Cold Water Supply and Back Siphonage Q5

Thomas Sim PEA2066

Q.
a) With fully labeled diagrams, explain the basic difference and advantages of the direct and indirect cold water supply system. (18 marks)

b) Back siphonage, is the back flow of water which may be contaminated, into the main water supply. Explain how back siphonage can be prevented from occurring. (7 marks)

(25 marks, 2011 Q5)

A.
(a) This question always come out in past years, as shown in link below.

2011 Q5a (This question)
2012 Q4b
2013 Q2b
2014 Q5a
2015 Q1b

Differences, Advantages Direct v Indirect Cold Water Supply System. See posting earlier on Indirect, Direct, Hot or Cold Water Supply here.

 

 

Indirect Cold (Hot) Water Supply System
The key difference is the storage tank and how the heater or boiler obtains the water source. For Indirect Water Supply, water from outside of the building is channeled to a storage tank (cistren) in the roofspace. This water storage tank then supply the heater/boiler with water. The heater/boiler would have specific copper piping to house the hot water. When it goes to the Bathtub or Shower Sink, there are two separate tabs. One for hot and it would be always hot when heater is on, and the other for cold, and always cold.
However, this is not the case for Direct Water Supply System. Water from outside building is channeled directly to the outlet to be used. Thus, water is channeled to the heater/boiler directly. So, the water which is heated up will occupy the same pipe where cold water is running (if heating is not needed).
Whereas, when it is indirect, the water for heating is separately channeled for hot water only. When hot water is needed, it is heated and delivered in this dedicated piping system for hot water - usually copper piping. When it is not needed, the cold pipe is still the same pipe to deliver the cold water.
Such structure of Indirect Hot Water System is familiar in Old Hotels where, mixing of hot and cold water is done in the bathtub. You do not see the water heater unit on the wall. It is usually hidden in the roofspace or centralised in a heater/boiler room. On the other hand, modern water heater is Direct Hot Water Supply System. This Direct Hot Water Supply System channels the cold water directly from outside the building to the heating filament in the heater, the thermostat would adjust how hot or warm the water should be and thereafter, channel out for consumption. There is no storage tank to house the water before it is channeled to the water heater. During the time hot water is not needed, the same piping system is use for Cold Water Supply.
Advantages of Indirect Cold Water Supply System.
1. provides a reserve of cold water from cistren (storage tank in the roofspace)
2. reduces the contamination to the main piping system by back syphonage.
3. since most of the sanitary appliances are fed from the cistren (storage tank), it is less pressure (compared to main pipes from outside) and reduced wear and tear.
4. as there is ready storage from the cistren, there will be less supply irregularity due to peak demand for water supply in the entire area.
Advantages of Direct Cold Water Supply System.

1. energy saving as it does not hold water in storage tank. Only when hot bath is required then there heating of the water. There is minimum hot water retained in the piping system unused.
2. there is no storage of extra water unused. Water tanks may collect deposits and hence requires periodical cleansing. There can be dead animals or insects inside the water tank and thus contaminates the water supply. This does not happen with Direct Cold Water Supply System.

Ref:
[x] Own account with reference to
Google Answers Jan 2004 at http://answers.google.com/answers/threadview?id=297355

(b) Back siphonage, how to prevent backsiphonage.

Similar question was asked in 2015 Q7(ii). You can read the complete explanation page from Kentucky Government - Energy and Environment Cabinet, Department for environmental protection here.

Backflow means the undesirable reversal of flow of a liquid, gas, or suspended solid into the potable water supply; a backflow preventer is designed to keep this from happening. Points at which a potable water system connects with a non-potable water system are called cross connections. Such connections occur naturally in appliances such as clothes washers and dishwashers, but they must be carefully designed and installed to prevent backflow. Another common location for a backflow preventer is the connection of a fire sprinkler system to a water main, to prevent pressurized water from flowing from the fire suppression system into the public water supply.

Back-siphonage occurs when higher pressure fluids, gases, or suspended solids move to an area of lower pressure fluids. For example, when a drinking straw is used to consume a beverage, suction reduces the pressure of fluid inside the straw, causing liquid to move from the cup to inside the straw and then into the drinker's mouth. This is an example of an indirect cross-connection, undesirable material being pulled into the system.

Backsiphonage - more reading here at this posting.

Back-pressure occurs for example when air is blown through the straw and bubbles begin to erupt at the submerged end. If instead of air, natural gas had been forced into a potable water tank, the gas in turn could be carried to a kitchen faucet. This is an example of a direct cross-connection, with undesirable material being pushed into the system.

Back pressure can force an undesirable contaminant to enter potable water piping. Sources of back pressure may be boilers, heat exchanging equipment, power washing equipment, fire sprinklers, or pumps in the water distribution system. In some cases there may be an almost continuous risk of overcoming the static water pressure in the piping. To reduce the risk of contamination, a backflow preventer can be fitted. A backflow preventer is also important when potentially toxic chemicals are used, for instance for commercial/industrial descaling of boilers, or when chemical bleaches are used for residential power washing.

A closely related device is the backwater valve, which is designed to prevent sewage from backing up into a building and causing basement flooding.[1]

Back siphonage, how to prevent it from happening.


Backflow Prevention

Backwater sanitary valves (also known as “check valves” or “backwater valves”), are mechanical devices that are designed to allow the flow of water in one direction only – away from your home. Used on a sanitary sewer lateral, they can offer a decrease in risk of sewage backup if installed properly and maintained adequately.


What is a backwater sanitary valve and how does it work?

A backwater sanitary valve is a type of check valve that is designed to only allow flow in one direction. Different backwater sanitary valves work in different ways, but in general, the type of device that is used in sanitary sewer scenarios works like this:

  • The valve is normally in an open position: the “gate” (or “flap”) is open.
  • When a backflow condition occurs, floats under the gate lift it up and start to block the backflow.
  • If the backflow condition increases, the gate closes against a gasket and creates a seal which does not allow water to pass in the backwards direction.
  • When the backflow condition ends, the gate falls back down due to gravity and returns to the open position to allow normal outflow of sewage from the home’s plumbing system.
How a backwater sanitary valve works

There are other methods of preventing backflow. Please see below.

Partial list of backflow prevention devices[edit]

  1. Air gap
  2. Atmospheric vacuum breaker (AVB)
  3. Check valve (usually not a legally approved method of backflow prevention)
  4. Chemigation valve (primarily used in agriculture)
  5. Double check valve, or double check valve assembly (DCVA)
  6. Pressure vacuum breaker assembly (PVB)
  7. Reduced pressure zone device (RPZ)
  8. Spill resistant pressure vacuum breaker assembly (SPVB)
  9. Vacuum breaker

An air gap, as it relates to the plumbing trade, is the unobstructed vertical space between the water outlet and the flood levelof a fixture.[1]

A simple example is the space between a wall mounted faucet and the sink rim (this space is the air gap). Water can easily flow from the faucet into the sink, but there is no way that water can flow from the sink into the faucet without modifying the system. This arrangement will prevent any contaminants in the sink from flowing into the potable water system by siphonage and is the least expensive form of backflow prevention.

A common use of the term "air gap" in home plumbing refers to a fixture that provides back-flow prevention for an installed dishwasher. This "air gap" is seen above the countertop as a small cylindrical fixture mounted parallel with the faucet. Below the countertop, the drain pipe of the dishwasher feeds the "top" of the air gap, and the "bottom" of the air gap is plumbed into the sink drain below the basket, or into a garbage disposal unit. When installed and maintained properly, the air gap works as described above, and prevents drain water from the sink from backing up into the dishwasher, possibly contaminating dishes.[2] Water flowing from the fixture into the sink indicates a need for maintenance or repair.[3]

To further illustrate the air gap, consider what could happen if the air gap were eliminated by attaching a hose to the faucet and lowering the hose into a sink full of contaminated water. Under the right conditions (if the water supply loses pressure and the sink is higher than the point at which the water supply enters the house, for instance), the dirty water in the sink will be siphoned into the water pipes through the hose and faucet. The dirty water then will be dispersed throughout the drinking water system.

One-Inch Brass Atmospheric Vacuum Breaker mounted on PVC Pipe

An Atmospheric Vacuum Breaker (AVB) is a backflow prevention device used in plumbing to prevent backflow of non-potable liquids into the drinking water system. It is usually constructed of brass and resembles a 90-degree elbow with a hood on its top to allow air to enter the water system if a siphon attempts to form. Inside this elbow is a poppet valve that is held "up" by the water pressure found in the system, closing the air entrance to the device. If the pressure in the "upstream side" is reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system, breaking the siphon.

These devices, since they work on atmospheric principles, cannot be installed in an enclosure containing air contaminants. Those contaminants could be drawn into the device, thus fouling the pipes. AVBs must be installed a minimum of 6 inches above the highest usage point in the system (i.e. sprinkler, drip emitter, etc.) Underground installation of AVBs is entirely ineffectual in providing backflow protection as groundwater in the underground vault could be drawn into the water system, contaminating it.

The AVB can be used in high hazard situations but not with continuous pressure, as the poppet would likely stick and the AVB would no longer function properly. A shutoff valve should never be placed downstream of any AVB, as this would result in continuous pressure on the AVB. The AVB is not a testable device.

A check valve, clack valve, non-return valve or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.

 
Chemigation - Sometimes called a check valve or a backflow preventer, a chemigation valve is an apparatus designed to protect water supplies from agricultural chemicals used during chemigation, the application of chemicals such as fertilizers and pesticides through irrigation water.[1] Most chemigation valves consist of a spring-loadedcheck valve, a low pressure drain, an air and vacuum relief valve, and an injection port for introducing the chemicals downstream of the check valve. Many chemigation valves also have a 4 inch inspection port so that a person can reach inside and feel if the check valve is still functional[citation needed]. Some governments require the use of two chemigation valves installed in series if hazardous chemicals are to be injected[citation needed].

 

A double check valve or double check assembly (DCA) is a backflow prevention device designed to protect water supplies from contamination. It is also a valve used in air brake systems on heavy trucks.

It consists of two check valves assembled in series.[1] This employs two operating principles: firstly one check valve will still act, even if the other is jammed wide open. Secondly the closure of one valve reduces the pressure differential across the other, allowing a more reliable seal and avoiding even minor leakage.

A pressure vacuum breaker (PVB) is a type of backflow prevention device, used to keep non-potable (or contaminated) water from entering the water supply. A PVB is similar to an atmospheric vacuum breaker (AVB), except that the PVB contains a spring-loaded poppet. This makes it acceptable for applications that are high hazard or where valves are downstream. Pressure vacuum breakers must be protected from freezing when installed outdoors. PVBs usually have test cocks, to which specially-calibrated gauges are attached, in order to ensure that they are functioning properly.

A RPZD is considered suitable for significant hazard applications,[note 1] that is, where the consequence of backflow into the water supply would cause significant harm, although not for the highest risks, such as human waste.[4] They are considered suitable because they prevent both back pressure and back-siphonage, because of a redundant design (even with two check valves broken the device still provides protection), and because they are testable to verify correct operation.

RPZDs are often chosen instead of an air gap since the head loss across the RPZD is much smaller.

A vacuum breaker is an attachment commonly placed on a bibcock valve or toilet or urinal flush valve, that prevents water from being siphoned backward into the public drinking water system. This prevents contamination should the public drinking water system's pressure drop.

A vacuum breaker typically contains a plastic disc that is pressed forward by water supply pressure, and covers small vent holes. Should the supply pressure drop, the disc springs back opening the vent holes (which let in outside air), and preventing backflow of water. A more complex valve that accomplishes much the same purpose is the backflow preventer.

Vacuum relief valves are sometimes known as vacuum breakers.

Ref:
Earlier post.
https://en.wikipedia.org/wiki/Backflow_prevention_device
https://utilitieskingston.com/Wastewater/BasementFlooding/Backflow
Various links provided under 'partial list of backflow prevention devices'