Gaskets compression stress

Thus, for a pressure of 1.4 GPa, the overpressure in the gaskets is 470 MPa, which is enough to make the softer parts of the gasket packing flow and plug any leak. The drawback is that the bore of the cylinder and the piston-head stem undergo a compressive stress of 1.4 X 1.33 = 1.9 GPa, or more than 2 GPa if friction is included. This may cause a pinch-off of the stem and permanent deformation of the bore. 

Mechanical failures occur when the part is exposed to some t5q)e of force that exceeds its capability. A part may be exposed to three different t5q)es of forces tensile, compression, and vacuum-generated stresses. Many processes require super- or subatmo-spheric pressure. In a fluoropol5mier-Iined vessel or a stand-alone vessel at elevated pressure, the walls are subjected to tensile stress. Compression stress develops in parts such as seals and gaskets where force is applied to the part, for instance, by placing it between bolted flanges. Vacuum can be a permanent or transient feature of a process and subjects a part to complex forces which could be a combination of tensile and compression. 

The gasket allows an even distribution of stress in the die taper and along the tapered anvil cone. In this way, both the die and anvil are supported by the resulting compressive stresses. 

Fa = allowable compressive stress, psi 12. Unit stress on a gasket, Sg. 

Different tests have been set up to develop industry standards and test evaluations that should be directly useful in their applications. For example, a gasket relaxometer applies a compressive stress to a flat annular specimen, similar to the way many gaskets are stressed in service (see ASTM F 38). This device is simple to operate, inexpensive, and capable of measuring the effects of such pertinent variables as stress relaxation in regard to time and environment. 

Compression Stress Relaxation The definition of compression stress relaxation is that when a constant strain is applied to the gasket sample, the force necessary to maintain that strain is not constant but decreases with time this behavior is called stress relaxation. The test apparatus used for compression stress relaxation measiuements is the Wykeham Farrance device. It provides information for the prediction of the service life of materials by measuring the sealing force decay of a sealing gasket sample as a function of time, temperature, and environment. 

Flange face areas experience stagnant conditions. Additionally, some gasket materials, such as asbestos fiber, contain leachable chloride ions. This creates crevice and stress corrosion cracking problems on sealing surfaces. Where necessary, flange faces that are at risk can be overlaid with nickel-based alloys. Alternatively, compressed asbestos fiber gaskets shrouded in PTFE may be used. 

Note For pumps with flange and pressure ratings in excess of the minimum values in 2.2.2, the gland stud size and circle may increase. Larger studs shall be furnished only if required to meet the stress requirements of Section VIII or Section II of the ASME Code, or to sufficiently compress spiral wound gaskets in accordance with manufacturer s specifications. 

Stress relaxation measurements can be made in compression, shear or tension, but in practice a distinction is made as regards the reason for making the test which is generally related to the mode of deformation. The most important type of product in which stress relaxation is a critical parameter is a seal or gasket. These usually operate in compression and, hence, stress relaxation measurements in compression are used to measure sealing efficiency. 

The number of bolts and the bolt size must be chosen such that the bolt load is less than the maximum allowable stress in the bolts. The bolt spacing must be selected to give a uniform compression of the gasket. It will not normally be less than 2.5 times... 

After answering these questions about the environment and performance requirements, the chemist can examine data from the various databases of material choices and make an informed decision. A successful seal design will ensure adequate compressive force while minimizing stresses acting upon the seal or gasket. 

The simplest form of compatibility test is to immerse standard ASTM coupons in a liquid or vapor in the lab. Coupons can be installed in the field in a stream. However making a coupon assembly for installation in a pipe or a vessel may be difficult and persuading plant operators even more so. Lab testing is easy to carry out but will not replicate real life conditions such as flow, agitation, stress (compression for gaskets) and one-sided exposure. For this reason, lab dunk tests are usually done for screening reasons. Application temperature is easy to achieve by space heaters. 

The first step in the project was to perform an extensive set of experimental tests on the actual glass-filled PTFE that was used in the gasket. Specifically, uniaxial tension and compression experiments at different strain rates and different final strain levels were performed. Both the loading and unloading behavior was examined. A few select stress relaxation experiments were also performed to directly probe the stress relaxation behavior of the material. Examples of the experimental data obtained from these tests are shown in Figs. 11.13 to 11.16. 

---