Amongst the various valve maintenance best practices to follow, grinding and lapping is high on the priority list as the procedure achieves a dense, flat surface. This effectively seals the valve seat surface in the valve body and on wedge/gate/globe/slide, to completely seal the valve’s metal-to-metal seat area.
By Allan-Taylor Barry, CEO/Global Business Development — Allap Technical Solutions LLC
An Essential Procedure
Valve reconditioning can be carried out in-situ, with the valve still fitted to the pipeline system, or in the workshop. In-situ valve lapping and repair tends to be more efficient, as it eliminates the need to remove the valve from the system. The only time workshop maintenance is more efficient is when a major valve rebuild is required.
In-situ valve maintenance requires that the bonnet is removed, and that sufficient space is available around and inside the valve body. To make the best use of the available space, ensure that the equipment you buy is light-weight, able to fit in tight spaces, as well as easy to attach. Your time onsite should be spent grinding the valve, not mounting the valve grinder.
Grinding equipment for use in the workshop needs to be easy to operate and adjust, versatile for use on several applications, and be able to extend capabilities with additional accessories for specialized applications.
Preparing for the Procedure
The following step-by-step procedure describes how to prepare the valve, as well as the basic steps to take in valve grinding and lapping:
1. Cover the area under and around the valve with plastic sheeting to prevent loss of parts and to contain dirt and grease.
2. Begin dismantling by removing any lagging. Clean the outside of the valve. Remove any rust and paint from nuts and bolts. Lubricate the bolts with penetration oil. Try to avoid any damage to the fasteners – even just one damaged nut will add an inordinate amount of downtime and cost.
3. Dismantle the interior parts of the valve. Clean, check and mark to ensure correct refitting. Clean the inside of the valve body, removing rust and any residual liquid. Check gaskets for cracks, corrosion, and defects.
4. Start the grinding by machining any large defects on the surface of the valve seat. Grind defects with portable valve grinders, using appropriate abrasives based on seat material and condition.
5. Remove grinding dust from the valve.
6. Leak-test the valve, ensuring it meets current standards.
7. Reassemble and refit the valve to its original position. Make sure the actuator still works.
8. Finally, write a report on the performed procedure.
Pinch/jam marks.
Cutting marks.
Erosion marks.
The seat is not flat.
Cracks in the valve seat.
Conical seat. A: Solid plate grinding centrifugally rotating. B: Solid plate grinding oscillating plate. C: Friction-driven head. D: Driven head grinding.
An example of both a concave, and a convex valve seat surface.
Valve Grinding Techniques and Equipment
A great number of techniques for valve grinding and lapping have evolved over the years. Individually driven grinding heads is the preferred technology today. However, other methods, including more traditional ones, are still used in many sectors across the industry.
Individually Driven Grinding Heads: Using these heads, the grinding discs are arranged in a planetary arrangement around a central driving head. This ensures the same speed is achieved on the inside and the outside of the seat. The discs can be positioned correctly on both flat and angled seats, forcing the grinding heads to grind equally over the whole surface of the seat. This achieves a perfectly flat seat with a fine surface and cross-hatch pattern in a short period of time (See Figure 1).
Grinding or Milling Machines for Workshop: These are high-speed machines that enable grinding or milling on the outside, inside, and the plane at any required angle. The mounting mechanism ensures the machine is accurately centered. The results are excellent, and the cutting speed is faster than any other method.
Grinding Machines with Friction-Driven Grinding Heads: These produce a fine surface structure with cross-hatch pattern. However, the cutting speed is relatively slow. On flat seats, the friction-driven grinding heads must be placed off-center to force rotating action. The friction can cause the grinding head to slow down or stop where the frictional forces are greatest. As a result, this technology will produce different speeds on the outside and the inside of the seat, potentially resulting in a concave seat.
Grinding Machine with Solid Flat or Angled Cast Iron Plates: This method may be used for flat seats and small gate valves when space is an issue. Use of adhesive grinding paper is recommended to prevent cast iron plates from becoming convex in the area where the plate is positioned. A solid flat plate will achieve higher speed along the outside of the perimeter than on the inside of the disc. Consequently, more material is removed from the outside of the seat, which becomes slightly convex. This method gives a grinding structure with a longitudinal scratch that results in an inaccurate flatness across the entire seat. If grinding paste is used, it will slowly migrate away from the center until all of the paste is on the outside of the seat. The cutting speed is slow.
Lapping with Rotating Plane Table: This provides good results when the work is carried out in the workshop; purpose-made tables are available for grinding valve seats or wedges. This method is, however, time-consuming.
Hand Lapping: This is alternative method is extremely labor intensive and can be messy if grinding paste is used.
TYPICAL DEFECTS FOUND ON SEAT SURFACES
1. Pinch/Jam Marks: These marks are caused by foreign matter in the pipelines. Usually, these marks are no deeper than 0.05 mm.
2. Cutting Marks: These typically occur as a result of sliding movement on gate valves or any valves with a wedge or plug, using a sliding action. Damage occurs between the seat and the wedge as the valve opens and shuts. It is usually more prevalent on the lower side of the valve, between the 4:00 and 8:00 positions. These are often deeper than 0.05 mm. If possible, cutting marks can be avoided by using a valve with a smaller radius.
3. Erosion Marks: These marks are visible at the location of the leak when the valve is closed. This can be found on all valve types. The marks are often deeper than 0.05 mm.
4. Seat is Not Flat: This can occur when the seat has been subjected to welding, when the seat has become worn or if it has been inaccurately ground or lapped. This is a defect that occurs most commonly on gate valves.
5. Cracks in Seat: This defect typically occurs on valves with a welded seat.
6. Conical Seat: The valve seat surface should normally be flat but can become concave or convex due to incorrect griding procedures.
Friction driven grinding heads frequently produce a concave surface, i.e., one that is slightly lower on the inside.
Single grinding discs frequently produce a convex surface, i.e., one that is slightly lower on the outside. With this type of grinder, the rotational speed is higher on the outside of the disc. In addition, grinding dust migrates to the periphery of the disc, contributing to additional grinding along the outside edge.
Remember, a dense, flat surface is particularly important in the case of pressure relief valves (PRV) and pressure release valves (SRV) safety relief valves, as they rely on spring tension to remain shut.
Final Thoughts
It is crucial for plant personnel to be informed and up-to-date on the current valve maintenance equipment and techniques. It is important to keep in mind that technicians using this equipment also require the skills and judgment that is ultimately responsible for results of the valve maintenance performed. Judgement and skill is an ongoing process that is enhanced by the individual’s knowledge of existing methods, procedures, equipment, and by being aware of the ongoing technology being developed.
Valve maintenance is not only critical for saves in time and money, but also for keeping the plant and operators within the plants, the safest they can be.
ABOUT THE AUTHOR
Allan Barry-Taylor worked as an Engineering Tradesman in the gold, platinum and diamond mines of South Africa, and progressed into Technical Sales. He has applied hands-on engineering experience in technical sales from industrial products and machine tools sales through complete plant maintenance projects in the process industry. Allan then became self employed as a distributor for several product brands, and developing those products, to global brands.
Finally, Allan ended in up in company startups and global business development and product coordination, specializing in flat lapping, polishing of metal-to-metal (and other exotic materials), valve seat repairs and pressure testing.
* All images provided by Allap Technical Solutions.