Backflow Prevention Assembly Enclosures
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.
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.
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.
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.
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.
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.
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.
Other 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.
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:
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.