The Core Function and Mechanism of a Check Valve
To determine the necessity of a check valve in a micro pump system, it is essential to first understand its fundamental role and how it operates.
The Definition and Purpose of a Check Valve
A check valve, also known as a non-return valve or one-way valve, is an automatic valve designed to permit the flow of fluid (liquid or gas) in one direction only, while automatically preventing backflow in the opposite direction.
Its primary functions in a fluid system are:
•To prevent the medium from flowing back toward the pump when the pump is shut off.
•To prevent the reverse rotation of the pump and its motor.
•To maintain system pressure or fluid column integrity.
The Operating Principle of a Check Valve
The operation of a check valve relies on the pressure differential across the valve. The valve opens when the upstream pressure exceeds the downstream pressure and the valve's cracking pressure (the minimum pressure required to open the valve). When the flow stops or reverses, the back pressure, combined with gravity or a spring mechanism, forces the valve to close, thereby stopping the backflow.
Differentiating Check Valve Needs by Micro Pump Type
The requirement for an external check valve is highly dependent on the inherent characteristics of the micro pump being used.
Positive Displacement Pumps (Diaphragm and Peristaltic)
These pump types are generally designed with a degree of internal sealing:
•Diaphragm Pumps: Micro diaphragm pumps typically incorporate internal check valves (such as duckbill or umbrella valves) at the inlet and outlet ports to control the flow direction. For most standard applications, these internal valves are sufficient, and an external check valve is not required.
•Peristaltic Pumps: Peristaltic pumps achieve flow by completely occluding the flexible tubing. This mechanism provides inherent self-sealing capability, making them highly resistant to backflow. Consequently, external check valves are usually unnecessary for peristaltic pumps unless the system requires extremely high back-pressure resistance.
Dynamic Pumps (Micro Centrifugal Pumps)
Micro centrifugal pumps operate differently and lack inherent sealing:
•Centrifugal Pump Characteristics: When a micro centrifugal pump stops, it does not create a seal. Fluid in the discharge line will immediately flow back due to gravity or system pressure.
•Necessity: In applications where the discharge point is significantly higher than the pump (high head) or where the fluid column must be maintained for priming, an external check valve is typically required on the discharge line to prevent backflow and siphoning.
Four Application Scenarios Determining Check Valve Installation
The decision to install an external check valve should be based on specific system requirements rather than a blanket rule.
Consideration 1: Preventing Siphoning and Maintaining Prime
•Scenario: If the pump's discharge point is lower than the fluid source, siphoning can occur when the pump is off. For non-self-priming pumps, losing the fluid column (prime) prevents the pump from restarting correctly.
•Solution: Installing a check valve on the discharge line prevents the fluid from draining back, thereby maintaining the pump's prime and preventing unwanted siphoning.
Consideration 2: System Pressure Maintenance and Protection
•Scenario: Applications involving pressure accumulators, dosing systems, or pressure-sensitive components require the system pressure to be held constant even when the pump is inactive.
•Solution: A check valve installed between the pump outlet and the pressure-holding component ensures that the system pressure is not released back through the pump mechanism.
Consideration 3: Multi-Pump Parallel or Series Operation
•Scenario: When multiple micro pumps operate in parallel to increase flow, a higher-pressure pump can force fluid back into a lower-pressure pump, potentially damaging it or disrupting flow.
•Solution: Installing a check valve immediately after the discharge of each pump isolates the pressure and flow of individual pumps, ensuring stable operation of the parallel system.
Consideration 4: Protecting Against Water Hammer
•Scenario: While less common in micro systems, rapid flow reversal can cause pressure surges (water hammer).
•Solution: A properly selected check valve can mitigate the effects of sudden flow stoppage, protecting the pump and sensitive downstream components.
Check Valve Selection Criteria
When an external check valve is deemed necessary, careful selection is crucial to avoid introducing unnecessary resistance to the micro pump system.
Three Critical Technical Parameters for Selection
•Cracking Pressure: For micro pump systems, select a check valve with the lowest possible cracking pressure (often measured in mbar or psi). A high cracking pressure forces the micro pump to work harder, reducing its efficiency and lifespan.
•Material Compatibility: Ensure the valve body and seal materials (e.g., PP, PVDF, Stainless Steel) are chemically compatible with the fluid being transferred.
•Size and Flow Characteristics: The valve's size must match the tubing diameter, and its flow path should minimize pressure drop to maintain the pump's intended output.
Conclusion: Check Valves are a System Design Choice
The question of whether a check valve is needed is not a simple yes or no, but rather a technical decision based on the specific fluid control system. While many micro pumps have internal valves, the need for an external check valve is determined by the application's requirements for backflow prevention, pressure maintenance, and system integrity.