6 pressure tanks... Too many?

Ok, I posted a question about my home last week and now the boss wants me to help at work.
Our building is on a well and one of the six pressure tanks has gone bad. (A 96 gal tank that is rusty and leaks) I drained it and it reads 0 psi. (It's one of two State 96gal tanks, 2 Wellxtroll wall mount tanks each measuring about 2 ½ ft in length and two unknown size tanks in the ceiling). Why in the ceiling I don’t know. The original plans only called for two 96 gal tanks and it is my thought that someone in the past has added the other tanks because the 96 gal tank went bad and they were too lazy to remove it.
I’ve looked in our invoices and see that we discharge into the city sewer about 14000 to 19000 gal per month or 600 gal per day. We have a large storage tank I could park my truck in with two 3hp pumps that feed the building. From what I’m told each pump will alternate usage so as to not wear one out. Each one has an hour meter I could use to see how much they run. My pressure switch says it’s a 30-50 but it reads anywhere from 70 to 75 all the time. Need a new one of these too. I drained the entire 96 gal pressure tank before the pump turned on. If I understand how this system works, the well pump fills the large storage tank and the other two pumps pressurize the building. I would think the storage tank would have floats to turn the pump on and off and would be far enough apart to keep the pump from turning on and off all the time. I don’t know what size the well pump is or how deep.

Here is my question: What would I need to do to figure out if this bad pressure tank is even needed anymore? Seems to me that if it takes close to a hundred gal to turn on one of the pressure pumps we may not need it?

600 gallons a day use and you have 100 gallon drawdown, you have plenty of storage under pressure without the tank, it would seem. Sounds like the system is way oversized for its present use.

It looks like you have what I call "Oversize Centrifugal Pump Syndrome".

Someone looked up the pump curves and decided that a centrifugal pump with a big impeller was needed to supply the pressure, so the smallest single-stage pump that would supply the pressure was a 3 HP pump. I'm surprised you don't have a 5 HP pump. I see that in a lot of systems.

Even if you had to pump 1000 gallons in one hour on peak days (extremely unlikely), you could do the job with a 1 HP multistage pump. A submersible in the big tank would do the job nicely. Two would provide the redundancy that you have.

The large array of tanks was probably installed to provide reasonable run time for the 3 HP pumps. I expect that each one delivers about 40 GPM. A pump like that should run for at least 2 minutes, so you should have a drawdown capacity of about 80 gallons. That requires 250 to 300 actual gallons of tank.

You should put a water meter on the system and check the peak-hour gallons to verify what size pump you need. You may already have one if you are getting bills based on flow. You should also put some good gauges on the system. Get the $30 liquid filled kind rather than the $5 kind that lasts about a year.

I would start by verifying what you need for water supply, and then determine what pumps you need to supply it. Then I would plan to replace stuff as required or as money is available to get to a reasonable system.

I suspect that you could buy a proper size pump for about the same price as a new tank, and a proper size pump wouldn't need a new tank. The duplex controller would work with the new pump.

If you have a 15 to 20 GPM pump at about 1 HP, you would need only about 100 gallons of storage.

If you want to replace the tank because it is quicker, then I would get a 119 gallon bladder tank and just plan to junk the other tanks when they fail. Then start the program to replace the badly misapplied pumps.

What kind of 3hp pumps are these? Why 3hp if you only use 600 gallons per day?

I'm sure there is a better system than what you have now. Get us some more info on what you have.

bob...

The two pumps inline after the large storage tank and before the pressure tanks are Paco pumps with Baldor motors. While I was out today looking for this information I could hear the well pump turn on a couple of times. It only ran for about 2 to 3 minutes. Rather that a float inside the tank there is some sort of rod (McDonald something) that points down at the water, copper maybe, and it can tell when the water level is low. Magic?

Slaz said:

Rather that a float inside the tank there is some sort of rod (McDonald something) that points down at the water, copper maybe, and it can tell when the water level is low. Magic?

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It's probably a probe, is there only one, or is there a low water turn on probe, and a tank full turn off probe?

Is it one of these?

http://www.mcdonnellmiller.com/Literature/LiquidLevel.pdf

Rancher

That is the probe, just a single rod on mine.

valveman said:

I do believe your pumps are way oversized for the use you have. Even so, you could use a Cycle Stop Valve to get the run time needed, and do the whole thing with only one pressure tank.

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I hate to think about the effect of running a 3 HP pump through a 1 or 2 GPM restriction.

The original poster has not identified the model of pump, but a 3 HP Goulds Model MCC, 1 x 1.25 x 6 puts out 40 GPM at the maximum efficiency point and in the zone of 2 GPM it is susceptible to surging.

Many centrifugal pumps have a characteristic that the head is lower at zero flow than at a few GPM, which can cause the pump to surge and hunt while trying to find a stable operating point. That kind of pump shouldn't be restricted below about 25 percent of its maximum efficiency point.

valveman said:

The CSV's that handle a 40 GPM pump have either a 3 or 5 GPM minimum flow. I know what you mean about an unstable pump curve. Those flat curve pumps were the pump companies way of trying to maintain a constant pressure with variable flow. To bad they did not have the CSV in mind when they designed them. They would not have had to screw up the pump curve. As long as you stay well below the lowest part of the curve, they will work fine with a CSV.

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I don't understand what you mean by staying "well below the lowest part of the curve". How do you get "below the lowest part of the curve"? Throttling the pump merely moves the operating point along the Flow/Head curve toward the zero end of the flow. That is where the surging problem is.

You can set the output pressure of the regulator at a lower pressure and throttle the flow, but the pump operates on the curve. When the pump is throttled below about 10 percent of the maximum efficiency flow point, it is operating in the region where surging occurs.

I don't think the people who designed centrifugal pumps were trying to get constant pressure with variable flow, because with those size pumps nobody ever wants to operate in that very low efficiency region. With a pump like that they want want maximum efficiency and the design point is generally about 75 to 80 percent of shutoff head where the slope of the Flow/Head curve assures stable operation.

I don't think you will find Paco pumps at a retail store. They were probably bought from someone who knows how to size pumps (unless they were bought at auction or something similar). Do the pumps have a nameplate with a model number? Paco may have curves online. The ones I have been around were typical end suction (a couple vertical) centrifugal pumps with fairly flat curves and less than stellar efficiency.

By the way, does the business have a big lawn to water or any other water use that doesn't go down the sewer?

We don't need to water more than about a 200sf area of grass and most of the time it doesn't need it. As some may have guessed this is a government building and we all know how these places get built. I will look into a model # if I can read the plate. Most of what has been discussed is way over my head. The building has about 25 employees and sometimes has meetings with 100 people for 8 hour time slots. I thought I would make a chart of the hours on the Paco pumps and see how much they are running but the actual well pump has nothing to monitor the flow or time.