High pressure screw plug (also called breech lock) exchangers are arguably the most complicated bolted connection in a refinery. After traveling around the world helping facilities turn these exchangers from chronic leakers (from internal tubesheet and external diaphragm and B style shell leaks) to one of the most reliable heat exchangers in the plant, it is very clear to me that manufacturers, engineers and mechanics struggle to correctly understand the interactions between gaskets, internal pressure, the two circles of external push bolts that are part of A style exchangers and all the internal parts.
Some manufactuers have given up on trying to make these connections operate leak free, and prefer to weld in the tubesheets and diaphragms. This significantly adds to the cost and time needed to open and close these exchangers, as special equipment is needed to machine out the parts and weld them back together again.
There are a handful of equipment manufacturers that build these exchangers, and while there are differences from one manufacturer to the next, once the basic design is understood, the reader should be able understand that they are all basically the same when it comes to the few critical steps that must be followed when opening and closing these exchangers.
In order to obtain reliable leak free performance, all the basic sealing tools must be employed, including:
1. A spreadsheet that will easily analyze the critical variables;
2. Correct gasket selection;
3. Correct assembly procedures that focus on important steps including proper spacing of internal parts and obtaining consistent thread friction;
4. And finally hot torqueing the exchangers after startup.
The goal of this paper to provide the end-user with a good understanding for how parts interact and how the bolted connections function, so that they might also achieve reliable, leak free performance. This paper will explain:
1. How the connections work;
2. Common misunderstanding about screw plug exchangers;
3. Pressure Testing Screw Plug Exchangers;
4. Analyzing the design and calculating the important variables;
5. The key assembly steps that must be included in all assembly procedures;
6. Important design considerations when building new exchangers.