Drainage Flashcards
What is the accepted technique used for drainage system designs today
The Discharge Unit Method
What is the purpose of the design of drainage system ventilation
To ensure that the water trap seals remain full during system use, by ensuring adequate air flow rates
What is the standard pipe size for a WC
100mm
What is the standard size pipe for a bath/shower
40mm
What is the standard size for a wash hand basin pipe
32mm
What is the main BS for drainage
BS EN 12056
What does the guidance on vent sizing try to ensure
That no more that +37.5mm water pressure are pressure is possible in a vented system. This limit ensures that water trap seals are maintained and offers some scope in the design of a system for the engineer
Where do air pressure transients in a drainage or vent system usually arise from
Events which occur in the system itself
In a typical drainage system what is the most frequent event inducing air flow
The formation of a water annulus in a vertical stack as a result of the discharge of an appliance or horizontal collection drain from an upper floor. The formation of water plugs and hydraulic jumps, when a horizontal pipe runs full due to either poor design or faulty installation, can briefly cause positive pressures of greater magnitude than those associated with stack downflow
What heavily influences the efficiency if air movement
The film thickness and velocity of the stack annular water flow
How quickly is Annular flow established
Typically within 1 metre
Where does the water fall within the vertical drainage pipe (Stack)
Most of it quickly forms an annulus and clings to the pipe wall. This happens within a few metres of entering the pipe
What is the main casual factor responsible for a high air flow rate
A high water flow rate
How may there be a complete blockage of the stack
A complete blockage of the stack may occur because the collection drain capacity has been exceeded
What causes sudden momentary stoppage of an induced air flow
Junction Flow
What develops under the circumstances of junction flow
High negative pressures will suddenly develop under the branch inlet, and positive pressures of similar magnitude will develop above, for the duration of the vertical stack blockage.
What can reduce back pressures arising from blockage of the air core at the stack base
Swept entry bends (i.e. larger radius bends) to horizontal collection drains
What is an issue with using swept entry bends
Higher air velocities down the stack and may increase the magnitude of pressure transients
What is the use of water traps in drainage systems
To ensure that the drainage system is isolate to the habitable space from the effects of waste
What ways can Pressure differentials that cause water loss from traps occur
Induced siphonage and back pressure effects
What is induced siphonage
Negative pressures in the system side of the water trap moves the water out of the trap. When insufficient water remains in the trap to provide a seal, an air path is created between the building drainage system and the habitable space.
How can negative pressures originate in the stack
Because of an established water and entrained air flow from other appliances or drains, or it may originate from another appliance downstream in the same collection drain, particularly if that appliance is discharging full bore and especially if it is unvented
What are back pressures
Where positive pressures are experienced at the system side of the trap, are the second cause of the trap seal loss specific to drainage network transients. The consequences of back pressure may be less sever than cases involving a negative pressure of similar magnitude, because water forced up from the trap into a basin or bath, for instance, will drain back into the trap. If the back pressure is removed suddenly, for instance if it is resulted in the first place from a temporary stack closure on horizontal branch discharge into an established flow lower in the stack, then trap oscillation will occur. If the magnitude of the oscillation is large enough then some water will inevitably be lost and the trap will again be depleted.
What is the solution to the objective that drainage pipe systems must be able to accept, with no storage delay, discharge from any sanitary appliance
All pipes must run downhill and sloped must be sufficient to ensure continuous flow and where possible continuous solid transport
