Controlling the load is essential to ensuring the gasketed joint will seal properly. Previous Sealing Sense articles have examined the types of gaskets to use, how flange finish affects gasket sealing and major pitfalls to avoid to properly assemble a gasketed joint. However, regardless of the type of gasket, controlling the load is probably the most important criteria for getting a gasketed joint to seal. A big problem is the load on the gasket cannot be measured directly and easily during installation.
However, applied torque on the flange bolts can be measured and controlled and is one of the most frequently used methods to control gasket load. This article explores bolt torque and the major considerations for converting measurable bolt torque into the gasket load necessary to seal a flanged connection.
Torque is the turning force measured in foot-pounds (ft-lb) or inch-pounds (in-lb) applied to tighten (turn) the nut on a bolt. Torque can be measured during flange assembly with a properly calibrated torque wrench. In a bolted flange, the applied torque generates the axial load in the bolt. The bolt acts like a spring. Tightening the nut stretches the bolt, which increases the load on the gasket. The relationship between torque, the turning force, axial bolt force and gasket load can be expressed by the simplified formula:
T=Torque in ft-lb
k=Dimensionless nut factor
f=axial force in pounds
d=Nominal bolt diameter in inches
The nut factor is a "modified" friction factor, but a nut factor involves more than just friction. It is more of a multiplier "in total," taking into account many other load losses. If the same torque is applied, a 0.1 nut factor would produce twice the axial force as a 0.2 nut factor. Small changes in the nut factor can result in large changes in the load experienced by the gasket. This illustrates the need for a well lubricated bolt, nut and washer.
Figure 1. Forces on a bolted connection
Forces on a Bolted Connection
As described above, the bolts act like springs pulling the flanges together. They need to be stretched enough to keep the load on the gasket as the system is pressurized and as pressure and temperature cycle during normal usage. Additional loading above the minimum load required to seal will give the bolted joint flexibility to absorb these load changes and a safety margin to maintain the seal as these system forces fluctuate.
Bolt yield strength is a measure of the load required to stretch the bolt to its maximum length or stretch and still allow it to spring back to its original length. If the bolt is overstretched and is loaded beyond its yield strength, the bolt will not "spring back" when the load is removed. Overloading bolts can cause them to stretch beyond their yield strength and actually result in lower loads exerted on the gasket, after additional external and application loads are applied to the joint. Continued tightening of the bolts will not necessarily increase gasket load or stop a gasketed joint leak, and may lead to bolt failure.
The bolt may also lose its compressive load if it is not stretched enough and the system relaxes beyond the amount the bolt has been stretched. It is often recommended that a bolt be loaded to approximately 50 to 60 percent of its yield strength to ensure the "spring" is stretched enough. However, this recommendation should be tempered by the amount of gasket stress and flange stresses generated; check to ensure that the applied load will not overload and damage the gasket or the flange.
Bolts come in a variety of grades, each with individual yield strength and properties. Proper bolt selection is critical to the proper assembly of a bolted flange joint.
Gasket Design Requirements
We have a torque wrench to measure the torque during assembly and a formula to calculate the torque based on the gasket load, but what is the gasket load needed to generate a seal? The f, or force, portion from our torque equation is composed of two major parts, as noted by the design rules for raised face pipe flanges in the Boiler and Pressure Vessel Code. First is the force needed to compress and hold the gasket in place. The load generated by the bolts has to compress the gasket so it conforms to the flange surfaces, and to "seat" the gasket into the flange. This involves the gasket unit seating load or factor, y.
Second, is the combined force needed to: