Observation

Most of the candidates did well in question 1(a), but many of them could not give correct explanations of how water seal is broken and could not state up to four reasons for installing storage cistern in a building. The required responses to these parts are as given below:

(1b)

(i) Evaporation - When the fitting is left unused for a long time, evaporation of the water in the trap occurs, causing the seal to be broken.

(ii) Induced siphonage - This is caused by the discharge of water from another sanitary appliance to the pipe. Water passing through the branch pipe connection may draw out air of the branch pipe creating a partial vacuum and thus causing siphonage of the trap.

(iii) Self siphonage - This is caused by a moving plug of water in the waste pipe connected to a trap

(iv) Momentum - The most usual cause of loss of trap seal by momentum is the sudden discharge of the bucket-full of water into a W.C. pan.

(v) Leakage - This is usually due to a faulty joint on the cleaning eye or a crack in the trap below the water level.

1(c) Cistern are needed in domestic water supply to promote the following needs:

(i) To guard against interruption of water supply due to repairs on water mains.

(ii) To reduce waste of water.

(iii) To prevent foul water from sanitary fitting being siphoned back into the main.

(iv) To reduce maximum rate of demand on the main during peak periods.

(v) To limit the pressure on distribution pipe and sanitary fittings.