Because there is a natural variation in any assembly method between calculated and actual compressive stress, most gasket manufactures will supply a maximum recommended stress that is safely below the actual crush test results. For example, if laboratory tests show damage to a gasket at 25,000 psi stress, the recommended maximum stress might be limited to 15,000 psi.
Target gasket stress (SgT) is the load that allows the gasket, as well as the entire joint, to operate at optimal performance and sealability. Additionally, the installation stress creates a preload in the joint that compensates for overall bolted joint relaxation after installation and during operation for the service life of the joint (with consideration given to joint integrity). ASME PCC-1-2010 Guidelines for Pressure Boundary Bolted Flange Joint Assembly recommends that the target stress should be as high as possible, “The target gasket stress should be selected to be towards the upper end of the acceptable gasket stress range, as this will give the most amount of buffer against joint leakage.”
Many reasons exist for using a high target gasket stress. In assemblies running at high pressures or flanges with large internal diameters, there will be significant unloading of the joint when pressurized. Studies conducted by the Pressure Vessel Research Council (PVRC) on pipe bending stresses showed that the bolt stress in the joint was a major factor in keeping the gasket from leaking. The higher the bolt stress, the more bending force the joint could potentially handle.
System fluctuations in pressure and temperature will affect the retained bolt load in a joint. Since these factors may reduce the load in the bolts, it is good practice to install the bolts at higher initial stresses, as long as the components are not damaged and bolt material yield is taken into account. Stresses in the bolts will have a direct impact on the stresses in the flanges, so these factors must all be considered when selecting the target gasket stress.
How does one choose the appropriate gasket installation stress? With equipment such as pumps, valves, actuators, sight glass assemblies, etc., the manufacturer of those components should be consulted. For standard plant piping, the designer or plant engineer will typically define the maximum bolt stress based on the bolt grade, operating temperature and flange design stresses. Note that this maximum bolt stress is NOT the same as the allowable stress in ASME design calculations, which is typically only 25 percent of yield. This stress limit is much higher because the ASME Code calculations are meant to force the design to have a significant safety factor, and the design stresses are therefore low.
Once the maximum bolt installation stress is known, the gasket supplier can provide the recommended gasket stress. They will need to know the service conditions for the assembly to select the correct gasket stress. The Y value from the ASME Code calculations should only be used to design the flanges.
The minimum gasket operating stress might be used if the system is going to run at very low pressures. For example, assemblies using pipe and flange materials with low compressive strengths might need to use the lowest possible gasket stress to affect a seal and avoid damage to the flanges. Flanges running at higher pressures and temperatures will use a stress higher than the minimum operating stress.
Target Gasket Stress
The simplest method of selecting the target gasket stress is to calculate the available compressive stress at the maximum bolt stress. This maximum bolt stress is typically determined by the plant engineer, and could vary from 40 percent of bolt yield to over 75 percent at some plants. As long as the available gasket stress at maximum bolt stress is below the maximum gasket stress (or crush strength of the gasket) and above the minimum recommended gasket stress for the operating conditions, that can be the target stress.
Another detail when discussing gasket stress and related available bolt load with the manufacturer is to be sure that both are considering gasket stress based on the same gasket compressed area. ASME Boiler and Pressure Vessel Code calculations for the initial bolt load requirement or operating condition (Wm1) and the gasket seating condition (Wm2) are based upon an effective gasket area, which in the case of Wm2 can be as little as half the ACTUAL compressed area. Many manufactures will use the actual compressed area when discussing gasket stress. This can often lead to misunderstandings if not taken into account.
Next Month: How can compression packing be applied in severe service?
We invite your questions on sealing issues and will provide best effort answers based on FSA publications. Please direct your questions to: firstname.lastname@example.org.
Figure 1. Y value ranges for gasket materials