Surface Water Pumps: Preventing Cavitation and Noise
©2003 Conergy
The most economical DC water pumps are surface (non-submersible) pumps. They can reduce pumping energy consumption by half or more, compared to conventional AC pumps. This article will help you to prevent a common problem that all pumps are susceptible to. The problem occurs primarily when a surface pump is mounted above the water source.
Excessive suction causes cavitation, which is the formation and collapse of bubbles. When water pressure is reduced beyond a critical point, water vapor and/or dissolved gases are released, like when you open a carbonated beverage. When a bubble reaches the pressure side of the pump, gas returns to the liquid state. Bubbles collapse in sudden implosion. This causes water to impact violently, like tiny hammer blows, against the working surfaces of the pump. Cavitation causes loud noise and excessive pump wear.
Cavitation is not the fault of the pump, but of the installation. To prevent cavitation, follow these precautions:
- Refer to the pump's specification sheet and instructions, and observe the limits of vertical suction lift.
- If feasible, eliminate suction lift by placing the pump below the water source. Otherwise, minimize the vertical distance from the source to the pump.
- Think of an intake line as a drain line in reverse. The water must flow easily.
- Use large intake pipe (larger than the pump's intake port). This is especially critical in cases of long intake piping. (See Pipe Sizing Chart)
- Avoid 90° elbows. Use pairs of 45° elbows to reduce friction.
- Carefully choose intake screens or intake filters for low friction, and make sure that they will be easy to clean.
- Work carefully to minimize the possibility of air leaks.
- Avoid creating high-points in the intake pipe. They can trap bubbles that will restrict the flow (like in a siphon). If a high-point cannot be avoided, install a pipe Tee at the highest point, with a cap or a ball valve above it. When water is poured in at the high point, it will displace all of the air to fully prime the intake line. This may need to be done periodically.
Most plumbers work only with pressurized distribution and are not aware of the critical nature of intake piping to a pump. Do not allow a plumber to install intake piping unless s/he reads this article or the pump's instructions.