Error 3
If a pump intake line is restricted, clogged, or capped off, the pump motor draws high current (the motor is under heavy load).
Answer
For pumps with Specific Speeds less than about Ns = 6000 and possibly higher, all pump intake problems cause a reduction in motor load, resulting in a reduction in amps drawn by the motor. This range of specific speeds covers most pumps less than 30 horsepower used in irrigation (excluding flood irrigation), pressure boosters for potable and re-use water. This fact can be used to diagnose low pressure problems.
- Connect an amp meter to the motor.
- Operate the system and create a high flow demand.
- Eliminate the possibility of mechanical interference. Any mechanical interference within the rotating assembly (including bearings, shaft, windings, and pump impeller) can cause high current draw and will make a loud noise that can be easily traced and located, (ignore cavitation sounds).
- If the amp meter reveals that the motor is drawing close to maximum current (FLA), and there are no mechanical interferences, then the low pressure problem is due to excessive flow in the field. Current draw close to the motor FLA proves that the intake line is working properly and providing sufficient water to allow the pump to work hard, which is why the motor draws maximum or close to maximum amperes.
- If the amp meter reveals that current is well below FLA, then the problem is either a restricted discharge or an intake problem.
- Prove that the discharge is not restricted by opening valves in the field or system. If water flow is normal then the discharge is not restricted.
- If the discharge is not restricted, then you have solid evidence perhaps even final proof that there is an intake line problem (restricted or suction leak).
If you prove that the intake line is the source of the low pressure problem you have just partially diagnosed the problem by the "divide & conquer" method. By eliminating high flow as a possible cause, you now have the pump intake and intake line as the only possible source of the problem.
Related Subjects on this Website:
Glossary - Best Efficiency Point (BEP)
Glossary - Power Characteristic
How Fluids Move Through Impellers
