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%, 50%, or 60% 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 – in so far 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 which show that 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. The “target” must be the gasket stress needed for optimal performance. Because of this, it is necessary for your gasket supplier to:
1. Evaluate the gasket area
2. Then evaluate the pressure interactions and the desired gasket stress
3. Calculate the stud stress that must be developed to achieve that end
To put it simply, any assembly specification that is based on achieving a specific stud stress is aiming at the wrong target. In fact, any such specification can only develop the optimal gasket stress by accident, or by coincidence. The only way to make sure that the correct gasket stress is achieved is by targeting gasket stress.
Click here to download a technical note on the importance of using gasket stress instead of stud stress when torquing your heat exchanger flanges.