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What is Trap?

by Swati
  • A trap is bent or depressed sanitary fitting which always remains full of water. 
  • Water which remains in it is water seal, which is measured as the vertical distance between crown and dip of a traps.
  • The depth of water seal in practice varies from 25 mm to 75 mm.


  1. The main function of a it in a drainage system is to prevent the passage of foul air or gases through it.
  2. Also it allows the sewage to flow through it. 


  • The traps are classified in the following two ways
  1. According to shape
  2. According to use.

1.According to shape:

  • Traps according to shapes are as follows:
  1. P-trap
  2. Q-trap
  3. S-trap

1. P-trap :

  • One which has shape of letter P is known as P-traps.
  • The legs of it are at right angles to each others.


  • One which has shape of letter Q is known as Q-traps.
  • The legs of traps meet at an angle other than right angle.


  • One which has shape of letter S is known as S-traps.
  • The legs of it are parallel.

2.According to Use:

  • Traps according to use are as follows:
  1. Floor trap
  2. Gully trap
  3. Intercepting trap

1.Floor traps:

  • Floor one is also known as nahni traps.
  • One provides a cast-iron cover with grating at top, so as to prevent entry of solid matter.
  • However One can remove cover for cleaning of the traps.
  • Location: Nahni traps are located in bathroom.

2.Gully traps :

  • One usually makes it of stoneware and provide a cast-iron grating at its top.
  • One provides water seal of a height of 60 to 70 mm.
  • One keeps it at floor or pavement or ground level.
  • It leads the sewage either to sewer or to inspection chamber or to manhole.
  • A CJ cover is provided at top so that cleaning can be done by lifting the cover.
  • Location:
  1. The gully one forms the starting point of horizontal flow of sewage.
  2. One locates it near the external face of wall and keeps it slightly higher than ground level.

3.Intercepting trap:

  • The main idea of providing the intercepting one is to prevent the entry of sewer gases from public sewer line into the house drains.
  • This one has water seal of 100 mm.
  • It contains an inspection arm for the purpose of cleaning or inspection.
  • One keeps the inspection arm closed by a plug or lid.
  • A fresh air inlet is also provided in the manhole containing the intercepting traps.
  • Location: Intercepting one is provided in the last manhole at the junction of house drain and public sewer line..

Good Qualities:

Following are the good qualities :

  1. Firstly, It should be of simple construction.
  2. It should possess adequate water seal.
  3. Lastly, One should clean it easily.

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