306A Drainage and Venting Systems 4 — Questions and Answers
Question 1: What causes self-siphonage in a plumbing trap?
- Excessive water pressure
- The momentum of water leaving the fixture creates a vacuum that pulls the trap seal out (Correct answer)
- Tree root infiltration
- A defective trap weir
Correct answer: The momentum of water leaving the fixture creates a vacuum that pulls the trap seal out
Self-siphonage occurs when the slug of water leaving the fixture creates negative pressure behind it in the trap arm, pulling the trap seal water out through siphonic action. Proper venting prevents this by admitting air behind the water slug.
Question 2: What is 'induced siphonage' in a drainage system?
- Siphonage caused by the fixture's own discharge
- Trap seal loss caused by the discharge of another fixture on the same branch (Correct answer)
- Siphonage caused by municipal sewer surcharging
- Intentional siphoning for drainage purposes
Correct answer: Trap seal loss caused by the discharge of another fixture on the same branch
Induced siphonage occurs when a nearby fixture discharges and creates negative pressure in the shared branch drain, pulling the trap seal from an adjacent, non-discharging fixture. Proper branch venting prevents this.
Question 3: What is the fixture unit rating for a shower stall in a residential building?
- 1 fixture unit
- 2 fixture units (Correct answer)
- 3 fixture units
- 4 fixture units
Correct answer: 2 fixture units
A residential shower stall is rated at 2 fixture units, reflecting its drainage flow rate and load on the drainage and venting system.
Question 4: What is the minimum pipe size for a building storm drain?
- 2 inch
- 3 inch
- 4 inch (Correct answer)
- 6 inch
Correct answer: 4 inch
The minimum building storm drain size is 4 inches to handle the volume of rainwater collected from roof areas, ensuring adequate capacity during heavy rainfall.
Question 5: What determines the size of a roof drain leader (rain water leader)?
- The height of the building only
- The area of roof being drained and the local rainfall intensity (Correct answer)
- The pipe material used
- The number of vent stacks in the building
Correct answer: The area of roof being drained and the local rainfall intensity
Roof drain leaders are sized based on the horizontal roof area being drained and the local rainfall intensity (typically in mm/hour for a 5-minute duration, 10-year return period), ensuring the pipe can handle peak storm flows.
Question 6: Why should storm drainage never be connected to the sanitary sewer system?
- Storm water is too dirty for the sanitary system
- Rainwater would dilute the sewage treatment process and surcharge the sanitary system during storms (Correct answer)
- It is impossible to connect them due to pipe sizes
- Storm water is always cleaner than sanitary waste
Correct answer: Rainwater would dilute the sewage treatment process and surcharge the sanitary system during storms
Connecting storm drains to the sanitary sewer causes massive surcharging during rain events, leading to sewage backups into buildings and raw sewage overflows into waterways. It also dilutes sewage, reducing treatment efficiency.
306A Drainage and Venting Systems 4
What causes self-siphonage in a plumbing trap?