Need knowledge of Backflow

[PurewaterGuy]

This is kind of a two-fold question here.

1) I would like to understand how a backflow preventer works.

2) Also, at a jobsite there is a backflow preventer installed prior to some pretreatment equipment I have, the town made the company have one installed. The other day I got a call stating the backflow preventer had dropped about 50 gallons of water on the floor. Since I did not install the backflow preventer I told them they needed to get their plumber in there who did. I'm looking for reasons why the device would drop that much water on the floor, I don't know anything about these BFP's and would like to learn how they work and what makes them fail as well.

 

[bimr]

Talk to the manufacturer, the backflow preventer is more or less 2 check valves in series with a reduced pressure zone placed between the check valves to prevent backflow.

http://www.watts.com/pro/_products.asp?catId=65

http://www.watts.com/pro/divisions/backflowprevention/learnabout/learnabout_usc.asp#heatcool

 

[PurewaterGuy]

Well, I found this,


There are two basic causes of backflow.

1)A sudden drop in mains pressure causes the water to siphon back into the system

My response to cause #1 ,, I have two Pretreatment units after the backflow preventor. I assuming what may have happened is the street pressure dropped significantly causing the two pretreatment units to siphon back, thereby tripping the Backflow preventor allowing it to release the siphoned water as it tried to enter the city supply side. It seems to me the amount of water that was released would be about the amount the pretreatment units hold, also this would indicate to me the BFP did the job as designed. Does that sound plausible?

2)The water supply is directly connected to equipment, such as a boiler or a private bore, which is at a higher pressure than the water supply system, forcing water into the supply system.

 

[PEDARRIN2]

City water should never be directly connected to a boiler. There should always be a backflow preventor upstream.

Since it seems you have a reduced pressure principle backflow preventor - the check valves on these occasionally have the tendancy to get fouled, which can cause them to spit some water. But from your description, it sounds like the pressure upstream of the BFP dropped and the equipment operated as it was supposed to operate.

It is good practice to put a deep body floor drain near these since these things can happene and they will spew a lot of water.

 

[cvg]

sounds like you have this setup:

http://www.cla-val.com/pdfs/TM-RP-4.pdf

 

[PurewaterGuy]

"But from your description, it sounds like the pressure upstream of the BFP dropped and the equipment operated as it was supposed to operate."

I'm not following where you get that impression from my description.

I'll describe the situation once again. The city water connects to the BFP, there is 4 ft of pipe from the outlet side of the BFP to a 12"x52" Multimedia Unit, the Multi media Unit is connected to a 14"x65" Carbon Unit, the Carbon Unit is connected to a Reverse Osmosis Unit.

It seems to me there was failure (Loss of pressure)on the city feed side of the BFP thereby causing the water in the idle Pretreatment Units to be siphoned BACK towards the BFP where the water was discharged to the floor rather than being allowed to re-enter the city water feed.

If I experienced a pressure drop after the BFP it would mean the equipment was pulling more water than the city was supplying at that time, correct? Even if that were the scenario, the water would flow in the direction of the demand and not back toward the city feed, So I surmise the problem would have to be the supply side was inadequate and the equipment not in any operational state at the time was having the water within the vessels siphoned backward.

What do you think?

 

[cvg]

read the description of the reduced pressure principle backflow preventor cla-val assembly I provided. It describes your situation very well. i think pedarrin2 got it right, city pressure (upstream of the bfp) dropped below the pressure of your equipment. the bfp check valve shut and the pressure differential relief valve opened. it did more than "spit" water, as this type of device releases enough water to reduce your system pressure below that of the city water main. In this case, it could have emptied your tanks on the floor. it seems that somebody forgot to install the drain as recommended.

 

[PurewaterGuy]

OK, I've read it a couple of times, again.

Under the maintenance portion is states:

"Intermittent discharge of water through the Pressure Differentila Relief Valve is generally caused by fluctuation of the inlet pressure and usually occurs during a period of no flow through the unit."

Should I suspect the BFP itself has malfunctioned? Possibly a failure in CV #1 ?

 

[cvg]

no, you missed it again. As you can see, under normal operation when the supply pressure reduces below your zone pressure or when your zone pressure increase above the supply pressure, the relief valve opens to compensate. when it opens, it discharges sufficient water to lower the zone pressure below the supply.

Under a no flow condition, both check valves are closed. When supply pressure drops to 2 psi above the “zone” pressure, then the relief valve discharges as necessary to maintain the “zone” pressure at 2 psi below supply pressure. When supply pressure drops below 2 psi above the “zone” pressure, then the relief valve opens fully.

Backflow Protection: The RP-1 Backflow Preventer provides
protection against backflow in the following manners:
If the supply pressure drops, the Pressure Differential Relief Valve acts to maintain a pressure in the zone between the first and second check valves of 2 psi lower than the supply pressure by releasing just sufficient water to maintain the required difference in pressure.

If the supply pressure becomes 2 psi or less, the Pressure Differential Relief Valve opens and the pressure in the zone becomes atmospheric. When the supply pressure drops below the downstream pressure, backflow is prevented by action of the second check valve.
Should the downstream pressure rise above the inlet pressure due to action of a pump or other means in the downstream system, backflow will be prevented by the second
check valve.

If a backflow condition occurs and operation of the second
check valve is impaired by damage, or tight closure is prevented by a foreign object, then the Pressure Differential Relief Valve will act to discharge leakage caused by such damage. The zone pressure will, under such conditions, be lower than the supply pressure.

 

[PurewaterGuy]

Let's approach it this way because I'm having a very difficult time understanding this for some reason, and I thank you for your patience.

If I may do it by scenario.

Lets say we have normal street pressure feeding the equipment. All is normal at this point. Now, one of the units goes into backwash. What would have to happen in order for the bfp to discharge water through the relief valve to the drain?

 

[cvg]

supply pressure less than 2 psi above zone pressure

 

[PurewaterGuy]

This is where I'm having my problem. When we say "zone" ,, are we speaking of the area between the two check valves?

 

[cvg]

yes

 

[PEDARRIN2]

The reduced pressure principle backflow preventor protects against backflow where the pressure in your system is reversed - for whatever reason.

It also protects against back siphonage when the pressure upstream of your backflow preventor is negative - think fire department hooks up to a hydrant and pulls lots of flow - or there is a water main break. Watts (the link by bimr) has excellent information about the different scenarios protected by backflow preventors.

 

[PurewaterGuy]

OK, I read over the material from both offerings. I think I get it. There were actually two Preventers that went off in the building, So, let's see if I have this right this time.

The city pressure dropped coming into the building. As the pressure rose again the BFP (2 of them) began to release water through the relief until the pressure stabilized throughtout.

Am I close?

This is one of the preventors:

http://www.watts.com/pro/_productsFull.asp?pid=895&ref=1

 

[PEDARRIN2]

The backflow preventor (do you have one or two) - the Watts unit you show is one backflow preventor - with two check valves - releases water when the pressure upstream of it decreased enough to cause flow to go in the opposite direction. The discharge would have been prior to the upstream pressure rising again.

For you to get discharge in the middle zone, the downstream check would be open, while the upstream check would be closed. This would be correct in a back siphonage situation where the upstream pressure decreases without a noticable backflow.

This would not be true in a backflow situation - which a backwash would initiate and the downstream check - the one closest to the treatment equipment would close.

These types of backflow preventors also get fouled at times which leads the unit to "spit".

In any event, if the owner is concerned, get the installing plumber or someone trained in testing backflow preventors to come out and check the unit. But it sounds like it worked like it should even though it created a mess.