Losses In Pipe Fittings

• An additional loss of head takes place in the course of flow through pipe fittings like valves, couplings and so on. In-general, more restricted the passage is, greater is the loss of head.
  
• For turbulent flow, the losses are proportional to the square of the average flow velocity and are usually expressed by , where V is the average velocity of flow. The value of K depends on the exact shape of the flow passages. Typical values of K are

  Approximate Loss Coefficients, K for Commercial Pipe Fittings .

  Type and position of fittings	Values of K
  Globe valve,wide open	10
  Gate valve, wide open	0.2
  Gate valve, three-quarters open	1.15
  Gate valve, half open	5.6
  Gate valve,  quarter open	24
  Pump foot valve	1.5
  90°elbow(threaded)	0.9
  45°elbow(threaded)	0.4
  Side outlet of T junction	1.8

  
  

  
  
• Since the eddies generated by fittings persist for some distance downstream, the total loss of head caused by two fittings close together is not necessarily the same as the sum of the losses which,each alone would cause.
  These losses are sometimes expressed in terms of an equivalent length of an unobstructed straight pipe in which an equal loss would occur for the same average flow velocity. That is

(37.5)

where, represents the equivalent length which is usually expressed in terms of the pipe diameter as given by Eq. (37.5). Thus depends upon the friction factor f , and therefore on the Reynolds number and roughness of the pipe.

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