Published on November 6, 2007
Pump Types and Pump Selection: Pump Types and Pump Selection Well Types: Well Types There are basically two types of wells: Shallow wells in which water must be lifted vertically 25 feet or less. Deep wells where vertical lift is greater than 25 feet. Ponds and rivers are generally utilized similar to a shallow well. Pump Types: Pump Types Non- positive displacement pumps have an impeller which spins to create pressure but does not have close tolerance to the casing.(1) This type of pump will not build pressure beyond its normal rating. Pump Types: Pump Types Positive displacement pumps use gears, pistons or helical rotors with tight tolerance to the casing so that pressure can build up beyond normal rating.(1) Pump Types: Pump Types Pressure relief valves are necessary in these systems to avoid system damage. Both positive displacement pumps and non-positive displacement pumps can be: Shallow or deep well Single or multi-stage Surface mounted or submersible Electric motor or engine driven Pump Types: Pump Types Pumps may also be single stage (meaning one impeller in one case) or multi-stage meaning two or more impellers enclosed in one casing to increase pressure like pumps connected in series.(1) Submersible pumps are designed to spend most of there life underwater, only being pulled out every several years for routine maintenance. Pump Types: Pump Types Surface mounted pumps have motors which are above ground- although deep well types may have pump parts hundreds of feet below the surface. Centrifugal pumps operate by spinning an impeller at rapid velocity in a circular case forcing water out one side of the case. The water being forced out one side creates a partial vacuum or suction on the other side drawing in more water. Pump Types: Pump Types Velocity of water being discharged is determined by the size of the casing and impeller as well as the speed of the impeller spinning inside the case. Velocity is used to increase pressure. A turbine pump is basically a series of centrifugal pumps all on one shaft. Pump Types: Pump Types Each separate section is called a stage the greater the number of stages, the greater the pressure or lift that is created. http://www.irrigationtutorials.com/pump.htm Pumps create flow by reducing atmospheric pressure on water (by creating a vacuum) Slide14: Turbine pump with wet well Slide17: Horizontal Centrifugal Pump Pump Types: Pump Types Other pump types less common in irrigation include: Helical rotor pumps which force water through with an auger type action. Reciprocating pumps (piston pumps) that force water out of a cylinder that looks like a piston from an engine. Rotary pumps which move water by forcing it between interlocking gears. Cavitation: Cavitation Cavitation occurs when a void forms within a pump when discharge of water is faster than intake. Air is taken into pump Cavitation can usually be heard- sounds like rocks passing through pump Cavitation: Cavitation Cavitation can lead to erosion or pitting of pump and volute if not corrected. NPSH — Net positive suction head. This is the pressure a pump requires for cavitation-free operation. Cavitation: Cavitation For example, to prevent cavitation, water must enter and exit a pump at the same pressure so that it enters and exits at the same speed. This prevents vaporization of the water as it enters the pump, thus preventing cavitation. Available NPSH is the pressure that exists at the suction inlet of the pump (which is dictated by the site, not the pump). Required NPSH is the pressure needed for pump operation (it varies by pump model). For a pump to operate correctly, available NPSH must be greater than the pump's required NPSH. Pumps Connected in Series: Pumps Connected in Series When pumps are connected together in series (or multi-staged) the total flow (GPM) will stay the same, while the pressures generated by the pumps will be additive. Example 3 pumps are connected in series, each one pumping 30 GPM at 30 PSI. What is the total output in flow volume and pressure? Total flow = 30 GPM, Total pressure =30+30+30= 90 PSI Pumps Connected in Parallel: Pumps Connected in Parallel When pumps are connected in parallel, the pressure stays the same, while the flow volume is additive. Example 3 pumps are connected in parallel, each one pumping 30 GPM at 30 PSI. What is the total output in flow volume and pressure? Total pressure = 30 PSI, Total flow =30+30+30= 90 GPM Booster Pumps and Jockey Pumps: Booster Pumps and Jockey Pumps Booster pumps are in line pumps (series connection) used to increase the operating pressure of the system. A jockey pump is a smaller pump which is used to refill a tank or otherwise maintain a lower flow for an extended period so large pumps do not cycle on and off. Pressure Tanks: Pressure Tanks Pressure tanks are often utilized to act as a pressure reservoir so that the pump doesn’t need to run each time there is a small demand for water. Pressure tanks utilize air trapped inside the tank to keep pressure on the water supply. Pressure Tanks: Pressure Tanks If there is too great a demand on water for a long period and systems pressure drops, the pressure tank can become water-logged (not enough air space) and the tank will not function properly. Many newer pressure tanks now have air bladders which are inside the tank, and will not allow the tank to become waterlogged. Pressure Tanks: Pressure Tanks Designing a pressure tank that will function correctly into a system requires the assistance of a expert in pump system design. Systems designed to operate without pressure tanks make use of pressure relief valves(Cla-val is the most widely used brand) that allow for by-pass of the extra water when low flow demand is created. Excess water pumped back into the reservoir so that pump cycles are moderated. Pump System Categories: Pump System Categories Bypass pumping systems have one or more pumps with fixed speed motors and a pressure relieving bypass that allows excess water to flow back into a wet well, or back into the suction source.(1) Pump System Categories: Pump System Categories The disadvantages of bypass systems are: Water hammer is likely to cause some damage to the system because pressure relief devices take some time to respond allowing some surge to occur prior to opening. (1) Much energy may be wasted whenever the bypass opens because water is being recycled and not pumped out. (1) Unless water use can be scheduled very precisely to keep demand constant and near pump capacity much energy will go into bypass water each day. Pump System Categories: Pump System Categories The conventional pumping system makes use of constant speed motors with horizontal centrifugal pumps or vertical turbine pumps that produce a constant volume of water at a constant pressure, with a control valve to maintain constant pressure as water volume demand changes.(1) Most larger systems also have multiple pumps that switch on and of as demand changes. Conventional Pump System Operation: Conventional Pump System Operation “LOGIC DIAGRAM: Irrigation begins Control valve detects downstream pressure drop and begins to open, releasing pressure from hydropneumatic tank. Pump Log Control (PLC) , via the pressure transducer detects upstream pressure dropping below normal and starts jockey pump. If after a preset time the upstream pressure does not return to a preset minimum the PLC starts the second pump. Pump station continues to start additional pumps as needed. Conventional Pump System Operation: Conventional Pump System Operation Irrigation ends. As flow decreases PLC sequentially cycles off pumps based on flow signal. Control valve continuously and independently opens and closes as required to maintain constant downstream pressure.” (1) Variable Frequency Drives: Variable Frequency Drives Variable frequency drives (VFDs) are becoming more and more popular in irrigation. VFDs, change output volume and pressure by changing the speed at which the motor runs to to maintain pressure and match demand for water. VFDs have the advantage of using electricity most efficiently, and the ability of using a single pump to meet varying demand levels effectively. Initial costs are much higher however. References: References 1) Edward S. Pira. A Guide to Golf Course Irrigation Design and Drainage. Chelsea, MI: Sleeping Bear Press, 1997. 2) Landscape Irrigation Design Manual. Rainbird Sprinkler Manufacturing Corporation,1998.