Diaphragms are important components not only for fluid pumps and metering valves, but they are also a key factor in the successful function of systems used in building services, plant engineering, and construction. Their proper function is critical to the success of the overall system — when the diaphragm component fails, the entire system fails.
For example, in building engineering and domestic installations, diaphragms play an important role in the control systems for drinking and wastewater, as well as heating systems. If not properly functioning, their failure could cause some unpleasant sanitary issues. Or, in their role in process technology for food and pharmaceutical engineering, a faulty diaphragm has the potential to violate critical Food and Drug Administration or United States Pharmacopeia regulations. Likewise, a diaphragm’s role in control and automation technology is critical when dealing with compressed air and the leakage of gasses and fluids.
Because of their extreme value to the overall system, as well as the increasing demands on longer service life and improved durability (even under severe operating conditions), a need has arisen to explore alternative materials to the traditionally used fabric reinforced nitrile (NBR), ethylene propylene (EPDM), or fluoroelastomer (FKM) rubber to improve the sealing capability of diaphragms. Along this line, recent testing revealed that thermoplastic polyurethane (TPU) is capable of tripling a diaphragm’s life span and considerably reducing life span-related costs.
Subtle seal — Big role
A diaphragm is a versatile dynamic seal that surpasses many of the limitations of other sealing methods. Typically affixed to a moving piston and/or actuated by differential pressure, diaphragms convert a mechanical force to a pneumatic or hydraulic force (or vice versa) by containing this force to one side of the diaphragm and transferring it to the other. For example, in a pump application, when the piston pushes on the diaphragm, it causes the diaphragm to expand, creating a hydraulic force.
The diaphragm also functions as a barrier, preventing the mixture of fluid and/or gas from migrating across the barrier. Elastomeric diaphragms are often used since they do not leak, offer little friction, can be constructed for low-pressure sensitivity, create minimum wear, are relatively maintenance-free, don’t create a stick slip effect, and have few requirements concerning surrounding metal (dimensions and surface finish).
Diaphragms are available in shapes ranging from flat to deep-draw styles. If the application requires only a separating member and a short stroke, a flat-cut diaphragm is usually sufficient. Should the application require a long stroke, or a constant effective area, then a rolling diaphragm is the best choice. With proper material choices, elastomeric diaphragms can withstand wide temperature and pressure variations without maintenance or lubrication. However, without the proper material choice, a diaphragm’s sealing ability can be greatly diminished.
Considering the environment in which the part will operate when choosing an elastomer is critical and will often override considerations taken for manufacturability. Relevant environmental considerations include: chemical contact; abrasive hardware or medium; applied loads; and health or federal regulations. If conditions allow, the elastomer will be selected for manufacturability based on its flow characteristics, cure system/rate, fabric adhesion and the ease of cut (generally described by the durometer).
In addition, certain fluids can adversely affect materials based on the chemical composition of each element. As an example, dosing pumps often require highly chemical-resistant materials or, in many cases, a polytetrafluoroethylene (PTFE) covering to prevent the fluid from attacking the diaphragm material. As a diaphragm is constantly flexing, keeping the material consistent over the life of the product is essential to function. A diaphragm that softens, hardens or cracks will not perform as expected and will eventually allow problematic fluid transfer. Adding the PTFE coating provides another challenge as PTFE does not stretch like an elastomer and cracking due to fatigue must be considered and avoided. Diaphragms used in valves and fittings often must be amine-resistant and/or health and safety compliant due to their contact with water at all levels of purification. Even everyday drinking water has the ability to attack materials depending upon the chemicals it may contain. The elastomer is expected to endure years of exposure to these chemicals, no matter how slight the chemical level (for example, a pool with chlorine in it).
Subtle seal — Big demands
To compound the aggressive application demands and severe operating conditions, customers are requiring longer life from diaphragms. Downtime for maintenance is costly and often very difficult to perform. Once the diaphragm is installed in a component, it isn’t always easy to access. To increase seal life and overall product strength, companies commonly use fabric-reinforced diaphragms, which may increase the difficulty in manufacturing a diaphragm, as well as decrease the elastic function of the part.
Further, since a fabric starts out as a sheet and has a square structure (like the threads in a cloth napkin), when they are used as reinforcement material for a diaphragm, these threads must be stretched into a round shape (like trying to put a cloth napkin over a glass without wrinkling the sides). Knit fabrics are often more difficult to predict because of the dissimilar stretch characteristics in each thread direction. For diaphragms with large height-to-bore ratios, a fabric with an open weave (larger thread spacing) must be used to allow the fabric threads to realign themselves. Design engineers at diaphragm manufacturing companies must keep in mind that shaping fabric essentially forces an initially square structure into a round structure.
Design engineers have the following fabric choices:
- Cotton – Used to some extent when special shape compliance is required and/or bulk is needed to make the cross-section thicker;
- Nylon and Polyester – Used for the majority of diaphragm applications. These fabrics are available in a variety of woven and knitted styles and are normally used for shallow draw and/or high-pressure diaphragms;
- Fiberglass – Available and used, but is not generally recommended for diaphragms because it can be extremely brittle;
- Nomex and Kevlar – Used in extremely high-temperature applications; and
- Other Fabric Types – Including silk for ultra-thin/sensitive diaphragms and non-woven or chopped fabric for reducing fluid wicking.
As applications evolve further, companies will seek new solutions to meet their changing needs. As a result of the limitations associated with fabric-reinforced diaphragms, an opportunity arose for a new solution.
An air-operated double diaphragm pump was chosen as a test apparatus, since it requires a precise amount of pressure (measured in pounds per square inch) and air (measured in cubic feet per minute) to deliver the proper amount of fluid. The pump operates by displacing fluid from one of its two liquid chambers during each stroke and contains only a few dynamic parts: the two diaphragms, which are connected by a common shaft; the two inlet valve balls; and the two outlet valve balls. Image courtesy of Flotronic Pumps Ltd.
Subtle seal — Big potential
Material selection relies on the knowledge and experience of a diaphragm supplier. Specifying the best material not only ensures the longest possible life of the diaphragm, but also avoids unnecessary costs incurred by over-designing for the application.
Alternative materials capable of extending the diaphragms’ lifespan in demanding environments over traditionally used elastomeric compounds. One high-potential candidate was the TPU material because when compared to NBR and EPDM materials, TPU materials have very high flexural strength, high wear-resistance and tensile strength, outstanding tear-resistance, very good pressure-resistance and excellent oxidation resistance. While the material’s characteristics make it an excellent candidate for longer life, it may have certain fluid compatibility issues. As such, one needs to check the material against the fluid.
Belonging to the thermoplastic elastomers (TPE) family, TPU materials close the gap between thermoplastic and elastomer materials regarding hardness, deforming behavior and processability. The TPU compound is made from three key ingredients: a disocyanate; a polyol; and a cross-linking agent. The kind, quantity and reaction precondition of these components define the characteristics of the polyurethane. Some urethanes are produced in bulk and sold commercially, others are completely customized and some are a customized mix of commercially produced components. Thus, "tuning" the TPU material to specific requirements is essential. In the case of this diaphragm application, a custom blend was required to meet the application conditions over a commercial blend. In some instances, the commercial blend actually decreased the diaphragm life.
An air-operated double-diaphragm (AODD) pump was chosen as a test apparatus, since it requires a precise amount of pressure (measured in pounds per square inch) and air (measured in cubic feet per minute) to deliver the proper amount of fluid. The pump operates by displacing fluid from one of its two liquid chambers during each stroke and contains only a few dynamic parts—the two diaphragms, which are connected by a common shaft; the two inlet valve balls; and the two outlet valve balls. A complete failure is easy to detect because the diaphragms act as a separation membrane between the compressed air supply and the liquid. For this experiment, the TPU diaphragms were tested on an air-driven, free-flow diaphragm pump operating at 50 cycles per minute in 80 C water and 8 bar pressure.
Overall, more than 60 tests were conducted against a baseline NBR diaphragm with fabric inlay for structural optimization. Initially, diaphragms made from various commercial and custom TPU material blends were used, but later tests also included multi-component diaphragms which contained bonding to a PTFE foil (which, as mentioned earlier, would be used for protection against highly aggressive media). Modern calculation and simulation tools used parallel to the test series achieved a comprehensive optimization of the diaphragm.
After examining a variety of TPU materials, it was determined that if the correct TPU material is used, it can triple a diaphragm’s lifespan without fabric reinforcement, which will considerably reduce failure and maintenance-related costs. Lifetime testing results for three TPU diaphragms, all using the same design, against the baseline fabric-reinforced NBR diaphragm are shown in the table to the right.
As demonstrated, using the proper TPU material in combination with a customized design has a major influence on the lifespan of the component, as well as lifecycle costs. The critical choice of material has the potential to save time, money and, most importantly, the worry that the system will fail unexpectedly.
Joel T. Johnson serves as global vice president of technology for Simrit, Freudenberg-NOK Sealing Technologies’ general industry sealing business. In this position, he is responsible for the identification and realization of technology needed to expand the company’s growth in target industrial markets and the integration of established technology into core markets. Johnson earned a bachelor’s degree in Mechanical Engineering from the University of Wisconsin in Platteville. He is a member of the Society of Automotive Engineers (SAE), has presented at the SAE Diesel Forum (Curitiba, Brazil 2006), IFPE 2008 (Wide Range Temperature, Pressure and Fluid Resistant Hydraulic Cylinder Sealing Systems), and is a two-time winner of Freudenberg-NOK’s Circle of Excellence award. Mr. Johnson can be reached at 847-428-1261 or at [email protected].
