.
In-situ valve maintenance requires removal of the valve cover and adequate space around and inside the valve body. To make the most of the available space, make sure you purchase equipment that is lightweight, can fit in tight spaces and is easy to install. Your time in the field should be spent grinding valves, not installing valve grinders.
Grinding equipment used in the shop needs to be easy to operate and adjust, versatile enough to be used in a variety of applications, and able to be expanded with additional accessories to accommodate specific applications.
Preparation Procedures
The following step-by-step procedure describes how to prepare the valve and the basic steps for valve grinding and lapping.
1. Cover the area under and around the valve with a plastic sheet to prevent the loss of parts and to contain dirt and grease.
2. Begin disassembly by removing any encasement. Clean the outside of the valve. Remove rust and paint from nuts and bolts. Lubricate bolts with penetrating oil. Try to avoid any damage to the fasteners - even just a damaged nut can add excessive downtime and cost.
3. Remove the internal parts of the valve. Clean, inspect, and mark to ensure proper reinstallation. Clean the inside of the valve body to remove rust and any residual fluid. Inspect the gasket for cracks, corrosion, and defects.
4. Begin grinding by machining any large defects in the valve seat surface. Use a portable valve grinder to grind defects, using the appropriate abrasive for the seat material and condition.
5. Remove grinding dust from the valve.
6. Perform a leak test on the valve to ensure it meets current standards.
7. Reassemble and reinstall the valve in its original position. Make sure the actuator is still working.
8. Finally, write a report on the actuation procedure.
Valve Grinding Techniques and Equipment
A large number of technologies for valve grinding and lapping have been developed over the years. Individually driven grinding heads are the technology of choice today. However, other methods, including more traditional methods, are still used in many sectors throughout the industry.
Individually driven grinding heads: With these grinding heads, the grinding discs are arranged in a planetary arrangement around a central drive head. This ensures that the speed is the same on the inside and outside of the seat. The grinding discs can be correctly positioned on the flat and inclined base, forcing the grinding head to grind evenly over the entire base surface. This allows perfectly flat seats with fine surfaces and cross-shadow patterns to be achieved in a short time (see Figure 1).
Shop floor grinders or milling machines: These are high-speed machines that can grind or mill at any desired angle on the exterior, interior and flat surfaces. The mounting mechanism ensures that the machine is accurately centered. The results are very good and the cutting speed is faster than any other method.
Grinders with friction-driven grinding heads: These grinders produce fine surface structures with cross-shadow patterns. However, the cutting speed is relatively slow. On a flat base, the friction-driven grinding head must be placed off-center to force rotation. Friction causes the grinding head to slow down or stop where the friction is greatest. As a result, this technique will produce different speeds on the outside and inside of the seat, which may cause the seat to recess.
Grinders with solid flat or beveled cast iron plates: This method can be used for flat seats and small gate valves when space is limited. The use of tacky sandpaper is recommended to prevent the cast iron plate from protruding in the area where the plate is located. A solid flat plate will achieve a higher speed on the outside of the perimeter than on the inside of the disc. As a result, more material is removed from the outside of the seat, which becomes slightly protruding. This method produces a grinding structure with longitudinal scratches, which results in the inaccurate flatness of the entire seat. If an abrasive paste is used, it slowly migrates from the center until all of the paste is on the outside of the seat. Slow cutting speed.
Grinding with a rotary plane table: Works well when the work is performed in a workshop; a special table is available for grinding valve seats or wedges. However, this method is time-consuming.
Hand grinding: This is a very labor-intensive alternative and can be dirty if the abrasive paste is used.
The valve pressure test bench is designed and manufactured with reference to ISO5028 "Pressure test of industrial valves" and GB/T13927-92 "General valve pressure test", and takes into full consideration the advantages and disadvantages of similar equipment at home and abroad, so that it can be continuously improved and optimized. The equipment is mainly used for the testing of various sealing performances of new valves before they come online or after the repair of old valves. These tests include internal sealing performance test, upper sealing performance test, valve body strength test, etc.
It is critical for plant personnel to be aware of the latest valve maintenance equipment and techniques. It is important to remember that the technician using the equipment also needs the skills and judgment that are ultimately responsible for performing the valve maintenance results. Judgment and skill is an ongoing process that is enhanced by personal knowledge of existing methods, procedures, and equipment, as well as knowledge of technologies being developed.
Valve maintenance is critical not only to saving time and money but also to keep the plant and the operators within the plant the safest possible.
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