New valve technologies overcome problems of leakage, valve life and heat transfer in medical and analytical applications
Everyday, new types of machines and equipment are being designed and developed that push the boundaries of medical science forward.
In this continuing offensive, specialist medical, biomedical and analytical valve products are playing a key role.
Steve Meadows of Asco Scientific looks at new developments in these areas and outlines how they benefit the market.
In the design of any analytical instrument (HPLC, GC, sample prep, DNA/protein sequencer, etc) one of the most critical components to consider are the solenoid valves used to control fluid flow.
These must be free of the problems of leakage that can cause instruments to produce inaccurate analysis results.
However, most of the inert solenoid valves on the market today, use a solid PTFE diaphragm for sealing, which has inherent leakage problems.
Since PTFE is not a very resilient material, it does not have good sealing characteristics and is very susceptible to scratching from particulate matter.
The problem with scratching is that it can create a leak path across the valve seat allowing fluid to flow in the closed position.
This leakage will cause the instrument to produce inaccurate results.
A new development by Asco Scientific greatly reduces the possibility of seat leakage in solenoid valves due to the action of particulate matter.
The development is an inert isolation valve (series 462) that uses a 'resilient-seal' against the valve seat.
The series 462 uses a soft Kalrez disc inserted into a PTFE diaphragm.
The PTFE diaphragm provides the rigid structure that isolates the fluid from the solenoid components while the Kalrez disc provides a very tight seal on the valve seat.
Since Kalrez is an elastomer, it has excellent sealing characteristics.
It is also a very inert material; therefore it is compatible with the majority of solvents and reagents used in analytical instruments.
In addition, the resilient nature of the Kalrez disc means that particulate matter does not easily scratch it, as is the case with a complete PTFE diaphragm.
Finally, if there is any particulate on the valve seat or deformation of the valve seat, the Kalrez disc will tend to conform around the particulate or deformation to provide a better seal than PTFE.
In addition to difficulties with sealing, another common problem that can spoil the accuracy of medical and analytical processes is heat transfer from a valve to the fluid medium that it is controlling.
This problem occurs when conventional solenoid operators with relatively high wattage coils are employed.
However, it can be overcome using piezo technology, as employed in Asco Scientific's new Piezotronic proportional valve.
The new Piezotronic valves use CMA (ceramic multilayer actuator) technology, which gives response times as low as 100microseconds, a practically unlimited service life (of greater than one billion cycles) and, crucially, extremely low power consumption at just 0.3mA at 24v (0.007W) and 0.9mA at 70V.
This means that there is no heat transfer from the valve into the fluid being controlled.
The CMA (ceramic multilayer actuator) technology employed by Piezotronic comprises a monolithic piezo-ceramic strip with integrated electrodes that are specially configured so that the element bends when a voltage between 20v and 40v is applied.
This bending action, which involves no moving parts, is key to providing the precise levels of proportional control demanded in such applications as gas analysis and the micro-injection of aqueous solutions into living cells.
The fact that there are no moving parts in the operation of Piezotronic offers several benefits: firstly, response times are exceptionally fast - 100 microseconds typically.
Secondly, service life is practically unlimited at around one billion cycles.
Thirdly, the unique operation of the Piezotronic valves also means that there are no inductive peaks when switching, so unlike conventional solenoid valves, no circuit protection is necessary.
Finally, no holding current is required for the valves to maintain position, so heat generation here is not a problem.
As well as overcoming actual operational problems that can adversely affect equipment performance, valve suppliers to the medical and analytical sectors are also increasingly required to provide the longest reliable life possible to optimise equipment costs.
In common with all areas of technology, manufacturers in the medical, bio-medical and analytical sectors are looking for products that they can effectively 'fit-and-forget'.
The key to achieving this objective is new designs, such as the flat spring technology employed on the Series 065 valves, from ASCO Scientific: these provide an operating life of one billion cycles when used with neutral gases.
The flat spring technology combines both valve spring and core functions in a unique flat part manufactured from a special stainless steel.
The result is a highly compact no-maintenance valve with only one moving part; a design, which, with no loss of core material due to friction, offers both high levels of process reliability and also integrity of the fluid controlled.
Intended for the control of gases and liquids, and vacuum (0.1bar abs) in the temperature range from 0C to 60C, the series 065 valves are AISI 303 stainless- steel- bodied- units with internals also of stainless steel, and seals and discs of high-performance FPM (fluoroelastomer/Viton).
This construction ensures the widest range of chemical compatibility, a feature that can be extended with the option of Kalrez (FFPM) seals for aggressive fluids.
