VALVE GRINDING
pattern caused by the abrasion , which should be criss-crossing itself , rather than run longitudinally along the seat . Even if the seat is lapped to a mirror finish , this pattern is always there , even though it may only be visible with a microscope . The cross-hatch pattern breaks up the capillary paths of the fluid and helps produce a better seal . When the grinding head is placed centrally above the valve seat , the desired geometry of the crosshatch pattern is easier to achieve . Having successfully ground the valve seat this way , the user ends up with a perfectly flat seat with a fine surface structure , achieved far quicker than any other method can provide .
Preparing the job
To ensure that dust can leave the work area , it is best to select a slightly oversized grinding tool . Any build up of dust will increase the temperature through increased friction . For instance , If the seat is 25mm ( 1 ”) wide , a Ø50mm ( 2 ”) grinding tool or larger could be used , providing the design of the valve body allows this . Before starting grinding , the valve seat should be examined for damage . Start with a medium to fine paper , just to clean
Discs with diamond abrasive are used to grind valves that have particularly hard seat material
the surface prior to examination . Do not use a coarse paper at this stage . If the valve seat damage is only slight , coarse paper will not be needed and you will only have made more work for yourself . Also , there is only a finite amount of material in the valve seat , so using a fine paper at this stage prolongs the life of the valve . If the seat damage turns out to be severe , proceed with coarse paper .
Otherwise , continue with fine or medium paper . Use progressively finer papers , moving into micron sizes . When the valve seat is completely smooth , finish off with fine abrasive , diamond paste or slurry , depending on the surface finish that is needed . For a gate valve , this may be a smooth , dull gray finish , whereas for safety valves and valves handling gas , a mirror finish is frequently required .
62 Valve World June 2024 www . valve-world . net