Energy costs are very high and are likely to remain so. Whether you are building a new irrigation system or using an old one, there are steps you can take to make sure that water is pumped for the least energy cost possible.
Pump basics
Many nurseries use horizontal-centrifugal pumps that pump from surface-water supplies. If the motor is electric, it is usually a close-coupled unit. When buying a new pump and motor, specify that it be three-phase, 460- (440-) volt. This uses less electricity than a three-phase 230- (220-) volt motor because 460 volt uses half the amperage of a 230-volt motor. Some electricity suppliers dispute this, but I was taught this many years ago by an electrical engineer who worked for Oklahoma Gas & Electric Co.
Some nurseries don’t have three-phase power available. This should always be a criterion when selecting a nursery site. If three-phase power is unavailable, you are confined to smaller electric pumps at a higher operating cost. Phase converters are available, but each costs almost as much as the pumping unit itself. Or you can use internal-combustion motors to power your pumps.
If you use horizontal-centrifugal pumps (whatever size and type), several factors have an effect on efficiency. First, always use an intake pipe or hose at least one pipe size larger than the pump-intake port. Two sizes larger is even better. This greatly reduces friction loss and increases the efficiency of the pump impeller. You should always have at least a 3-foot straight run of pipe, without any fittings or other restrictions, directly into the pump-intake port. This calms turbulence in the water stream before it enters the pump. Water turbulence entering the impeller makes the pump less efficient.
Choose a reducer
Since the intake pipe should be larger than the pump-intake port, use an eccentric reducer to attach the intake pipe to the pump. This eliminates trapping air at the top of the fitting where the intake pipe enters the pump. Trapped air will cause impeller cavitation, making the pump less efficient, and also causes excessive impeller wear.
An eccentric reducer is one on which all of the offset (or difference in size) is on the topside of the fitting. It is better to use a flanged (bolt-on) reducer than a threaded one. The taper on a flanged reducer is more gradual and smoother. It’s difficult to determine when a threaded eccentric reducer is top dead center and when it’s tight enough to prevent air leaks.
On the discharge port of the pump, use a concentric reducer to immediately step up the discharge pipe to one or two pipe sizes larger than the pump discharge port. This reduces friction loss and energy consumption.
Always buy a pumping unit with flanged ports so you can use flanged fittings on the pump unit. The water stream is more efficient, and, in case of pump trouble, it is far easier to unbolt the flanges and remove the pump for repairs.
If old pumps have threaded ports, they can easily be converted to flanged ports using “companion flanges” in combination with close (all-thread) pipe nipples. Insert the close nipples into the pump intake and discharge ports using pipe dope to eliminate air or water leaks. Companion flanges come in sizes to fit almost any situation.
Since you do not have to worry about trapped air on the discharge side, the companion flange can be screwed direct to the threaded pump port. The size of the discharge pipe used should depend on the distance of the mainline run and the gallons per minute pumped. The larger the pipe, up to a point, the less friction loss you will have. This translates to less pressure loss and energy consumption.
PVC pipe fittings
If you attach PVC pipe to the flanged fittings on your pump, Schedule 80 PVC flanged fittings can be glued to the pipe. They come in fixed holes or the “Van Stone” type. I recommend the Van Stone type because the outer ring of the flange with the bolt holes rotates. This allows easier lineup of the bolt holes if a flange you add is slightly off top dead center.
Always use a full-face, 1/8-inch-thick, red rubber gasket on each flange joint. Thinner gaskets are available but do not offer as good a cushion of safety when tightening PVC flanges. Ring gaskets are difficult to line up. PVC flanges may be cracked if bolts are not tightened evenly, a little at a time. Don’t exceed the manufacturer’s specifications (usually 125 foot-pounds).
When selecting a screen for the intake pipe, the screen needs enough capacity to meet or exceed the capacity of your pump. If you run overhead sprinklers, an intake screen with 1/8-inch holes is a good idea. This will greatly reduce stopped-up sprinkler heads. Most suction screens have larger holes.
Screens can be special ordered from an irrigation dealer since many companies don’t sell direct to the end user. Perfection Sprinkler Co. in
Insist on efficiency
When buying an electric-powered pump, insist on knowing the efficiency curve. Don’t buy a pump that is less than 80-percent efficient. Give your supplier all required information. Here are some data that might be required:
* The lift from water surface to the pump (not more than 10 feet).
* The lift from the pump to the highest point in the system.
* Required gpm.
* Pressure needed at the pump discharge.
Don’t forget to include friction loss through the mainlines to the farthest point of distribution. This is a function of pipe size and gpm.
If you buy a pump powered by an internal-combustion engine, consider two efficiency curves: the pump unit and the engine. Both will have rpms at which they are most efficient. The closer you can match the two efficiency curves, the more efficient your water pumping will be.
If your pumps are old and have had heavy use, have them checked for impeller wear. This can contribute to inefficient operation.
- Ben F. Davis II
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Ben F. Davis II operates Davis Nursery Consulting in Grand Saline, Texas, (903) 962-7712; b37p41d@earthlink.net.
April 2008