Tube fittings and valve service supports design of advanced decompression chambers
Methods
Tube fittings and valve service supports design of advanced decompression chambers
KeywordsDecompression chambers, Parker Hannifin, Valves
A new generation of computer-controlled decompression chambers developed by MSI-Defence Systems has standardised on Parker Hannifin valves and fittings, following joint testing to select the optimum materials to cope with the hazards posed by this application(Plate 6).
The decompression chambers will upgrade the diving support systems on the Royal Navy's minesweepers. MSI-Defence Systems' chamber design provides major new features including computer-controlled regulation of the atmosphere, and therapeutic capabilities to support diving operations.
On top of this advanced functional specification, MSI had to design a chamber with low magnetic characteristics, and protection against the combustion hazards associated with an oxygen-rich atmosphere at input pressures of 207 bar all in a form which will be easily maintainable over a projected lifecycle of over 20 years.
The choice of materials for the gas supply systems is crucial to the reliability of the system, but this is tempered by the need to choose components which are also economic today, and easy to source in the future. MSI wanted to base the gas system on tubing fabricated from the copper alloy, tungum, because of its anti-corrosion properties and inert nature. But although tungum tubing in appropriate thicknesses is readily available, it was not possible to identify suitable compatible fittings to cope with very high pressures.
Designing custom fittings would have substantially lengthened the development process, and made the chambers much more expensive to produce. The solution was provided by Parker Hannifin's Instrumentation Products Division, who helped MSI's engineers evaluate combinations of tungum tubes, with twin ferrule A-LOK fittings (and valves with integral tube connections) fabricated from industry-standard materials such as brass, stainless steel and monel, to ensure reliable performance under projected conditions.
The tests were performed by Parker at its European design and manufacturing headquarters in Barnstaple. These included burst pressure tests at up to 16,000psi to allow for a safety factor of typically four times maximum pressure and pressure testing with thermal cycling over extended temperature ranges. The experiments proved to MSI that brass fittings were not adequate for use with the relatively hard tungum tubing, and with the help of Parker's applications engineers, the company evaluated different combinations of other standard fitting materials and tungum to find the optimum solutions.
Plate 6 Parker Hannifin valves and fittings have been chosen by MSI-Defence Systems for its new generation of computer-controlled decompression chambers
The detailed knowledge gained from these exercises allowed MSI to specify the gas control system using standard fittings and valves which are manufactured in volume for a wide range of high-integrity applications, and which can be supported indefinitely. The final combinations include copper/brass, tungum/monel, and tungum/stainless steel, depending on the media and pressures encountered in various parts of the system.
Another key factor behind the choice of Parker as the project supplier was the company's willingness to support MSI's manufacturing efficiency by supplying parts in kit form, as chambers go through the manufacturing process. This service is provided via Parker's local distributor, SES of Great Yarmouth. Parker also applies a number of custom processes during manufacture to help MSI, including cleaning for oxygen media, applying compatible lubricants, and special marking.
"Parker's applications engineering support helped us to speed the development of this system, and minimise lifecycle costs," notes Chris Watts, MSI-Defence Systems' project engineer. "Its willingness to invest and share the development in this fashion is of enormous value to prime contractors like us."
Over 300 A-LOK metric twin ferrule compression tube fittings which are approved by numerous organisations including Lloyd's Register of Shipping, DNV, DVGW and RINA are required for each decompression chamber. There are also around 140 metric valves, to support all of the different gas supply criteria, and meet safety requirements including duplication of controls to allow operation from inside or outside the chamber. The majority of the valves are ball types for speed of operation of the gas supply systems, but needle and check types are also used for specific functions. All of these valves include integral compression fittings, machined into the body, a feature which eliminates two connections(and potential leak paths) for MSI and saves space, weight and installation labour.
The first three decompression chambers have been delivered to the Royal Navy. In total, 40 will be supplied. These will be refurbished at regular intervals.
Details from Parker Hannifin Instrumentation Products Division. Tel: +44 (0)1271 313131; Fax: +44(0)1271 373636; E-mail: mchapman@parker-h.comAt MSI-Defence Systems. Tel: +44 (0)1603 484065; Fax: +44 (0)1603 415649.
