Threaded Pipe and Pump Connections
Irrigation Craft uses flange or groove connected pumps, except as follows:
- Pumps 5 horsepower and less are often difficult to obtain with flanged or grooved connections. Irrigation Craft diligently searches for small pumps with flanged or grooved connections, but at times Irrigation Craft may be forced to use threaded pumps in those small sizes. In pump sizes larger than 5hp, Irrigation uses pumps with either flanged or grooved connections.
- Line Shaft and Submersible Turbine pumps must have threaded connections due to casing diameter restrictions, therefore all turbine pumps must be thread connected.
Why Irrigation Craft Avoids Threaded Pump Connections
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Most centrifugal pumps have relatively small suction and discharge nozzles as a necessary element of good pump design. Small connection size to a pump results in higher fluid velocities near the pump than in pipe further away from the pump.
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The thread cutting process removes up to one-half of the material thickness of the pipe at the leading edge of the thread. Also, during the thread cutting process, the protective zinc coating (if the pipe is galvanized), is removed at the leading edge of the thread (where the material is thinnest), and on the outside of the pipe where the threads are cut into the pipe.
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High velocity flows corrode and deteriorate pipe faster than low velocity flows.
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Therefore, where corrosion and the rate of pipe deterioration is greatest (at the pump connections), the pipe material is thinnest and corrosion protection has been removed.
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The result is early failure of the threaded pipe connecting to the threaded pump.
Why are Pump Connections Often Relatively Small ?
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Low Specific Speed (Ns < ~8000) pump connections are usually smaller than the pipe system connected to the pump.
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Suction nozzle size is determined by impeller design, particularly the diameter of the impeller eye. Fluid flow into the impeller eye must be a smooth laminar flow.
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Discharge nozzle size is a function primarily of impeller and volute design. Volute discharge nozzle size may be determined by the pump designer's attempts to control the following: head flow curve geometry, discharge recirculation and cavitation, radial thrust, pump efficiency, and more.
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Suction and Discharge nozzle size is NOT the manufacturer's recommended pipe manifold size, and in many or most installations, a quality installation increases pipe sizes a few feet before and after a pump connection.
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General exceptions (large connection sizes), are pumps with diffusers (i.e. self priming and multistage), pumps with circumferential flow (i.e. multistage turbines), high specific speed pumps (Ns > ~8000), and high suction specific speed pumps (Nss > ~9000).
