For a great marksman, hitting the center of a target is not a problem. You set up a target, and he can hit it. In fact, he can hit it all day long, because this is what he does. However, all his skill and confidence won’t help him a bit if he’s presented with two targets and not told which one he is to hit. Which one should he focus on, and which one should he ignore?
Such is the problem with many flange torqueing standards, especially in regard to heat exchangers. In the majority of cases, existing assembly procedures for heat exchangers call out a specific stud stress that should be targeted. That stress level may be 40 percent, 50 percent or 60 percent of yield, and that same value is applied — almost without exception — to all heat exchangers in the plant, regardless of pressure ratings, the number and size of the studs, the gasket size or a host of other factors. Success — insofar as tightening the exchanger is concerned — is defined by how much a stud is stretched. It’s an easy target to hit. But is it the right target?
Many studies have been completed that show long-term sealing of bolted connections is all about gasket stress. To be more precise, it is about attaining the needed stress and maintaining that stress in the face of the relaxation that can be anticipated in the joint. The ruling parameter in determining the assembly torque required for a given joint must always be gasket stress, not stud stress.
Studs don’t seal flanges; gaskets do. Since the “target” must be the gasket stress needed for optimal performance, it’s necessary for your gasket supplier to: 1. Evaluate the gasket area, 2. Evaluate pressure interactions and desired gasket stress, and 3. Calculate the stud stress needed to achieve that end.
Put simply, any assembly specification based on achieving a specific stud stress is aiming at the wrong target. Any such specification can only develop the optimal gasket stress by accident or by coincidence. The only way to make sure the correct gasket stress is achieved is by targeting gasket stress.
There are complete technical notes available that include examples of exchangers that were chosen at random from the hundreds of heat exchangers asked to be analyzed. They were not selected because they are extremes and simply demonstrate the natural variations that exist among code-designed heat exchangers.
For more information, call Advanced Sealing at (562) 802-7782. To download the complete technical notes on this study, visit http://info.Lewis-goetz.com/gasket-stress.
