Sometimes the parts you need aren’t sold on the shelves. Rexarc ran into this problem trying to find the best fittings for our stainless steel pressure vessels, ones that would meet ASME standards for the very high pressures our vessels are designed for. How did we get our customers the best possible fittings if no one was selling them?
Fittings for pressure vessels are small components added to the vessel itself so that plumbing can be added. Pressure vessels need to have a way to add and remove fluids from their interior space that meet several requirements:
Some larger, permanently installed pressure vessels might do better with permanent plumbing, but in many cases it’s helpful for both project requirements and maintenance if the vessel can be disconnected.
The Society of Automotive Engineers (SAE) defines standards used for thousands of components used in the aerospace industry, fittings included. SAE fittings meet their standards, which means they’re designed to be safe and functional by meeting the requirements set by panels of experts. Standardized components also mean it’s easier to design modular systems and swap pieces out for repairs or other modifications.
Commercially available, off-the-shelf SAE fittings aren’t designed for the high pressures (>3000 psi) most of our pressure vessels are optimized for. There aren’t too many other manufacturers designing and building custom high pressure stainless steel vessels, and other companies made do with fittings that weren’t reusable or simply left it up to the customer to get the fittings installed.
The fittings our customers were asking for use a replaceable O-ring to form a seal instead of the fitting threads themselves. The O-ring presses against a flat surface on each fitting, molding itself to fill the gaps between them. The threads are used to apply pressure to the O-ring, so these fittings can form a better, reusable seal.
Since no one was selling the fittings our customers wanted we decided to design and fabricate our own, and set one of our top engineers to the task. They designed multiple new fittings, some for our stainless steel vessels, and some for our carbon steel ones.
There’s a lot of factors that have to be accounted for when coming up with a fresh design for any component, and fittings are no exception. Here’s some details our engineer considered:
Matching the material to the vessel is important so the ends of the fittings can be properly attached to the vessel, as welding dissimilar metals introduces additional complexities like warping, altered material properties, and a decrease in corrosion resistance.
Perhaps one of the biggest design considerations is determining what pressures the fittings can withstand. In this case, we knew what pressure ranges we wanted the fittings to work with, so worked backwards from there to create the rest of the design.
The way the fittings are machined can limit the design. Many small components like fittings are cut from a larger piece of material using a CNC machine, which takes a 3D computer model of a part, translates it into a string of coded actions, and removes material from the original material to create the new part. Some machines have more limitations than others, whether it’s different tolerances (how much the completed cuts can vary from the stated dimensions), cutting angles and speeds, how many times the piece of material has to be readjusted, and more. The part has to be designed with the manufacturing process in mind so it can meet the required specifications.
Threads are another critical area to be machined. They’re mainly cut using taps and dies, taps for internal threads, and dies for external threads. They each have their own tolerances to be accounted for, and the part has to be designed so that the taps and dies can have enough clearance to cut the threads.
Once the design was finished, we sent it off to be machined so we could test that it met our expectations, and the results were a success!
Our new SAE fittings are designed, built, and tested, and now we’re excited to offer them to our customers.
Rexarc improves what we offer every day, and we’re always looking for more input! Feel free to leave us a note or ask a question here. Want to learn more about our stainless steel pressure vessels? Click here.
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