Contents
Understanding Backflow Prevention
Operating Principles of a Reduced Pressure Zone Device RPZD
Operating Principles of a Double Check valve
General Knowledge
Backflow Incidents
Understanding Backflow Prevention
Let us help you to understand exactly what is backflow and why it is so important.
The term backflow means any unintended reversal of flow of contaminated or non potable water (water that is unable to be used for human consumption) into the distribution system. The direction of flow under these conditions is in the reverse direction from that intended by the system and normally assumed by the owner of the system.
Backflow may be caused by numerous specific conditions; but, basically the reverse pressure gradient may be due to either a loss of pressure in the supply main called backsiphonage, or by the flow from a consumer’s pressured system through an unprotected cross-connection, which is know as backpressure. The term Backflow covers both a backsiphonage condition and/or a backpressure condition.
A reversal of flow in a distribution main or in the consumer’s system can be created by any change of system pressure where the pressure at the supply point becomes lower than the pressure at the point of use. When this happens in an unprotected situation the water at the point of use will be siphoned back into the system; thus, potentially polluting or contaminating the remainder of the consumer’s system. It is also possible that the contaminated or polluted water could continue to backflow into the public distribution system.
The point at which it is possible for a non-potable substance to come in contact with the potable drinking water system is called a cross-connection.
To prevent backflow from occurring at the point of a cross-connection a Backflow Prevention Assembly must be installed.
However, it is important the backflow prevention assembly match the particular conditions at that location and is suitable to protect against the degree of hazard present.
Because there are a number of different types of contaminated water these have been designated into three different categories. HIGH, MEDIUM & LOW. With each different category, there are a number of different types of Backflow Valves that must be used. Backflow installations call for a specialist as a number of factors must be considered when choosing and installing the Backflow Prevention Assembly.
Operating Principles of a Reduced Pressure Zone Device RPZD
An RPZ device works on the principle of differential pressures between three main chambers of the device and a diaphragm and spring that forces the relief port open.
The differential between the incoming water pressure and the pressure in the second chamber is what keeps the relief valve shut for a properly functioning RPZ valve.
To obtain the differential pressures in the second chamber, kinetic energy in the form of water flowing through the device is converted to potential energy and stored in the spring of the first check. Therefore water pressure in the second chamber is lower than the incoming water pressure, equal to the amount of the crack pressure of the first check plus the dynamic force exerted to compress the spring to a fully open check. As the flow of water is slowly reduced and comes to a complete stop the spring pushes the check valve shut. The pressure drop in the second chamber is now equal to the crack pressure of the first check (ie: the amount of force required to open the check) the pressure difference between these two chambers must be above or equal to 35kpa (5.08psi) for the valve to pass.
The second chamber also contains a diaphragm which pushes the the connected to stem and washer open on the relief port. The force acting on the diaphragm in the second chamber is the combined water pressure in the second chamber plus spring tension trying to force open the relief port. The spring tension has a minimum force valve of 14kpa (2.03psi).
The force acting on the other side of the diaphragm is the incoming water pressure.
Because the second chamber has a lower combined pressure then the first chamber the diaphragm pushes the stem and washer closed against the seat of the relief valve. If at any time this principle is violated, (ie: the first check is damaged) the pressure in the second chamber will be the same as the pressure in the first chamber and thus the relief port will open..
To obtain the differential pressures in the third chamber, kinetic energy in the form of water flowing through the device is converted to potential energy and stored in the spring of the second check. Therefore water pressure in the third chamber is lower than the incoming water pressure, equal to the amount of the crack pressure of the second check plus the dynamic force exerted to compress the spring to a fully open check. As the flow of water is slowly reduced and comes to a complete stop the spring pushes the check valve shut. The pressure drop in the third chamber is now equal to the crack pressure of the second check (ie the amount of force required to open the check) the pressure difference between these two chambers must be above or equal to 7kpa (1.06psi) for the valve to pass.
If the water pressure in the third chamber is higher than or equal to the incoming water pressure or at a value of 14kpa (2.03psi) less then the incoming supply pressure or the spring tension of the relief valve and the second check is damaged, the pressure in the third chamber will be and second chamber will equalise. This will result in the relief valve opening.
Operating Principles of a Double Check valve
A double check valve works on the principle of differential pressures between 2 main chambers.
To obtain the differential pressures in the second chamber, kinetic energy in the form of water flowing through the device is converted to potential energy and stored in the spring of the first check. Therefore water pressure in the second chamber is lower than the incoming water pressure, equal to the amount of the crack pressure of the first check plus the dynamic force exerted to compress the spring to a fully open check. As the flow of water is slowly reduced and comes to a complete stop the spring pushes the check valve shut. The pressure drop in the second chamber is now equal to the crack pressure of the first check (ie: the amount of force required to open the check) the pressure difference between these two chambers must be above or equal to 7kpa (1.06psi) for the valve to pass.
To obtain the differential pressures in the third chamber, kinetic energy in the form of water flowing through the device is converted to potential energy and stored in the spring of the second check. Therefore water pressure in the third chamber is lower than the incoming water pressure, equal to the amount of the crack pressure of the second check plus the dynamic force exerted to compress the spring to a fully open check. As the flow of water is slowly reduced and comes to a complete stop the spring pushes the check valve shut. The pressure drop in the third chamber is now equal to the crack pressure of the second check (ie: the amount of force required to open the check) the pressure difference between these two chambers must be above or equal to 7kpa (1.06psi) for the valve to pass.