Creep is a phenomenon that occurs in the materials used for the manufacture of flat gaskets and which is not given sufficient importance.
To understand what the creep of a flat gasket means, let's assume you have installed a gasket like the one in the image above, with the recommended tightening torque which in this case is 100 N-m on each bolt.
Two days later and before starting up the installation, following the plant safety protocol, you proceed to verify the tightness of the gaskets. To your surprise, the bolts on the flange where you installed the gasket show a tightening torque of 90 N-m and not the torque applied during assembly two days before, so you have to retighten it. To begin with, a piece of advice: never do it when hot, as the bolts do not have the same elasticity as when cold.
But what happened? Human error? No.
What has happened is that the gasket material has not been able to maintain its thickness under the tightening of the bolts. In other words, the joint has relaxed with an effect similar to that of the bolt nuts loosening. The joint is then said to have suffered from creep.
Gasket materials require a minimum surface pressure to seal the flanged joint. This pressure is provided by the orthogonal force applied (torque) to the joint area.
If it is the case that the thickness of the gasket material suffers and yields to the torque being applied to the flanged joint, automatically the gasket aŕea increases and, consequently, the surface pressure exerted decreases proportionally. If it is reduced below the minimum value of the same, then the flanged joint leaks.
As a user of virgin PTFE gaskets, this problem will not sound new to you. When using virgin PTFE gaskets, you must frequently retighten the flanges because of the high creep of the gasket material. In fact, virgin PTFE is probably the worst gasket material and is only used for its excellent chemical resistance.
The following table shows some creep values as a function of gasket material and temperature. Creep is expressed as loss of initial tightening (%), at the indicated temperature and for the same time base in all cases.
The solution to the creep problem of a material is by no means simple. Among the possible joint materials, you should choose the one whose manufacturer publishes the lowest creep/relaxation value.
The thickness of the joint also plays an important role. The thinner the joint, the lower the creep of the material
Whenever possible, evaluate the surface pressure that the joint will make at the torque reduced by the creep of the material and make sure that this surface pressure is higher than the minimum required by the material.
On the other hand, in the case of installing PTFE gaskets, the addition of fillers to virgin PTFE significantly reduces the creep of the material. The additives act as barriers that contain the deformation of the PTFE. In addition, when starting from restructured PTFE (Gylon®), the results improve considerably.
The image below shows PTFE gaskets with loads, before and after the creep test. The gaskets are subjected to 260 °C for 1 hour and compressed to a pressure of 140 bar. Creep is manifested by a loss of structural integrity of the sample:
Samples 1, 2 and 3 are from Gylon® with barium sulfate, glass beads and silica fillers, respectively. Sample 4 is virgin PTFE.
Test tube no. 3 of Gylon® with silica has the lowest creep, unlike the Virgin PTFE specimen which is clearly deformed and unrecoverable.
From EPIDOR SRT, Our Technical Department is at your disposal to advise you properly. Ask us about your problem and we will find the most appropriate solution to your needs.