The Case for Dedicated Laboratory Vacuum Systems

Getting Pumped Up on Diaphragms by Design
(Process Pumps)

by Richard J. Aerts, Process Products Engineer for KNF Neuberger, Inc.

All diaphragm process pumps for use with gases and vapors share certain fundamental characteristics, including relatively simple construction, oil-free operation without maintenance, high gas tightness, and uncontaminated delivery of the gas. Beyond the basics, though, specialized performance can be realized from diaphragm pumps due to their design versatility and application adaptability.
Diaphragm pumps transfer, compress, recirculate, or evacuate gases or vapors in industry and research applications for medical technology, analytical instrumentation, control engineering, chemical and process engineering, or in the laboratory, among others. The proper design and selection of a pump’s diaphragm and how effectively a pump can be customized to handle the demands ultimately will govern success in an application.

Diaphragms by design
The diaphragm primarily functions to displace the working gases from the pump’s compression chamber. Integration into a pump is relatively simple: The elastic diaphragm is clamped pressure-tight between the pump head and the housing to separate the transfer compartment from the housing’s interior. The diaphragm then is connected pressure-tight to a connecting rod.
In operation, the drive in the interior of the housing reciprocates the connecting rod, which causes the diaphragm to move up and down. In the downward stroke, the suction created in the pump chamber causes the inlet valve to open, allowing flow into the chamber. In the upward stroke, the pressure caused by the rising diaphragm causes the outlet valve to open, allowing flow out of the chamber.

The most common standard diaphragm designs include flat, molded, and structured.
The simples and least expensive is the flat diaphragm, which is essentially an elastomer disk. The connection between the diaphragm and the connecting rod is provided by a clamping disk (usually metal) and a screw guided through a hole in the center of the diaphragm. Pumps with flat diaphragms provide high compression strength, because the connecting rod and the diaphragm-support disk contribute support.
Among tradeoffs, however, pumps with flat diaphragms will not typically achieve optimal vacuums; uncoated metal parts will be prone to corrosive or aggressive gases; and relatively poor gas tightness can be expected, leading to higher leakage rates (normally 1 mbar 1/s) and restricting their application potential in the areas of analysis, chemistry, and medical technology.

Molded diaphragms represent a significant improvement by fully enclosing the side of the diaphragm located in the pumping chamber. This is accomplished by vulcanizing the metal stud (required to actuate the diaphragm) into the center of the diaphragm, which forms a rigid zone at that point and eliminates any need for a rigid clamping disk (and possible leakage path).
With this design the pumping chamber adapts to the contour of the actuated diaphragm and the clearance volume of the pump is reduced without the risk of the diaphragm striking the pump head at the upper turning point of the movement. An ideal vacuum is created and the closed surface of the diaphragm promotes superior gas tightness for the pump.

In addition, this design inherently allows for the development of chemically resistant versions without a need to coat metal parts; the vulcanized metal stud of the pump is already covered by elastomer. For applications that will experience corrosive or aggressive gases or vapors, added protection for the diaphragm can be provided with an appropriately enabling coating.
One noteworthy tradeoff: The lack of the flat diaphragm’s rigid clamping disk and other support can restrict the compression strength critical for compressor applications.
Structured diaphragms combine the advantages of both the flat and molded types and dispatch many of the drawbacks. As with the molded diaphragm, the metal stud required to actuate the diaphragm is vulcanized centrally into the diaphragm, where it forms a rigid zone, and the side of the diaphragm located in the pumping chamber becomes entirely closed.

The difference with the patented structured diaphragm is that its underside is ribbed to accommodate the particular load to be imposed and the diaphragm is stiffened in the center. The outcome: Reduced mechanical loading (and less wear), smaller size (for more compact designs), comparably higher compression strength, and good flow capacity. Standard structured diaphragms exhibit extremely high gas tightness (which can be improved upon with special designs) and will demonstrate significantly lower leakage rates (reduced by a factor of 100 compared with flat diaphragm pumps).

Variations on a theme
Regardless of industry, evolving applications for process pumps have imposed increasing demands on diaphragms and their capability to satisfy ever-burgeoning mechanical, chemical, and thermal loading requirements. In response, special versions can offer properties tailored to application parameters. Among them:
“Gas tight” diaphragm pumps seal exposed areas with O-rings to achieve dramatically lower leak rates (5 x 10-3 mbar l/s to 5 x 10-6 mbar/s). These can prove especially useful in applications involving poisonous or radioactive gases, whose traces in the surrounding air could endanger workers and the environment.

Corrosion-Resistant Considerations

Corrosion-resistant diaphragm pumps benefit from combining high-grade steels and solid PTFE (or other inert materials for the wetted head portion) with a laminated layer of corrosion-resistant material over the diaphragm. Such a combination imparts mechanical and thermal resistance, resistance to corrosion, and high tensile strength and resistance to pressure. By laminating the PTFE, pumps can become more flexible and exhibit longer service life. The high-grade steels will equip the suction channel and the output channel of the head parts with robust threads. With secure and reliable screw joint connections, pumps can greatly resist pressure and significantly lower the potential for leaks.

Heated-Head Sampling Pumps

Heated diaphragm pumps will be specified where a small cooling down of the working gas leads to “condensing out” of parts of the gases, which can distort measurement results if the gases are transferred as samples. In order to prevent such condensation, the sample gas must be guided via a heated pipeline and the pump head, too, must be heated. An electric heating element installed in the pump head does the job. (The current supply to the heating element can be switched off using a thermal switch attached to the head or, as a recent innovation, temperature sensors can be mounted on the head to regulate electronically.) The heated head offers a corollary advantage by keeping the gas dry and preventing the formation of corrosive compounds in the pump chamber.
Diaphragm pumps with explosion-proof AC motors offer solutions in potentially hazardous locations, such as for applications in the chemical, mining, hydrocarbon processing, plastics, and petroleum industries. Specialty pumps for compliance with Class 1, Division 1, Groups C & D and ATEX hazardous locations have been engineered to deliver high performance and long service life during continuous, heavy-duty operation.

Double Diaphragm Safety Pumps
Double diaphragm pumps pair a “safety” diaphragm with a “working” diaphragm to safeguard applications where hazardous, toxic, or otherwise harmful (or valuable) gases must be transferred. Such applications raise the bar on the demands for gas tightness and leakage prevention and have been commonly engaged for decades to monitor emissions at nuclear power plants.
Together with additional sealing rings, a double diaphragm system’s arrangement enhances gas tightness with leak rates as low as < 6 x 10-6 mbar l/s. (In very rare cases, should the working diaphragm become damaged, the pumped medium will not escape but will be captured in the intermediate space between the two diaphragms.) The “safety” diaphragm is subject only to low mechanical and thermal loads during pump operation; the “working” diaphragm is elastically distorted and warmed by the compression process.

A rupture of the working diaphragm will easily be detected through a sudden and dramatic drop in the pump’s pumping or compression capacity. If the pump is generating only low pressure, a sensor can be fitted to monitor the intermediate area between the working diaphragm and safety diaphragm to detect any damage to the working diaphragm. Suitable pressure and gas sensors can be specified for this task.
Footnote: As all these designs suggest, the simple rubber-component diaphragm has evolved into a device supported by finite-element calculations and complex manufacturing processes. With all the choices and capabilities to customize, users can benefit from partnering at the outset with an experienced manufacturer to develop the best-suited diaphragm pump for the application.

Richard J. Aerts is Process Products Engineer for KNF Neuberger, Inc., 2 Black Forest Road, Trenton, NJ 08691-1810 USA. Phone: 609-890-8600. Email: raerts@knf.com Web site: www.KNFProcess.com