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Bolted Flange

5 min read

A bolted flange holds the door with a ring of bolts that together must carry the whole pressure force on the opening plus the load needed to squeeze the gasket, so the bolts, flange thickness and spacing must come from a recognised design and never be guessed. Tighten the bolts gradually in a star pattern, count every one before each cycle, and never tighten or loosen a bolt while the vessel holds pressure.

Part of Door and Closure Mechanism

The simplest and slowest autoclave closure: a welded flange on the vessel, a matching flange on the door, a gasket between them and a ring of bolts.

The Bolts Carry The Pressure Force And The Gasket Load

The carrier’s procedure is to weld a flange ring to the vessel opening, machine a matching flange for the door, drill 12 to 16 bolt holes for a 400 mm opening, put a gasket between, and tighten the bolts evenly in a star pattern. It is simple and strong, and slow, because opening the door means undoing every bolt.

The load the bolts carry has two parts:

  1. The hydrostatic end force, the pressure acting on the whole opening trying to push the door away. For the 400 mm opening at 1 bar gauge, from the Door and Closure Mechanism article, that is 12,566 N.
  2. The gasket load, the extra clamping needed to squeeze the gasket enough to seal. It is set by the gasket material and area, it exists even with no pressure inside, and in practice it can be a large share of the total (a recalled design principle; the figure belongs to the gasket and the code).

Only the first part can be computed from what this corpus holds. Per bolt, with the carrier’s range of bolt counts:

  • 12 bolts: 12,566 ÷ 12 = about 1,047 N each.
  • 16 bolts: 12,566 ÷ 16 = about 785 N each.
  • At the carrier’s relief setting of 110% (13,823 N): 13,823 ÷ 12 = about 1,152 N, and ÷ 16 = about 864 N.

These are the pressure term only. The real bolt tension is higher, which is why the bolt size, grade and number come from the code drawing or the certified vessel and not from this arithmetic. The figures here show the order of magnitude and why a single missing bolt matters: the remaining bolts each pick up its share.

What You Need

  • Bolts or studs of known steel and grade, with full thread engagement. A stud with nuts both sides is better than a bolt into a tapped hole in the vessel wall, which wears out.
  • Flange faces machined flat and finished (Gasket Seating and Flange Finish).
  • A flange thick enough not to dish. A thin flange rotates under bolt load, opens at the inside edge and leaks even though the bolts are tight.
  • A numbered layout on the flange so every bolt can be counted and tightened in order.
  • A suitable gasket (Gasket Types).

Method: Closing A Bolted Flange

Step 1: Clean and inspect both faces. Any grit, old gasket or scratch across the sealing surface leaks.

Step 2: Seat the gasket dry and centred, with no folds, and fit the door so it sits level on the gasket.

Step 3: Run every nut on finger-tight, then check that the door is not tilted: the gap should be equal all round.

Step 4: Tighten in a star pattern, not around the ring, moving to the bolt opposite each time, in several passes of increasing tightness. Going round in order pulls the door over to one side and leaves the far side loose.

Step 5: Check the gap again with a feeler at several points. If it is not even, loosen and start over.

Step 6: Count. Say the count of tightened bolts aloud against the numbered layout before applying heat.

Step 7: Re-check cold after the first few cycles. The Gaskets and Seals article notes that gasket bolts should be retightened after 24 hours as the gasket settles. Do this only with the vessel cold and at zero pressure.

Common Mistakes

  • Tightening around the ring in order. This skews the door.
  • Leaving a bolt out “because the rest are tight”. The load on the others rises and the gasket compresses unevenly.
  • Using whatever bolt fits. An unknown-steel bolt may be soft or brittle.
  • Wide bolt spacing. The gasket between bolts is under-compressed and leaks first there.
  • Trying to stop a leak by tightening under pressure. Never; release pressure first.

Safety

A loosened or missing bolt turns the door into a projectile

Never tighten, loosen or remove a flange bolt while the vessel holds pressure or is hot enough to contain steam. A leak at the flange edge is the first warning that the closure is losing the fight; shut off the heat, let the vessel cool to zero pressure, and then find the cause.

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