Posted by Pierce Kiltoff CWD/PI on 2/5/2011
to How To
Submersible pump curves are how we determine the performance of a pump in a variety of situations. Learning to properly read pump curves will tell you whether a pump will last in a situation, what its performance will actually be in that situation, and whether or not it will develop the flow/pressure required for your intended use.
Pressure vs Flow
The first thing to recognize is that Flow and Pressure, while related, are not mutually inclusive. One could pump 500 Gallons Per Minute ('GPM') at near zero pressure. We can also move 1 gallon per minute at 25,000 Pounds Per Square Inch ('PSI') to cut steel. Most residential pumps are sized to pump about 10 GPM at between 30 and 60 PSI.
Most plumbing codes do not allow pressure to exceed 80 PSI in residential systems.
Feet of Head
Most water based hydraulic systems measure pressure in 'feet of head'. A foot of head represents the pressure at the bottom of a one foot tall column of water - converted, one foot of head equals 0.43 Pounds Per Square Inch ('PSI'), and 1 PSI equals 2.31 feet of head.
Feet of head is how we calculate how much pressure the pump must produce to deliver the pressure at our delivery point. For example, if the pump needs to lift the water 100 feet from the top of the pumping level in the well, plus another 50 feet up a hill, plus up 20 feet to the highest faucet in the house - we know we have to at least make 100+50+20 = 170 feet of head, plus whatever delivery pressure we want. So - if we want the well pump to deliver 60 PSI at the highest faucet, we need 170 feet of head, plus 60 PSI. 60 PSI converts to about 140 feet of head - so we need a total of 140+170 feet, or 310 feet of head.
There are a couple of ways to determine your flow rate for any given type of pump system. Fixture counts, animal consumption, peak flow requirements, etc.
If you're trying to determine flow rates required for a single family house, it's good to have an idea of your total daily usage (typically 100 gallons per day per person in the home), divide this by two to determine peak loads (usually from 6am to 8am, and from 6pm to 8pm). This would mean that for a 4 person home, you'd need to deliver 200 gallons in the morning, and another 200 gallons at night, both times intermittently over a 2 hour period.
Combining the above information with a fixture count can give you a good idea of what your peak flow rate is. A fixture count generally involves adding up all the fixtures, but assuming that some fixtures use slightly more water than others - use a fixture count of 1 for sinks, 1.5 for toilets, 2 for showers, 0.5 for dishwashers, and and 1.5 for laundry machines. By adding together the fixture count, you can get a rough number for the well pump flow rate - for example, if you have a kitchen sink, two bathroom sinks, two toilets, two showers, a dishwasher, and a laundry machine, the flow rate at the head required should equal 12 GPM, ideally.
For irrigation systems, well pumps should be sized to deliver a consistent pressure based on the flow rates of each individual zone. Ideally, a well pump is sized for the well, and then the irrigation is designed around the limitations of the well system. For example, you'd like to install a 30 GPM pump to deliver 40 PSI at the irrigation main, but you won't be able to do this if you have a 15 GPM well, without install cisterns or large storage tanks & booster pumps.
For livestock watering, assuming that your animals consume a certain amount of water per day, and design the pump system around that requirement. The livestock watering requirements are generally well documented, or you can contact your local agricultural department for information.
Reading The Curve
Once you've determine flow rates and head requirements, you can match the two points on the graph and draw a line from each to where they meet on the curve. If the head requirements vs the flow requirements don't line up on the main part of the curve, close to the ideal operating point, try another curve/model of pump - running pumps to the left/right of the curve (for example, running a 10 GPM 1/2 HP pump at full flow with 300 feet of head, or running a 10 GPM 5 HP pump at 15 GPM with no head pressure) can cause premature wear/failure in pump end/motor components. Sometimes you can't find the exact right pump, in which case you can use Orifice Valves/Restrictors to control the flow rate and force a pump to run somewhere else on the curve by choking it back. Generally, all pumps will operate well as long as they have some head pressure and are in the middle of the operation curve.