Quiz & test questions Grade 12 Engineering & Design
Thornquay Cross-Connection Hazards: A Backflow Control Exam
A grade 12 exam on backflow protection for a fictional district, mixing hazard classification with head-to-pressure and life-cycle cost calculations.
Objectives
- Students will distinguish backsiphonage from backpressure and name the condition that causes each.
- Students will select a backflow protection assembly appropriate to a stated hazard level.
- Students will convert elevation head to pressure and evaluate a pressure against the regulatory minimum.
Vocabulary
- Cross-connection
- A cross-connection is any place where drinking water pipes are linked to something dirty, like a garden hose left sitting in a bucket of soapy water. If the flow reverses, the dirty water can get sucked in.
Also: backsiphonage, backpressure, reduced pressure zone assembly, air gap, static pressure
Teacher overview
This 40-minute exam is worth 30 points and covers cross-connection control for the fictional Thornquay Water District. It mixes three multiple-choice items, four computation items, and two short-answer items, so students have to both classify hazards and carry numbers through the head-to-pressure conversion. The misconception this exam targets is the belief that the water main always holds more pressure than the building it serves, so water cannot flow backward into the system. A booster pump, a boiler loop, or a pressurized tank can all raise building pressure above the main, and a main break or a heavy fire flow can drop the main below the building, which is why both backpressure and backsiphonage appear in the item set.
Assessment
The fictional Thornquay Water District maintains one elevated tank with an overflow at elevation 218 feet. Use 0.433 psi for every foot of head, and remember that most states require a minimum of 20 psi in the distribution system.
- (2 points) Backsiphonage occurs when
- A) a booster pump raises building pressure above the pressure in the main
- B) pressure in the water main drops below the pressure in the customer's plumbing
- C) a check valve is installed facing the wrong direction
- D) a main is disinfected after a repair
- (3 points) A hose sits submerged in a tank of mixing chemicals at a fictional Thornquay industrial customer. Which protection is acceptable for that connection?
- A) a dual check valve under the nearest sink
- B) a pressure vacuum breaker on the hose bib
- C) a reduced pressure zone assembly, or an air gap at the tank
- D) a water meter fitted with an internal check
- (4 points) A boiler loop at the fictional Thornquay Regional Hospital operates at 85 psi. The water main at the building's service connection holds 62 psi. Find the backpressure acting toward the main.
- (4 points) Thornquay's tank overflow sits at elevation 218 feet. A customer's meter sits at elevation 96 feet. Using 0.433 psi per foot of head, find the static pressure at that meter.
- (4 points) During a fire flow, pressure at that same meter drops 39 psi below its static value of 52.8 psi. Find the pressure at the meter during the fire flow.
- (3 points) Most states require at least 20 psi in the distribution system. Using the fire-flow pressure of 13.8 psi from item 5, state whether that requirement is met and name the specific hazard the shortfall creates.
- (4 points) A reduced pressure zone assembly costs $1,850 installed, and annual testing costs $95 per year. Find the total cost of installing one assembly and testing it every year for 10 years.
- (3 points) Which of these four fictional Thornquay sites presents the highest hazard to the public water system?
- A) an office building with restrooms and drinking fountains only
- B) a single-family home with an outside hose bib
- C) a school with a rooftop cooling tower dosed with a biocide
- D) a barber shop with a hand sink and a shampoo bowl
- (3 points) Explain why an air gap requires no annual test while a reduced pressure zone assembly must be tested every year.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: B) pressure in the water main drops below the pressure in the customer's plumbing. That reversal, caused by a main break or a heavy fire flow, pulls water backward out of the building. Choice A describes backpressure rather than backsiphonage, and choices C and D describe installation and maintenance events, not the pressure reversal itself.
- Answer: C) a reduced pressure zone assembly, or an air gap at the tank. A submerged hose in a chemical tank is a high-hazard connection, and only those two protections are accepted for high hazard. A dual check and a pressure vacuum breaker are not rated for a health hazard under backpressure, and a meter check is not a backflow assembly at all.
- Answer: 23 psi. At the fictional hospital, 85 psi in the boiler loop minus 62 psi in the main equals 23 psi of backpressure pushing toward the main.
- Answer: 52.8 psi. The head is 218 feet minus 96 feet, or 122 feet. Then 122 times 0.433 equals 52.826, which rounds to 52.8 psi.
- Answer: 13.8 psi. 52.8 psi static minus a 39 psi drop equals 13.8 psi at the meter during the fire flow.
- Answer: No. At 13.8 psi the meter sits below the 20 psi minimum. Accept any explanation noting that once pressure falls this low, the main can no longer be assumed to hold back building plumbing, so any unprotected cross-connection on that line can siphon backward into the distribution system.
- Answer: $2,800. Ten years of testing at $95 per year equals $950, and $1,850 installed plus $950 equals $2,800 over the ten years.
- Answer: C) a school with a rooftop cooling tower dosed with a biocide. The tower holds biocide-treated water and connects to the potable supply for makeup water, so a backflow event could carry a health hazard into the main. The other three sites carry only low-hazard fixtures.
- Answer: An air gap has no moving parts. Accept any answer explaining that an air gap is a physical separation between the supply outlet and the flood rim, so it cannot fail mechanically and only has to be inspected for a bypass, while a reduced pressure zone assembly relies on check valves and a relief valve that wear, foul, and can fail without any outward sign.
Differentiation
Support
Provide items 4 and 5 with the subtraction step already written out, so students begin from 122 feet rather than from two elevations. Supply a reference card listing the four common assemblies with a one-line hazard rating for each, and allow it during items 2 and 8.
Extension
Ask these students to size the protection for one more fictional Thornquay customer: a car wash with a chemical injector on the rinse line and a booster pump that runs at 105 psi against a 62 psi main. Have them name the assembly, compute the backpressure, and state which of the two conditions, backpressure or backsiphonage, drives the choice.
ELL supports
Provide a two-panel diagram showing water moving the normal direction and then reversing, with the words backpressure and backsiphonage labeling the two causes. Offer sentence frames for items 6 and 9: "The pressure is ___ psi, which is ___ the 20 psi minimum, so ___," and "An air gap does not need testing because ___."
Extensions
Suppose the fictional Thornquay district discovers 340 unprotected commercial connections during a survey and can inspect 60 per year. Ask students to propose an order of work based on hazard rather than address, then to calculate how many years the program takes and to name the record the district would need to defend that order to its state regulator.