Valve grinding procedure
Individually driven grinding heads – the key to flat valve seats
The objective of valve grinding and lapping is to achieve a dense, flat surface. The LarsLap system, with its individually driven grinding heads, achieves just this.
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.
A dense, flat surface is particularly important in the case of safety valves, as these rely on spring tension to remain shut.
Friction is the key to all grinding. A successful result depends on the right amount of friction – this is what causes abrasion. The appropriate abrasion is decided by the material of the valve seat, the required amount of material removal and the machine speed. The single most significant factor in this equation is the friction, i.e. the grade of the abrasion material. This means grinding discs need to be selected with care.
On-site valve lapping and repair
When preparing to grind a valve seat, the seat first needs to be wiped and any moisture removed. The seat surface should then be cleaned more thoroughly by lightly grinding it, using a portable valve grinding machine with a fine to medium paper, 120-180 grit. Inspect the surface carefully and assess the extent of any damage.
If only small scratches are present, continue with fine grit. Do not start with coarse grit without first having used fine grit – this will only make the process take longer.
If the seat has deep scratches, use coarse grit.
Check progress regularly during the grinding process. When most of the damage has been removed, change to a paper with finer grit.
When the surface is smooth, the process is finished. Do not grind more than necessary – this will only shorten the life of the valve. Some marks may be left, providing the do not interfere with the sealing action of the valve.
Valve seat surface defects
It may be useful to make notes on the type of damage in the valve seat, to try to establish the root cause of the problem.
Pinch or jam marks are caused by foreign matter in the pipeline. Usually, these marks are no deeper than 0.05 mm.
Cutting marks usually occur as a result of sliding movement on gate valves or any valves with a wedge or plug using 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 o’clock and 8 o’clock positions. These are often deeper than 0.05 mm. If possible, cutting marks can be avoided by using a valve with a smaller radius.
Erosion 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.
Seat is not flat – this can happen when the seat has been subjected to welding, when the seat becomes worn or when it has been inaccurately ground or lapped. This is a defect that occurs most commonly on gate valves.
Cracks in seat – this typically occurs on valves with a welded seat.
Conical seat – the surface should normally be flat but can become concave or convex due to incorrect grinding procedures.
Repairing leaking valves
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 on site 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 specialised applications.
Before dismantling the valve in preparation for maintenance, the area around the valve should be covered with plastic sheets to prevent loss of parts and unnecessary spread of dirt and grease.
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.
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.
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. Read more about seat materials here.
Remove grinding dust from the valve.
Leak test the valve, ensuring it meets current standards.
Reassemble and refit the valve to its original position. Make sure the actuator still works.
Finally, write a report on the procedure carried out
A great number of techniques for valve grinding and lapping have evolved over the years. LarsLap pioneered the technology with individually driven grinding heads, because the methods that existed previously were cumbersome and produced inaccurate results.
Individually driven grinding heads is the preferred technology today. However, other methods, including traditional ones, are still used in many places across the industry.
When using individually driven grinding 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 space of time.
Grinding or milling machines for workshop use 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 centred. The results are excellent and the cutting speed is faster than any other method.
Grinding machines with friction driven grinding heads 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-centre 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 centre until all of the paste is on the outside of the seat. The cutting speed is slow.
Lapping with rotating plane table 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 is also an alternative, however this method is extremely labour intensive and can be messy if grinding paste is used.
Read more about how to select the right valve grinding materials.