USC 10th Edition Hydraulics Backflow Notes

These notes serve as reminders of concepts previously learned from classes or books. If any part is unclear, refer back to your course materials or textbooks, such as the USC 10th Edition Cross-Connection Control—a must-have resource for anyone involved in backflow testing.

Backflow Prevention Hydraulics Basics

Water

Understanding cross-connection control requires basic knowledge of hydraulics and water properties. Water is unique because it exists as a solid, liquid, and gas under normal conditions. Unlike most substances, water expands when it freezes, making ice less dense and able to float.

Water Pressure & PSI Calculation

• 1 cubic foot of water weighs 62.4 lbs and has a footprint of 144 square inches.
• The pressure exerted by this cubic foot of water is 0.433 psi per 1 foot of height.
• To achieve 1.0 psi, the water column must be 2.31 feet (or 27 ¾ inches) high.
• The diameter of the water column does not affect the pressure at the bottom; only height matters.
• Other water numbers:  1 gallon of water weighs 8.34 pounds.  7.48 gallons in one cubic foot.

Types of Pressure

• Gauge Pressure (psig): Measures pressure above atmospheric pressure.
• Absolute Pressure (psia): The sum of gauge pressure and atmospheric pressure.
• Atmospheric Pressure: At sea level, it is 14.7 psia.
• Negative (Sub-Atmospheric) Pressure: Pressure below atmospheric pressure, often creating a vacuum effect.

Effects of Pressure Equalization

• Water moves from high pressure to low pressure to seek equilibrium.
• A vacuum can pull water up to a theoretical height of 33.9 feet at sea level.
• If a hose is filled with water and each end is submerged in separate buckets, water will not flow until a pressure difference is introduced (e.g., by lowering one bucket).
• This fundamental understanding of water pressure, movement, and equalization is critical for cross-connection control and backflow prevention.

Siphoning and Dynamic Pressure Changes

• Once one bucket is lowered and water begins flowing through a hose, dynamic pressure changes slightly.
• Siphoning action continues until either the upper container is emptied or air enters the upper end of the hose.
• This same siphoning principle is used when emptying a fish tank or transferring fuel from a gas tank.
• Water naturally flows from higher to lower elevation, as seen when a garden hose is used.

Backflow and Hydraulic Anomalies

Understanding Backflow

• Backflow is the undesirable reversal of flow in a water system.
• It occurs when normal hydraulic conditions change, allowing contaminants to enter the potable water supply.
• Backflow can occur due to:
  • Backsiphonage – A sudden drop in system pressure creates a vacuum that pulls non-potable substances into the water system.
  • Backpressure – When downstream pressure exceeds supply pressure, forcing non-potable substances into the potable system.

Causes of Backsiphonage

• Water main breaks lower pressure in the system, allowing water to flow in reverse.
• Open fire hydrants can introduce atmospheric pressure into the system, pulling non-potable water from connected sources.
• When a hose is left submerged in a bucket containing chemicals, there is a risk of backflow due to the siphoning effect caused by a backsiphonage event, which can allow contaminants to enter the drinking water supply.

Aspirator Effect and Venturi Principle

• Aspirator Effect: When water flows rapidly through a pipe, pressure decreases as velocity increases, creating a siphon effect at TEES connecting another pipe.  Water always flows from high pressure to low pressure.  Water moving through a pipe loses pressure and can cause water to be siphoned out of the attached pipe.
  • I have a question for you. So, this aspirator effect affects connected pipes, but what about a pipe with a leak in the ground, and you have a fast-flowing water through the damaged pipe? Is that potentially doing an aspirator effect and pulling in contaminants from outside the pipes through that leak?
    • Yes, a leak in a pipe combined with a high flow rate can create a situation similar to the aspirator effect. When there’s a significant pressure drop inside the pipe due to the fast-moving water, it can create a partial vacuum. This vacuum can potentially draw in contaminants from the surrounding soil or groundwater through the leak, especially if the pressure outside the pipe is higher than inside.
• Venturi Principle: A venturi is a narrow pipe section (preceded and followed by a wider pipe) that increases velocity and decreases pressure, creating a siphon. (an aspirator effect by design)
  • Common Applications:
    • Laboratory equipment using suction.
    • Chemical and fertilizer injectors that siphon liquid into irrigation water.                                                                                               

Backpressure and Cross-Connections

• Backpressure occurs when downstream pressure exceeds supply pressure, potentially forcing contaminants into the potable system.
• Cross-Connections are pathways through which backflow may occur:
  • Indirect cross-connection – Subject to backsiphonage only (e.g., a garden hose left in a bucket of chemicals).
  • Direct cross-connection – Subject to both backsiphonage and backpressure (e.g., a water makeup line to a boiler system with chemically treated water).

Degree of Hazard: Pollutants vs. Contaminants

• Pollutants (Non-Health Hazard):
  • Aesthetic issues (taste, odor, color) but do not cause illness or death.
• Contaminants (Health Hazard):
  • Substances that can cause illness or death if introduced into the potable water supply.
  • Lethal hazards include radioactive materials and raw sewage.

Backflow Incident Conditions

For a backflow incident to occur, three conditions must be present:

1. A cross-connection – A direct or indirect connection between potable water and a non-potable source.
2. A hazard – A pollutant, contaminant, or lethal hazard present in the non-potable source.
3. A hydraulic change – Either backsiphonage or backpressure creating conditions for backflow.

When these three conditions align, a backflow incident occurs, posing serious health risks to the water system.

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Backflow Prevention Methods

Air Gap

• A physical separation between the supply pipe and the receiving vessel, preventing contaminants from re-entering the system.
• The “Air Gap” must be twice the diameter of the supply pipe but never less than one inch.
• The most effective backflow prevention method, especially against lethal hazards.

Double Check Valve Assemblies (DC) 

• DC features two internally loaded check valves arranged in series, along with two resilient-seated shutoff valves and four resilient-seated test cocks for field testing.
• Resilient seating means the sealing surface is made of a non-metallic material, ensuring a tighter seal compared to metal-to-metal contact.
• If one check valve fails to seal properly due to debris or other factors, the second check valve serves as a backup, helping to prevent backflow. This redundancy is why a single check valve is not considered a reliable backflow prevention device.
• DC assemblies protect against pollutants (Not contaminents/Health hazards) under backsiphonage and backpressure conditions. 

Reduced Pressure Principle Assembly (RP)

• RP includes two internally loaded check valves in series and a mechanically independent differential pressure relief valve positioned between them.
• This assembly also features two resilient-seated shutoff valves and four resilient-seated test cocks for field testing. While the relief valve operates independently from the rest of the assembly mechanically, it is hydraulically dependent on the pressure difference across the first check valve.
• If the pressure upstream of the first check valve drops to less than 2.0 psi above the downstream pressure, the relief valve activates and discharges water, ensuring proper backflow prevention.
• Protects against both pollutants and contaminants under backsiphonage and backpressure conditions.

Vacuum Breakers

• Atmospheric Vacuum Breakers (AVB):
  • Consists of an air inlet valve, air inlet port, and a check seat.
  • Prevents backsiphonage only.
  • Protects against pollutants and Contaminants.
  • Cannot be used under continuous pressure.  Can only be used for 12 hours in any 24 hour period.
  • Must be installed at least 6 inches above downstream piping and can not have any shutoff or control valves downstream.
• Pressure Vacuum Breakers (PVB):
  • The PVB includes an internally loaded check valve, a spring-loaded air inlet valve, two resilient-seated shutoff valves, and two resilient-seated test cocks for field testing and maintenance.
  • Protect against backsiphonage of pollutants and contaminants.
  • Can be used under continuous pressure.
  • Must be installed at least 12 inches above downstream piping.
• Spill-Resistant Pressure Vacuum Breakers (SVB):
  • The SVB features an internally loaded check valve, a spring-loaded air inlet valve, two resilient-seated shutoff valves, one test cock, and one vent valve (screw).
  • SVB is designed so that when the assembly is pressurized, the air inlet valve closes before the check valve opens, reducing unnecessary water discharge.
  • Protect against backsiphonage of pollutants and contaminants.
  • Can be used under continuous pressure.
  • Must be installed at least 12 inches above downstream piping.

These backflow prevention devices help maintain safe potable water supplies by reducing the risk of contamination due to pressure changes and cross-connections.

To determine What backflow device is needed ask these 3 questions:

1. Is it an Indirect or a Direct cross connection?
2. Is it a Contaminant or a Pollutant?
3. Is it Continous or Non-Continous pressure (not exceeding 12 hours in 24)