Maximum % Oveflow Possible for a Water Meter

[smithranch]

I have a nominal 3/4" water meter connected 50' away from a 12" main with a 1" pipe from a 1" diameter tap into the 12" main. The static pressure at this meter location is 90 psi.

This is on a ranch outside of city limits. I have not done a hydrant flow test to see what gpm this meter can really flow and with what residual pressure. The water utility company says to expect 26-30 gpm from this meter setup and that is plenty for normal uses.

However, for some fire protection capability and from the perspective of either using an automatic sprinkler system or just some outside hose stream allowance, I would like to be able to flow 60 gpm using a sufficiently sized booster pump in the event of emergency.

My question is this. Can a typical 3/4" water meter be made to flow double its normal capacity given the conditions I mentioned above. I realize that the meter would not read correctly at that flow rate and would create a significant pressure drop while flowing double capacity, but would it allow such a large overflow rate for a period of 30-60 minutes without seizing or otherwise retarding the flow? What do you think is the maximum possible percent overflow rate for a small meter in good condition?

I realize that the utility company might not approve of such a strategy, but for an event that may never happen it seems reasonable to consider.

 

[dicksewerrat]

Is this tap a 1 inch ID? Or is the hole in the main 1 inch? And the ID is 3/4? A 1 inch tap used to take a 1 1/4 inch line. But I think if you want to use something for a little fire prtection, get a 2,000 gal. tank, a generator to run your fire suppression pump and let it go at that. I don't think you can suck more water out of the main with a pump.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

You will not get 60 gpm through a 3/4" water meter.

Here is a brochure from Badger, a water meter supplier:

http://badgermeter.com/Water/Water-Meters/Recordall-Disc-Series--1-.aspx

The brochure says a 3/4" meter will do 35 gpm.

If you are so concerned about the fire flow, why don't you request (and pay for) a larger meter and water service from the water utility?

 

[smithranch]

Dicksewerrat, I was just told there is a 1" tap with a 1" pipe leading to the 3/4" meter. It was tapped like that in case of an upgrade to a 1" meter.

I have seen numerous web sites that list the typical maximum flow rate of a 3/4" meter between 26 and 35 gpm. However, those gpm values are based on just pressurized flow from a water main with no consideration of a pump with lift capability near the meter. Such a pump will cause atmospheric pressure to assist the pressure already available at the meter. The meter will definitely flow more than it otherwise would without the pump present. Just how much more is what I am interested in being able to predict. For my conditions, and assuming I install a pump optimized for what I am trying to do, just how much more flow capability could be achieved through a 3/4" meter? If not 60 gpm, maybe 40 or 50 gpm?

I had originally thought I would upgrade to a 1" or even a 1.5" meter with associated expanded tap sizes. Unfortunately I did not realize that my rural water utility required huge impact fees and a large minimum monthly bill for meters greater than 3/4". A 1.5" meter requires a $10,500 impact fee and a $125 minimum monthly bill. A 1" meter is not as costly but still many thousands more than a 3/4" meter.

When a 3/4" meter can easily service my normal needs, the savings in impact fees with only a $24 minimum monthly bill gives a lot of justification for investigating ways to flow 50-60 gpm only in emergencies.




I am wondering if anyone has experience with

 

[bimr]

At 60 gpm, the 50 feet of 1" pipe alone would have 88 psig head loss. Add to that the meter and fittings headloss.

A pump would not work as a minimum water pressure (NPSH) is required on the pump suction side.

dicksewerrat provides a reasonable solution; a water tank.

 

[smithranch]

bimr, even if using a thick wall type K copper service tube I only calculate a friction loss of 64.27 psi for 60 gpm flow through a 50' long copper tube.

On page 2 at this site

http://badgermeter.com/getdoc/05fe8bcd-ef3d-4933-a8f6-0563937ec917/rd-t-34-pdf.aspx

you can see that a 3/4" meter has a fairly constant loss until after about 15 gpm and then then pressure drop increases fairly rapidly.

Even with substantial meter loss the atmospheric pressure suction boost from using the pump (about another 10 psi towards positive suction) means that a pump could easily have enough suction head available to operate at 60 gpm without cavitation.

Several meter manufacturers are calling me back with some info based on their tests to destruction with high driving pressures. All their nominal published data is based on insuring the meter survives with being able to meter correctly thereafter. With enough pressure, the meters can flow way more than their nominal ratings. I don't mind replacing an inaccurate meter after some emergency high flow event.

Concerning the tank solution, that still requires a pump and also only gives a limited high flow time. Overdriving a meter in a rare high flow emergency requirement seems like a cost effective solution.

 

[bimr]

I looked at this again using Crane's Paper 401. It is probably best to model this using the orifice equation.

You can probably get 52 gpm. That assumes 57 feet of headloss across the meter and 152 feet of headloss across the piping.

http://files.engineering.com/getfil...4a92-420d-a82d-f6a40177628f&file=headloss.pdf

Is it a good idea? That is your call.

1. Who will be exercising the pump to make sure that it will work in an emergency?

2. Residential piping is typically not designed for these conditions. The maximum recommended velocity to minimize erosion in copper tubing is 8 ft/sec. You also have water hammer considerations.

"To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 feet per second for cold water and 5 feet per second in hot water up to approximately 140°F."

http://www.copper.org/applications/plumbing/techref/cth/cth_3design_size.html

 

[smithranch]

Bimr, I have been modeling this scenario with my own company's (Elite Software) hydraulic fire protection software. I have full control of the pressure drop on the piping downstream of the meter and I can even pay a one time fee to upgrade the tap size and tap line. As long as I keep the meter size 3/4", I can enjoy the minimum impact fee and the monthly minimum monthly rate of $24

I might look at paying for a 1.5" tap and tap line size with the idea that I might upgrade to a larger meter in the future. So I have full control and high predictability on the piping losses.

The issue I am still wondering about is the loss through the meter at 60 gpm. Using the web link above and extrapolating from that graph, a 25 psi loss (agrees with your 57 feet of head loss) is reasonable. Could be even higher though and hopefully I will hear soon from a meter factory that has actually flowed 3/4" meters to extreme degrees in their factory testing.

As a personal system, I will be the one to exercise and monitor the pump system. This is just a for what its worth system that is not intended to confer the full benefits of an NFPA approved commercial system although it will well exceed what an NFPA13R residential system can provide.

Concerning the water velocity issues, no doubt the velocities from such high flows would be detrimental and shorten the life of the piping system if they occurred on a routine basis. However, such high flows may never happen in the life time of the system so those potential high velocities are not a significant risk to the long term reliability of the piping system.

 

[smithranch]

I have talked with people at three different meter manufacturers. Its very hard to get by low level tech support to people who have actually performed tests to destruction on meters. These people will all admit that the meters can survive for some time at flows much higher than their published rating.

But only one of the three people I talked to would give an extensive opinion on the subject and that was Scott Bruno at Master Meter in Texas. Although not endorsed and not warrantied for such use, Scott said a 3/4" Master Meter could likely run for a week at 50-60 gpm and not only survive but still meter accurately after the event.

A lot depends on the quality and type of meter on how well it can handle a large flow event. Of course, good practice is not to subject water meters to flow rates beyond their rated design limits. But it is good to know that a quality water meter can likely handle such an isolated event without catastrophic failure.