A positive displacement pump is turning at the expected speed, but the flowmeter shows less product reaching the process than the displacement calculation predicts. That gap doesn’t automatically mean the pump is worn out. It may reflect normal internal leakage, changed fluid properties, incomplete filling, or flow returning through a bypass.
Positive displacement pump slip is fluid leaking internally from the higher-pressure side toward the lower-pressure side instead of reaching the discharge. In pumps with running clearances, slip generally increases as differential pressure rises and decreases as viscosity rises. That relationship explains why a fixed-speed pump can deliver different flow rates on different products—or on the same product at different temperatures.
The practical question is whether the missing flow is expected for the operating conditions or evidence of a problem.
Theoretical Displacement Versus Delivered Flow
For a rotary positive displacement pump, theoretical flow comes from the volume displaced per revolution multiplied by shaft speed:
Theoretical flow = displacement per revolution × revolutions per unit time
Use consistent units. Displacement in gallons per revolution multiplied by revolutions per minute gives gallons per minute. Use pump shaft speed, not motor nameplate speed when a gearbox or other drive changes the ratio.
For a fully filled pump handling a substantially incompressible liquid, with no external bypass flow:
Delivered flow ≈ theoretical flow − internal slip
Volumetric efficiency expresses the relationship:
Volumetric efficiency = delivered flow ÷ theoretical flow × 100%
This is not overall pump efficiency. It doesn’t account for mechanical friction, viscous drag, or motor losses.
Also, a calculated flow deficit isn’t proof of internal slip. Entrained gas, incomplete chamber filling, leaking check valves, bypass flow, and measurement error can all produce a similar result. Where the flowmeter sits matters: downstream of a return branch, it measures process delivery rather than total pump discharge.
Why Higher Pressure Produces More Slip
Gear, screw, and rotary lobe pumps move liquid through spaces formed by their pumping elements and casing. Running clearances allow those parts to operate without unwanted contact. Those same clearances provide leakage paths.
The pressure difference across the pump drives liquid backward through those paths. With viscosity, speed, and clearances otherwise unchanged, increasing differential pressure generally increases leakage and reduces net delivery.
Differential pressure means discharge pressure minus suction pressure—not discharge pressure alone. A pump drawing from a pressurized vessel operates differently from one drawing under vacuum, even when their discharge gauges read the same.
A loading filter, partially closed valve, or higher receiving-vessel pressure can raise discharge resistance. The pump builds more pressure to move the liquid, and slip may increase. If a relief valve begins opening, delivered flow can fall much more sharply because liquid now has a separate return path.
That distinction matters. Internal slip and relief-valve bypass are different problems, even if both show up as low process flow.
Never close the discharge to test a positive displacement pump. Appropriate overpressure protection is required; ordinary flow controls are not a substitute.
Why Viscosity Changes the Answer
A thinner liquid passes through internal clearances more readily than a thicker one. Under comparable operating conditions, lower viscosity usually means more slip. Higher viscosity usually reduces internal leakage.
This is why a water-based performance test may not represent service on a viscous product. Conversely, a pump selected around a thick product may deliver less than expected during a thin-product campaign.
Temperature can change flow without changing speed
For many liquids, viscosity decreases as temperature rises. A heated product may therefore leak backward through the pump more readily, reducing delivered flow at the same speed and differential pressure.
Consider a hypothetical Memphis blending operation transferring a temperature-sensitive liquid from an outdoor tank. Summer product temperatures could lower viscosity enough to change delivery. In colder weather, reduced slip might help volumetric performance—but thicker product could create excessive suction losses instead. Actual product temperature and viscosity are more useful than the weather forecast.
Thicker liquid doesn’t always mean more delivered flow
Higher viscosity also increases resistance through suction piping, strainers, fittings, and valves. If liquid cannot enter fast enough, pumping chambers won’t fill completely. Flow then falls even though internal slip may be lower.
That’s an inlet-filling problem, not evidence that viscosity increased leakage. Review inlet pressure, available net positive suction head against manufacturer requirements, and any technology-specific inlet limitations. Thick liquids can also raise torque and power requirements.
For shear-thinning or other non-Newtonian products, one viscosity number may be inadequate. Selection should reflect temperature, shear behavior, and the operating range—not just a laboratory value taken under unrelated conditions.
Wear, Speed, and Pump Design Change Volumetric Performance
Wear can enlarge internal leakage paths. Abrasive solids, corrosion, or contact caused by unsuitable operating conditions may increase clearances and reduce delivery. Leakage can be highly sensitive to clearance changes, so a modest dimensional change may produce a noticeable performance loss.
However, comparing today’s hot, thin product with last month’s cold, thick product won’t establish wear. Compare performance at similar speed, differential pressure, temperature, and viscosity.
Speed also matters. Theoretical displacement increases with shaft speed, while slip doesn’t necessarily increase in the same proportion. At low speed, leakage can represent a larger fraction of theoretical flow. Increasing speed may recover output, but it can also exceed inlet-filling, torque, wear, or manufacturer speed limits.
Not every positive displacement design follows the same leakage model:
Gear, screw, and rotary lobe pumps: Running-clearance leakage is a major part of their pressure-flow behavior.
Progressive cavity pumps: Rotor-stator sealing, elastomer condition, interference, pressure per stage, and fluid properties affect slip.
Diaphragm pumps: Check-valve sealing, chamber filling, and actual cycling conditions influence delivered volume.
Peristaltic pumps: Tube or hose occlusion, recovery, wear, and pressure-related deformation influence delivery rather than ordinary metal running-clearance leakage.
Use current manufacturer performance data for the actual pump design. There is no universal “normal slip percentage” for all positive displacement pumps.
How to Investigate Missing Flow Without Guessing
Start with a simultaneous operating snapshot. Measurements collected across different batches or valve positions can point the investigation in the wrong direction.
Verify actual speed and measurement quality. Check pump shaft speed through approved methods. Confirm the flowmeter suits the liquid, viscosity, gas content, and pulsation present.
Record suction and discharge pressure together. Calculate differential pressure and note gauge locations. Remote readings may include significant piping losses.
Record fluid temperature and properties. Compare against the selection basis. Check for dilution, recipe changes, entrained air, and solids.
Review inlet conditions. Look at tank level, approved valve-position indications, strainer differential pressure, and available suction-pressure readings.
Account for return paths. Review relief valves, recirculation lines, and control bypasses. Don’t assume all displaced liquid reaches the process.
Compare with the correct curve and baseline. A repeatable decline under comparable conditions supports further inspection for wear or internal damage.
New noise, vibration, or unstable flow can support an inlet-condition investigation, but none proves cavitation by itself. Likewise, a normal-looking motor doesn’t establish that the pump is filling properly.
Operating observations must follow facility procedures. Internal inspection, clearance measurement, and component servicing require proper shutdown, isolation, lockout/tagout, depressurization, and chemical-handling precautions. Controlled performance testing belongs with qualified personnel using an approved procedure.
What to Request Before Repairing or Replacing the Pump
Ask the supplier to review delivered flow across the expected pressure, temperature, viscosity, and speed range—not just one rated point. Identify whether published flow is theoretical displacement or tested delivery, and what test liquid and conditions apply.
For variable-speed service, review low-speed volumetric performance as well as high-speed inlet requirements. Flow feedback can compensate for changing delivery within approved operating limits, but it won’t repair worn internals or resolve a restricted suction line.
If pump repair is being considered, request inspection findings tied to manufacturer tolerances. Replacing the same pump repeatedly won’t solve a recurring inlet restriction or a downstream pressure increase.
Bottom Line
Separate internal leakage from incomplete filling, bypass flow, and measurement error before deciding what to change. Then compare delivered flow against manufacturer data at the actual differential pressure, viscosity, temperature, and speed.
Process & Power can help facilities throughout Tennessee, Arkansas, and Mississippi review positive displacement pump performance, investigate the surrounding system, and discuss repair or selection options.
Call Process & Power, Inc. at 901-362-5500 or visit us at 1721 Corporate Avenue, Memphis, TN 38132 for help with industrial air compressors, pumps, blowers, vacuum systems, and compressed air equipment and service throughout Memphis and the surrounding area.
