Well & Pump Design for Energy Efficiency and Mechanical Longevity
Over the last two years we've implemented a policy of pump testing every well we drill. We've found that the normal practice of 'air testing' or 'bailer testing' a well just isn't accurate - we've seen pump testing results that are 350% higher than 'air testing' results. Not only do we determine that wells yield much better results after performing a pump testing, we also have found that proper well testing and pump system design can easily affect the operating costs of the well & the length of time a pump will last in a given environment. Given the high level of costs involved with installing such a system, it's good to have a designer who actually takes the time to make sure the system is set up right from the beginning.
Case Study - New Irrigation Well
Let's say we've just drilled an irrigation well for a home owner that had an irrigation system designed ahead of the well (in this case it worked, normally this is the wrong way to go). The driller on site drills to 100 feet, sets a screen, and 'air tests' the well using his drill rig, determining that the well makes 25 GPM with the air stem set at a depth of 100 feet. Additionally, the Static Water Level (SWL) of the well is 20 feet (this is the depth that the water comes up to when the well is not actively being pumped - see the diagram on the right).
The designed irrigation system runs from 12 AM to 4 AM, every day, using just 25 Gallons Per Minute (GPM), or 6000 gallons per day (actually, this is fairly conservative for an irrigation system). The owner already owns two 90 gallon pressure tanks, which will have a combined total of 53 gallons of usable draw down at the designed system pressure, and a 40/60 PSI pressure switch (the pressure drops to 40 PSI, turns the pump on, and when the pressure comes back to 60 PSI, the switch turns the pump off, and the 'cycle' repeats).
Without Testing The Well....
Not performing a draw down test, someone who only looked at the drill log, would see that the well made 25 GPM at a depth of 100 feet, and would therefore make the assumption that the water in the well would pump down completely at 25 GPM. To meet the 25 gallon per minute flow requirement, one could use a 25 gallon per minute pump, and with a 100 foot well depth & draw down level, this pump would need to have at least a 2 horsepower motor attached - if you make the assumption that the well draws down completely! In other words - if you don't do a pump test, you'll never know that this well really only draws down to 60 feet!
But, Why Does Draw Down Level Matter?
When this well is actually tested, we find that the well only draws down to 60 feet under a 25 GPM load. This means that we're only actually pumping from 60 feet, if we're pumping 25 GPM continuously. With 60 feet of 'head pressure', plus the system pressure of 40 to 60 PSI, we can calculate that the pump only needs to have a 1.5 HP motor!
1.5 HP vs 2 HP
All pumps operate on a specific 'Pump Curve', which is a measure of the amount of pressure the well pump can produce versus the amount of flow it can produce. Generally speaking, pumps prodoce lots of pressure, or lots of flow, but not both without increasing the Horse Power requirements. In this case, we're looking at a 25 GPM pump with either a 1.5 HP motor, or a 2 HP motor. A 1.5 HP motor uses approximately 1660 Watts (1.66 KWH) for every hour of run time, while a 2 HP motor uses 2060 Watts (2.06 KWH) for every hour of run time.
Given the pressure tank and switch the owner wants to use, once the pressure drops to 40 PSI, the 25 GPM 2 HP pump will turn on and run for 103 seconds, then it will shut off for 128 seconds, before repeating the cycle. During the 4 hour irrigation period, this 2 HP pump would go through a total of 63 cycles, running a total of 108 minutes, and be off for 132 minutes. The pump would use a total of 3.708 kilowatts, which results in $0.38 of electrical costs (assuming $0.10 per kilowatt hour).
If the well is pump tested and the results are that the water level only falls to 60 feet under a 25 GPM load, we can then install a 1.5 HP pump instead of a 2 HP pump, resulting in a lower cost in electricity. For example - the 1.5 HP pump would run a total of 114 seconds, and be off for 128 seconds, per cycle. This means the pump is a full 1/2 HP smaller than the 2 HP pump and only runs for an additional 11 seconds per cycle. The 1.5 HP pump would run an approximate total of 60 cycles, 114 minutes of running time, and 128 minutes of off time. The result is that this pump would use only 3.154 KWH, equivalent to $0.32 in electrical costs, per irrigation cycle.
This all results in a difference of $21.90 per year in electrical costs. Also, the 1.5 HP pump would see 3 less start cycles per evening, resulting in the motor theoretically starting 1095 fewer times per year, a 5% difference in life time expectency. Obviously, if you're just using water in the house, there is less water being used, and less cost savings in power.
