6.3 Removing and Installing Valves: Flange Bolting and Torque Sequence

Before the first bolt comes off

Removing a valve is a hazardous energy task. The sequence from Chapter 2 applies in full: isolate, lock, bleed, verify. Beyond that, valve-specific preparation:

1. Confirm the line is drained and depressurized at a bleed or vent, not by cracking a flange.
2. Break the flange on the far side from your body, loosening the bottom bolts last so any trapped liquid drains away from the mechanic.
3. Support the valve before the last bolts come out. A 6-inch Class 300 gate valve is a two-person or rigging load, and the assessment expects the load to be estimated, not guessed.
4. Check the replacement against the old valve: size, class, facing, end connection, body and trim material, and flow direction.

Matching flange facings and gaskets

The rule that produces items: never bolt a raised-face steel flange directly to a flat-face cast iron flange. The raised face concentrates the bolt load on a narrow annulus, and the resulting bending moment cracks the brittle iron flange — often days later, under pressure. The correct fixes are to machine the raised face off the steel flange and use a full-face gasket, or to install an adapter.

Spiral wound gaskets require the correct inner and outer ring for the flange type and must never be reused. Sheet gaskets are cut with the bolt holes punched, not burned, and the gasket must not project into the bore where it will be eroded away.

The bolt-up sequence

Flange bolts must load the gasket evenly. Tightening in a circle crushes one side and leaves the other open.

1. Clean and inspect both flange faces for scratches, radial gouges, and corrosion. A radial scratch across the seating surface is a leak path; a circumferential mark usually is not.
2. Check flange alignment before bolting. Flanges must meet parallel and concentric without being pulled together by the bolts. Using bolts to close a gap builds a permanent pipe strain into the equipment — the direct cause of misalignment and seal failure on the connected pump.
3. Lubricate the bolt threads and nut faces with the specified compound. Torque values assume a friction condition.
4. Install the gasket dry unless the gasket manufacturer specifies otherwise, centered on the raised face.
5. Run all nuts up hand tight, then tighten in a star or crisscross pattern, in graduated passes: commonly 30 percent, then 60 percent, then 100 percent of final torque, followed by at least one full circular (clockwise) pass at 100 percent to pick up relaxation.
6. Recheck after the system reaches operating temperature if the specification calls for a hot re-torque.

The crisscross numbering for an 8-bolt flange runs 1, 5, 3, 7, 2, 6, 4, 8 — always jumping to the bolt most nearly opposite the one just tightened.

Orientation rules

• Follow the flow arrow. Check valves, globe valves, and control valves are directional. An arrow cast into the body is not advisory.
• Stem orientation. Install gate and globe valves with the stem vertical, or at least above horizontal. A stem below horizontal lets solids and condensate settle into the stuffing box, where they score the stem and destroy packing. A stem pointing straight down is the worst case.
• Accessibility. The handwheel must be reachable, and the bonnet must have room to be removed with the valve in place.
• Butterfly valves need clearance for the disc to swing outside the flange faces on some designs; check before bolting a wafer body between two flanges with a reducing bushing behind it.
• Welded-in valves. For a metal-seated valve, weld with the valve closed so weld spatter cannot reach the seat; for a soft-seated valve, weld with the valve open and take steps to limit heat, because the seat material will be destroyed by the temperature otherwise. Always follow the valve manufacturer's instruction, which is the governing document.

After installation

Open a newly installed valve slowly, watching the packing and the flange joints. Full opening of a gate valve is followed by backing the handwheel a quarter to half turn off the backseat, so thermal expansion does not jam the stem in the fully open position. Record the valve tag, size, class, and materials in the maintenance record — the next mechanic will need it.