Engineers and designers who want to control flow without contaminating media such as blood, pharmaceuticals, reagents, or even water can specify isolation valves. There are three general types of isolation valves (so named for their ability to isolate media): rocker, diaphragm, and pinch valves. They are commonly used in many types of medical applications, including those that require precise, repeatable dispensing of media in analytical, diagnostic, and therapeutic equipment. Here’s a quick look at the characteristics, differences, and advantages of these valves.
Isolation Valve Basics
Any of these three valves can be set up to separate the activation mechanisms from the media being moved. They can also be configured as simple two-way devices or as multi-port selector/diverters, and are typically used in applications where a simple on/off function is needed rather than the ability to gradually modulate flow.
There are two main types of “on/off” media isolation valves: the rocker and diaphragm varieties. Although not formally considered isolation valves, pinch valves meet the definition and can be used for the same purpose.
Here are a few key concepts that describe important characteristics of media 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.
Rocker Isolation Valves
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 (see figure below). Rocker valves can be configured as simple two-way devices or as multi-port selector/diverters.
Rocker valves are generally smaller and more compact than diaphragm valves, making them well suited for applications with space limitations. Other benefits include low internal volumes and fast actuation times, along with relatively low costs. However, despite their low internal volume, rocker valves have more dead volume and are less well swept than diaphragm valves. This means they have more carryover (media left inside when valve closes), which can be problematic for certain applications as it increases the risk of cross-contamination. Another factor to consider is that rocker valves include elastomeric seals, giving them a shorter lifespan when used with corrosive chemicals and making them less chemically compatible than some diaphragm-style valves. Rocker valves are frequently used in industry for dispensing materials or to drive other larger valves.
Diaphragm Isolation Valves
A diaphragm isolation valve—also known as a “membrane valve”—is a solenoid-operated device that uses a diaphragm that extends and retracts to seal the valve seat and isolate the flow path. Like rocker valves, diaphragm valves can be configured as simple on/off two-way or three-way devices.
Compared to the rocker valves, diaphragm valves have much longer life, are better swept, and have much less dead volume—some as little as zero. Another benefit of these valves is that the diaphragm can be made from non-elastomer materials such as PTFE, which eliminates the need for seals and provides more chemical compatibility. This type of valve can be made of inert materials, including an entirely inert fluid path. This makes them ideal for applications involving corrosive media.
The NPV Series Miniature Pinch Valve from Clippard is available with one or two tubes. The single tube versions act as standard on/off two-way valves and can be normally-open or normally-closed. The two-tube version have a normally-open tube and a normally-closed tube, letting them function as three-way valves.
When selecting this type of isolation valve, it is important to consider the diaphragm material and the media that will be used. Some diaphragm valves use elastomeric membrane such as FKM or EPDM. These materials are highly flexible, which lets them tolerate small amounts of fine particles, but common chemicals such as methyl alcohol or ammonia can damage them. For chemotherapy treatments or other applications involving corrosive media, valves need more chemically resistant diaphragms made of materials such as PTFE.
When choosing a valve for use with corrosive media, it is important to ensure all wetted materials are inert. For the longest lifespan and lowest risk of cross-contamination, the flow path and all wetted areas of the valve should be constructed of an inert material compatible with the media that will be used.
A pinch valve is an open valve, and closes the flow path by pinching a removable, disposable tube with a mechanical device that drops down to pinch the valve closed or lifts to open the valve. Pinch valves can be configured with single tubes as simple on/off two-way devices or with several tubes as multi-port selector/diverters.