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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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