Solenoid Valve Innovations Provide Alternatives
to Traditional Proportional Controls

by Andrew Harris, Vice-President of Technical Marketing, Burkert USA

For years, plant engineers have wrestled between system cost and process control accuracy when choosing between traditional pneumatic or hydraulic proportional fluid control systems and simpler on/off fluid control systems. Today, innovations in valve coil designs and plug-on smart controllers can incrementally control the solenoid valve position, delivering, for the first time, true proportional control at low cost, changing these price/performance tradeoffs. These valves, with a smart controller mounted directly onto them, cost 1/3 to 1/2 of a traditional proportional control valve and offer a compact control valve solution.

Tried and True

On/off systems are inexpensive, reliable, easy-to-control systems consisting of a pneumatically- or electrically-controlled valve with a programmable switch or relay. These simple control loops are typically used in applications where variable control is not required, such as tank filling, continuous flame detection, and container/bottle fill.

However, there are many applications such as annealing, pasteurization and chemical coating where the process engineer wants finer control to maintain product quality or production, but a traditional proportional control system, with its added components, plumbing, control lines and electrical connections, is too costly to justify. Even in those processes that require increased accuracy and performance, the facilities manager would often prefer a simpler design for the sake of maintenance and installation. Sometimes adding one more proportional control system involves added secondary changes, such as a larger air or fluid supply, additional electrical drops, new cable trays, etc.

The Next Generation

Recently, new families of solenoid control valves were introduced with plug-on controllers. These valve/controller combinations directly link to process sensors to form a precision continuous process control loop with the control of proportional systems, but are priced and configured similar to an ON/OFF control loop.

Table 1 lists the components of a traditional proportional and a continuous solenoid control valve (CSCV) system. It is apparent that the continuous solenoid control simplifies system components, reducing cost while making it much easier to install and maintain. The lower number of parts also means that controllers are lighter and smaller. In addition, CSCV controllers are electrical and do not require air or hydraulic plumbing for operation. Plus, the controllers are intelligent and have programmable set points (internal or external), eliminating the need for a PLC in many applications.

The most substantial advantage of the new control loops, however, is the much lower cost to create a proportional control loop with technically acceptable precision. This is possible because of the advancements in solenoid valve design that now allow "pulse width modulation" (PWM) position control of the valve seat coupled with a direct plug-on PI controller. The positioner, controller and actuator all become one self-contained "plug and play" unit with these continuous solenoid control valves.

Applications

These valve systems that directly connect valves and sensors are designed for multiple process applications. The units can be used to supply precise control (0.5%) for flow control, level control, temperature control, pressure control, pH control or conductivity control. They are ideally suited for industries such as water treatment, chemical processing, utilities, semiconductor manufacturing, food and beverage, and pharmaceutical production.

One example would be in the machine sheet metal industry, where cooling is essential for industrial lasers. If the laser head elongates due to heat build-up, the cutting beam is no longer in focus, which results in bad cuts of sheet metal. A customer's original design involved a simple pump driving fluid through the head. However, the pump had pressure fluctuations ranging from 56 to 140 psig resulting in excessive head heating. This caused material to be scrapped and time lost while the operator made manual focus adjustments. By using a CSCV and a miniature pressure sensor mounted on the laser-cutting head, the customer was able to precisely control the pressure and thus achieve consistent flow of cooling fluid. With the beam in continuous focus, the new laser cutting systems delivered more consistent cuts, less scrap and higher throughput -- at only slightly higher installed cost with significant production improvements.

Another example involves testing of hot water heaters on a company's production line. Precise flow must be maintained during testing, although the flow setpoint varies with the number of units being tested. The previous setup used a central PLC, programmed with the number of units, to monitor a flow meter and adjust a proportional control valve system. The replacement system, using a CSCV with intelligent controller and a direct connection to a flow meter, gives better control of the minimum flow without the use of a PLC. Operation is also simpler -- the operator just programs the plug-on PI-controller at the valve with the required number of units and it automatically adjusts the flow control. The replaced system saved 50% in actual parts costs while providing similar control.

New Continuous Solenoid Control system
with valve, intelligent controller and direct I or F
connection to sensor

Producing paper is a very complex process. To ensure paper quality, a measuring head is used to detect thickness, square weight, evenness, and moisture from a hot, fast moving line of paper. This measuring head must be kept to a stable temperature using temperature-controlled water. In one instance, circuit pressure was required to be less than 60 psig while a second heat-exchange circuit assured the cooling water entering the measurement head was 70°F. Since paper quality was essential, the cost of a proportional control system was not an issue. This customer's system had a pressure control valve set to activate between 45 and 60 psig, and a temperature sensor tied to a PLC, which in turn regulated the cooling water flowing through the heat exchange. The problem was that the system control could only deliver a measurement accuracy of 10%, resulting in lower paper quality. Replacing the current system with a smart temperature sensor and pressure sensor gave more precise control over pressure in the cooling loop. Additionally, the direct connection from the temperature sensor and the heat exchanger meant less fluctuation in the 70°F water and eliminated the need for a costly PLC. This new cooling circuit resulted in 2% paper measurement accuracy.

Conclusion

Since electronically-controlled continuous solenoid valves are typically limited to 1/2- to 2-inch line sizes (due to power limitations of the solenoid coils), traditional (pneumatic) proportional control systems still have their place in the larger line sizes. Non-electric valves are also presently better suited for explosion-proof environments. Continuous solenoid valves can be used with both liquids and gases, but solids or dirty liquids may tend to quickly foul the valves. The electric valves also have inherent slower response time and may not be suitable for fail-safe operations.

The main benefit of continuous solenoid valve systems is their ability to deliver technically-acceptable process control precision to a desired set point, at 1/3 to 1/2 the cost of traditional proportional control loops. These systems are finding wide acceptance for controlling flow, temperature, pressure, pH, and conductivity in water, utility, waste and process systems.

About the Author:

Andrew Harris has a B.S. in Chemical Engineering from the University of Arizona and an MBA from the University of Southern California. He has more than 13 years of industrial engineering experience in Fluid Controls and Applications as a Chemical Engineer employed with Exxon Company USA, Dow Chemical, BetzDearborn, Hercules Chemical and Burkert. He is currently Vice-President of Technical Marketing at Burkert-USA .

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