Using Pumps in Groups to Increase Flow
In most pumping situations when a larger flow rate is needed from a pump most designers will select a “bigger” pump. This usually means a pump that has a higher flow capacity. To make a pump produce higher flow the manufacturer will either increase the motor size from, say, a 1/2 horsepower motor to a 3/4 horsepower motor. Or they could combine the bigger motor with a different “wet end” (the part of the pump that moves the water). As the pump motors are increased in size, it may be necessary to increase the voltage requirements from 120 volts to 230 volts, and maybe to a 480-volt three phase motor. But there is a downside to increasing the pump capacity in this way.
First, a 230- or 480-volt pump is not as readily available as a standard 120-volt, 1/2 horsepower pump. When these larger pumps fail, there may be a delay in being able to replace them. Second, the corresponding control and electrical supply requirements are increased, causing the price of the system to also increase. Third, the increased voltage can increase the potential of serious electrical shock (not that 120-volt shock is fun, but from my experience it usually does not cause serious injury). And fourth the increased cost of the pump. So, what to do?
One option is to increase the number of 120-volt pumps that are used. We call this option running pumps in parallel. Instead of running one pump at a time we have multiple pumps run at the same time. The result of this arrangement is that we can double the flow rate with two pumps. What we can’t do is increase the amount of head the pumps can produce. In other words, we can’t increase the height the pump(s) can pump water to. As an example, if a pump can pump water up to a 50-foot elevation height we say that the pump has a 50-foot head capacity. If we attach a 50-foot pipe and point it straight up, no water will come out the end. If the pipe is cut to 49 feet, a little water will come out. Conversely, if we cut the pipe at 5 feet high (a five-foot head requirement) there will be a large flow of water leaving the pipe.
If we connected two identical 50-foot head pumps and turned them both on, we would not get any water out the 50-foot-tall pipe. But, at 49 feet, we would get twice as much water as one pump. The same would be true with the 5-foot pipe. By increasing the number of pumps, we increase the flow proportionally. This can be very helpful with subsurface drip systems. As a rule of thumb, we need to have 1/2 the flow during the dose cycle as the flushing cycles. By alternating the pumps between doses, we have the hydraulic performance we need. When the flush cycle is called for, we can have both pumps run, giving the correct flow for both cycles without over pressurizing during the dose cycle.
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