Page 437 - Chemical process engineering design and economics
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418 Chapters
PIPE FITTINGS
The major functions of pipe fittings are to:
1. change the direction of flow
2. reduce or enlarge pipe size
3. split or combine fluid streams
4. facilitate disconnecting piping from equipment
5. access the flow system for temperature, pressure, flow rate, and liquid level
measurements, and for sampling process streams
Several common threaded pipe fittings are shown in Figure 8.1, but welded
fittings and piping are frequently used. In plants, threaded piping is mostly used
for water, steam, and natural gas [31]. For threaded fittings, a thread sealant must
be used to prevent leakage. For welded fittings, flanges are used to connect pipe to
equipment. In this case, gaskets are needed for sealing. Because welded connec-
tions are less likely to leak, process piping is always welded [31].
Ninety-degree, forty-five degree, and the street elbow, shown in Figure 8.1,
change the direction of flow. The reducing coupling or reducer and bushing
change the pipe size, and a coupling joins two lengths of piping of the same size.
The pipe tee and the pipe cross combine or split fluid streams. They are also used
to gain access to the flow system for sampling the fluid and to measure process
variables. When removing equipment from the flow system for repair or replace-
ment, pipe unions are required for threaded piping and flanges for welded piping.
Even if removal of equipment is not necessary, a little reflection will show that for
threaded piping a union is a necessity when making a connection between two
fixed points. If we do not use a union, one end of a pipe will unscrew while
screwing the other end into a fitting.
VALVE TYPE
Before selecting a valve, the function of each valve type must be considered first.
Several valve types, listed in Table 8.1, are used for on-off service, prevention of
back flow, and throttling. Figure 8.2 to 8.4 shows only a few examples of valve
types. For a discussion of many other valve types see Reference 8.2.
The simplest valve function is on-off service. Examples of this valve type
are gate and ball valves, shown in Figure 8.2. A ball valve is used for tight shut-
off One application is a drain valve on a tank, where it is required to have the
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