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Integrating components such as isolation valves into plastic manifolds or other products is integral for achieving precise fluid control and enhancing system functionality. This chapter explores various valve types, including rocker and diaphragm valves, and offers insights into their features and applications to assist in effective plastic design.
In this section, we will review the different valve types that plastic products can use to enhance function. There are two main types of media isolation valves: rocker and diaphragm varieties. Rocker style valves are typically smaller and have the ability to manifold mount much easier and as such have become a standard in the industry for bonded manifold applications.
ISOLATION VALVE BASICS
Isolation valves separate the activation mechanisms from the moving media. They can be configured as simple 2-way devices or as 3-port selector/diverters. Engineers typically use these in applications where they need a simple on/off function or diversion of liquid flow rather than the ability to gradually modulate flow.
A few key concepts describe important considerations for these isolation valves:
Dead volume: The volume inside the valve that cannot be flushed during normal operations. Minimizing or eliminating dead volume is essential in applications where cross-contamination is an issue, such as drawing diagnostic samples from several patients.
Internal Volume: The volume trapped inside the valve assembly when the valve is closed.
Swept volume: The volume of the flow path within the valve assembly. A streamlined flow path where swept volume is equal to internal volume means zero dead volume.
Wetted materials: Any material that contacts the media flowing through the valve. Selecting appropriate materials that contact the media flowing through the valve ensures compatibility and longevity.
A rocker isolation valve is a solenoid operated device that uses a pivoting rocker mechanism to seal the valve seat and isolates the flow path. Rocker valves can be configured as simple two-way way devices or as multi-port selectors/diverters.
Rocker valves are generally smaller and more compact than diaphragm valves, making them well suited for applications with space limitations and manifold mounting.
Other benefits include low internal volumes and fast actuation times, along with relatively low costs.
Rocker style valves are available from several global suppliers. Some provide a full range of orifice/valve sizes with standard and optional features like power-saving capabilities, LEDs, plug-in electrical connectors, and more.
Built-in Power Savings Circuit
If the designer expects the solenoid valves to be continuously energized for extended periods of time, we recommend to use valves with power saving circuits to minimize the amount of heat released by the coil.
Plug Connector with Light/Surge Voltage Suppressor
Using the incorporated LEDs and plug-in option for media isolation valves integrates the electrical harness directly to the solenoid valves. This cleans up the assembly and reduces the cost of each valve termination. LEDs serve as a great feature, and their plug-in connections are our default on all new designs.
When designing a valve manifold, consider the valve assembly and how the manifold design can eliminate mounting a valve in a reverse orientation. Designing with valves with a molded male pin in its base and its clearance in the manifold simplifies and prevents the valve from incorrect installation. We use this new molded male location pin on all new designs we create.
While fluid contact is isolated from the solenoid portion of Rocker style valves, it does come into contact with the valve base and the seals. Most valve suppliers offer their valves with a few different seal materials. These include EPDM (most common), FKM, and Kalrez. These decisions are based on the chemical compatibility of the fluids on these seals, with EPDM being the least expensive option and Kalrez being the most expensive.
PNEUMATIC VALVE BASICS
Two of the most common designs for pneumatic control valves are the poppet and the spool. A poppet valve consists of an orifice that is opened and closed by raising and lowering a sealing surface onto the orifice. It usually has an internal spring that holds the valve in the closed position. When the spring force is overcome, the sealing surface lifts off the orifice and the valve opens. A spool valve consists of a shaft with a series of O-rings inside of a barrel. As the shaft moves back and forth, the O-rings shift position to open and close different flow paths.
Both valve types can be manifold mounted and valve manufacturers have standard valve manifold designs for mounting pneumatic valves (Both solenoid and air piloted) onto multi-station manifolds. When designs call for valve flow to interact with each other it is common to use pneumatic valves on custom bonded manifolds.
Here are a few key concepts that describe important characteristics of pneumatic valves:
Spool Valve: A spool is a cylindrical valve element that opens and closes ports when moved axially within the valve bore. Most spool valves use elastomers to seal between flow paths. Compared to poppet valves, spool valves provide certain advantages and disadvantages which make them better suited for vacuum applications, applications which require holding pressure downstream, selector valve applications, and applications which require consistent response time.
Direct Acting Solenoid Valve: Direct-Acting Solenoid Valves utilize the force generated by the magnetic field of the solenoid to operate the valve. There is no minimum pressure required which makes them also a great choice for low pressure and vacuum applications. When the electrical current is removed, a mechanical spring returns the valve to its original position.
Indirect Acting Solenoid Valve: To conserve power, indirect or pilot assisting solenoid valves utilize air pressure to shift the valve. They often use a miniature 3-way solenoid valve vs a bigger valve solenoid and can operate at very low wattage draw. In doing so it limits the valves minimum operating pressure to approximately 20 psi and therefore not suitable for low pressure or vacuum applications
DIGITAL VALVES
Digital valves generally have an "on-off" style switch with digital readouts and other capabilities. Several manufacturers market manifold mount valves in sizes as small as 5mm in width and with various flow rates.
PROPORTIONAL VALVES
Unlike digital valves, proportional valves provide air or gas flow control. They vary the output flow based on the current or voltage input to the solenoid.
When choosing a valve, always pay close attention to its advertised leakage rate. Confirm this with your valve manufacturer as not all publish the leak specs for all components. Many valves have leak rates of 15-100 sccm, which can be fine for many applications. Life Science applications often utilize on board pressure or vacuum pumps and storage accumulators. A good design practice is to reduce the on/off cycles of the pumps to reduce noise, vibration etc. These applications call for low leak valves in the 0.1-1.0 sccm range when possible.
It is widely accepted that "bubble-tight" refers to a valve that, when fully closed, does not pass any bubbles when one side of the valve seat is pressurized with air/nitrogen, and the other side of the seat is submerged in water. Actual test procedures can vary from manufacturer to manufacturer. There is no formal standard to define bubble-tight.For example, most tires are tested to 1x10-5atm-cc/s, units commonly seen from a high vacuum helium leak detector. On an air pressure decay style leak detector, the equivalent leak rate would typically be stated as 0.0002 sccm. And a bubble leak test would take 10 minutes before the first bubble would escape to the atmosphere.
Low leak valves feature large cross sectional O-rings, minimal leak points, and proven poppet designs. All mounting hardware is outside of the flow path and no internal parts are threaded during assembly to reduce the possibility of contamination.
MANUAL REGULATORS
Often an application requires a regulator to obtain a lower pressure circuit. Manifold mount is a convenient location to control the pressure. There are a few manufacturers that meet the criteria we look for in such regulators; size, accuracy, repeatability, flow, low leakage to atmosphere, manifold sealing surface and cost.
Electronic proportional controllers offer precise, linear digital pressure control within a closed-loop system with ultra high resolution and repeatability. Highly-customizable and available for pressure or flow control of gas applications. It has a great manifold mount interface for your manifold designs
Proportional pressure or vacuum control is often used with fluidic manifolds to precisely control the flow of samples, reagents, and other fluids through the instrument.
Single valve models achieve excellent results in dynamic applications like control of carrier gasses, blanketing gasses or dynamic dispensing applications with the phenomenal high resolution, high accuracy, and repeatability.
PROPORTIONAL ISOLATION VALVES
There are a limited number of manufacturers of proportional isolation valves for direct manifold mounting. Manifold mounted with zero dead volume, this valve design is ideal in critical applications for liquid and gas delivery, medical and analytical applications requiring ultra fine resolution and excellent repeatability.
LIQUID DISPENSE PUMP-SOLENOID/DIAPHRAGM STYLE MANIFOLD MOUNT
Diaphragm solenoid operated pumps are usually derived from a diaphragm-poppet style solenoid valve, where inlet and outlet check valves are added to the valve body. This type of pump utilizes electromagnetism to apply a force to the poppet-diaphragm to positively dispense liquid. The movement of the diaphragm-poppet forces liquid through the one-way (check) valves. The volume dispensed is determined by the stroke of the diaphragm-poppet and the internal geometry of the valve body. Variations in flow rates are achieved by simply varying the frequency input.
Diaphragm Solenoid-Operated Pumps ADVANTAGES
| Diaphragm Solenoid-Operated Pumps DISADVANTAGES
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Users mark positive displacement pumps as the clear choice when trying to design compact, multi-pump fluidic assemblies. As these can be mounted onto manifolds, this style of pump will often be more precise than a syringe pump without the expense of replacing a syringe. Available in a variety of sizes, they correlate to the volume of fluid that is drawn into the pump and then dispensed.
Other considerations to take into account when selecting a pump style:
Users can configure displacement pumps easily to optimize the most important aspects of the system’s fluidic requirements. The broad range of pump volumes along with the ceramic piston and pump head options provide many opportunities that can increase a manifold’s function.
Air displacement pumps and auto-pipettors are used for aspirating and dispensing samples with zero carryover. They are a great choice for benchtop instruments that work with disposable tips. Many customize these further for modular designs that accommodate a wide variety of pipette tips, tip adaptors, liquid level sensors, and pneumatic valves.
Additionally, their compact and lightweight nature allow them to mount directly on most small robotic workstations. This, in turn, makes auto-pipettors suitable for OEM instruments and production line operation.