Building drainage

Drainage system

• All the piping within the private and public premises which conveys sewage, rainwater and other liquid waste to a point of disposal. A drainage system does not include the mains of public sewer systems or a private or a public sewage treatment or disposal plan.

• Stack: a general term used for any vertical line of soil, waste or vent piping.

• Soil pipe: a pipe that conveys the discharge of water closets or similar fixtures containing fecal matter, with or without the discharge of other fixtures to the building drain or building sewer

• Waste pipe:a pipe that conveys only liquid waste free of fecal matter.A waste pipe is generally smaller than a soil pipe because of the nature of matter being discharged into the system.

• Branch: any part of the piping system other than the main, or stack. • Building/house drain:

that part of the lowest horizontal piping of a plumbing system which receives the discharge from soil, waste and other drainage pipes inside a building and conveys it to the building sewer/ house sewer.

• Building/ house sewer That part of the drainage system that extends from the end of the building drain and conveys its discharge to the public sewer, private sewer, individual sewage disposal system, or other appropriate point of disposal.

TRAPS

• The trap most commonly used with plumbing fixtures is the P-trap. The P-trap gets its name because of its general shape-that of the letter P.

• Traps are required because they prevent sewer gases from entering a building and causing serious illness or death.

• The term trap seal refers to the water being held in the bent portion of a fixture trap. The trap seal forms a seal against the passage of sewer gases through the trap and into the building.

Examples of P-Traps

Failure of water seals in traps

1. Self-siphonage2. Induced siphonage3. Compression4. Evaporation5. Oscillation6. Capillary attraction7. Poor warkmanship8. Momentum

Self-siphonage

• Full bore flow will cause brake of the water seal

• The smaller the bore and the longer the pipes, the greater will be the possibility of the effect

Induced siphonage

• Water flawing past the lower branch pipe tends to draw the air out of the branch pipe and so induce siphonage. Main branch pipe

Compression

• Compression can occur when previous flow of waste water flows down

Evaporation

• Loss of water seal by evaporation is possible when the sanitary fixture is left without use for a long time

Oscillation

• Water seal can be lost due to varying pressure conditions (wind gusts or atmospheric pressure) Vent

Varying pressure conditions

Capillary attraction

• Pieces of fabric or hair stacked on the weir can drain water from the trap and break the seal

Momentum

• Water seal removed by momentum caused by long drop

Other considerations

• Materials – expansion allowance (PVC pipes has a high coefficient of expansion)

• Inspection and cleaning eyes to be provided• Size – fixture’s waste pipe maximum 40mm, main branch pipe size

depends on number of fixtures is serves

house drain

• A house drain is the pipe that receives all waste and water discharged by the soil stacks and waste lines.

• This house drain is laid from a point just outside the building foundation wall where it connects to the house sewer, then through the wall, and either along or under the cellar floor to the point where connection with the soil stack is made.

• Before laying this drain, determine its overall length and how much pitch to give it so that it will drain as it should.

Cleanouts

• A plumbing cleanout is a cleanout fitting with a removable plug that is found in a roughed in waste system. It is designed to help keep clear any type of debris that could cause any type of stoppage in the water drain lines.

• Cleanouts are usually placed at the connection point between the sewer lines and the drain lines where the base is located of a vertical stack and at all places were the pipe direction changes at 90 degrees.

VENTS

• To prevent the siphonage of a trap seal in fixture traps and allow gravity

flow of drainage, you must let atmospheric air from outside the building into the piping system to the outlet (or discharge) end of the trap. The air is supplied through pipes called VENTS. This air provides pressure on the outlet end of the seal equal to pressure on the inlet end.

• Since the air supplied by the vent to the outlet end provides a pressure equal to that at the inlet end of the trap, the trap seal cannot escape through siphonage.

• All vent systems should be provided with a main vent or vent stack and a main soil and waste vent. A “main vent” may be defined as the principal artery of the venting system, and vent branches may be connected to the main vent and run undiminished in size as directly as possible from the building drain to the open air above the roof.

• The term main soil and waste vent, or soil stack vent, refers to the portion of the stack extending above the highest fixture branch. In the figure, this vent extends through the roof. Actually, it is an extension of the main soil and waste stack.

Plumbing symbols

Requirements of a well designed system

the main requirement of a well designed system is that it should operate without the user being aware of its existence. However, this is a tall order and there is therefore a need to more fully specify some requirements which can lead to the ‘invisible system’. The following requirements are essential in achieving a safe, usable and reliable drainage system;• The system should remove all waste as quickly as possible• Long horizontal pipe runs must be self-cleansing• There must be minimal loss of water trap seal to ensure there is a barrier for the ingress of sewer gasesOther requirements which are less critical are• Minimal noise from the system• Minimal Odor from the appliance side • Ease of maintenance

How to overcome pressure transients?• The need to communicate an increase or decrease in the air flow and

the finite time that this takes is central to the requirements of providing a safely engineered drainage system. The absolute key to maintaining a state of equilibrium in a drainage system is to provide pressure relief as close to the source of an event as possible.

Alleviating negative transients

negative transients are the system’s way of communicating the need for more air. This call for air can be caused by a number of phenomenon;• A branch pipe filling up with water (full bore flow) cause siphonic action to produce a vacuum into which the water from a trap seal is sucked.• The pressure losses associated with water falling down a vertical stack will induce negative transients which will propagate around the system at the speed of sound. Some of these transients can be of sufficient suction pressure to evacuate water from a trap seal (induced siphonage).• Any increase in airflow (for whatever reason) will produce negative air pressure transients in the system as the need for more air is communicated to the termination (where the air comes from).• Air leaving the system will cause a negative transient (either into the sewer or from any other interface point e.g. the top of the stack)

• The most efficient way of dealing with this call for increased airflow is to simply answer it as quickly as possible. This means providing the extra air as quickly as possible. If a trap is 30 ft away from an air inlet to the system then it will delay the arrival of air and quite possibly compromise a water trap seal.

Alleviating Positive Pressure Transients

• If negative pressure transients are a call for more air then positive pressure transients are a call to stop sending air. Because pressure transient analysis follows a set of well defined rules, their source can be established and are given below;

• Changes in the water/air flow rate produce positive as well as negative air pressure transients• A sudden closure at a system termination, for example a surcharge in the sewer, resulting in a stoppage of the airflow out of the system will cause a positive pressure wave to be produced and propagate throughout the system• A Blockage or major clog in the system

• Positive pressure transients travel at the same speed as negative pressure transients, the speed of sound, and represent a deceleration force on air and water in its path. So, the consequences of a positive air pressure transient reaching a water trap seal would be that air is blown through the trap into the building (at best) or all the water in the trap is forced into the habitable space.

• a positive pressure wave, produced at the base of a drainage stack, will not be alleviated by an open top on the stack. This is because the pressure wave must travel the length of the stack in order to escape the building at the top. It will meet water traps on the way which, if it has sufficient pressure, will blow and so relieve the system into the habitable space.

• Again the best way to provide relief against positive air pressure transients is to locate a pressure relief device as close to the source as possible. So in them case of a transient produced at the base of a stack, relief is needed at the bottom, not at the top. Parallel vent pipes only divert a portion of the wave and will provide best relief if the diameter of the vent pipe is equivalent to the diameter of the stack. But this will only reduce the magnitude of the pressure by 1/3.