# backflow question



## irishplumber29 (Jun 23, 2010)

if i have a 4" water feed then it reduces to 2" for a water meter than after the water meter it increases back to 4", i have to put at least a 4: backflow on that system, am i right?


----------



## mongo (Jun 26, 2010)

Based on what? Code? Local Ordinance?


----------



## Ishmael (Dec 9, 2009)

Love the tag line, Mongo!:thumbup:


----------



## jitr64 (Sep 30, 2010)

we always go same size as meter,i would check with the water authority or whoever is requiring a backflow though.


----------



## plbgbiz (Aug 27, 2010)

I'd put 2" BFP after meter THEN increase to 4".


----------



## Tommy plumber (Feb 19, 2010)

Sounds like to me that the meter is under-sized, you'd better check with the bldg. official and water utilities dept.


----------



## plbgbiz (Aug 27, 2010)

Tommy plumber said:


> Sounds like to me that the meter is under-sized, you'd better check with the bldg. official and water utilities dept.


Good point. Why a 2" meter if there is a need for 4" water?


----------



## futz (Sep 17, 2009)

plbgbiz said:


> Good point. Why a 2" meter if there is a need for 4" water?


Usually because in situations like that the 4" main is sized that way much more to keep velocity down than for capacity reasons. If you didn't care about velocity and the problems it causes long term then 2" would probably feed the building just fine. 

High velocity through the restrictive 2" brass won't cause the problems that it would flowing through 2" copper, plus it's totally accessible for repair if there is a problem. Noise isn't usually a concern in the mechanical room either. A (possibly noisy) undersized line failing in a ceiling or wall is a much bigger problem. 

We undersize water meters (everywhere) and PRV stations (on commercial, at least) routinely here and the inspectors are fine with it. Undersizing them can save a LOT of money. For BFPs I suspect you can do at least some undersize (it's been too long since I did a big one for me to remember). If you want to know how much your inspectors will let you undersize, just ask them. They'll almost always let you drop at least one size - maybe more.


----------



## hydronicsbob (Oct 6, 2010)

*taken from Zurn on RPZ (reduced Pressure zone) BFP*

*link to website

How do I size an RPZ BFP?* Downstream demand (gpm) will always be the overriding factor in sizing water control valves. Calculate the desired flows and the maximum allowable head loss. Review the head loss curves for the most appropriate size. Don't forget to consider an appropriate velocity (feet per second).


----------



## plumjoe (Oct 21, 2009)

why do you need a backflow? you adding chemicals, sprinkler system, water softner?


----------



## ASUPERTECH (Jun 22, 2008)

plbgbiz said:


> Good point. Why a 2" meter if there is a need for 4" water?


 
2" meter has far better water/ sewer rates than 4". If at 1 time or another the property had lets say it's own fire hydrant, then you may have needed the 4" line, if the municipality is now feeding a new hydrant on the property with reclaimed, then you would no longer need 4", other demands may have also been eliminated, lower flush toilets, no longer irrigation on ptable, etc.. I would for sure check w/ municipality before buying 2" PRZ cha ching big difference between 2-4"


----------



## sprinklertech (Oct 24, 2010)

plbgbiz said:


> Good point. Why a 2" meter if there is a need for 4" water?


Size alone doesn't mean a whole lot.

Bear with me, this will be worth your time.

*And don't panic, it is easier than it first looks.*

Assume a business that is set back from the road 500' and needs 40 gpm delivered at 45 psi during peak times of use. Checking the available city pressure we find 60 psi is available.

Checking on our tapping fees we find we're tapping into the city well system and the fees are as follows:

For that portion of the Village Served by the well system:
Size of Water Tap Charge
3/4" $2,170.00
1" $3,900.00
1-1/2" $8,568.00
2" $15,400.00
3" $34,500.00
4” $47,775.00

But that isn't all, to this we must add sewer tap fees which are based on the size of the water tap.

Size of Water Tap Sewer Tap Charge
3/4" $5,594.00
1" $9,458.00
1-1/2" $18,817.00
2" $32,034.00
3" $69,714.00
4" $123,455.00

I know what you are thinking... "He's crazy, no way tapping fees are that high anywhere in the world" right? Check out these *water tap fees* on page 4 

Obviously we can all assume the owner can get all the water he wants using 4" but he might bark at that combined $171,230.00 combined water and sewer tap fee. That is just the permits to tap.. the city does not do the physical tap, road repair or bringing the line to the property line which you pay in addition to the tap fee. The tap fee gives you the piece of paper that gives you permission to tap into the city water.

In this example even the difference between 1 1/2" and 2" is $27,373.00 vs $47,434.00 is substantial.

Using the *Hazen Williams Formula* where:

Pf=Friction Loss in PSI/Linear Foot
G=Flow in Gallons Per Minute
C=Roughness Coefficient. Use 150 for plastic and Copper, 140 for cement lined underground and between 80 to 120 for all others to be safe. For example if you have some 100 year old unlined sand cast you might want to use a roughness coefficient of 80.
D=Inside Diameter in factional inches... 2" steel pipe would be 2.067"









Where
Pd = pressure drop in pounds per square inch / foot
Q = flow in gallons per minute
d = inside pipe diameter (inch)
C= roughness coefficient or C factor

Typical _C_ factors used in design, which take into account some increase in roughness as pipe ages are as follows

Material C Factor low / C Factor high
Asbestos-cement 140 140 
Cast iron 100 140 
Cement-Mortar Lined Ductile Iron Pipe 140 140
Concrete 100 140
Copper 130 140 
Steel 90 110 
Galvanized iron 120 120 
Polyethylene 140 140
Polyvinyl chloride (PVC) 130 130 
Fibre-reinforced plastic (FRP) 150 150

I disagree on the old Cast iron... if old (100 years) unlined sand cast you might want to use a C factor of 80.

Type L copper has the following inside diameters

3/4"=0.785
1"=1.025"
1 1/4"=1.265
1 1/2"=1.505
2"=2.009"

If you have a computer there's an easy to use calculator at *Engineering Toolbox*. Simply plug in your values and you'll get your answer. See? Told you not to panic.

Using the toolbox calculator flowing 40 gpm

What we are looking for is a combination loss through the total length of underground plus loss through meters and backflow prevention assemblies that, in our example, will not be more than 60psi-40psi=*20psi* used in our example.

Through 3/4" the head Loss (psi / 100 ft pipe): 145.6 psi or 1.456 psi/linear foot or a whopping 727.9 psi through the entire 500' length. Obviously it won't work.

Through 1" the head Loss (psi / 100 ft pipe): 39.48 psi or .395 psi/linear foot or 197.4 psi through the entire 500' length. Doesn't look like this will work either.

Through 1 1/2" the head Loss (psi / 100 ft pipe): 6.08 psi or .0.61 psi/linear foot or 30.41 psi through the entire 500' length. We got 60 psi available at the city main and need 45 psi. 60.00-30.41 leaves 29.59 psi without subtracting the loss for the meter and backflow so this doesn't look like this will work either. 

Finally, through 2" the head Loss (psi / 100 ft pipe): 1.49 psi or .0.015 psi/linear foot or 7.45 psi through the entire 500' length. 

Looking at a *Watts 2" backflow preventor *the loss is about 4.0 psi. Loss through a *2" Neptune meter* flowing 40 gpm will will be about 1.0 psi.

We got 60 psi available at the city main and need 45 psi. 60.00-(7.45+4.0+1.0) leaves 47.55 psi available at the building when flowing 40 gpm.

The 2" will work but you figure the owner might whine about the $47,434.00 tap fee?

We know that 1 1/2" won't work but what if we had the city do a a 1 1/2" tap, run 60' to the property line where we would install a 1 1/2" backflow preventor and water meter then run increase the size running 2" or even 2 1/2" to the building?

Flowing 40 gpm the loss would be:

60' 1 1/2" = 0.061*60=3.65 psi
1 1/2" meter =1.80 psi
1 1/2" Watts Backflow = 4.30 psi
440' 2" to Building = 6.55 psi

Add them all together and we have 16.30 psi. 60.00 less 16.30 leaves us with 43.7 psi but we need 45.0 so this doesn't make it.

Ok, as a final step leave it as is, do the 1 1/2" tap, water meter and backflow but instead of running 2" 440' to the building why not run 2 1/2" having an ID of 2.465"?

The friction or head loss though the 440' is only 0.006/ft for a total of only 2.42 psi. Now if we add everything up we have a loss of 12.17 psi flowing 40 gpm and 60.00 less 12.17=47.83 psi. This works.

Yeah, the 2 1/2" costs more than the 2" but something tells me the $47,434.00 difference between the 1 1/2" and 2" tap fees will more than cover the extra cost of larger diameter pipe.

Now look close at the total friction loss numbers between running everything 2" (47.55 psi available @ 40 gpm) and our combination 1 1/2" and 2 1/2" (47.83 psi available @ 40 gpm) and we can say the 1 1/2" and 2 1/2" scheme *performs better* at flows between 0 to 40 gpm!

Who would have thunk it?

Notes: Be careful, if in doubt how to do this the couple hundred you pay to a professional engineer might be worth it. It isn't rocket science but you don't want to make a mistake either.

I did not use equivalent fitting lengths for any fittings because my post is long enough already.

A decent enough *equivalent fitting length chart*. here Let's say on that 60' of 1 1/2" from the city main to the meter you have two (2) 1 1/2" elbows, a tee and gate (tapping corp stop) valve. To be more accurate you should add the equivalent fitting lengths to the actual length of pipe for more accurate results.

One 1 1/2" tee (flow changes direction 90 degrees) = 9.9 feet.
Two 1 1/2" 90 deg elbows @ 7.4 feet = 14.8 feet.
One 1 1/2" gate valve = 1.2 feet.

The combined equivalent fitting lengths are 29.9 feet giving the total equivalent pipe and fitting length of 89.9 feet. We had 60' 1 1/2" = 0.061*60=3.65 psi but to be more accurate we should use 89.9' 1 1/2" = 0.061*89.9=5.48 psi. In our case this would still work but where you got a lot of fittings you want to be careful. 

And another note... go ahead and round up. In fact I would add 30% to everything on that fitting length chart just to be on the safe side it was was critical. The chart assumes clean fittings and we know how fittings get gunked up. Another area you need to watch is city water taps. All of us know these are not tees.... you got tapping saddles often treated like tees for making calculations but they do act like tees. I one had a tap where the the equivalent fitting length should have been around 80' but actual flow tests indicated the actual measured equivalent fitting length was 250' or 3 times what a "T" fitting was expected to be.

Newly laid copper will have a C-value closer to 150 instead of the 140 we used providing us with a little bit of safety factor. Not much but it is there.

Gee, that was fun.


----------



## plbgbiz (Aug 27, 2010)

I should have poured the first cup of coffee before I started reading your post. Good info. :thumbsup:


----------



## plbgbiz (Aug 27, 2010)

futz said:


> Usually because in situations like that the 4" main is sized that way much more to keep velocity down than for capacity reasons. If you didn't care about velocity and the problems it causes long term then 2" would probably feed the building just fine.


 Now that's an eye opener. Why didn't think of that? Thanks Futz. Just because a 2" service CAN deliver the water, doesn't mean it SHOULD. Higher velocity in copper would certainly result in faster erosion near the turbulence of fittings.

The 4" may have been installed just to help assure there would be no need to dig it up in less than 100 years or so.


----------



## sprinklertech (Oct 24, 2010)

plbgbiz said:


> Now that's an eye opener. Why didn't think of that? Thanks Futz. Just because a 2" service CAN deliver the water, doesn't mean it SHOULD. Higher velocity in copper would certainly result in faster erosion near the turbulence of fittings.
> 
> The 4" may have been installed just to help assure there would be no need to dig it up in less than 100 years or so.


Again from *engineering toobox*

The water velocity in _*copper pipes*_ should be kept within certain limits to avoid erosion, corrosion and excessive noise generation.

* in cold water systems the velocities should not exceed 8 feet per second (2.4 m/s)
* in hot water systems with temperatures below 140 oF (60 oC) the velocities should not exceed 5 feet per second (1.5 m/s)
* in hot water systems with temperatures above 140 oF (60 oC) the velocities should not exceed 2 feet per second (0.6 m/s)

Many companies, have in their process piping go-bys or rules-of-thumb, references to a maximum velocity of fluids in piping. This maximum recommended velocity is referred to as ‘erosional velocity’ and is typically given as some constant divided by the square root of the density of the fluid. The constant will vary from company to company but generally ranges between 100 and 160 for systems operating continuously. Higher constants are sometimes used for intermittent service. *In actuality, this value has no engineering basis when dealing with clean, single-phase fluids that are not corrosive to steel.*

The background of erosional velocity stems from piping failures (circa 1940’s) due to loss of metal in elbows that had been subjected to many years of constant high velocity fluids. Research has revealed that this metal loss was primarily due to two primary factors:

1) two-phase fluids in high velocity service (liquid droplets in gas service or solid particles in liquid service) OR

2) a high velocity fluid that is corrosive to steel.

Why the 40's? The war years and industry used whatever they could get their hands on even if it wasn't appropriate. There's an old Pure Oil Company refinery on Lake Erie that was build in 1942 and 43 that has a 12" fire main look over 1 1/2 mile long made of brass pipe. Note tube but pipe... this stuff has a 3/8" wall thickness. Go figure, they could get brass but not steel.

*In the case of clean, single-phase fluids, there is ample evidence in the physical world that there is no appreciable metal loss to steel exposed to high velocity fluids, even at velocities 10 or more times that of ‘erosional’ velocity.* Some examples are impellers of pumps and seats on control valves. In clean, single-phase service, most of the damage to these devices is due to pitting caused from cavitation, which is caused by sonic velocities. However, any solid particles present in the fluid can cause rapid deterioration of steel due to the impingement of the particles on the metal surface at a change in direction (typically, at elbows). With corrosive fluids, the impact of the high velocity fluid and the eddy currents created by changes in direction, tend to concentrate the loss of metal on elbows.


----------



## plbgbiz (Aug 27, 2010)

sprinklertech said:


> ...made of brass pipe. Note tube but pipe... this stuff has a 3/8" wall thickness. Go figure, they could get brass but not steel...


Maybe they used brass pipe for the same reason they sometimes use brass wrenches.


----------



## plumbpro (Mar 10, 2010)

plbgbiz said:


> Now that's an eye opener. Why didn't think of that? Thanks Futz. Just because a 2" service CAN deliver the water, doesn't mean it SHOULD. Higher velocity in copper would certainly result in faster erosion near the turbulence of fittings.
> 
> The 4" may have been installed just to help assure there would be no need to dig it up in less than 100 years or so.


Good point, the IPC inlcludes table to size water lines. I have seen undersized copper lines break at the fittings because they were paper thin.
I would recommend sizing the system and see what size water line is required and go by that, then check with the inspector. Typically, here anyway, the inspector doesn't know much about proper plumbing. They only seem to know a couple of no no's that others have tried to get away with.


----------



## futz (Sep 17, 2009)

plbgbiz said:


> Now that's an eye opener. Why didn't think of that? Thanks Futz. Just because a 2" service CAN deliver the water, doesn't mean it SHOULD. Higher velocity in copper would certainly result in faster erosion near the turbulence of fittings.


Don't forget potential water hammer problems too.


----------

