Bolting Torque and Tightening Sequence for Valve Flanges

From Wrench to Gasket: Following the Load Path

A torque wrench does not tighten a gasket. It twists a nut, which loads a stud, which stretches, which squeezes a flange, which compresses a gasket. Every step in that chain loses some of the input — most of it to friction. In a typical unlubricated or poorly lubricated joint, the majority of the applied torque is consumed fighting friction under the nut and between the threads, and only a fraction arrives at the gasket as clamp load.

That loss is why flange bolting is technique, not arithmetic. The formula connecting torque to bolt preload is simple, but its friction terms are unstable in field conditions, so the discipline that matters is consistent lubrication, consistent procedure and staged tightening. Plants with mature bolting programmes do not get better numbers from better formulas; they get better joints from doing the same things every time.

The Torque Calculation and Its Assumptions

The working relationship most sites use is the classic torque-preload equation: bolt preload equals torque divided by a factor that lumps thread geometry and the friction coefficients together. That factor — often called the nut factor — is where lubrication enters.

• Dry, unlubricated B7 studs can carry a nut factor high enough that achieving target gasket stress requires wrenches beyond what crews can realistically deliver — or worse, drives crews to overtorque in frustration and gall the nuts.
• A consistent anti-gall and lubricant compound lowers and, crucially, stabilises the nut factor. Molybdenum-disulfide and nickel-based pastes are the usual choices, chosen for the temperature range; the lubricant that is perfect at ambient can be a friction disaster at 350 °C, and vice versa.
• Because the nut factor swings joint-to-joint, target torque is usually set to land the achieved gasket stress comfortably between the gasket's minimum seating requirement and its crush limit, rather than at some precise point.

The inputs to any torque table are therefore: bolt size and grade, target gasket stress from the gasket datasheet, the assumed lubrication condition, and the number of studs. Change any one of them and the table changes. Using a torque table printed for dry studs while crews apply paste is one of the quieter ways plants end up with both leaking and snapped studs in the same line.

Tightening Sequence: Why Cross-Pattern and Stages

Tightening one bolt fully before touching the next tilts the flange and crushes the gasket into a wedge. The corrective pattern is crosswise — work opposite pairs, never neighbours — and staged, meaning the full torque is reached in passes.

• Pass one brings all studs to roughly a quarter of final torque, just enough to seat the gasket uniformly.
• Pass two moves to around half torque, again crosswise.
• Pass three reaches full torque, crosswise.
• A final circular pass at full torque confirms every nut has stopped turning; any nut that rotates again has been overtaken by its neighbours' relaxation and gets re-torqued until the circular pass is quiet.

On large-bore flanges, a fifth pass checks the studs against each other one more time after a wait, because gasket relaxation continues for minutes after assembly. Skipping the circular confirmation pass is the most common shortcut in the field, and it is precisely the step that catches uneven joints before they are pressurised.

Tools: Torque Wrenches and Tensioners

Calibrated torque wrenches cover the majority of work, but two upgrades are worth knowing. Hydraulic torque wrenches deliver the higher values that large studs demand, with reaction arms instead of muscle. Stud tensioners bypass the friction problem entirely: they stretch the stud directly with hydraulic pressure and spin the nut down finger-tight, so the achieved preload depends on hydraulic gauge accuracy rather than on a nut factor nobody can measure in the field. For critical joints — large RTJ flanges, high-pressure Class 900 and above — tensioning both sides of the stud simultaneously gives the most even gasket loading available. The trade-off is protruding stud length: tensioners need a defined amount of thread above the nut, a detail worth checking before ordering studs, not at the workface. How much thread and what bolting class each valve family expects is listed on the manufacturer's ball valve and flanged product pages, so alignment between order and site practice starts before shipment.

Bolting Materials and What They Can Take

Stud selection is a pressure-temperature decision, not just a strength decision.

• ASTM A193 B7 studs with A194 2H nuts are the default for carbon-steel flange work through moderate temperatures.
• B7M is the low-strength, hardness-limited version mandated in sour service under NACE MR0175 — it exists because high-strength steels are susceptible to sulfide stress cracking. Substituting ordinary B7 into a sour-service line is a real and repeated field error.
• At elevated temperature, chromium-molybdenum grades such as B16 retain strength where B7 relaxes; at cryogenic temperature, toughness becomes the criterion and austenitic stainless or other low-temperature grades are selected, following the same logic as BS 6364 low-temperature valve testing.

Relaxation deserves its own mention: at high temperature, studs creep and gaskets relax, so bolt load drops during the first thermal cycles. This is not a defect — it is material behaviour — and it is managed with hot torqueing rather than with heavier initial torque.

Hot Torqueing and Re-Tightening

A joint assembled cold at correct gasket stress will lose a portion of that stress at operating temperature. The established response is a scheduled hot torqueing pass — re-tightening to the specified torque with the line at or near operating temperature, using equipment and procedures designed for the hot environment and done only where safe to do so. Done at the right time (typically after the first sustained run at temperature) and to the right pattern, it restores gasket stress and dramatically reduces the leak count at the first turnaround inspection. Note the sequence discipline from cold assembly carries over: crosswise and staged, never one nut to full torque in isolation.

Records, Reuse and Discipline

Two closing disciplines separate professional bolting from hopeful bolting. First, records: which joints were tightened, to what torque, with which lubricant, in what sequence, and what the confirmation pass showed. When a joint leaks later, that record is the difference between diagnosing the cause and guessing. Second, reuse rules: studs and nuts that have been stretched to yield, gall-damaged or corroded in the threads do not go back into a critical joint, and gaskets are never re-used — a spiral wound that has been compressed once has work-hardened seating surfaces and will not reseal to specification even if it looks intact. Reference material on flange classes and bolting across a full valve range can support the site procedure drafting.

An experienced industrial valve maker producing the complete valve programme in-house — ball valves, control valves, gate valves, globe valves, check valves, strainers, knife gate valves, plug valves and butterfly valves — supplies flanged assemblies with bolt torque guidance, gasket recommendations and, where required, pre-matched bolting sets for the service temperature. Send your Valve Schedule or the flange class and gasket details to the manufacturer's engineering team, and they will return bolting recommendations and a quotation matched to your line classes.