9.1 Cross-Connections, Backsiphonage, Backpressure & Degree of Hazard

Definitions (IPC Chapter 2)

• Cross connection: any physical connection or arrangement between two otherwise separate piping systems, one containing potable water and the other containing water of unknown or questionable safety, steam, gas or chemicals, whereby flow could pass from one system to the other depending on the pressure differential.
• Backflow: the reversal of flow into the potable supply.
• Backsiphonage: backflow of potentially contaminated water into the potable system because the pressure in the potable system falls below atmospheric pressure at the connected fixtures, pools, tanks or vats.
• Backpressure: pressure, from any source, in the distribution system that exceeds the pressure in the supply mains and creates a potential for backflow.
• Backpressure, low head: a backpressure of 4.33 psi or less, which is the pressure of a 10-foot column of water.
• Contamination (high hazard): an impairment of potable water quality that creates an actual hazard to public health through poisoning or the spread of disease by sewage, industrial fluids or waste.
• Pollution (low hazard): an impairment of potable water quality that does not create a hazard to public health, but adversely and unreasonably affects its aesthetic qualities, such as taste, color or odor.
• Critical level (C-L): the reference elevation that sets the minimum height of a backflow preventer or vacuum breaker above the flood level rim. If a device has no C-L mark, the bottom of the device is the critical level.
• Vacuum breaker: a backflow preventer for openings under normal atmospheric pressure that admits air to the discharge side to break a siphon.

Cross-connection programs also use the terms direct connection (a pressurized piped link, which can see backpressure or backsiphonage) and indirect connection (an outlet that could be submerged, which sees backsiphonage). These are industry terms, not IPC definitions.

How backflow happens

Backsiphonage happens when supply pressure drops below atmospheric pressure, so atmospheric pressure pushes water from an open vessel into the pipe. Common causes:

• a water main break, or heavy draw for firefighting;
• high-velocity flow through an undersized main or riser, which creates suction at branches;
• draining a building's piping for repairs while upper outlets are open;
• a booster pump failure in a tall building.

Backpressure happens when downstream pressure exceeds supply pressure, even while the main is at normal pressure. Common causes:

• pumps on the customer side, such as boosters, chemical injectors and fire pumps;
• boilers and pressure vessels;
• thermal expansion in a closed system;
• elevated tanks or piping with height. Each foot of water column exerts 0.433 psi, so a tank 150 feet above a connection exerts about 65 psi;
• interconnection with an auxiliary pumped supply.

Matching protection to hazard (IPC 608.1 and Table 608.1)

608.1: a potable water system must be designed, installed and maintained to prevent contamination from nonpotable liquids, solids or gases entering through cross connections. Backflow preventers are applied according to Table 608.1, except where 608.2 to 608.17.10 say otherwise.

The selection logic:

1. Backpressure + high hazard: air gap or RP only. Section 608.17.6 requires an RP where a potable connection goes to a nonpotable line, fixture, tank, vat, pump or equipment subject to high-hazard backpressure.
2. Backpressure + low hazard: air gap, RP, double check, or a low-hazard device listed for the application, such as ASSE 1012 or 1024.
3. Backsiphonage only: add the vacuum breakers (AVB, PVB, SVB), which are acceptable at high or low hazard but never under backpressure.

Containment and isolation

• Containment: a backflow assembly at the service entrance protects the public water system from everything in the building. Water purveyors require containment devices through their cross-connection control programs.
• Isolation (point-of-use protection): a device at each fixture or piece of equipment protects the building's own occupants, as well as the public. This is the protection the IPC requires at fixtures and equipment.

Containment alone does not protect people inside the building from a cross connection in their own piping, so the two are used together.

General protection rules (IPC 608.2 to 608.13)

• 608.2: fixture supply lines and fittings must prevent backflow. Fixture fittings provide backflow protection under ASME A112.18.1/CSA B125.1.
• 608.3: devices for sterilization, distillation, processing, cooling, or storing ice or food that connect to the water supply need backflow protection. 608.3.1: hospital fixtures are protected by an RP, an atmospheric or spill-resistant vacuum breaker, or an air gap. Bedpan washer hose vacuum breakers must be at least 5 feet above the floor, and hose connection vacuum breakers in health care or laboratory areas at least 6 feet.
• 608.4: water treatment equipment (pumps, filters, softeners, tanks) must be located to prevent contamination. Overflow, relief and waste discharge pipes must terminate through an air gap.
• 608.6: chemicals and substances that make water toxic, or affect its taste, odor or color, must not be introduced into the potable system.
• 608.7: cross connections are prohibited except where approved backflow prevention assemblies, devices or other methods protect the potable supply. 608.7.1: cross connections between a private water supply and a public supply are prohibited.
• 608.8: potable outlets and combination stop-and-waste valves may not be installed underground. A freezeproof yard hydrant that drains into the ground is allowed only if its supply is protected by an RP or a PVB and the hydrant and its piping are identified as nonpotable.
• 608.9: nonpotable water systems are identified in purple, with "CAUTION: NONPOTABLE WATER — DO NOT DRINK" signage, repeated at least every 25 feet and at wall, floor and roof penetrations.
• 608.10: water used to heat or cool equipment or processes may not be returned to the potable system.
• 608.11: piping used for anything other than potable water may not later be used for potable water.
• 608.12: tanks, coatings and liners in contact with drinking water must meet NSF 61.