The Pulsation Problem
Positive-displacement pumps — reciprocating, diaphragm, gear, and peristaltic types — produce pulsating flow rather than the steady flow of centrifugal pumps. Each pump stroke creates a pressure and velocity surge that causes the flow switch paddle to oscillate rapidly.
Without damping, this oscillation causes the micro-switch to chatter — rapidly opening and closing — which generates false alarms, wears out the micro-switch prematurely, and can damage downstream control equipment.
Damping Options — Which to Choose
Dash Pot
DetailsUse when: Moderate pulsation; delay period of 5–30 seconds required
A hydraulic dashpot attached to the paddle stem provides time-delayed actuation. The paddle must deflect and hold for the full delay period before the switch actuates — filtering out brief surges.
Stem Dampener
DetailsUse when: Severe pulsation; immediate response still required
A mechanical dampener acts directly on the paddle stem to absorb rapid oscillations without introducing a time delay. Preferred when the system needs fast response to a genuine no-flow condition.
Turbulent Flow / Hinged Connection
DetailsUse when: Violent, turbulent, or cavitating flow
A hinged paddle connection allows 90° of free travel, preventing the paddle from being damaged by violent flow surges while still detecting the presence or absence of flow.
Pump Protection — Dry-Run Detection
Beyond pulsation, flow switches are commonly used to protect pumps from dry-run conditions. Install the flow switch on the pump suction or discharge line to detect loss of prime. Wire the switch to shut down the pump motor when flow drops below the actuation threshold.
For pump discharge protection, install the switch at least 10 pipe diameters downstream of the pump discharge flange to allow the turbulent discharge flow to stabilize before reaching the paddle.