In most cases a great deal of design consideration goes into the choice of cylinders, valves and other active system components; but can the same be said for the fittings?
The fittings, with the pipework, form the connection between all components and can have a big influence on the safety, efficiency and energy consumption of the system. This article reviews different fitting types and reinforces some best practice.
What are the basic types of fitting?
Firstly, the conduit between fittings can be a hose, which usually are constructed of multiple layers, a pipe (measured by the internal diameter) or a tube (measured by the outer diameter); for simplicity here, we will refer to the conduit as pipe. There are various types of fitting construction, the major types shown below:
Push in fittings - The pipe is secured by pushing through a grab ring or collet and easily released by pushing a release collar; pressure rating of plastic fittings is around 10 bar and for nickel plated brass 18 bar. Stainless steel and versions for specialist applications are also available.
Push on fittings - The pipe is pushed over a barbed end in the fitting to a stop and then secured by tightening down a knurled nut onto the pipe and barb assembly; they are ideal for use on vacuum systems and pressure up to around 15 bar.
Compression fittings - Pipe is passed through a sleeve, or olive, and pushed into the fitting until it bottoms; as the nut is tightened onto the fitting the sleeve is compressed against the pipe to form the seal. The operating pressure and temperature will be limited by the pipe used.
BSP/Hose fittings - Hose is pushed onto a barbed adaptor and usually secured with a hose clamp (jubilee clip); the adaptor is screwed into appropriate bright nickel plated brass BSP fittings to make the system. All fitting types are generally offered in parallel or tapered thread, both male and female.
What shapes are available?
Shapes are available to ensure pneumatic systems can be constructed effectively. Elbows allow a change in flow direction without the risk of kinking the piping; tees and crosses can split or combine flows. Various adaptors and unions can increase or reduce the pipe diameter or ensure a neat continuity from one fitting to another. Bulkhead fittings allow the pipework to pass through machine structures without energy wasting detours. Banjo fittings can be used to control flow, either 'in' or 'out'.
What are the considerations when choosing a fitting?
Piping and fittings should be designed to allow the necessary flow without introducing significant pressure drops; as a general rule keeping the pipework as simple as possible will reduce energy loss though the system - for example don't use elbows if straight runs are possible. The following should be considered in choosing the correct fitting type:
Working Pressure
Working Pemperature
Working Environment - both possible contaminants and ease of access for construction and maintenance
Construction and Maintenance - any approvals required for the application, for example use in the food industry.
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