Why Is My Pump Running but Not Pumping? 7 Things to Check
You turn on the pump, the motor is running or the pump is cycling, but little to no fluid is coming out. It’s a common industrial pump problem, and it doesn’t necessarily mean the pump has failed.
A pump can appear to operate normally while something in the suction line, piping, valves, internal components, or application prevents it from moving fluid. Before replacing the pump, work through these seven common causes.
Safety first: Before inspecting or disassembling a pump, shut it down, isolate its power or air supply, relieve system pressure, and follow the manufacturer’s service procedures. When pumping chemicals or hazardous fluids, use the appropriate PPE and review the fluid’s SDS before opening the system.
1. Check That the Pump Has a Fluid Supply
Start with the simplest possibility: is there actually enough liquid reaching the pump?
Check the tank or supply level and make sure the suction inlet is adequately submerged. Low fluid levels can allow the pump to pull in air or create a vortex at the inlet instead of drawing a steady stream of liquid. Also check that any suction-side isolation valves are fully open. A pump can continue running even when its liquid supply has been interrupted, so verifying the source is an easy first step before investigating the pump itself.
2. Make Sure the Pump Is Properly Primed
Some pump designs, particularly centrifugal pumps, must have liquid in the pump casing and suction line before they can create useful flow. If air enters the system or the pump loses prime, the motor may continue spinning while little or no liquid moves.
If your pump requires priming, verify that the casing and suction line are filled according to the manufacturer's instructions. If a pump repeatedly loses prime after startup, don't just keep repriming it. That can indicate an underlying problem such as an air leak, inadequate submergence, excessive suction lift, or a faulty foot or check valve.
3. Inspect the Suction Line for Restrictions or Air Leaks
The pump can't discharge fluid it can't successfully pull in.
Check the suction line for clogged strainers, closed or partially closed valves, kinked hoses, buildup, undersized piping, and other restrictions. Then inspect fittings, gaskets, seals, connections, and hoses for places where air could enter.
A suction-side air leak can be particularly deceptive because it may not leak liquid outward. Instead, the pump pulls air inward while operating. The result can be reduced flow, intermittent pumping, loss of prime, or no flow at all. Suction restrictions and air leaks are common troubleshooting points across several types of pumps.
4. Check for a Blockage Inside the Pump
If liquid can reach the pump but still isn't moving through it, something inside the pump may be obstructed.
What you inspect depends on the pump design. A centrifugal pump may have debris or buildup around its impeller. A diaphragm or other positive-displacement pump may have a check valve that is clogged, stuck, damaged, or unable to seal properly. Fluids that contain solids or that crystallize, harden, or leave residue can make these problems more likely.
If cleaning doesn't solve the problem, worn internal components may need to be replaced. Depending on the pump, this could mean replacing check valves, diaphragms, seals, O-rings, an impeller, or using a manufacturer-specific repair or wet-end kit.
5. Check the Discharge Side of the System
Don't focus exclusively on the inlet. The pump also needs somewhere for the fluid to go.
Confirm that discharge valves are open and look for clogged filters, blocked piping, stuck check valves, excessive system pressure, or another downstream restriction. A pump operating against more pressure or head than it was designed to overcome may produce very little flow even though its driver continues operating.
If the system or piping has recently changed, compare the pump's operating range with the new conditions. The problem may be with the system rather than a failed pump.
6. Look for Cavitation or Poor Suction Conditions
Unusual noise, vibration, fluctuating flow, or a sound sometimes compared to gravel moving through the pump can point toward cavitation.
Cavitation occurs when pressure within the liquid drops low enough for vapor bubbles to form and then collapse. In centrifugal applications, insufficient available NPSH is a common cause. Long or restrictive suction piping, excessive suction lift, high liquid temperature, high flow demand, and partially blocked suction lines can all contribute to poor suction conditions.
Cavitation isn't just a flow problem. If allowed to continue, it can damage pump components, so the underlying suction conditions should be corrected rather than simply allowing the pump to continue running.
7. Check for Worn, Damaged, or Incorrectly Operating Components
If the piping and operating conditions check out, it may be time to look more closely at the pump itself.
Worn impellers, diaphragms, seals, check valves, vanes, or other internal components can reduce the pump's ability to move fluid. Some pump types can also experience little or no flow if rotation is incorrect, speed is too low, or the available air or power supply is insufficient for the application. For AODD pumps specifically, adequate air pressure alone isn't always enough—the pump also needs sufficient air volume to achieve the required cycling rate and flow.
When internal wear is the problem, replacing the affected components or installing the appropriate repair kit can often restore performance without replacing the entire pump.
Still Not Getting Flow?
A running pump with no flow doesn't automatically mean you need a new pump. Start at the fluid source and work through the system: verify supply, prime, suction conditions, internal flow paths, discharge conditions, and finally the pump's internal components.