As manufacturing processes become increasingly complex, it is common for fluid control applications to involve increasingly demanding requirements. These requirements can include extremely low or extremely high differential pressure (DP), wide flow coefficient ranges, two-phase flow and supercritical states. As a result, choosing the right flow control solution can be a much more complicated task than it used to be.
A wide variety of flow control valves are available to meet specific requirements. Popular choices include rising stem globe-style valves, rotating ball valves and slide gates. Each class of valve offers customized variants for extreme pressure, extreme temperatures, corrosion resistance and specific flow ranges. Even so, chemical engineers often encounter problems finding the perfect flow control solution for a particular combination of circumstances.
A new approach
Direct-sealing diaphragm valves with dome-loaded actuation have been used to control back pressure for more than 10 years. They are especially well-suited for processes requiring wide flow range, high precision, two-phase flow and other specific demands. Accordingly, the devices have been adopted in applications including catalysis research, core sampling, gas chromatography and supercritical extraction.
Recently, innovative engineers and scientists have begun adapting direct-sealing diaphragm valves to control flow rate in complex applications. They have discovered that these devices offer the same advantages for flow control that they offer for pressure control, making them suitable for scenarios that cannot be solved by traditional methods. Some of the demanding requirements involved in these applications include:
- Flow coefficient (Cv) ranges wider than traditional valves (>100:1)
- Extremely low DP and extremely high DP
- Two-phase, phase-change and supercritical states
How the direct-sealing valve works
Diaphragms are frequently used in pressure regulators to sense pressure and actuate valve movement. Standard diaphragm valves use a mechanically actuated diaphragm to seal against a weir, typically for on/off service. The direct-sealing diaphragm valve (as shown in Figure 1) takes a completely different approach by using multiple orifices and pneumatic pilot pressure on top of the diaphragm to create precise control across a wide range of flow.
As shown in Figure 2, the device works in conjunction with a pilot pressure controller and a flowmeter in a control loop. A proportional-integral-derivative (PID) controller monitors the flow from a flow transmitter (FT) and adjusts the pilot pressure to bring the flow under control. An electro pneumatic transducer (E/P) translates the electronic signal into a pressure signal for the pilot controller. Flow is decreased by raising the pilot pressure up to the media supply pressure. Flow is increased by lowering the pilot pressure below the media supply pressure.
The PID controller must be switched to direct mode instead of the more common inverse mode, as pressure must be increased in response to an increase in flow.
See Figure 3 for a comparison of a traditional globe control valve and a direct-sealing diaphragm valve.
Application example #1: Phase-change gases
Berlin-based Integrated Lab Solutions, Gmbh, (ILS) develops automated laboratory testing systems worldwide. ILS systems allow customers to conduct process research and development (R&D) with a strong focus on catalyst and materials R&D. Laboratory R&D systems like these require enormous flexibility in flow rate, pressure, temperature and chemical compatibility.
ILS faced a specific problem in the dosing of propylene for a particular R&D system. As a liquefied gas, propylene undergoes strong Joule-Thomson cooling and phase change during the expansion and pressure drop of the dosing valve. These effects, combined with the tendency to swell elastomeric seals, make flow control of this fluid especially difficult. ILS previously used needle-type globe control valves. These control valves are a proven technology where steady-state conditions with minimal downtime are of primary importance. However, ILS found the 15:1 flow coefficient turndown ratio of this type of valve extremely limiting for such R&D applications where the ability to test a wide parameter space is of utmost importance.The founder and creative force behind ILS, Dr. Anton Nagy, developed and successfully implemented a dosing system using a direct-sealing diaphragm valve to meet the application’s broad requirements — one of the first commercial applications of these valves for flow control. The system uses an Equilibar direct-sealing diaphragm control valve downstream of a manual Swagelok pressure-reducing regulator and a Bronkhorst mass flow controller (as shown in Figure 4). The nitrogen pilot pressure for the diaphragm valve was controlled by redeploying the flow control valve supplied with the mass flow controller. The ILS system was able to control the propylene dosing across wide ranges of absolute pressures, differential pressures and flow rates.
"We dosed liquefied gases both with and without flashing in the valve and saw no effect on flow control, which is amazing," Nagy said. "We can say conclusively that the implementation of a direct-sealing diaphragm valve gave us equal or superior performance for all the conditions we tested."
Application example #2: Ultra-wide Cv range
Armand Bergsma of Pressure Control Solutions (PCS), a fluid control specialty company in Veenendaal, the Netherlands, also recognized the advantages of using direct-sealing diaphragm valves to address wide-ranging flow control applications. PCS works extensively with research scientists throughout Northern and Western Europe who are testing the boundaries.
"Experimental setups are often well-engineered processes built to explore a broad range of process conditions," Bergsma said. "Often, the process conditions are only roughly known; therefore, these conditions need to be flexible but kept as constant as possible during experiments. We often receive inquiries for applications that require instrumentation that can operate over a large range."
For one particular client application, PCS needed to develop a system capable of dosing both H2 and Cl2 gases with inlet pressures from 100 bar to 5 bar and with flow rates from 5 gram/hour up to 700 gram/hour (see Figure 5). Such an application carries an impressive range of flow Cv from 1E-5 to 2E-2 and a turndown ratio of approximately 2500:1. Bergsma demonstrated the system using the Bronkhorst Coriolis-type mass flowmeter and proportional control valve together with a direct-sealing diaphragm valve from Equilibar.
In most chemical process flow control applications, a traditional globe or needle-style control valve will be the most economical and convenient solution. Advantages of these methods include:
- Highly proven, rugged design
- Industry approvals such as American Petroleum Institute
- Convenient actuation (3 to 15 psig air or electric motor actuation)
- Defined fail-open and fail-closed spring states
While the direct-sealing diaphragm valve approach offers distinct advantages, it is more complex than many traditional methods. Implementing this approach involves the following requirements:
- An instrument gas supply, such as air or nitrogen, is required equal to the supply pressure of the fluid being controlled. For higher pressure applications, this can require using bottled nitrogen.
- A high-resolution pressure controller is required to control pilot pressure in the same range as the fluid supply pressure. Electronic pressure controllers can be more expensive for higher pressure ranges.
- Depending on the configuration of the pilot pressure controller, the PID controller may need to be switched to direct mode. In some cases, further modifications may be required to the PID configuration.
- A block valve may be required to prevent high flow in the event of loss of pneumatic supply pressure.
Advantages for demanding application requirements
Process engineers should consider direct-sealing diaphragm valves as a possible solution when application parameters exceed the ranges commercially available from traditional control valves. These direct-sealing diaphragm control valves have been proven to perform in the following conditions:
- Wide range of required flow Cv
- Highly corrosive gases and liquids (available in exotic alloy bodies and diaphragms with FFKM)
- High temperature (seals available up to 500ºC)
- Sanitary and biopharmaceutical applications (available with USP Class VI diaphragms)
- Extremely low flow rates (controls Cv down to 1E-9)
Jeff Jennings, PE, is founder and president of Equilibar, LLC, which designs and manufactures direct diaphragm sealing back pressure regulators and flow control valves near Asheville, North Carolina. Jennings has more than 30 years of engineering experience including 16 years with The DuPont Company. He holds several international patents and continues active research in the field of fluid controls. He may be reached at [email protected].