RUNAWAY SPEED R023-1
Reverse runaway speed occurs when a pump runs in uncontrolled reverse direction due to the reverse flow of the pumping liquid
The most common occurrence is having a power failure while pumping from a deep well with a vertical turbine pump and a discharge valve, check valve or foot valve fails to close when the pump stops.
The pump then acts as a hydraulic turbine.
- If the pump is driven by a prime mover offering little resistance while running backwards, the reverse speed may approach a maximum or runaway speed.
- If the runaway speed exceeds the normal pump speed, it may impose high mechanical stresses on the rotating parts of both pump and driver. These stresses could lead to premature shaft and/or mechanical seal failure
- Water hammer often accompanies this problem.
- Maximum reversed speed appears to increase with efficiency and specific speed
Vertical pump drivers can be equipped with non-reverse ratchets to prevent reverse rotation, but you seldom find them installed
Check valves on the pump discharge and/or suction may help, but unfortunately many of these check valves stick open due to wear, or solids carried in the pumping fluid.