Factors that Affect Torque Loss in Gaskets

07 August 2019

A ring of bolted fasteners typically encircles a pair of flange faces. Ideally, the bolts are tightened in a pattern so that they deliver a fixed amount of torque. In this way, a pipe joint or system fitting maintains a leak-free coupling. Even if the fluid flowing through a coupling is highly pressurized, the properly tightened ring of fasteners holds firm, at least that’s the general assumption.

Rejecting Flange-Fitted Conjectures

Engineers never make assumptions. Apart from anything else, bolts suffer from torque loss hardships. Even a mere 24 hours after the tightening force has been applied, bolts and nuts will relax a little. And that’s the first pertinent causative factor, the fact that bolts don’t hold their shape. They stretch a little because heat-treated metals are tempered and malleable. If that weren’t the case, hardened fasteners would be so brittle, so frangible, that they’d fracture. To compensate for alloy malleability issues, installation technicians return to gasket fitting sites and re-torque relaxing bolts.

Concerning Gasket Creep Challenges

Less compression-capable sealing materials have trouble conforming to extreme flanging pressures. As the torque applied to a joint surpasses a gasket material’s compressibility limitations, it flattens out and loses its shape. This time around, instead of the bolts stretching, it’s the gap between those anchored fasteners that diminishes. At any rate, the effect is much the same. With that flange and gasket space contracting, torque loss problems blossom. Offsetting such challenges, gasket materials are creep tested and assigned relaxation resistance ratings. Take note, creep relaxation performance can also be influenced by in-system and environmental temperature fluctuations.

Avoiding Installation Mistakes

Wrongly applied tightening patterns complicate matters. Worsened when a technician doesn’t use a second or third re-torquing pass, bolted flanges suffer from seating stress on one side of the gasket while the opposing side presents as a low-compression sealing area. Left like this, a gasket blowout becomes a distinct possibility. Referring back to creep relaxation performance issues, thicker gaskets and more elastomeric materials are particularly sensitive to seating stress hazards. Incidentally, as another point of installation concern, pipe parallelism is a known torque loss troublemaker. If flange faces aren’t parallel, aren’t closely aligned, then a coupling’s positional characteristics will inevitably suffer.

Words like “creep” and “relaxation” become especially relevant when talking about torque loss difficulties. Subsequently, leakage and blowout conditions become high probability events. Let’s add compressibility and plasticity to the list of engineering terms. Compressibility, the way a gasket thins when it’s torque-tightened, is a factor that’s influenced by the selected gasket material and the thickness of that seal, among other things. On the other hand, plasticity problems ensue when malleable fastener metals stretch and deform.

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