A gasket seals only if the two flange faces are flat to a small fraction of the gasket’s thickness and carry no scratch, pit or tool mark running across the sealing band, because any such mark is a leak path that the gasket cannot fill. Prepare the faces by machining or scraping them flat, check with a straightedge and feeler leaves, finish them to the gasket maker’s requirement, and clean them before every assembly.
Part of Sealing Systems
Preparing the metal faces that a steam gasket seals against, and seating a new gasket correctly.
The Gasket Can Only Fill Gaps Smaller Than It Can Squash
A gasket is compressed by a fraction of its thickness. Everything the gasket has to fill, including waviness across the flange, a high spot at a weld, or a groove left by a tool, must be smaller than that squash. Larger imperfections leave a channel that steam finds.
The carrier asks for flange faces machined smooth and flat, with concentric grooves of 0.8-1.6 μm Ra surface roughness, a flatness within 0.1 mm across the full face, and no scratches or pitting across the sealing surface. The carrier does not say where its roughness and flatness numbers come from, and I have not been able to confirm them: concentric-groove finishes for gaskets are commonly specified rougher than that, and the roughness that works depends on the gasket material (softer gaskets tolerate finer faces, harder or metal gaskets need grooves to bite into). The finish should therefore come from the gasket’s maker or the vessel’s drawing, and the carrier’s figures are an indication, not a specification.
The flatness figure can at least be checked against the gasket thickness, since the carrier gives its gasket range as 3-5 mm. A 0.1 mm flatness error is then 0.1 ÷ 3 = 3.3% of a 3 mm gasket and 0.1 ÷ 5 = 2% of a 5 mm one. Those are small enough to be absorbed. A face that is out of flat by a whole millimetre would not be, and neither would a single deep radial scratch.
What You Need
- A straightedge and feeler leaves. Standard feeler leaves run from 0.05 to 1.00 mm (Thickness Gauges).
- A surface plate and marking compound to find high spots on a face you are scraping or lapping (Surface Plates).
- A lathe or a milling setup for machining the face, or a flat file and scraper for hand work.
- Clean, lint-free rags and a solvent that evaporates fully.
- The gasket and its supplier’s finish requirement.
Method: Preparing And Seating
Step 1: Check flatness. Lay the straightedge across the face in several directions. Slide feeler leaves under it and note any gap. Look also for a lip around the bolt holes where bolts have raised metal.
Step 2: Remove high spots. Mark high spots with compound against a flat reference, then scrape, file or machine them away. Do not chase flatness by removing so much metal that the flange thins.
Step 3: Finish the face to the specified roughness, working in the circumferential direction, never straight across the sealing band. A scratch that runs across the band is a leak path from inside to outside.
Step 4: Clean everything. Remove oil, old gasket residue, rust and grit. Residue left from the previous gasket is the commonest cause of a leak on a fresh one.
Step 5: Seat the gasket dry, centred and flat, without sealing compounds unless the gasket maker requires them.
Step 6: Tighten evenly. Follow the star pattern and stepped tightening for a bolted flange (Bolted Flange), and let a bayonet door turn to its stop (Bayonet Lock).
Step 7: Run a water-only trial and look for weeping at the joint. If it leaks, release, open, and inspect the face and the gasket for the mark that caused it.
Common Mistakes
- Polishing the face to a mirror finish “to be sure”. Some gaskets need a little tooth; follow the gasket’s requirement.
- Tooling marks across the band. Always run the finish round the band, not across it.
- Gasket sealant smeared on “for good measure”. It can squeeze out and block a vent port.
- Forcing a gasket down onto a dirty face. Residue shows as a leak at the first heat.
- Machining so much off the flange that it no longer carries the bolts. Flange thickness is part of the design.
Safety
A leaking joint under pressure is an early failure
Never try to fix a face leak by tightening bolts with pressure on. Release to zero, open, and correct the face or the gasket. Wear eye protection when scraping, filing or machining, and do not breathe dust from old gasket material.
What’s Next
- Learn next: Safety Devices — what protects the vessel when sealing and design are not enough
- Combine with: Gasket Types — the material the faces must suit