Losses in fittings.

Pipe work is commonly made up from lengths of straight pipe. Only when some special requirement has to be met are pipes bent. Clearly some system of making joints is required and these joints have to serve two functions. The first is to make a leak proof joint and the second is to provide some mechanical strength between lengths of pipe.

 

Suitable joints may be made by welding for steel and plastic. Steel pipe may also be joined by cutting screw threads on the ends of the pipe and screwing the pipe into a screwed sleeve. Copper and brass pipes are frequently joined by sliding the ends of the pipes into standard fittings and running solder into the gap between pipe and fitting so providing a seal and mechanical strength. Copper and brass pipes are also joined with compression fittings in which a soft metal ring is squeezed against the wall of the pipe by tightening a suitably shaped screwed nut on to the fitting. There are special joints for plastic plumbing pipes that are push-to-fit, need no jointing compound and can be dismantled very easily Flanged joints are very commonly used on middle and large size pipes. They may be cast with the pipe or welded to it if it is steel. There are other systems of making joints in pipes for use underground, for pipes carrying fluid at high pressure and for very large pipes. Cast pipes are often joined with spigot and socket ends rather like rainwater fittings on houses. Often underground pipes are lined with bitumen.

 

Here we are interested in the joints that cause a loss that is greater than that which would occur if the pipe were to be continuous and without a joint. Clearly straight joints that produce an effectively continuous surface do not add to the loss and it should be the aim of the designer of the joint and of the person who makes the joint to make a smooth joint. In practice, unless special care is taken, the many joints in a pipe system vary widely in standard and there is little point in trying to quantify the loss in a typical straight connector. The fittings that cause a loss that can be quantified, are those in which a change in direction or a change in area of cross-section of the flow occurs. These are in bends, elbows and tees and in reducers (for changes in diameter up or down), sharp entries, fully open valves and partially open valves.