In this article we will discuss about the requirements and types of traps.
Requirements of Traps:
Foul gases produced in the sewers, drains, waste-pipes may cause nuisance by entering in houses through house-connecting pipes, if their passage is not checked by some suitable devices. The devices which are used to stop the escape of foul gases inside or outside the houses are known as traps.
The traps generally consist of a bend tube which provides a water seal between the atmosphere and the sewer gas. The efficiency of the traps depends on the depth of water seal, deeper the seal more efficient will be the trap.
The following are the requirements of a good trap:
ADVERTISEMENTS:
(i) It should be made of non-absorbent material.
(ii) It should provide sufficient depth of water seal all times (about 50 mm) having large surface area.
(iii) It should be self-cleaning and should not obstruct the flow of sewage.
(iv) It should be provided with access door for cleaning.
ADVERTISEMENTS:
The water seal of the trap can break under the following conditions:
(i) If there is any crack in the bottom of seal or the joint is faulty.
(ii) If for a long time the seal is not in use, its water will evaporate in the atmosphere.
(iii) If due to blockage or any other reason there is increase in the pressure of the sewer gases, it will pass through the water of seal.
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(iv) If partial vacuum is created in the sewer fittings, it will suck up the seal water. To avoid the breakage due to this reason, the portion between the trap and the soil pipe should be connected to the vent pipe.
Types of Traps:
The following types of traps are most commonly used in practice:
(a) P, Q and S-Traps:
These traps are classified according to their shape. Fig. 24.14 shows these traps. They essentially consist of a U-tube which retains water acting as a seal between the foul gas atmosphere.
(b) Gully-Traps:
This trap is provided at different places in the drain pipes. Waste water from sinks, bath etc. enters in through back inlet and unfoul water from the sweeping of rooms, courtyards etc. enters from the top, where a coarser screen grating is fitted to check the solid matter. Fig. 24.15 illustrates a gully trap.
(c) An Intercepting Trap:
ADVERTISEMENTS:
The sewage from every house goes in street sewers which carry it away from the city. The street sewers contain foul gases in it and if their passage are not checked from street sewers to the house. They may enter in the house drain and pollute the atmosphere.
For this purpose a trap in one inspection chamber is provided outside the houses, which is called an intercepting trap. This trap is provided at top with a cleaning eye with a plug. Fig. 24.16 shows this type of trap.
(d) Anti-D Trap:
P, Q and Straps are largely used for baths, sinks and lavatories. In such cases, they are made with enlarged mouth so that the waste pipe may be thoroughly flushed out. But in these traps full bore of the trap is not interfered with by the discharge. These traps are made of ordinary circular sections.
Anti-D trap is an improvement over the above traps which was made by Mr. Melleyer of England. By a series of experiments Mr. Melleyer found that the driven out of water by momentum of the change from the trap can be prevented by so shaping the trap that the water-holding portion is contracted and the outgo is large and square is in section. This trap also prevents siphonage action.
Fig. 24.17 shows the pictorial view of this type of trap. The water-way in the anti-D trap is reduced, which ensures the removal of all refuse, while the outlet being larger prevents the pipe from filling full and causing siphonic action.
(e) Anti-Siphon Trap:
There are several types of anti-siphon traps in the market, which are also called re-called trap. These traps avoid the connection to the vent pipe and reduce this expensive work. Grevak trap which is most common is shown in Fig. 24.18.
The construction of this trap is such that when water seal is subjected to the pull due to siphonic action, the heavier atmospheric pressure on the inlet side presses the water down and the air can pass from by – pass tube B as shown in Fig. 24.18 (a) and the water is stored in trough (‘, when the pressure on both sides becomes equal, the water stored in trough C, falls back in the tube and seals it as shown in Fig. 24.18 (b).
