The three pillars of loop control for process automation are the field transmitter that measures and reports the significant variable, the field controller that takes that signal and turns it into a control signal, and the final control element, most often a control valve. The control valve takes the control signal and performs an operation based on the signal it gets. Field transmitters and field controllers have been getting smarter, faster, smaller and cheaper for decades. So what’s been happening to control valves?
First, let’s take a little detour and look at the control loop itself. For years, we’ve been doing what has been known as “single-loop control.”
Most people, when I ask this at meetings, simply no longer remember why it is that we do single-loop control.
Why?
The reason is that when Edgar Bristol, Sr., designed the first pneumatic loop controller nearly a hundred years ago, it was way too expensive and physically nearly impossible to use more than one variable to control the final element—the valve. So we got into the habit of picking the one variable that had the most impact on the process, and that was the variable we measured for the loop.
Even today, nearly all loop-controller algorithms have those peculiar delay times we called “huff and puff damping”—because pneumatics do not react at the speed of light, and there was delay between the sender and the receiver.
But in the future, we can expect that the single-loop controller and even the single-loop control function in the plant automation system (that’s newspeak for what we used to call a DCS) will be replaced with multivariable and advanced process control schemes. These control schemes will make controllers smarter, valves smarter, and put lots of the control decisions into the loop devices themselves.
Now what about valves?
Valves have been getting smarter and more efficient for years. Smart valves from multiple vendors, which can actually serve as the entire control loop all by themselves, have been on the market for a while now.
They can measure flow, do single-loop control functions on that measured variable and manipulate the actuator they’re attached to, as well as send a signal, either an analog 4-20 mA, with or without HART data, or a Profibus or Foundation fieldbus signal back to the control room.
This is what we used to call “control on the wire,” and it is what these smart valves do.
There’s a fly in the ointment, though. While control valves do a fantastic job of restricting, channeling and metering the flow of fluids in the process environment, they do so by dissipating and wasting enormous amounts of energy.
One of the trends we see for the next few years, as being “green” becomes more and more important and the cost of energy rises, is the use of more and more pumps and variable- speed drives instead of control valves in many batch and continuous process applications.
Indeed, motors connected to pumps, compressors and fans consume an estimated 60% of all industrial energy. In a typical control loop, an oversized motor and pump are run at full speed; the control valve in conjunction with a flowmeter effectively controls the flow rate, but also wastes energy.
A more practical–and today, possible–approach to meeting the varied load requirements of motor-driven flows is to vary the speed of the equipment by precisely controlling the speed of the motor.
And by controlling the speed of the motor to meet the needs of the process, variable-speed or variable-frequency drives (VFDs) can eliminate the need for a control valve, as well as boost energy savings and operating efficiency.
Smarter control loops, smarter and more efficient control valves, and the increased use of greener variable-speed drives…that’s what we see in the future of final control elements.
