FAQ’s
Frequently Asked Questions about Backflow Prevention
F.A.Q.
All You Ever Wanted To Know About Backflow Prevention
What Does a Backflow Look Like?
How Do Backflow Prevention Assemblies Work?
*This Information is curtesy of Bavco.comIn order to repair any assembly, RP, DC, PVB or SVB, it is important that the repair technician first understand how the assemblies are supposed to work, so that when they are not working, the problem can be properly identified. The purpose of the repair process is to return the assembly back into its original factory specifications. When we are field testing any of these assemblies, we are making a diagnostic analysis at one point in space and time. From this data, we cannot say how the assembly worked yesterday or if it will work tomorrow, only what it is doing right now. When we perform the field test, we are generating data on the workings of the assembly, which we must compare to the minimum acceptable performance data as established in our accepted test procedure. For example, two (2.0) PSID is the minimum acceptable relief valve opening point, but few assemblies are designed to open at 2.0 PSID. For this reason, when we get a performance value from a field test, whether it be a relief valve opening point, or a check valve value, we must understand what is happening in the assembly. Reduced Pressure Principle Backflow Prevention Assembly (RP) To understand what happens inside an RP, let us flow water through a generic assembly. An RP consists of inlet and outlet shut-off valves, four properly located test cocks, a first and second check valve component, and a relief valve component. Let us hook up our RP to a water source, which delivers 100 PSI, and begin to pressurize the assembly. Relief Valve As the inlet shut-off is opened, water enters the upstream side of the assembly body ahead of the 1st check valve. Once in this area, the water enters a relief valve sensing line. Some sensing lines are external hose or pipe, and some utilize an internal passageway. The water travels through this sensing line to the elastic element in the relief valve. The elastic element is either a diaphragm, or a rolling diaphragm. This pressure will build up on the high pressure side of the elastic element, which will deflect and cause the relief valve stem to compress the relief valve spring, and move the relief valve disc to seal against the relief valve seat. The function of the relief valve spring is to constantly try to open the relief valve. If the pressure downstream of the 1st check rises to where it is a minimum of 2.0 PSI less than the inlet pressure upstream of the 1st check, then the 2.0 PSI loading from the relief valve spring would cause the relief valve to open. In a properly working relief valve, the opening point can be anywhere from 2.0 Ð 5.0 PSID depending on the manufacturer, model and size. Once the water pressure has closed the relief valve, then the pressure will increase to the next point, which will cause the first check to open, and allow the water to travel past the first check valve. 1st Check Valve The water pressure will be reduced by the amount of pressure it takes to open the first check spring loading. The 1st check of an RP can have a loading of anywhere between 5-15 PSI depending on the make, model and size. In our generic RP, we will assume our relief valve spring generates a 2.0 PSID and the first check spring has a 10.0 PSI Loading. Once the 1st check opens, water will travel past and pressurize the area between the 1st and 2nd checks. When this area is pressurized, it will also pressurize the low pressure side of our relief valve. The higher inlet pressure (100 PSI) is placed on the high pressure side of the elastic element in the relief valve, and the lower pressure past the 1st check (90 PSI) is placed against the low pressure side of the elastic element. 2nd Check Valve Once this area between the two checks and the low pressure side of the relief valve is pressurized, the pressure will now cause the 2nd check to open. The loading of the 2nd check spring will be anywhere between 1-5 PSI depending on the make, model, and size. The pressure after the 2nd check will be reduced by the amount of pressure it takes to open the 2nd check. For our illustration, we will give the 2nd check a 5.0 PSI loading. The upstream pressure (100PSI) of the RP in our illustration is reduced by the combined pressure load of the 1st and 2nd check spring (100 Ð 10 Ð 5 = 85PSI), producing our downstream pressure of 85 PSI into our piping system. Don’t forget that the relief valve spring is continually trying to open the relief valve, which the inlet pressure is keeping closed, and that a properly working relief valve can only open when the pressure downstream of the 1st check plus the relief valve spring load is greater than the upstream pressure to our RP. In our illustration, the pressure past the 1st check (90 PSI) must increase to 98.1 PSI, so that the value of the relief valve opening point (2.0 PSID) and the pressure after the 1st check (90 PSI) is now greater (98.1 + 2.0 > 100PSI) than the inlet pressure. During the normal flow of water through a properly working RP, the relief valve will be pressured closed, and the check valves will modulate between an opened and closed position to fill the water demand of the plumbing system. The two check valves (1st & 2nd Check) will have different spring loadings. The check valves in an RP will open when something downstream of the RP in the piping system opens a water-using fixture, and as the water begins to flow to that fixture, the pressure drops. The pressure upstream of the fixture begins to drop as the water flows through the fixture. Because of this flow of water, the pressure upstream of the RP is now great enough to cause the check valve to open and flow water to the fixture demanding water. Check valves only open enough to fill the demand for water; they do not open fully, but usually modulate to fill the demand for water. Backflow is the hydraulic condition that can cause an RP to stop working in the described normal flow pattern. Backflow can happen by either backpressure and or backsiphonage. Backpressure is a condition where a greater pressure is generated on the downstream than the upstream side of the assembly. This condition can happen for many reasons, pumps, thermal expansion, etc. Let us assume we have a proper working RP, and we apply backpressure to the outlet side of our RP. In our illustration, the normal upstream pressure is 100 PSI and the downstream pressure is 85 PSI. If the starting downstream pressure (85 PSI) increases to105PSI for example, and the second check is working properly, the 2nd check closes and keeps the 105PSI pressure from migrating into the area (90 PSI) between the 2 check valves. Even if we have a working 2nd check, and backpressure is applied, we can get a discharge from our relief valve. A condition called disc compression can cause discharge from a properly working RP. When backpressure occurs, this increase in pressure placed on the downstream side of the 2nd check causes the 2nd check disc to embed farther into the 2nd check seat. The volume of water in the body between the 2 checks is being squeezed as the 2nd check disc embeds farther into the seat. Water is a not a compressible fluid in these pressure ranges, so this squeezing of this water causes an increase in pressure in the area between the 1st and 2nd check valves. If this increase in the pressure between the 2 checks, which started at 90 PSI in our illustration, increases to the point where it is greater than the inlet pressure minus the relief valve spring loading (100 Ð 2.0 = 98), the relief valve will open. In our field test procedures, when we perform the 2nd check test of an RP, we are simulating a backpressure condition by bringing the higher inlet pressure (100 PSI) around to test cock 4 (85 PSI). If you remember from your field test procedures, when an apparent 2nd check failure is observed, you are required to open your low side bleed valve on your test kit. This will draw the elevated pressure from the area between the 2 check valves, while the second check disc stays embedded into the 2nd check seat from the applied backpressure. When the low bleed is opened, you are reestablishing the pressure in the area between the 2 checks back to its normal pressure of 90 PSI while the elevated 100 PSI is maintained after the second check. Disc compression is one of the most common errors made by a backflow assembly tester when performing a field test. Once the relief valve discharges when testing the second check, you must open the low bleed one more time to determine if the 2nd check is actually working or not. A disc compression scenario may occur, and the tester may incorrectly assume the 2nd check is not working. Let us see what happens when we apply backpressure to a non-working 2nd check. Once the pressure begins to increase on the outlet of our assembly, the 2nd check cannot maintain the separation of pressures between the inlet and outlet of the 2nd check, and the pressure will equalize on both sides. As the pressure increase begins (from 85 PSI in our illustration) the area between the 2 checks will also increase. Remember that the area before the 2nd check is where our low pressure is applied to the low pressure side of our relief valve elastic element. As the pressure increases above our starting pressure of 85 PSI, and goes to the point equal to the inlet pressure minus the relief valve opening (100 PSI Ð 2.0 PSI = 98) of 98 PSI, the relief valve will open. Backsiphonage Condition Backsiphonage is a condition that causes a sub-atmospheric pressure to be applied to the upstream side of the assembly. Backsiphonage can happen for several reasons; one of the more common is excessive water demand in the distribution system. Let us examine the effect of backsiphonage on our RP and see how it functions. When the inlet pressure to our assembly (100 PSI in our illustration) goes down to sub-atmospheric, or negative, the 100 PSI is reduced to a negative pressure. The pressure at the inlet of the RP is what keeps the relief valve closed. When the pressure at the high pressure side of the elastic element in the relief valve is reduced to a negative, the relief valve will open because of the relief valve spring load, and any pressure remaining in the area between the 2 check valves is now applied to the low pressure side of the relief valve elastic element. The relief valve of an RP can only open for the two backflow conditions of backpressure or backsiphonage or the simulation of these two conditions. One of the more common simulations of backpressure happens when there is a pressure fluctuation at the inlet of the RP. If there is no flow going through the assembly, and then the upstream pressure drops quickly from 100 PSI down to 80 PSI, this can cause the relief valve to open. This would happen because there would be higher pressure on the low pressure side of the relief valve elastic element versus the high pressure side (high pressure side would be 80 PSI and the low pressure side would be 90 PSI plus the relief valve spring loading). If a water hammer condition happens on the downside of a properly working RP because of a quick closing solenoid, this increase in pressure would create a backpressure condition, which could cause the relief valve to open by disc compression. Just because a relief valve discharges water, does not always mean the RP is not working. Pressure fluctuations can simulate conditions that can lead a person to assume the assembly is not working properly. To be sure whether the assembly is working or not, a test kit must be attached and proper test procedures applied to determine the working condition of the assembly. Faulty Component Diagnosis Now let us look at how an RP reacts when components of the assembly are not working properly and how we can diagnose the condition. Let us assume we perform a field test on an RP and generate a 1.5 relief valve opening point. Does this mean the assembly is Leaking or will not prevent backflow? The answer is probably no. We know from our tester training that 2.0PSID is the minimum acceptable relief valve opening point. If the relief valve opens at 1.5PSID, the relief valve will open and keep the pressure in the area between the two checks 1.5 PSI lower than the upstream pressure, if subjected to a backflow condition. An RP with a 1.5 PSID relief valve opening point will still prevent backflow, but does it at a value lower than the minimum accepted value of 2.0 PSID. For the assembly to perform optimally, it must operate at or above this minimum standard, in this case 2.0 PSID. Low Relief Valve Opening Point The cause of a relief valve opening below the 2.0 PSID minimum can vary greatly between different models. Usually, the incorrect assumption is made that a spring has worn out and that is why the relief valve will not open. The most common cause of low relief valve opening points is a restriction on the travel of the relief valve stem mechanism. Either the guide of the relief valve becomes damaged, or a scale or corrosion will cause the guide to not travel optimally leading to a low relief valve opening point. High Relief Valve Opening Point What happens when our field test data presents an excessively high relief valve opening point, something above 5.0 PSID? A high relief valve opening point can happen for different reasons depending on the make, model and size. The most common cause of a high relief valve opening point is when the relief valve disc does not completely embed itself into the relief valve seat, usually because the relief valve stem assembly is not traveling its full length; for example, if a rolling diaphragm in the relief valve is pinched or twisted, it will restrict the relief valve from traveling its full designed length. If the stem does not travel its full length, the relief valve disc cannot fully embed into the relief valve seat. If this disc is just barely touching the relief valve seat, and not fully embedded, then the relief valve opening point will be excessively high. Faulty First Check Valve Let us see what happens when the check valves are not working properly. Let us talk a little about the first check. In this example we show an upstream pressure of 100 PSI. The pressure downstream of the first check shows us 90 PSI, which means we have an 10 PSID across the 1st check valve. This is the load the first check is generating on a properly working first check. If the first check was completely fouled and there was no differential produced, that means we would have 100 PSI before and after the first check (0 PSID); then the relief valve spring would cause the relief valve to stay open. The first check rarely fails where there is no differential. The usual case is that instead of a 10 PSID, as shown in our example, the differential begins to fall as the first check begins to wear out. Let us assume we know our relief valve has a 2.1 PSID opening point. Let us add further that our first check is starting to degrade and it can only generate a 2 PSID. In other words our upstream pressure is still 100 PSI and the downstream pressure of the first check is 98 PSI and we know we have a 2.1 relief valve opening point, what would happen to our relief valve? The relief valve would open up and begin to discharge. If we have a 100 PSI inlet pressure and a pressure of 98 PSI after the first check, you can see where the 98 PSI along with the 2.1 PSI from the relief valve spring loading would cause the diaphragm to move, causing the relief valve to open, because there is a greater pressure on the downstream side of the relief valve diaphragm (98 +2.1 =100.1 PSI) than on the upstream side (100 PSI). Some administrative authorities require the loading on the first check to have a minimum of 3.0 PSID higher value than the relief valve opening. By having a buffer greater than 3.0 PSID, this would help minimize relief valve discharge from a small pressure fluctuation in a static condition. This would mean that if our relief valve opening point is 2.1 PSID than we would have to have a first check loading of at least 5.1 PSID to pass the field test. If a 3.0 PSID buffer was not required in your area, then any first check value greater (above 2.1 PSID) than the relief valve opening point would keep the relief valve closed and would be a passing check value. The cause of check failure tends to be due to the failure of the disc to seal against the check seat easily. Many times the check spring is blamed for a check failure but this is usually not true. The more common causes are dirt and debris on the disc, disc degradation where the disc will not seal, or a check guide restricting the travel of the check component. Faulty Second Check Valve The criteria for the workings of the second check, like the first check, must maintain a higher pressure upstream of the check than the downstream pressure. This differential is established by the spring loading of the second check spring, which is designed to be a minimum of 1.0 PSID. The test procedure for the second check is different than the first check. The 2nd check test is a backpressure test, while the 1st check is a direction of flow test. In our field test of the 2nd check, we take the higher inlet pressure form test cock #2 upstream of the first check (100 PSI), and with needle valves and hoses, place it into our number four test cock (85 PSI) causing the pressure on the downstream side of the second check to rise until it is higher than the upstream side of the second check. When the field test is performed on a working 2nd check, the check disc will be embedded farther into the check seat. This can cause a condition known as disc compression as previously mentioned. When the second check fails, the higher pressure would go past the leaking second check into the area between the two checks. As the pressure in this area increases, the relief valve senses the differential. When the pressure in the area between the two checks increased to 98.0 PSI (relief valve opening point 2.1 PSID), then the diaphragm would move, causing the relief valve to open. The causes of failure on a second check are similar to the first check. In conclusion, the field test is the way we generate the data needed to determine which part of the assembly is performing below the accepted minimal standard. When the numbers fall below the minimum standards established by the accepted test procedure, a repair must be facilitated to bring the working condition of the assembly above the minimum standards and back to its original factory working specifications. The generation of accurate data is very important and this means using an accurate test kit and proper test procedures and techniques to ensure that the data we generate properly reflects the working condition of the assembly. Double Check Valve Assembly – DC A DC is simply two approved independently operating check valves that can hold a minimum of 1 PSI in the direction of flow with the outlet of the check valve open to atmosphere. These checks must be located between an inlet and outlet shut-off and have 4 properly located test cocks. The check valves in a DC must hold a minimum pressure (1.0 PSI minimum) in the direction of flow. The two check valves (1st & 2nd Check) will have similar spring loadings. The check valves in a DC will open when something downstream of the DC in the piping system opens a water-using fixture, and as the water begins to flow to that fixture, the pressure drops. The pressure upstream of the fixture begins to drop as the water flows through the fixture. Because of this flow of water, the pressure upstream of the DC is now great enough to cause the check valve to open and flow water to the fixture demanding water. Check valves only open enough to fill the demand for water; they do not open fully but usually modulate to fill the demand for water. If a check valve is holding less than 1.0 PSI, for example, 0.5 PSI, testers have been known to incorrectly state, The check valve is leaking. This leads some people to believe that this check valve would not stop a backflow condition. In fact, the check valve is not leaking at 0.5 PSI because it is still sealing off the area upstream and downstream of the check valve with a 0.5 PSI loading; however, the check valve is performing below the minimum criteria as established in the test procedure (1.0 PSI). The minimum criteria in a test procedure is set at a point that will trigger a repair before the assembly can degrade to the point where it cannot prevent backflow (0.0 PSI). As long as our check valve has a positive loading, it can prevent backflow, but only when it is above 1.0 PSI does it meet the minimum criteria as established in the test procedure. So once the test procedure generates data that the check is maintaining less than 1.0 PSI, we must repair the check valve and return it to its original working specifications. Conditions that can cause a check to perform below its optimum level are many. The main cause of check failure is due to the failure of the disc to seal with adequate force against the check seat. Many times the check spring is blamed for this lack of force but this is usually not true. The more common causes of failure are dirt and debris between the disc and seat. Another common problem is disc degradation where the disc will not seal tightly against the check seat. The third common cause is a restriction in the travel of the guide, limiting the movement of the check valve and prohibiting it from sealing properly. There is a variation of a DC called a (DCDA). This is a double check created for fire sprinkler applications. A DCDA consists of an approved DC with a bypass arrangement that consists of a by-pass water meter and an approved by-pass DC. This by-pass is piped around the mainline DC. The purpose of this by-pass arrangement is to detect and register the first 3 gallons per minute (GPM) of flow across the backflow presenter into the fire system. Many testers think a DCDA is simply any small by-pass DC piped around any main line DC with a meter attached, and because the by-pass DC is smaller, the first flow will go through it. This is not true. In order for the by-pass to detect and register 3 GPM, the two DC’s and the water meter must be engineered so that the larger assembly will have a slightly higher differential at the low flow condition of 0-3 GPM. This will ensure that the first 3 GPM travel through and are registered by the water meter in the by-pass. Then, if the fire system demands more than 3 GPM, the main line assembly will open up and flow will commence up to the designed flow requirements of the system. An inexperienced installer may install a mainline DC and pipe in a small by-pass that looks similar to the DCDA. These unapproved DCDA’s cannot guarantee that they will detect this first 3 GPM because they are not factory engineered assemblies with the proper pressure differentials, but rather a collection of 2 DC’s and a water meter assembled to look like a DCDA. Most fire protection systems do not have a mainline water meter at their point of service from the water purveyor, and that is why it is important to detect this low flow of water. Because a fire system is an emergency connection, water purveyors do not want the expense or extra flow loss of going through a full size water meter. Because a fire system is considered an emergency connection, there should be no flow to detect across a mainline water meter anyway. Water purveyors use this bypass to ensure that water users with fire systems do not flow water through this emergency connection, and with the ability of the bypass to detect small flows, they can also detect if there are any small leaks that may be underground and out of sight. There is also a Reduced Pressure Principle Detector Assembly (RPDA). This has a main line RP with a bypass arrangement containing a smaller RP and detector meter to register the 1st 3 GPM of flow. These RPDA’s are used on high hazard fire systems, and the DCDA is used for low hazard fire systems, when the detector function is needed. The check valves of a DCDA are similar to the DC and the repair process will be similar. In many cases, the spring loading of the mainline assembly or the by-pass may be different from the standard DC, but the repair procedures and the test procedures to diagnose its workings are the same. Before we can repair any assembly, it is important to have correct data on the workings of the assembly to be sure we know what we are fixing, and just as importantly, that it really does need to be repaired. Pressure Vacuum Breaker Assembly (PVB) and the Spill Resistant Pressure Vacuum Breaker (SVB) The PVB consists of an inlet and outlet shut off, two test cocks, a check valve, and an air inlet component. The normal flow of water goes from the upstream side of the body into the check valve. The check valve (which is spring biased closed) is designed to hold 1 PSI in the direction of flow, similar to the check in a DC. The check valve opens, and water travels past the check valve, and causes an air inlet poppet (spring biased open) to travel up an air inlet guide, compressing an air inlet loading (it’s not always a spring), which is designed to generate a load of at least 1 PSI. The air inlet is kept closed by the normal water pressure in the piping system and is designed to open when the force from the air inlet (1.0 PSI Minimum) is greater than the water pressure in the area downstream of the check valve. The PVB is designed to prevent backflow from backsiphonage only, and must be installed 12Ó above the highest point of use or piping on the downstream of the assembly. PVB Check Valves Conditions that can cause the check in a PVB to perform below its optimum level are similar to the check failures in an RP or DC. In addition, there is a common cause of failure unique to the PVB and SVB, which has to do with the alignment of the check spring. Many models require the spring to be installed with a spring retainer that, if not properly installed, will exert a side pressure on the spring, preventing it from delivering the proper load to the check valve. Air Inlet Sometimes the air inlet will fail, in which it will not fully unseat itself when the water pressure in the body past the check valve is below 1.0 PSI. This can occur when the air inlet disc adheres to the air inlet seat, caused by temperature or water quality conditions. In other cases, the canopy that covers the bonnet is missing, which can also cause direct sunlight onto the air inlet, also causing a problem with deterioration from the ultraviolet rays of the sun. Lastly, On some models of PVB’s, the air inlet spring can easily be removed or inserted in such a way as to lower its loading below the 1.0 PSI minimum requirement. There is one brand of PVB that does not use a mechanical spring in the usual sense but rather a fold of rubber on the poppet that generates the load, and if you are not familiar with this brand, you could erroneously assume the spring is missing. Sometimes the air inlet poppet will not seal on the air inlet seat completely, and will leak. This unwanted discharge from the air inlet can be caused by several reasons. The usual is when some dirt or debris is located between the air inlet poppet disc and the air inlet seat. If the disc becomes damaged from this debris or becomes worn for other reasons, it could inhibit its ability to seal. Another cause of leakage can happen if the air inlet guide is damaged in such a way as to not allow the air inlet poppet to seat squarely on the air inlet seat. SVB There is a variation of the PVB called an SVB. The SVB has an inlet and outlet shut off, a check valve and an air inlet valve, a single test cock, and a bleed screw. The SVB performs similarly to the PVB except when initially pressurized. The normal path of water for a PVB is for water to enter the body, then open the check valve with a minimum 1.0 PSI loading, proceed past the check valve, and seal the air inlet with a minimum 1.0 PSI loading. The SVB is a little different. The check and air inlet have the same 1.0 PSI check and air inlet minimum loading requirements, but the order of the operation of the components is different. When water first enters the SVB, instead of causing the check valve to open first, as in a PVB, the air inlet closes before the check valve opens. This is accomplished by the air inlet having a lighter loading (1.0 PSI minimum) than the check valve (1.0 PSI minimum). Even though they have the same minimum loading requirement, there will always be a pressure differential between the load of the check and the air inlet, with the check valve having a higher loading than the air inlet. In this way, water seeking the path of least resistance will close the air inlet before it can open the check valve, thus minimizing any discharge (spill resistance) from the air inlet. In the SVB, water does not have to travel past the check valve to pressurize the air inlet as it does in the PVB. For this reason, the SVB will not discharge from the air inlet on initial start up. Once the SVB is pressurized, the SVB will perform similar to a PVB. The causes of failure of a SVB are similar to those of a PVB as discussed above. In order to determine how well a backflow prevention assembly is working, it is important to understand how it operates when it is in a working condition and a non-working condition. It is important that we can diagnose various conditions by performing an accurate field test with an accurate test kit to generate accurate data to make a true assessment of the workings of our assemblies. |
What is Backflow Prevention?
Backflow is the unwanted reverse flow of water or other substances into a potable water system. Backflow preventers are installed to prevent the potable water from being contaminated if a backflow incident occurs. This can happen for two hydraulic reasons: backpressure and/or backsiphonage. This need to assure that water in an irrigation system stays separated from the potable water is critical. The water in an irrigation system comes in contact with pesticides, fertilizers and other residues and is therefore unfit for human consumption.
For this reason, all plumbing codes recognize irrigation systems as a hazard which requires a separation from the potable drinking water. This separation involves the use of backflow prevention assemblies. These assemblies are installed to ensure that the potable water is not contaminated. These assemblies are required to be periodically tested and maintained by personnel with special certifications to ensure that the assemblies continue to provide needed protection. These assemblies also have very specific installation criteria to ensure continuous functioning. This installation criteria requires that the assemblies must be installed above grade and usually outside. During the winter months, most irrigation systems and their backflow assemblies are shut off and the sprinkler system is drained of water or winterized. In some installations, the water must remain on during the winter months. This leads to the need to place the backflow assemblies in an enclosure to protect the assembly from the adverse effects of the environment.
What are Backflow Prevention Assembly Enclosures?
Checklist
Be sure the enclosure you choose protects the assembly from the hazard the installation site presents. Here are some of the many ways to accomplish this task:
- When evaluating an enclosure for a backflow assembly, first look to the installation site and see if there are any unusual challenges such as temperature, wind or durability.
- Be sure the enclosure is sized to enclose all necessary parts.
- Be sure insulation and heat requirements are properly evaluated.
- Be sure the wall materials will have the desired strength for the installation site.
- Always give yourself a good installation pad to assure the enclosure will stay strong and stand up to regular use and maintenance.
Choosing an enclosure for a backflow preventer is similar to buying insurance. When you buy insurance, you never know how much to buy, you hope you never have a claim, and if you do, you hope you bought the right type of coverage. The same variables take place when choosing an enclosure, and just like insurance, the enclosure is really needed to protect against unusual or adverse conditions — and you hopefully bought the right type.
The definition of enclose is to protect or maintain and to isolate from the environment.This is what backflow preventer enclosures are designed to do, that is to isolate the water inside the piping from a low temperature which could freeze it or protect from vandalism or theft.
Types of Enclosures
Another type of enclosure is used for security only and does not protect against freezing. These may be desirable in areas where the assembly is either winterized or freezing is not a concern, but vandalism is an issue. These enclosures are commonly called cages and do not have to have solid walls as enclosures do. Many cages are made of a honeycomb steel called expanded metal. The metal can be either steel or stainless steel. The steel cages are coated with a paint or other type of rust resistant coating. They are usually attached to a permanent installation pad and locked to the pad to keep unwanted hands off the assembly. There are brands of expanded metal cages that wrap the metal cage around the assembly and do not mount to an installation pad. The expanded metal cages, because of their honeycomb construction, allow a visual inspection of the assembly without opening a door mechanism. There are some manufacturers that produce solid walled cages for vandal protection.
How is water affected by freezing?
Water is basically an incompressible fluid. In other words, squeezing it does not make it smaller; however, its volume can be expanded by either heating or freezing. Water can only be frozen in a static or non-flowing condition and when water does freeze, it will expand approximately 8% in volume. Problems arise when this expansion occurs in a fixed vessel like a piping system. This increase in volume translates to an increase in pressure that can deform or rupture the piping system at its weakest point.
People, like backflow preventers, need to be protected from the adverse conditions of the weather. People use houses that maintain an adequate environment protecting them from the cold outside temperatures, or if they must go outside, people will wear coats to protect themselves. The houses and coats both insulate people from the adverse weather. A backflow preventer enclosure provides the same type of protection.
Insulation Minimizes Heat Loss
Insulation is designed to minimize heat loss. insulation is made up of many layers of material that create small air pockets within these layers. The layers minimize the escape of the warm air. Insulation effectiveness is measured by its R factor. The higher the R factor the more heat that can be retained, because there are more of these air pockets to trap the warmer air.
Types of Insulation
Insulation can be either compressible or rigid in construction. Compressible insulation is like the fiberglass material found in the walls of houses which must maintain its thickness to retain its R factor. If the insulation is squeezed or compressed, there is less room to keep those pockets of air therefore less heat retention. Rigid insulation is different because as the name implies it does not easily compress. Rigid insulation is available in either sheet form or in a spray on type of material.
Besides compressible or rigid, insulation can be made of either closed or open cell construction. Closed cell insulation does not absorb moisture while the open cell design can absorb moisture in those air pockets which reduces its ability to retain heat. Another important element when evaluating insulation is the placement of a vapor barrier. When using insulation you will have an area of higher and lower temperatures. When these areas come in contact with each other, vapor can form. The purpose of this vapor barrier is to keep the formation of the vapor on the barrier not on the insulation which could reduce its ability to prevent heat loss.
How Much Insulation is Enough?
A coat will keep us warm on a cold winter’s day because the amount of heat loss through our coat can be made up by the warmth of our body. If our body cannot make up the heat as fast as it is lost, we must get a bigger coat or go inside the house to warm back up. In other words, the amount or thickness of the insulation needed is dependent on two factors: the intensity and duration of the low temperature.
For example, when it is 30 degrees for one hour, the amount of insulation needed would be different than if it was 30 degrees for 24 hours. Also, the amount of insulation would be different for 30 degrees versus -30 degrees.
Air Voids
When we go outside on a winter’s day, we button up our coats to keep the warm air inside. If our insulation was applied where there are openings or voids between the pieces of insulation, then like our unbuttoned coats, hot air could escape. The thickness of insulation is important as we mentioned, but also important is ensuring that no openings or voids exist between the pieces of insulation.
Other Factors of Heat Loss
Heat loss can also occur from contact with cold items such as the floor or any exposed piping outside the enclosure. If an RP or PVB assembly are installed, the enclosure must have adequate space for the discharge from the relief or air inlet valve. The size of the discharge must be large enough to ensure water does not collect in the enclosure.
Heat Sources
For all the reasons mentioned above, a heat source will usually be needed if the temperature is excessively low or long in duration to replace the heat lost through the insulation. There are two basic heat sources used in backflow preventer enclosures: One type is an area heater that heats the space within an insulated enclosure. The second type is a radiant heater that heats an object like the piping or an installation pad under the enclosure. Be sure you allow for the installation of electricity at the enclosure to handle the needs of the heater.
Enclosure Walls
The next item to evaluate in the enclosure is the construction of the walls and what they are made of. By being outdoors, the interior and exterior of the enclosure must be able to handle moisture, wind, ultraviolet deterioration and other problems from the installation site. Enclosures on the market today are made of aluminum, steel, stainless steel or fiberglass and come in many colors and textures. The main purpose of the walls is to provide strength. Strength is also needed to hold in place the insulation, and the inside heat source. Strength is also needed to withstand outside elements such as wind force or the weight of accumulated snow or debris. The skin of the walls may not be strong enough and support braces may be positioned inside the enclosure, especially on larger enclosures. These braces should be analyzed in your choices because the braces can be made of wood or metal and attached by screws, nails or rivets. Enclosures must have a large enough access in the walls for annual testing or maintenance of the backflow preventer contained inside. These access doors must not compromise the strength of the walls. The door closing mechanism must have strong rust resistant hinges to hold the weight of the doors without them falling on the technician working on the assembly. The hinges should be able to hold the weight of the door without stressing the wall material it is attached to.
Installation Pad
The next element that should be evaluated is the installation pad on which the enclosure will be mounted. Most enclosure manufacturers recommend a permanent and rigid pad such as concrete. The stability of the pad ensures that the walls and fasteners of the enclosure do not become stressed. A good installation pad also helps with security.
Appearance
Enclosures come mostly in a box shape and are placed over the assembly and its piping and then attached to an installation pad. An architect may create a beautiful landscape only to have this box in an area that may detract from it. To help with this issue, a group of aesthetic enclosures are available which are designed to be less obtrusive. They will usually have the appearance of a rock, but there are other custom designs to complement specific architectural features.
Other Ways to Prevent Freezing
Two other methods utilized to prevent freezing other than enclosures are insulated bags or thermostatic relief valves. The insulated bags are a plastic or canvas type material that is filled with a compressible insulation and is wrapped around the assembly. The thermostatic relief valves are connected to the assembly and monitor the temperature of the water in the pipe. When the temperature falls to a preset temperature like 35 degrees, the thermostatic relief valve will open. This allows a stream of colder water to flow from the relief valve and be replaced by a warmer water coming from the piping system in the ground which was not exposed to the adverse weather conditions. Once the warmer water travels through the relief valve the relief valve will sense the higher temperature and close.
What is a Backflow Device?
A backflow prevention device is used to protect water supplies from contamination or pollution. Many types of backflow prevention devices also have test cocks so that they can be tested or examined to ensure that they are functioning properly.
In the United States, the Environmental Protection Agency (EPA) holds local water suppliers responsible for maintaining a certain amount of purity in potable water systems. Many states and/or local municipalities require annual testing of backflow prevention assemblies. A check valve is a common form of backflow prevention.
Backflow prevention protects the potable water system from minor, moderate, and severe hazards. There are over 10,000 reported cases of backflow contamination each year. Some cases can be fatal. Backflow devices are required by law where needed and must be installed in accordance with plumbing or building codes. A backflow assembly has test cocks and shut-off valves and must be tested each year, if relocated or repaired, and when installed.
The simplest, and most effective way to provide backflow prevention is to provide an air gap. An air gap is simply a space between any device that opens to a plumbing system (like a valve or faucet) and any place where water can collect or pool.
Partial list of some backflow prevention devices
- Air gap
- Atmospheric vacuum breaker (AVB)
- Chemigation valve (primarily used in agriculture)
- Check valve although not a legally approved method of backflow prevention.
- Double check valve, or double check valve assembly (DCVA)
- Pressure vacuum breaker assembly (PVB)
- Reduced Pressure Principle Backflow Prevention Assembly (RP), RP Assembly, or RPZ (Reduced Pressure Zone)
External links
- IAPMO
- Drinking Water & Backflow Prevention magazine
- ICC
- ASSE
- The American Backflow Prevention Association
- Foundation for Cross-Connection Control and Hydraulic Research
- Backflow Prevention TechZone
- Australian Backflow Prevention expert
All the text above is licensed under the GNU Free Documentation License. To see the full article with more information, visit the Wikipedia article “Backflow Prevention“.
Why do I need a Backflow device?
City and state code require annual testing of every backflow assembly device. You should receive a letter from your municipality stating the due date and the device must be tested within 30 days of the due date or the city may turn off your water. After testing the device, City Wide completes and remits the proper certification paperwork to the city.
Enclosures for your backflow device will add another layer of protection against vandalism or theft of the device. City Wide will customize an enclosure for your particular assembly location.
Typical Arizona City Rules and Procedures:
(excerpt taken from: City of Chandler)
ARTICLE 4 : Backflow and Cross-Connection Control Program
The adoption of Code in this chapter does not authorize any use or the continuation of any use of a structure or premises in violation of any City Ordinance on the effective date of this Code. Where discrepancies occur between Unified Development Manual text and Municode text, Municode text shall prevail.
52-34. Cross-connection control; purpose.
[The purpose of cross-connection control is:]
A. To protect the public potable water supply of the City of Chandler from the possibility of contamination or pollution by isolating within its customers’ internal distribution system(s) or its customers’ private water system(s) such contaminants or pollutants which could backflow into the public water supply system;
B. To promote the elimination or control of existing cross-connections, actual or potential, between the customers’ in-plant potable water system(s) and nonpotable water systems, plumbing fixtures and industrial piping systems; and
C. To provide for the maintenance of a continuing program of cross-connection control which will systematically and effectively prevent the contamination or pollution of all potable water systems.
52-34.1. Responsibility.
The Director of Public Works shall be responsible for the protection of the public potable water distribution system from contamination or pollution due to the backflow of contaminants or pollutants through the water service connection.
52-34.2. Requirements.
If, in the judgment of the Director of Public Works, an approved backflow prevention device is required at the City’s water service connection to any customer’s premises for the safety of the water system, the Director of Public Works or his/her designated agent shall give notice in writing to the customer to install, operate, test, and maintain such an approved backflow prevention device at each service connection to his/her premises. The customer shall immediately install such approved device or devices at his/her own expense; and failure, refusal or inability on the part of the customer to install the device or devices immediately shall constitute a ground for discontinuing water service to the premises until such device or devices have been properly installed and tested for proper working order.
52-34.3. Exceptions.
In the case where residential homes incorporate the use of reclaimed wastewater as a water conservation measure, a backflow prevention device shall be installed by the City at the homebuilder’s expense at the time of water meter installation. The device shall be installed in City right-of-way at the downstream side of the City water meter. The device shall be tested and maintained by the City. A reduced pressure principle device is the approved backflow prevention device where reclaimed wastewater is available as an auxiliary water supply.
(Ord. No. 2149, § 1.B, 9-13-90; Ord. No. 3060, § 2, 11-18-99)
52-35. Water system.
The water system shall be considered as made up of two (2) parts: the utility system and the customer system.
A. The utility system shall consist of the source facilities and the distribution system and shall include all those facilities of the water system under the complete control of the utility up to the point where the customer’s system begins.
1. The source shall include all components of the facilities utilized in the production, treatment, storage and delivery of water to the distribution system.
2. The distribution system shall include the network of conduits used for the delivery of water from the source to the customer’s system.
B. The customer’s system shall include those parts of the facilities beyond the termination of the utility distribution system which are utilized in conveying utility delivered domestic water to points of use.
(Ord. No. 2149, § 1.B, 9-13-90; Ord. No. 3060, § 2, 11-18-99)
52-36. Policy.
A. No water service connection to any premises shall be installed or maintained by the City unless the water supply is protected as required by State laws and regulations and this article. Service of water to any premises may be discontinued by the City if a backflow prevention device required by this article is not installed, tested, and maintained; or if it is found that a backflow prevention device has been removed or by-passed; or if an unprotected cross-connection exists on the premises. Service will not be restored until such conditions or defects are corrected.
B. The customer’s system should be open for inspection at all reasonable times to authorized representatives of the City to determine whether cross-connections or other structural or sanitary hazards, including violations of these regulations, exist. When such a condition becomes known, the Director of Public Works may deny or immediately discontinue service to the premises by providing for a physical break in the service line until the customer has corrected the condition(s) in conformance with the State and City statutes relating to plumbing and water supplies and the regulations adopted pursuant thereto.
C. An approved backflow prevention device shall also be installed on each service line to a customer’s water system in all cases before the first branch line leading off the service line wherever the following conditions exist:
1. In the case of premises having an auxiliary water supply which is not or may not be of safe bacteriological or chemical quality and which is not acceptable as an additional source by the Director of Public Works, the public water system shall be protected against backflow from the premises by installing a backflow prevention device in the service line appropriate to the degree of hazard.
2. In the case of premises on which any industrial fluids or any other objectionable substance is handled in such a fashion as to create an actual or potential hazard to the public water system, the public system shall be protected against backflow from the premises by installing a backflow prevention device in the service line appropriate to the degree of hazard. This shall include the handling of processed waters and waters originating from the utility system which have been subject to deterioration in quality.
3. In the case of premises having a) internal cross-connections that cannot be permanently corrected and controlled, or b) intricate plumbing and piping arrangements or where entry to all portions of the premises is not readily accessible for inspection purposes, making it impracticable or impossible to ascertain whether or not dangerous cross-connections exist, the public water system shall be protected against backflow from the premises by installing a backflow prevention device in the service line.
D. The type of protective device required shall depend upon the degree of hazard which exists as follows:
1. In the case of any premises where there is an auxiliary water supply and it is not subject to any of the following rules, the public water system shall be protected by an approved air-gap separation or an approved reduced pressure principle backflow prevention device.
2. In the case of any premises where there is water or substance that would be objectionable but not hazardous to health, if introduced into the public water system, the public water system shall be protected by an approved double check valve assembly.
3. In the case of any premises where there is any material dangerous to health which is handled in such a fashion as to create an actual or potential hazard to the public water system, the public water system shall be protected by an approved air-gap separation or an approved reduced pressure principle backflow prevention device. Examples of premises where these conditions will exist include chemical manufacturing plants, hospitals, mortuaries, plating plants, sewage treatment plants and sewage pumping stations.
4. In the case of any premises where there are “uncontrolled” cross-connections, either actual or potential, the public water system shall be protected by an approved air-gap separation or an approved reduced pressure principle backflow prevention device at the service connection.
5. In the case of any premises where, because of security requirements or other prohibitions or restrictions it is impossible or impracticable to make a complete in-plant cross-connection survey, the public water system shall be protected against backflow from the premises by the installation of a backflow prevention device in the service line. In this case, maximum protection will be required; that is, an approved air-gap separation or an approved reduced pressure principle backflow prevention device shall be installed on each service to the premises.
6. In the case of water tankers and spray tanks, an air-gap separation or an approved reduced pressure principle backflow prevention device is required.
E. Backflow prevention assemblies shall have at least the same cross-sectional area as the water service and/or meter. If the water supply cannot be temporarily interrupted for the testing of assemblies, in those instances where a continuous water supply is necessary, two (2) sets of backflow prevention assemblies shall be installed in parallel.
F. Any backflow prevention device required herein shall be of a model and size approved by the Director of Public Works. The term “approved backflow prevention device” shall mean a device that has been manufactured in full conformance with the standards established by the American Water Works Association (AWWA) entitled “AWWA C506-78 Standards for Reduced Pressure Principle and Double Check Valve Backflow Prevention Devices”; and have met completely the laboratory and field performance specifications of the Foundation for Cross-Connection Control and Hydraulic Research (FCCC & HR) of the University of Southern California; and has a local parts and service center. Backflow prevention assemblies connected to fire lines must also carry UL-FM approval.
The AWWA and FCCC & HR standard and specifications have been adopted by the Director of Public Works. Final approval shall be evidenced by a certificate of approval issued by an approved testing laboratory certifying full compliance with the AWWA standards and FCCC & HR specifications. The following testing laboratory has been qualified by the Director of Public Works to test and certify backflow prevention assemblies:
Foundation for Cross-Connection Control and Hydraulic Research
University of Southern California
University Park
Los Angeles, California 90007
Backflow prevention assemblies which may be subjected to back-pressure or back-siphonage that have been fully tested, and have been granted a certificate of approval by FCCC & HR may be listed on the current list of “Approved Backflow Prevention Assemblies,” which will be made available upon written request to the Department.
G. It shall be the duty of the customer/user at any premises where backflow prevention devices are installed to have certified inspections and operational tests and repairs made at least once per year. In those instances where the Director of Public Works deems the hazard to be great enough, he/she may require certified inspections at more frequent intervals. These inspections, tests and repairs shall be at the expense of the water user and shall be performed by the device manufacturer’s representative, or by a certified tester approved by the Director of Public Works.
H. Assemblies shall be specified and located on the construction plans for all new buildings, additions with new services, and changes of use of existing buildings. Approval shall be obtained prior to issuance of the building permit. This section does not apply to building permits applied for prior to the effective date of this article [November 1, 1990].
I. All presently installed backflow prevention devices which do not meet the requirements of this section but were approved devices for the purposes described herein at the time of installation and which have been properly maintained shall, except for the inspection and maintenance requirements, be excluded from the requirements of these rules so long as the Director of Public Works is assured that they will satisfactorily protect the utility system. Whenever the existing device is moved from the present location or requires more than minimum maintenance or when the Director of Public Works finds that the maintenance constitutes a hazard to health, the unit shall be replaced by a backflow prevention device meeting the requirements of this section.
J. The Director of Public Works or his/her designated representative shall maintain a list of approved backflow prevention assemblies, by type and manufacturer, and a list of certified, approved and recognized individuals to test or perform servicing of those assemblies. The list shall be furnished to any customer required to install or maintain a backflow prevention assembly.
(Ord. No. 2149, § 1.B, 9-13-90; Ord. No. 3060, § 2, 11-18-99)
52-37. Inspections and testing.
A. In the case where a facility handles, stores or uses hazardous materials which may pose a risk to the water system, the Director of Public Works shall require that certified inspections and testing of the backflow assemblies be performed by an independent agent selected by the City.
B. Inspections and tests performed by such independent agent shall verify that the backflow prevention assembly is in full conformance with performance standards set by FCC & HR, and the independent agent shall be paid for the performance tests by the City. The City shall assess the water user a surcharge to recover the costs associated with performance testing. A schedule of fees set by the Director of Public Works shall be assessed at the time of performance testing and shall be based upon City costs for such testing.
C. Should repairs or replacement of the backflow prevention assembly be needed to achieve compliance with the performance standards set by the FCC & HR, the customer/user, at his/her expense, shall select from a list of certified testers approved by the Director of Public Works to have the work done. The certified tester performing repairs or replacement shall retest the assembly prior to returning the assembly to service operation.
D. Failure to perform testing, repairs or replacement of a backflow prevention assembly not in compliance shall be grounds for discontinuance of water service at the discretion of the Director of Public Works.
E. It shall be the duty of the Director of Public Works to see that these timely tests are made. The customer/user shall notify the Director of Public Works in advance when the tests are to be undertaken so that he/she or his/her representative may witness the test, if so desired.
F. Records of such tests, repairs and overhaul shall be kept and made available to the Director of Public Works.
52-37.1. Exceptions.
A. When performing testing to ensure a backflow prevention assembly is in compliance with FCC & HR performance standards, the customer/user shall be notified by the Water Department prior to the commencement of such work. Scheduling of tests shall be done in such a manner to minimize service interruption to the facility and the customer/user.
B. In the case where backflow prevention assemblies are tested, repaired or replaced on fire lines, the Fire Chief or his/her designated representative shall be notified prior to the commencement of such work. These devices shall be repaired, overhauled or replaced by a certified individual or agency at the expense of the customer user whenever such devices are found to be defective.
(Ord. No. 2149, § 1.B, 9-13-90; Ord. No. 3060, § 2, 11-18-99)
52-38. Discontinuance of service.
Whenever in this article reference is made to discontinuance of service, the customer/user shall receive written notice of the violation and be advised in writing of the opportunity to meet with designated personnel to present any objections; however, discontinuance may occur immediately if there is an immediate danger to the public health, safety or welfare in which situation notice and an opportunity to be heard will be given as soon as practical after discontinuance.
(Ord. No. 2149, § 1.B, 9-13-90; Ord. No. 3060, § 2, 11-18-99)
52-39. Existing water connections.
All existing water connections which may be deemed by the Director of Public Works to be subject to backflow prevention will, upon written notice, have an appropriate backflow prevention device installed and inspected within sixty (60) days of notification.
(Ord. No. 2149, § 1.B, 9-13-90; Ord. No. 3060, § 2, 11-18-99)
What Do all These Plumbing Terms Mean?
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