Pressure Drop After Pump Shutdown? Distinguishing Between Leakage and Backflow in NIDEC Diaphragm Pumps
In real-world applications of diaphragm pumps, we frequently receive similar feedback from customers: after pressurizing a sealed container to the target value and shutting down the pump, a gradual pressure drop is observed within the container, leading to suspicions of an air leakage defect in the pump itself. In fact, pressure decay does not necessarily indicate a product defect. Beyond "leakage" caused by seal failure, the structural "backflow" inherent to diaphragm pumps can also result in post-shutdown pressure loss. To help users accurately distinguish between these two scenarios with vastly different root causes—and to prevent misjudgments that could impact production or user experience—this article systematically outlines the essential differences, diagnostic logic, and solutions for leakage versus backflow.
Leakage (Product Quality Issue)
Definition: Refers to gas leakage caused by seal failure in the pump body itself, typically occurring at the pump body seals (indicated by the blue arrow in the figure above).
Acceptance Criteria: NIDEC factories implement 100% automated inspection for this parameter: a 500 ml container is pressurized to 40 kPa, and if the pressure drop is ≥26 Pa within 3.5 seconds, the unit is rejected as non-conforming.
Control Measures: The factory manages this via standardized leak inspection procedures. Non-conforming products are placed in designated red-labeled reject bins, and any anomalies are escalated through the proper reporting channels.
Backflow (Inherent Pump Characteristic, Not a Quality Issue)
Root Cause: NIDEC diaphragm pumps feature three umbrella valves distributed at 120° intervals. At any given moment, one valve is closed while another is open. Although these valves function similarly to check valves, they enter a state of relative equilibrium and lose control when the differential pressure between the inlet and outlet is low. This results in backflow (indicated by the red arrow in the figure above).
Characteristics: Backflow is intermittent and unpredictable. It occurs more rapidly under low differential pressure and slows down as the differential pressure increases.
Recommended Solutions: As this is an inherent structural characteristic of the pump, it can be mitigated by installing an additional check valve or solenoid valve in the external pneumatic circuit.
Core Differences
| Dimension | Leakage | Backflow |
|---|---|---|
| Nature | Seal defect; a product quality issue | Inherent structural characteristic; not a quality issue |
| Root Cause | Failure of seals (e.g., O-rings) | Loss of control of umbrella valves under low differential pressure |
| Diagnosis | Quantifiable rejection criteria; 100% factory inspection | No quality rejection criteria; a normal physical phenomenon |
| Solution | Replace with a conforming product | Install a check valve or solenoid valve in the line |