Most flange leaks do not happen at start-up. They happen weeks or months later, after the joint has quietly lost bolt load. Live loading adds spring travel to the bolting, usually with conical (Belleville) spring washers under the nuts, so the joint keeps more of its load as the gasket relaxes and the flange heats and cools.
Why flange joints lose load
A gasketed joint leaks when the stress on the gasket drops below what it needs to seal. Assembly errors cause some leaks, and a written procedure such as ASME PCC-1 addresses those. The harder cases are joints that were put together correctly and still lose load over time. Six mechanisms do most of the damage:
Embedment and gasket creep are the reason many procedures call for a final check pass, and sometimes a retorque before or after start-up. See Relaxation and Retorque Passes. Elastic interaction is managed with the tightening pattern and multiple passes. See Tightening Patterns Explained. Those steps help a joint reach the right load. They do nothing to help it hold that load in service, and that is where live loading comes in.
How a spring washer keeps the load on
A tightened stud is a stiff spring. A 1-8 B7 stud with about 6 in between nuts, tightened to 45,000 psi, stretches only about 0.009 in. Every thousandth of an inch the gasket relaxes comes straight off that stretch. So if the gasket relaxes 0.003 in, the stud has lost about a third of its load.
Conical spring washers under the nuts add their own deflection to the stud’s stretch. The joint now stores more elastic travel, so the same 0.003 in of relaxation is a much smaller share of the total and the joint keeps more of its load.
Figure 1
Bolt load left after the gasket relaxes
Longer studs with spacers or extension collars, and reduced-shank (waisted) studs, add flexibility in the same way. Spring washers are popular because they fit standard flanges and need only a small amount of extra stud length.
Choosing and installing spring washers
- Match the stiffness to the assembly bolt stress. Flange spring washers have traditionally been sized for common bolt stresses such as 30, 45 or 60 ksi. Today the target is more often set from the gasket stress, as in ASME PCC-1 Appendix O, so pick the washer for the assembly stress you will actually use.
- Expect less than the rating. PCC-1 notes that washers are often chosen to be fully compressed at assembly. Fully compressed, their effective stiffness is about twice the nominal value, so they are about half as effective as the rating suggests. Follow the washer maker’s data.
- Get the orientation right. The inside edge bears on the nut and the outside edge bears on the flange. An upside-down washer gives little or no spring action.
- Stack for load or travel. Nested washers (parallel) carry more load. Washers placed face to face (series) give more travel. Lubricate parallel stacks to reduce friction between the washers.
- Allow for the stack in the stud length. Add the stack height when sizing studs. Our stud bolt length calculator can help.
Figure 2
Common spring washer arrangements
What can go wrong
PCC-1 is clear that spring washers are not a cure-all. Problems it lists include environmental cracking, fatigue, creep, yielding or loss of spring properties, any of which can lead to a complete loss of bolt load. Others are installation errors in orientation or stacking, the wrong number or stiffness of washers, and stacks that shift sideways when the washer bore is loose on the stud. Our Belleville Washers and Live Loading sheet summarizes these points.
Spring washer materials
| Washer material | Typical use |
|---|---|
| H13 tool steel | The most common flange live-loading material. High strength, typically used from ambient to about 1,100 °F. |
| 17-7PH stainless | Lighter loads and cryogenic service, typically about −400 to 550 °F. Not for chloride or fluoride environments. |
| Inconel 718, X-750 | Higher temperatures and corrosive service where tool steel or 17-7PH will not hold up. |
On an uninsulated flange, the washers usually run cooler than the process, sometimes much cooler. Measure or estimate the actual washer temperature before you choose a material, and match the washer to the bolting and the environment.
Live loading a joint? Patriot Bolt supplies spring washers, including flange live-loading washers, together with the stud bolts, heavy hex nuts and flange washers on the same order, with mill test reports. Send us the bolt size, assembly stress and service conditions. Request a quote or call (281) 581-0176.
