SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Hi Wetboots,

Regarding your comment about township inspectors not standing behind the homeowner after passing an inspection... I'm thinking that the importance of the inspectors comes in after any loss occurs and the homeowners' insurance company might seek to escape payment by claiming that the homeowner did not properly obtain inspections.

If the town were to pass inspection on the AVB and someone were harmed, presumably the insurance policy would have to pay. The same might not be the case otherwise.

However, the liability that could occur from a backflow event could potentially be so great that the liability coverage on a typical homeowners' policy might not do much good. In that case, I'd doubt that an appropriate inspection certificate would be much help (i.e., for losses in excess of the policy coverage).

Homeowners may be led to assume that proper inspection is some kind of guarantee by the municipality but I believe that is simply not so. The case you cited is particularly troubling because of the number of individuals who might be harmed by the chlordane etc. that backflowed into the municipal water system. There is a virtually unlimited liability there.

Presumably, in the case we read, the medical tort was agreed to be slight enough to be covered by the corporate insurance (judging from the fact that the company continued to operate).

I would appreciate any other links or references to cases of backflow damage.

Meanwhile, I will figure out how to correct my problem this week. Since it's rainy and the seed's not actually in the ground as of yet I can shut off the supply valve to the irrigation system.

Picked up a Wilkins Model 720A Pressure Vacuum Breaker today for a bit over $60 including tax (non-ferrous metal construction -- brass or bronze -- as you suggest).

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Another web based resource from the folks at the University of Southern California
http://www.usc.edu/dept/fccchr/intro.html

Trying to understand the functioning of the Pressure Vacuum Breaker... seems a bit subtle. Will post when I find a better explanation.

The main problem for the residential sprinkler irrigation system seems to be backsiphonage which can open up the solenoid valves when the system is open to the potable water supply for months during the growing season.

The atmospheric vacuum breaker seems to be nominally capable of protecting the potable water system but fails when the atmospheric valve sticks in the closed position owing to months of being forced against its seat whenever the system is pressurized upstream of the solenoids, which is all the time.

The pressure vacuum breaker seems to add additional mechanisms that guarantee the atmospheric valve will indeed open. The nature of these mechanisms is what I'm still trying to understand... does the atmospheric valve open in normal conditions when the solenoid valves close?

The other attribute of the pressure vacuum breaker is that there is a check valve which should close the potable water supply line off except when there is positive flow in the correct direction.

Of course all this assumes that whatever bad stuff is in the water is not able to swim upstream or diffuse upstream quickly enough to ride backwards into the potable water system when the irrigation is functioning normally (think of salmon swimming upstream -- if we had any tiny salmon of sufficient fortitude to swim against the rushing high velocity water stream that would be a problem.)

To continue that thought, *drip irrigation* would seem to have an inherent drawback that it cannot possibly be prevented against minute quantites of backflow by organisms able to swim upstream or chemicals able to diffuse upstream against the tiny flow through a tiny orifice. something to think about.

The problem with the AVB seems

After studying the Wilkins valve I have now understood the following.

Essentially the pressure vacuum breaker PVB consists of two separate valves in one. The first valve is similar to what is found in an atmospheric vacuum breaker (AVB) with one exception -- the poppet which adds some weight to encourage the atmospheric valve contained in both PVBs and AVBs to open. Both the atmospheric vacuum breaker and the pressure vacuum breaker have a seal to atmosphere (the "atmospheric valve" I call it) which is closed whenever the interior of the valve is pressurized. Both rely on gravity to open the valve when pressure is absent. I.e., in the absence of internal pressure owing to water pressure from the potable water system, a disk drops under the influence of gravity, leaving open a gap around the perimeter of the disk. That gap allows atmospheric gases to enter the valve body, intended to release any "siphon" effect on the water in the system *downstream* of the valve.

So if water were trying to siphon out of the valve, either an AVB or PVB valve, this disc would permit airflow to occur and allow the siphoning to proceed. Without it, no air could enter the lines and the water would be trapped.

For water to siphon, the valve must be higher than the outlet through which the water would escape the sprinkler/irrigation system. In addition, any valves downstream of the AVB or PVB would need to be closed. The latter condition is not possible in an installation where solenoid valves are located downstream from the AVB or PVB because the solenoid valves close off any flow that might otherwise occur out of the outlets.

One difference between the PVB and AVB is that the PVB uses a weighted poppet to open the atmospheric valve. When operating properly, the atmospheric vent will open whenever the pressure inside the valve is below 1 psi. But that vend doesn't open at all if the solenoid valves are closed and the main shut-off valve inside the home is open.

In theory, the PVB is designed to protect the potable water supply from a vacuum condition in the potable water supply. The literature supplied with the PVB states that it is not supposed to protect the potable water system from "back-pressure." The PVB does this by using a check valve which prevents backflow whenever the source pressure is less than 1 psi over the downstream pressure (the pressure on the downstream connection leading from the PVB to the irrigation system). That's the real difference for the PVB that gives it an edge over the AVB. Theoretically, once the check valve is closed, the atmospheric valve has a chance to open if the downstream system has a chance to drain. But the main protection is from the check valve which is spring-loaded in the body of the PVB and which is totally absent in the case of an AVB.

Considering what would happen if the pressure in the potable water system were to suddenly be reduced. This could happen if there were a fire and the fire truck were to pump water out of a nearby fire hydrant, which might be located right in front of the home. The fire truck might actually create a vacuum in the line (pressure below atmospheric pressure). But a simple reduction in pressure also happens if water is simply being used within the house for a shower, for laundry, etc.

Under the pressure drop condition where the pressure still remains over 1 psi, the atmospheric valve in either the AVB or the PVB will be useless. It is not intended to open at pressures above 1 psi! Yet, the sprinkler system is still pressurized, so there is going to be a tendency to backflow water trapped in the sprinkler system to the house water supply!!!

Is that water possibly contaminated? It should not be if the solenoids are closed properly... but there is a possibility that the sprinkler heads are holding off a certain amount of pressure greater than 1 psi, and the lines going from the solenoid valves to those heads are sufficiently pressurized to push the solenoid valves open in the reverse direction. Those solenoid valves might not be

[corrected an error in previous posting]

After studying the Wilkins valve I have now understood the following.

Essentially the pressure vacuum breaker PVB consists of two separate valves in one. The first valve is similar to what is found in an atmospheric vacuum breaker (AVB) with one exception -- the poppet which adds some weight to encourage the atmospheric valve contained in both PVBs and AVBs to open. Both the atmospheric vacuum breaker and the pressure vacuum breaker have a seal to atmosphere (the "atmospheric valve" I call it) which is closed whenever the interior of the valve is pressurized. Both rely on gravity to open the valve when pressure is absent. I.e., in the absence of internal pressure owing to water pressure from the potable water system, a disk drops under the influence of gravity, leaving open a gap around the perimeter of the disk. That gap allows atmospheric gases to enter the valve body, intended to release any "siphon" effect on the water in the system *downstream* of the valve.

So if water were trying to siphon out of the valve, either an AVB or PVB valve, this disc would permit airflow to occur and allow the siphoning to proceed. Without it, no air could enter the lines and the water would be trapped.

For water to siphon, the valve must be higher than the outlet through which the water would escape the sprinkler/irrigation system. In addition, any valves downstream of the AVB or PVB would need to be *open* (<--- corrected an error in original text). The latter condition is not possible in an installation where solenoid valves are located downstream from the AVB or PVB -- because the solenoid valves close off any flow that might otherwise occur out of the outlets.

One difference between the PVB and AVB is that the PVB uses a weighted poppet to open the atmospheric valve. When operating properly, the atmospheric vent will open whenever the pressure inside the valve is below 1 psi. But that vent [*] doesn't open at all if the solenoid valves are closed and the main shut-off valve inside the home is open.

In theory, the PVB is designed to protect the potable water supply from a vacuum condition in the potable water supply. The literature supplied with the PVB expressly states that it is not supposed to protect the potable water system from "back-pressure." The PVB does this by using a check valve which prevents backflow whenever the source pressure is less than 1 psi over the downstream pressure (the pressure on the downstream connection leading from the PVB to the irrigation system). That's the real difference for the PVB that gives it an edge over the AVB. Theoretically, once the check valve is closed, the atmospheric valve has a chance to open if the downstream system has a chance to drain (which it does not if the solenoid valves are closed). But the main protection is from the check valve which is spring-loaded in the body of the PVB and which is totally absent in the case of an AVB.

Considering what would happen if the pressure in the potable water system were to suddenly be reduced. This could happen if there were a fire and the fire truck were to pump water out of a nearby fire hydrant, which might be located right in front of the home. The fire truck might actually create a vacuum in the line (pressure below atmospheric pressure). But a simple reduction in pressure also happens if water is simply being used within the house for a shower, for laundry, etc.

Under the pressure drop condition where the pressure still remains over 1 psi, the atmospheric valve in either the AVB or the PVB will be useless. It is not intended to open at pressures above 1 psi! Yet, the sprinkler system is still pressurized, so there is going to be a tendency to backflow water trapped in the sprinkler system to the house water supply!!!

Is that water possibly contaminated? It should not be if the solenoids are closed properly... but there is a possibility that the sprinkler heads are holding off a certain amount of pressure greater than 1 psi, and the lines going fr