Enhancing Media Compatibility - Material Selection for Pressure Instrument Wetted Components
Barksdale Control Products
Control every move with high quality process flow instrumentation with 75 years of industry experience.
PRESSURE INSTRUMENTS
This white paper discusses the selection of appropriate materials for the wetted components of pressure instruments such as pressure switches and pressure transducers. It provides an overview of the functioning of different pressure instruments and emphasizes the importance of material compatibility with the media and environmental factors. The paper explores various material options, such as rubber elastomer, polymers and different types of stainless steel, highlighting their properties and suitability for specific pressure and media conditions.
INTRODUCTION
Pressure switches serve as an On-Off switches that activate and deactivate in response to changes in pressure. As opposed to switches, a pressure transducer continuously monitors a pressure and converts it to digital or analog output. These pressure instruments utilize a combination of components, including O-ring, diaphragm, piston, housing, process connection and microswitch.
This paper focuses on the selection of appropriate materials for the O-rings, diaphragms and process connections, considering their critical roles in protecting internal components and maintaining accuracy.
O-RING SELECTION
O-rings are commonly made of elastomers and offer flexibility and sealing capabilities. Different elastomer compounds are available, with the most prevalent options being Buna-N (Nitrile), Viton (Fluorocarbon), and Ethylene Propylene (EPDM). Material selection depends on media compatibility, as these pressure instruments may encounter various fluids such as water, oil, and gases such as carbon dioxide and hydrogen. Buna-N is commonly used since it is particularly resistant to oil and prevents material corrosion. Temperature considerations also influence O-ring selection. These O-Rings need to be able to handle both extreme high and low temperatures without becoming brittle or losing their sealing properties.
DIAPHRAGM AND PROCESS CONNECTION SELECTION
The process connection and diaphragm serves as the first layer of measuring element between the media and internal components which can be manufactured from different materials such as aluminum, brass, and stainless steel and various elastomers or polymers. Among these, stainless steel offers superior anti-corrosive properties and tensile strength. Various types of stainless steel, such as 303, 304, 316, and 17-4pH, provide different qualities in terms of durability and strength. Polymers also possess unique properties among other materials because of their insulation properties and corrosion resistance. Application-specific requirements dictate the choice of diaphragm material, with higher pressure applications favoring stronger materials and lower pressure applications benefiting from greater flexibility.
SELECTING THE RIGHT MATERIAL
The selection of wetted materials for pressure switches and transducers involves careful consideration of media compatibility, environmental factors, and application requirements. The chosen materials must resist corrosion, maintain sealing integrity, and ensure accurate pressure measurements. Factors such as temperature, media type, pressure range, and durability requirements should guide material selection decisions.
This paper is going to focus about how Barksdale provides the best solutions for the three following media: Water, Compressed Natural Gas (CNG) and Hydrogen.
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WATER
Water, a fundamental and versatile substance, has long played a pivotal role in various industries. Among that, Barksdale has incorporated a range of products into agricultural industry, achieving seamless integration. It is important that the materials used are anti-corrosive and a compatible elastomer for the sealant that’s able to withstand the wide range of temperatures without deformations. 316L Stainless has proven itself to be one of the most common metal that’s used as wetted material for these pressure instruments. It’s low carbon content (compared to 316), high level of chromium and nickel is what helps the material fight against corrosion. Barksdale helps mitigate corrosion with a process called passivation. It is a process where stainless steel is exposed to nitric acid to remove excess foreign iron compounds from the surface. On other systems where strength is not as critical, Barksdale also offers a polysulfone process connection to completely eliminate corrosion. They are not only corrosion resistant, but they also provide dielectric protection for electrical components. As far as sealants and elastomer diaphragms, Nitrile (Buna-N) is the most common for water because it is able to withstand wide range of temperatures (-35 to 250 F) without risk of embrittlement as there is freezing tendency for water.
COMPRESSED NATURAL GAS (CNG)
CNG is an alternative fuel source that’s more economical, environmentally friendly and also safer to use if monitored properly. It can be used as an energy source for power generators, HVAC, residential cooking, etc. Some of the most important uses of CNG fuel that is emerging are in the uses for public transportation. Some of the largest public transit agencies, such as New York City Transit, LA Metro, Chicago Transit, etc., have started using CNG as a primary source of fuel. Material selection also holds significance in the monitoring of fuel. In the case of compressed natural gas (CNG), a notable challenge arises due to its susceptibility to expansion with temperature variations. Consequently, the chosen material must possess heightened strength to endure these conditions without any embrittlement risk at extreme temperatures. Barksdale has done extensive research and testing on material compatibility for CNG. To solve on balancing tensile strength while limiting embrittlement, Barksdale uses a special grade of 316 as its wetted material to mitigate corrosion resistance as well as a special grade of FKM (Fluorocarbon Elastomer) identified through extensive testing to be able to withstand extremely low temperatures as well as enhanced material compatibility. Barksdale has also qualified their transducers to NGV 3.1, proving the safety and reliability via third party certification.
HYDROGEN
Hydrogen is also becoming an emerging energy source used for fuel cells. However, it is tricky to monitor since it has a smaller molecular size than other gasses. Therefore, it is crucial that the right material is selected in the components to attenuate that issue. Barksdale has experimented with different materials that would be compatible with hydrogen. Over a low pressure, 316 Stainless Steel will be able to withstand the permeability of hydrogen. At higher pressure, hydrogen has a greater risk. To mitigate that, Barksdale has chosen to use gold to reduce the risk at a higher pressure. It is implemented through a process called gold plating where we put an extra layer on the diaphragm. Gold has a higher molecular density compared to stainless steel, hence preventing permeability through the gold layer.
Since hydrogen is mostly used in a fuel cell application, it has to also withstand extreme temperatures without deformation and embrittlement, therefore, Nitrile (Buna-N) or FKM is one of the best materials to withstand the environment.
Contact us at insidesales@barksdale.com for more information and to speak to our product experts.