All instruments connect to their respective processes and to each other by means of a pipe, tube, and/or wires. An improper connection may lead to huge problems in the process. Pipe are hollow structures used to lead process fluid to the desired destination. Pipes are not single cast but different pieces and equipment like valves and the different meter should be installed, hence many joints are needed. This section discusses some of the more common methods for joining pipes together. There are different types of common pipe fitting techniques used:
- Flanged pipe fittings
- Tapered thread pipe fitting
- Parallel thread pipe fitting
- Sanitary pipe fitting
Flanged pipe fitting:
A pipe “flange” is a metal ring, usually welded to the end of a pipe, with holes drilled parallel to the centerline of the pipe to accept several bolts:
The flange joints are press fit by inserting a donut-shaped joint between the pairs of flanges. The gaskets are made with softer materials than the flange material. When it is sandwiched between a pair of flanges, the joint will “crush” between them to seal all possible leakage paths. Flanges and pipe components must have matching flange ratings and sizes to function properly. The symbol used for flanged connection in P&ID diagrams:
Flanges differ with regard to their sealing design and required gasket type. For example raised-face (RF) flange, ring-type joint (RTJ).
A special provision of flanged pipe connections is the ability to install a blank metal plate called a blind over or between flange faces, thereby preventing flow. This is useful when a pipe must be blocked in a semi-permanent fashion, for decommissioning and maintenance purposes.
Tapered thread pipe fittings:
Tapered thread pipe fittings are used for very small pipe fittings. A very common design of threaded pipe fitting is the tapered pipe thread design. The intent of a tapered thread is to allow the pipe and fitting to “wedge” together when engaged, creating a joint that is both mechanically rugged and leak-free.
Based on the angle of taper and angle of thread, there are several standards in the tapered thread pipe fitting. National pipe taper device in united states, and BSPT, or British Standard Pipe Tapered.
NPT threads have an angle of 60o and a taper of 1o 47’ (1.7833o). BSPT threads have a narrower thread angle than NPT threads (55o instead of 60o) but the same taper of 1o 47’ (1.7833o).
Parallel thread pipe fittings:
An alternative to tapered wires in pipe joints is the use of parallel threads, similar to screw threads and machine bolts. As the parallel threads are unable to form a pressure tight seal by themselves, the sealing action of a parallel threaded pipe fitting must be achieved in another way. This function is generally met with an O-ring or gasket.
Another standard of parallel thread pipe is BSPP or British Standard Pipe Parallel. Like the BSPT (conical) standard, the thread angle of BSPP is 55o. Like the SAE standard with parallel thread, the sealing is achieved by an O-ring that is compressed against the shoulder of the corresponding female fittings:
Sanitary pipe fittings:
Food processing, pharmaceuticals manufacturing, and biological research processes are naturally sensitive to the presence of micro-organisms such as bacteria, fungi, and algae. For quality control and to ensure the purity of the product sanitary pipe fittings are used.
In sanitary pipe fitting, any pressure gauge must use an isolating diaphragm, where the process fluid pressure is transferred to the gauge mechanism through a sterile “fill fluid” that never contacts the process fluid.
Standard pipe fittings are problematic in sanitary systems, as small gaps between the coupling threads of male and female pipe fittings can provide shelter for microorganisms. To avoid this problem, special sanitary fittings are used instead. These connections consist of a pair of combined flanges, joined by an external clamp.