By: Clyde H. Zelch
I will start with the assumption that most, not all, but most treatment plant operators take their work quite seriously and make their best effort to produce good quality water; and many deep wells provide safe drinking water that requires NO treatment of any kind, but once that water leaves the plant or well house and moves into distribution and storage that quality is often squandered.
Most water systems are trying to provide first class quality domestic use DRINKING WATER and a flow capacity that is capable of meeting at least reasonable, or minimal, fire flows. For normal single family houses spaced 100’ or more apart that is generally considered to be 500 GPM. As the houses or buildings get larger or closer together the needed fire flow goes up quickly.
There is a very real difference between having enough clean water at needed pressures for domestic needs (this must focus on QUALITY) and having flow capacities sufficient for effective fire suppression (this takes QUANTITY). These are not necessarily compatible since high flow capacity for any extended time requires large diameter and/or well circulated piping to deliver water to the scene, substantial storage to draw from and reasonable production into the system.
That’s just fine for fire suppression and insurance rates but that large diameter well circulated pipe system means that during NORMAL use the water moves VERY slow and time in piping can be several days, even weeks. That substantial storage tank, or tanks, can mean serious ice problems in winter and thermal stratification in summer, and the same water staying in storage from the last cool spring nights to the first frost of fall, and in the deep-south it’s year round. Both of these can be very damaging to water quality and whatever reasonable production may be, it is different for every system.
I have more than 40 years experience in storage tank maintenance, 35+ years involvement with fire departments, distribution piping and hydrants needed for reasonable fire protection. I have worked with several systems on various water quality problems in storage and have done my share of line flushing, flow testing and color coding of hydrants. I have read many articles on these issues, most have several good points, some miss the point, and some are just a little misleading.
You CANNOT simply wash away all water quality problems by flushing hydrants and a tank level recorder will not assure water turn over in storage. Most older tanks were built with a single pipe that brought water both in and out at the tank bottom, whether you have a single pedestal or leg tank with wet or dry riser, or a standpipe changes nothing. A level recorder is actually recording system pressure at the tank and just because it shows a pressure change of X lbs., or X ft. in no way means you moved water THROUGH storage, just in and out but NOT THROUGH, and the difference is like day and night. So, if you have single piped storage re-pipe it with a properly designed retention circulation system to put water in high and take it out low, or put it in on one side and out on the other, but retrofit it to assure movement THROUGH storage. If your building new tanks double pipe them to achieve the same thing. There are many fringe benefits to this, but better quality water coming out of storage is the first one.
Next, check the vent or vents on your tanks. Most old vents and some of the new ones used by builders or painting contractors are little more than dangerous, unsanitary junk. They are not frost proof, not insect tight and they do not keep out air borne dust to the extent possible with good venting, as required by Ten State Standards, and some are entirely too short with air intake less than 1’ above the roof.
Then check your roof access hatch. Many of them, even on NEW tanks, do not fit close enough to keep out insects or air borne dust. Now your asking, “Just where’s this dust your talking about come from?” It’s simple, tank roofs are reasonably flat and birds crap all over some of them, it dries up, the wind blows and you have AIR BORNE DUST that is really bird droppings; and this gets under poorly fitted roof hatches and short vents in a hurry. Most hatches need a gasket to be bug tight and that is very important.
Next is the overflow. They should all come to near grade and be fitted with a ¼” mesh screen inside of a close fitting flap gate. Not either/or, but both and yes, I know that AWWA D-100 Standards don’t say that. I also know that some states still call for a 24 mesh screen or flap gate on overflows. Fine mesh screens are quickly clogged with rust from inside the overflow pipe, the screen mesh MUST be large enough to let fine rust or sand pass through without clogging. That makes it large enough for insects, wasps, flies, dust, grass clippings and many other things to pass through and be drawn up by the flue effect, very unsanitary. Put on a good flap gate with screen and you have stopped most of it.
There may be other sanitary problems with your storage but EVERY TANK has these three, so address them first and you have made real progress in water quality. This will also reduce disinfection byproducts since they result from chlorine reacting with organics. Keep out the organics.
Now your thinking of line flushing, good, but remember when your flushing there are only two places that water can come from, production and storage. I am still assuming you produce good water but if your storage has not been cleaned in 5 years or less then stop and clean it out first.
You CANNOT clean up your piping by running dirty water from storage through it. I have seen this tried more than once and many people got sick, some even died. Don’t make that mistake.
I don’t believe in just flushing or dumping water from a hydrant to simply move new water into a section of piping. You should take static pressure readings first, then flow the hydrant, flush the main and take flow pressure readings which, with a little calculating will give you GPM available at that hydrant. So what do you need? A good hydrant wrench and since most systems operate between 40 to 80 PSI static pressure use a hydrant cap with 100 PSI gauge. If your system has higher pressure have another cap with a 150 or 200 PSI gauge, use gauges that can operate in their mid ranges. You do not have to use a pitot tube to get reasonably accurate test results but I do prefer one. Nothing fancy, just a simple hand held tube fitted with a 2 ½” or 3” diameter 30 PSI gauge. You don’t need a higher pressure gauge if one 2 ½” hydrant outlet can flow more than 30 PSI. Then flow two of them and try to keep your flow pressure between 15 to 25 PSI, under 15 your putting part of your system at too low of a pressure and above 25 the flowing water can become rather destructive.
You will need at least one flow deflector. I have two, made them both, very low cost but functional. One has a fixed 2 ½” opening, the other is adjustable at 2”, 1 7/8”, 1 9/16”, 1 3/8” and 1 1/16”, and it can go smaller if needed for some reason, all made from scrap or simple
hardware store parts, this gives me a controlled flow of about 120 GPM at 15 PSI through the 1 1/16” opening up to 1375 GPM at 25 PSI through both the 2 ½” and the 2” opening; and 1505 GPM at 30 PSI. Most smaller systems have few, if any, hydrants that will flow more than 1500 GPM.
It’s generally accepted that a water speed of about 2.5 to 3 F.P.S will begin moving sediment in piping and 4 F.P.S, or a little more, will do a respectable job of removing most sediment from PVC or cement lined ductile piping, but not rust barnacles from unlined cast or steel. So, I want a water speed of 4 F.P.S or more to clean out most piping.
Is there a quick and easy way to figure water speed without a calculator? YES. Take any pipe diameter and square it, than add one 0, you now have GPM at 4 F.P.S.
Examples: 6 x 6 = 36 + 0 = 360 GPM at 4 F.P.S.
2 x 2 = 4 + 0 = 40 GPM at 4 F.P.S.
40 divided by 4 = 10 GPM at 1 F.P.S, it does not change.
12 x 12 = 144 + 0 = 1440 GPM at 4 or 360 GPM at 1 F.P.S.
Is this exact? NO. Is it close enough for what we need? YES, it’s also simple and fast.
You also need to know how to calculate flow in GPM from a hydrant regardless of the opening diameter you are flowing. With a cap gauge on one outlet and flowing the other a pitot tube reading of the flow pressure will be very close to the cap gauge reading, so you can get reasonably accurate flows without a pitot tube. You have diameter of the opening and cap gauge pressure, now take square root of pressure X diameter squared, X 29.8, X flow efficiency of opening (generally considered to be .9).
Cap gauge or flow pressure is 16 PSI √4 , diameter is 2 ½ squared = 6.25
4 x 6.25 = 25 x 29.8 = 745 x .9 = 670 GPM
On a rather low flow hydrant you could reduce the opening to 1 ½” with a cap gauge pressure of 22 PSI √4.69 .
1.5 x 1.5 = 2.25, 4.69 x 2.25 = 10.55 x 29.8 = 314 x .9 = 283, round that off to 285 GPM.
If your flowing this through a dead end 4” line take 285 divided by 40 = 7.125 F.P.S water speed, that line should clean out nicely and if this hydrant was 650’ from the last one you flowed, tested and flushed clean it will take about 90 to 100 seconds to clean 650’ of pipe, and use about 450 to 500 gallons. Your flowing about 5 gallons per second, 5 x 90 = 450 ; 5 x 100 = 500.
That’s not lost or wasted water, you know exactly where it went and it was put to good use helping to maintain water quality in your system. I have read articles saying you should not flush at a speed fast enough to stir up any sediment. If your not even going to stir up and remove sediment you are wasting water. If the system is that dirty it needs a THOROUGH flushing. Quite often to get adequate flows in the direction needed some parts of distribution will have to be valved off to prevent distributing sediment that could not quickly be flushed out. You must have a directed flow of adequate velocity to clean and flow long enough to clear it up. Some systems are so poorly valved, or they are broken or buried under 6” of blacktop that properly directed flow is not possible. If a system is really dirty you may want to flush and flow test from about 10:00 at night to 5:00 in the morning, most small systems or residential areas would have very little use at that time and minimal chance of customer complaints.
If that’s what it takes, then do it. Your job is to provide your customers with CLEAN DRINKING WATER, not just something to flush the toilet and you cannot do it with dirty, unsanitary storage tanks and distribution piping lined with sediment. Their job is to pay a fair price for this service and clean water at a reasonable pressure delivered to their house costs money. YOU MUST CHARGE FOR IT. All towns and water districts need to take a close look at rates. You MUST charge enough to pay your OWN way. Don’t depend on grants.
No government has ONE DIME they didn’t first take from someone else, and they have no right to take money from the customers of a well run system that pays their own way to give it to a system that’s poorly managed, operated and wasteful. Hire capable people that can and will work, and make your system self sufficient.
I am perfectly willing to help you but it is not my job to support your system.
I have my own to pay for.