back-pressure:
Back-pressure is the reverse from normal flow direction within a piping system as the result of the downstream pressure being higher than the supply pressure. This reduction in supply pressure occurs whenever the amount of water being used exceeds the amount of water being supplied (such as during water-line flushing, fire-fighting, or breaks in water mains).

back-siphonage:

Back-siphonage is the reverse from normal flow direction within a piping system that is caused by negative pressure in the supply piping (i.e., the reversal of normal flow in a system caused by a vacuum or partial vacuum within the water supply piping). Back-siphonage can occur when there is a high velocity in a pipe line, when there is a line repair or break that is lower than a service point, or when there is lowered main pressure due to high-water withdrawal rate (such as during fire-fighting or water-main flushing).

• The AVB must be at least 6 inches above any higher point downstream of the device. For this reason, they can never be installed below grade. Even if they are installed 6 inches above grade, inspectors should make sure that they are not installed less than 6 inches above some other point in the system downstream of the device.
  
  
• The AVB cannot be installed in an enclosure containing air contaminants. If contaminated air enters the water piping, it can poison the potable water supply.
  
  
• A shut-off valve should never be placed downstream of any AVB, as this would result in continuous pressure on the AVB.
  
  
• AVBs cannot be subject to continuous pressure for 12 hours in any 24-hour period or they may malfunction.
  
  
• Spillage of water from the top of the AVB is an indication that the device has failed and needs to be replaced.

• double check valves:  These are commonly used in elevated tanks and non-toxic boilers. Double check-valve assemblies are effective against backflow caused by back-pressure and back-siphonage and are used to protect the potable water system from low-hazard substances. Double-checks consist of two positive-seating check valves installed as a unit between two tightly closing shut-off valves, and are fitted with testcocks.
  
  
• reduced pressure principle assemblies:  These are commonly used in industrial plants, hospitals, morgues, chemical plants, irrigation systems, boilers, and fire sprinkler systems. Reduced pressure principle assemblies (RPs) protect against back-pressure and back-siphonage of pollutants and contaminants. The assembly is comprised of two internally loaded, independently operating check valves with a mechanically independent, hydraulically dependent relief valve between them.
  
  
• pressure vacuum breakers:  These are commonly used in industrial plants, cooling towers, laboratories, laundries, swimming pools, lawn sprinkler systems, and fire sprinkler systems. Pressure vacuum breakers use a check valve designed to close with the aid of a spring when water flow stops. Its air-inlet valve opens when the internal pressure is one psi above atmospheric pressure, preventing non-potable water from being siphoned back into the potable system. The assembly includes resilient, seated shut-off valves and testcocks.