What is valve lapping? Procedure for lapping

Lapping is the process of cleaning the control valve for maintenance purposes. The lapping is done with compounds that has roughness, and used to clean valve disc and nozzle seats etc.,


Every seat and disc from a relief valve or block valve, in general, must be subjected to lapping.

How to lap control valve:

The disc and nozzle relief valve that has been checked if it looks scratched or the surface of the seat is not flat then the seat or nozzle must be turned first, to smooth the results of the lathe then we do lapping.

We take the cast iron lapping plate according to the size of the disc or the nozzle relief valve then we take the compound for the first stage we use compound grid F (100).

After we have lapped the part of the disc seat that is even grey, then we continue lapping until it is even, after flattening the surface of the seat and disc then we replace it with a rather fine compound or grid 2A (400), then we continue with lapping again if it looks flat and there are no scratches on the disc or nozzle we replace it with the most subtle grid then we get very smooth results and no scratches and we can be sure that the results of the lapping we think are finished.

Procedure for lapping:

First step:

  • Unlock the bonnet bolt, after the bolt is removed all, lift the whole, then we tie to the vagina. Open gland packing bolt, when gland packing is open then remove packing stem.
  • Open the disc stem nut from the disc plug
  • Open the handwheel.
  • Release the threaded stem
  • Remove the Bonnet gasket.

Second step:

  • After all is open, then we do steaming, Brushing then we wash with turpentine oil, we check the disc plug and seat ring if it is on the seat ring or the disc plug is damaged or scratched then the seat ring and plug we lathe. Finish our Disc Plug and Seat Ring lapping together, Seat Ring and Plug Disc pitted directly.

  • After pitting directly, we check the settings again so that the Beat Disc and Plug holder is really in contact.

  • We attach the stem to Bonnet bushings.

  • Install the Hand Wheel and Nut Stopper

  • Change Packing

  • Tie Gland Packing

  • Replace Bonnet Gaskets

  • Reinstall Bonnet and Nut, tie until smooth and strong

  • then we do Hydrotest manually.

  • Place the valve in the position of the Test Clamp

  • Press the Clamp on the Valve (Adjust Hydroulic Clamp Pressure)

  • Raise pressure by using a pump directed by air and water fluid (Adjust Hydro test Booster Pump)

  • If there is a bubble in the water inside the valve, then the valve is leaked, if there is no bubbles in the valve then the valve is good.

Compounds used for lapping:

The compound used are listed below with roughness: Grid (in the format)

Extremely Fine

7A = 1200 6A = 1000 5A = 800

Fery Fine

4A = 600 3A = 400 2A = 300


1A = 320 A = 280

Medium Fine

B = 240 C = 220


D = 180 Ne = 150


E = 120 F = 100 G = 80