Submersible turbine pump lifespan formula?

[Pawpaw]

Has anyone ever encountered a formula for estimating submersible turbine pump lifespan?

For example, I've heard that in good conditions, diaphragm pressure tanks are considered cycle-limited. Thus one can roughly approximate their life by dividing the cycle limit by the cycle rate of the specific application.

1) A pump is obviously much different but does even a crude approximation exist? For example, a pump has X units of life, a start depletes Y units, and 1 hour of running counts for Z units?
2) Is it considered likely that a pump will fail just beyond warranty period if started the maximum times per day according to the manufacturer?
3) How does everyone estimate pump lifespan?

(This thread is an auxiliary to: Short-cycling pump: optimum strategy for particular system)

 

[Valveman]

All submersible pumps are designed with a planned obsolescence date or fail date of about 7 years. That means some will last 30 years, and some only 30 days. There are lots of variables. Not only does the number of cycles matter, but the amount of run and off time is important. Pumps can cycle a greater number of times when they stay on longer and off longer than when compared to short run and off times.

The temperature of the water and the amp draw are also important. Larger pumps can survive fewer cycles than smaller pumps. 1HP and smaller can take 300 cycles per day, where 1.5HP and up can only survive 100 cycle per day. I have heard from many people with lightly used water systems that have had pumps last 40 years or more. At the same time I have heard from many people with heat pumps that cause lots of pump cycles, and their pumps won't last but a year or two.

I think you are about right to say a pump can survive the max number of cycles per day for seven years. Break it down into the actual number of pump cycles per day, and you will know about how many days your pump will last.

 

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[VAWellDriller]

Has anyone ever encountered a formula for estimating submersible turbine pump lifespan?

For example, I've heard that in good conditions, diaphragm pressure tanks are considered cycle-limited. Thus one can roughly approximate their life by dividing the cycle limit by the cycle rate of the specific application.

1) A pump is obviously much different but does even a crude approximation exist? For example, a pump has X units of life, a start depletes Y units, and 1 hour of running counts for Z units?
2) Is it considered likely that a pump will fail just beyond warranty period if started the maximum times per day according to the manufacturer?
3) How does everyone estimate pump lifespan?

(This thread is an auxiliary to: Short-cycling pump: optimum strategy for particular system)

You are really overthinking this. There are too many variables to consider, you've named most of them, but you don't have the answers for the x, y, and z's....no one really does -- each and every well system is slightly different (not to mention when you throw in water quality differences).

You should buy pump sized correctly to run in the meat of its curve, a CSV and forget about it. If you really wanted to get cute, you could put a 1HP motor on a 3/4HP pump end (which would greatly lighten the load on the motor and keep it away from operating in SF amp draw). We do this regularly, especially on larger pumps/motors that are running at/near SF due to pump design.

All the pump manufacturers brag these days about the higher flow rates they can achieve with the same HP....but all they are doing is overloading the motors and running them at the peak of SF amps all the time....greatly reducing motor life.

I own my own company and could have anything I want at my house...so I'll tell you what I installed.
I have open loop geo system.....(2) 3 ton heat pumps with 2 stage compressors. I ignore the stage of the compressor and give the heat pump the same flow any time it runs...there really is no need to over complicate things and vary the water flow rate any on and open loop since the water temp is always constant. I have a 5 gpm dole valve on each heat pump, so they are getting 1.66 gpm/ton. I am running a Franklin Series V 1.5HP 20 gpm well pump. (static water level 160', pumping level at 20gpm is 165'). I have a 50 psi CSV and a 20 gallon tank. This system also feeds several outdoor hydrants and an outbuilding with bath and small kitchen. I could get by with a 1HP 10, but I wanted to have good flow on the hydrants with the heat pumps running. Have been here 6 years with no problems yet.

 

[Pawpaw]

The reason I'm asking is because (as Valveman pointed out here), pump lifespan (& hence cost) could be considerably different between various solutions to my short-cycling issue. I expected the complete answer would be "it's complicated", however, without even a rough way to predict pump lifespan, I'm not sure how to compare the larger-pressure-tank option versus the new-pump-plus-CSV option.

If you really wanted to get cute, you could put a 1HP motor on a 3/4HP pump end (which would greatly lighten the load on the motor and keep it away from operating in SF amp draw). We do this regularly, especially on larger pumps/motors that are running at/near SF due to pump design.

All the pump manufacturers brag these days about the higher flow rates they can achieve with the same HP....but all they are doing is overloading the motors and running them at the peak of SF amps all the time....greatly reducing motor life.

Interesting, so you're essentially creating the higher lifespan pump of yesteryear. Motor efficiency would be sacrificed when running the bigger motor with lighter load, right? Still, I can imagine there are situations where the longer lifespan outweighs the higher electricity costs. You said you do this primarily on larger pumps; are they specifically pumps that have long runtimes or frequent starts? Are they commercial/industrial pumps where downtime is very costly?

 

[Valveman]

Cycle Stop Valves have been around for about 28 years. Most of the pumps installed with CSV's back then are still running. There were valves similar to the CSV years earlier. However, the pump manufacturers who made the Aqua-Genie and Hydroservant stopped making them because it was cutting into their pump replacement business. Here is a picture of a pump system with a Hydroservant that someone posted and said the pump lasted 52 years. The CSV is even easier on the pump than the Hydroservant or Aqua Genie. That is why pump manufacturers do not sell Cycle Stop Valves and will do anything they can to keep you from using one.

 

[Valveman]

Submersible motors have been shortened up and cheapened up to try and more closely follow the 7 year obsolescence the way they plan. A 1HP motor is now shorter than a 3/4HP of the past. Putting a 1HP motor on a 3/4HP pump end does not make it draw a 1HP load. It will still draw a 3/4HP load, the motor will just run cooler.

 

[VAWellDriller]

The reason I'm asking is because (as Valveman pointed out here), pump lifespan (& hence cost) could be considerably different between various solutions to my short-cycling issue. I expected the complete answer would be "it's complicated", however, without even a rough way to predict pump lifespan, I'm not sure how to compare the larger-pressure-tank option versus the new-pump-plus-CSV option.


Interesting, so you're essentially creating the higher lifespan pump of yesteryear. Motor efficiency would be sacrificed when running the bigger motor with lighter load, right? Still, I can imagine there are situations where the longer lifespan outweighs the higher electricity costs. You said you do this primarily on larger pumps; are they specifically pumps that have long runtimes or frequent starts? Are they commercial/industrial pumps where downtime is very costly?

Yes, normally on larger pumps....where getting everything just right is all that more important. They are public water supply or industrial, and most either run all the time, or have a handful of cycles per day. Just installed one last week, and put a 50HP wet end with a 60 HP motor. It's so important to know the conditions of the well; the pumping level at design flow. In this case, due to well conditions and available wet ends, the 60 HP motor was drawing the same power as the 50. We are running the 60 HP at 78 amps which is just under published full load conditions. With the same wet end a 50 HP would also draw 78 amps and be at the maximum of its rated service factor (and be doing well over a 50HP calculated load). Time will tell, but its my opinion that the slightly underloaded 60 will last much longer than the overloaded 50. For larger pumps, a HP demand curve is available, which will show you exactly how much you load the motor.

If you knew your pumping level, you may realize that a 1/2 HP will do your job just fine....and really on something so small, we are talking peanuts of savings from one pump/motor to the next....so longevity is key. Simple is better, put in the CSV and forget about it.

 

[Valveman]

Like VA says putting a smaller HP pump end on a larger motor makes it draw lower than max amps and run cooler. This is called "de-rating" the motor load. This is also done when pumping hot water and is described in pump/motor manuals under "hot water applications". De-rating the motor load and making the motor run cooler is also what a CSV does, which is one reason the CSV makes pump/motors last longer as well.