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Lesson plan Grade 12 Science

Cross-Connection Control and Backflow at Ashbury School

A grade 12 science lesson on how pressure loss lets contaminated water enter a distribution main, with handout problems converting feet of head to psi.

Objectives

  1. Students will explain how backsiphonage and backpressure move water the wrong way through a service line.
  2. Students will convert between feet of water column and pounds per square inch in a distribution setting.
  3. Students will select an appropriate backflow protection for a stated hazard and justify the choice.

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, air gap, reduced-pressure principle assembly, head

Teacher overview

A distribution system keeps contamination out mainly by staying under pressure, which is why most states require a minimum of 20 psi everywhere in the system at all times. When pressure drops, as it does during a main break or heavy fire flow, water can move backward through any point where the drinking water system touches something else. This lesson uses the fictional Ashbury School District to work through that mechanism and the two protections used against it.

The misconception to name directly is the belief that water in a pressurized main can only travel outward. Students picture pressure as a permanent property of the pipe rather than as a difference between two points, so they conclude that contamination physically cannot enter. When the main falls below the pressure of a hose sitting in a mop sink, the pressure difference reverses and water moves back toward the main.

Procedure

Hook (6 min): Show a clear cup of colored water with one end of a length of tubing submerged in it and the other end held above. Ask students what would have to be true for the colored water to travel up the tube. Take answers, then tell them the fictional Ashbury School District found exactly that arrangement in a custodial closet last summer.

Direct instruction (14 min): Define a cross-connection as any point where the drinking water system can meet a non-drinking source. Distinguish the two ways water reverses. Backsiphonage happens when pressure in the main falls below the pressure at the connection, usually during a break or a fire flow. Backpressure happens when a pump, a boiler, or an elevated tank on the customer's side pushes harder than the main. Introduce the relationships students will use: 1 foot of water column produces 0.433 psi, and 1 psi corresponds to 2.31 feet of head. Note that distribution systems normally run at 40 to 80 psi and that most states set a floor of 20 psi.

Demonstration (8 min): Fill one cup with colored water and one with clear water, connect them with tubing full of clear water, and lower the clear cup below the colored one so flow reverses. Ask students to name what changed. The answer is the pressure difference between the two ends, not anything about the tubing itself. Point out that a hose lying in a bucket is the same arrangement without the tubing being obvious.

Worked example (10 min): Model the conversion with numbers that are not on the handout so students learn the method rather than the answers. A storage tank whose water surface sits 96 feet above a school's meter produces 96 x 0.433 = 41.57 psi at the meter. If a break drops that pressure to 5 psi, the equivalent head is 5 x 2.31 = 11.55 feet, meaning any liquid standing higher than about 11.6 feet above the meter could push back into the service. Walk through both directions of the conversion aloud.

Independent handout (12 min): Students complete the handout individually, converting head and pressure for the fictional Ashbury service, deciding whether backsiphonage would occur, and choosing a protection for a stated hazard.

Exit ticket and formative check (5 min): Pose one question: a hose is left lying in a bucket of floor-stripping chemical in a school hallway during a main break. Ask each student to write the one-sentence reason this is a hazard and to name a protection. The check is met when at least 80 percent of responses identify the pressure reversal rather than only saying the water is dirty.

Cross-Connection Control and Backflow at Ashbury School — Grade 12 Science
Name: ____________________________ Date: ______________

Student Handout

Use these values for the fictional Ashbury School District. Show your work with units.

  • Conversions: 1 foot of head = 0.433 psi; 1 psi = 2.31 feet of head
  • Most states require a minimum of 20 psi throughout the distribution system
  • The district's storage tank water surface sits 140 feet above the school's service connection
  • A cleaning solution stands 4.0 feet above the service line in a mop sink where a hose has been left submerged
  • During a main break, the pressure at the school service falls to 3 psi, and briefly to -2.0 psi
  • The fictional Ashbury plumbing code requires an air gap of at least twice the supply pipe diameter
  • The mop sink faucet is supplied by a 0.75-inch pipe

  1. Calculate the pressure at the school's service connection, in psi, produced by 140 feet of head. Round to one decimal place.

  2. Convert the 3 psi reading during the main break into feet of head. Round to two decimal places.

  3. Is the 3 psi reading above the 20 psi minimum most states require? Answer yes or no.

  4. Calculate the pressure, in psi, produced by the 4.0-foot column of cleaning solution standing above the service line. Round to two decimal places.

  5. During the worst moment of the break the main reads -2.0 psi. Compare that with your answer to question 4 and state whether water would move from the mop sink into the service line. Answer yes or no.

  6. Using the fictional Ashbury code, calculate the minimum air gap required above the mop sink rim for a 0.75-inch supply pipe, in inches.

  7. The school's boiler feed line carries a chemical that would cause illness if it reached a drinking fountain, and the boiler pump can push above main pressure. State whether an air gap or a reduced-pressure principle assembly is the appropriate protection there, and give one reason.

  8. Explain why holding at least 20 psi everywhere in the system is itself a barrier against contamination, even at connections that have no assembly installed.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: 60.6 psi.
    140 feet x 0.433 psi per foot = 60.62 psi, which rounds to 60.6 psi. That sits inside the normal 40 to 80 psi range for a distribution system.

  2. Answer: 6.93 feet.
    3 psi x 2.31 feet per psi = 6.93 feet of head. Anything standing higher than about seven feet above the service would out-push the main at that moment.

  3. Answer: No.
    3 psi is far below the 20 psi minimum. Once pressure falls under that floor, the system has lost the protection that pressure alone provides.

  4. Answer: 1.73 psi.
    4.0 feet x 0.433 psi per foot = 1.732 psi, which rounds to 1.73 psi. The column of cleaning solution is not pushing hard, yet it does not have to push hard once the main is lower.

  5. Answer: Yes.
    The main at -2.0 psi is below the 1.73 psi produced by the solution column, a difference of 3.73 psi in the wrong direction. Water and dissolved chemical move from the mop sink toward the main, which is backsiphonage.

  6. Answer: 1.5 inches.
    Twice the 0.75-inch supply pipe diameter is 2 x 0.75 = 1.5 inches. The faucet outlet must sit at least that far above the flood rim of the sink, with nothing bridging the space.

  7. Answer: A reduced-pressure principle assembly.
    An air gap would stop both backsiphonage and backpressure, but it cannot be maintained on a closed, pressurized boiler feed line, which needs to stay connected. The hazard is a health hazard rather than a nuisance, and backpressure from the boiler pump is possible, so the assembly that holds against both directions and can be tested annually is the correct choice.

  8. Answer: Pressure inside the main keeps the pressure difference pointed outward at every connection.
    Flow follows the pressure difference. As long as the main is higher than anything attached to it, the only direction water can move is out of the system. Holding a floor of 20 psi means an ordinary hose in a bucket, a boiler, or a sprinkler line cannot overcome the main under normal conditions. Assemblies exist for the cases where that floor fails or where the customer's equipment can push harder, which is why they are required at high-hazard sites rather than everywhere.

Differentiation

Support

Give students a two-column conversion card with feet on one side and psi on the other, and three worked pairs already filled in from the fictional Ashbury values. Complete handout questions 1 and 2 together as a class. For question 5, provide a number line marked from -5 to 5 psi so students can place the main pressure and the solution column and see which side is higher.

Extension

Ask students to calculate the tank water level, in feet above the service, that would be needed to hold 20 psi at the fictional Ashbury school if the tank were the system's only source, then to explain why a distribution system with tall buildings usually needs booster pumps rather than a taller tank. Students may also survey their own school building on paper and list three plausible cross-connections with the protection each would require.

ELL supports

Provide a labeled diagram showing the main, the service line, the mop sink, and the arrows for normal flow and reversed flow, with the six vocabulary terms placed on the diagram. Give a sentence frame for questions 7 and 8, such as "The correct protection is ___ because the hazard is ___ and the pressure can ___." Allow students to answer question 5 by marking a diagram before writing the yes or no.

Extensions

Students can research whether their state requires annual testing of backflow assemblies and summarize who performs that testing and what the certification takes, connecting the lesson to a licensed occupation. A second option is to compare the fictional Ashbury code's air gap rule with the rule in a published plumbing code and explain why codes express the gap as a multiple of pipe diameter rather than as a single fixed distance.