1660251956938 Valves

Local and independent control possible with top-down technology

July 19, 2001
Top-down technology reduces ownership costs and lets you control valves locally and and independently

Never before has the business environment in the process industries been so competitive and demanding. Maintaining a competitive edge requires upgrading process systems constantly with more efficient devices and methods. Fortunately, technological advances in control valves are helping plants maintain that edge.

Typically, control valves are directed by controllers programmed to maintain pressure, flow, level, temperature or analytical parameters. Control valves are expected to provide smooth, precise, repeatable changes in media flow.

Most plants use large, costly distributed control systems (DCS) to regulate control valves and processes. The input-output (I/O) process consists of receiving scaled signals from multiple sensors, comparing  the signals with predetermined setpoints maintained in a central host processor (a PLC, DCS or PC), and making adjustments to valves and actuators to compensate for deviations. While there may be some local processing, it usually relates to faster or improved communications with the central host.

Lightweight and compact, smart valves can deliver decentralized, intelligent process control for Y-pattern, globe, diaphragm and quarter-turn valves.

However, this is changing with the development of single-point, intelligent, programmable I/O. Connected to the valve directly, it allows measurement and control functions to be performed locally and independently. These “smart valve” systems use a compact pneumatic actuator with an integrated electropneumatic positioner mounted on top of the process valve. By using a direct link to the process sensor (flow, temperature, level, pressure, etc.), the smart valve automatically adjusts process variables to maintain setpoint.

The process control loop is the most important “basic cell” in automation. Its configuration influences the behavior and cost of the entire system. By eliminating dependency on a central processor, a top-down or “top control” valve technology enhances control flexibility, while reducing costs for installation, operation and maintenance.

Decentralized control

Lightweight and compact, smart valves can deliver decentralized, intelligent process control for Y-pattern, globe, diaphragm and quarter-turn valves.

As position controllers, they accept analog or digital signal inputs to establish the valve set position. In this mode, an integrated position controller compares the actual valve position reading against the setpoint. The valve position is measured by an integrated potentiometer. Adjustments are then made by activating internal pilot valves.

As process controllers, the position control loop works as a secondary service control loop. The process controller in the main control loop has a PID (proportional, integral, derivative) algorithmic function.

In this case, the setpoint is compared against  the actual process variable sensor signal and the valve position is adjusted accordingly. The actual value is determined by a sensor signal (frequency, Pt 100 or 4-20 mA representing the process measured variable). The setpoint may be established internally at the valve using an integrated keypad, or externally via a standard analog or digital input signal.

Reduced wiring improves valve reliability

Traditional schemes require significant wiring. For example, wiring must be installed to and from the local distribution panel, control room, power supplies and analog input and output modules of the controller. Furthermore,  these components must be assembled.

Imagine a conventional DCS with 1,000 sensors. Each sensor (which may have as many as four wires) needs to be wired to a termination panel, which is, in turn, connected to a multiplexed I/O signal conditioner. The conditioner is then wired to the central host computer via a network. This process signal is then acted upon by the controller, and signals are routed to the control valve device.

By contrast, decentralized intelligent controllers, which are mounted on a valve and linked to a sensor directly, create an easy local control loop with enhanced communication capabilities. Requiring only a power source and a pneumatic connection, they can work as standalone devices with no additional infrastructure. As a result, wiring is simplified, thereby improving reliability. Wiring between the smart valve and sensor is minimized. Top-down valve control excels in applications requiring an advanced level of process automation, such as textile, semiconductor, water treatment, chemical and pharmaceutical manufacturing, and food and beverage processing.

Another significant problem with DCS occurs when the I/O is separated from the central host processor. If communications are lost, the I/O becomes relatively ineffectual. Smart valves overcome this problem, because they do not require a central host processor or a network to do their functions.

Moreover, DCS or PLC-based software is complex, expensive and, in many cases, can only be modified by an engineer. By contrast, third-party software in a decentralized, local control loop is nonexistent.

Smart valves on a network

Linked directly to sensors, smart valves provide economical solutions for measuring many parameters. As a general rule, the specific application dictates the best two-point connection scheme. Systems can be assembled for flow, level, pressure, temperature and analytical control.

In fact, it’s estimated that more than half of all control loops could be considerably enhanced by improving valve performance. For example, smart top control valves can be easily programmed to overcome the inherent control problems associated with the characteristics of diaphragm valves. Besides lower ownership costs, smart valves provides a number of benefits for plants having a standardized bus protocol.

Depending on the application, data capture can be as simple as a chart recorder that takes a 4-20 mA signal or as sophisticated as a data collection system gathering detailed information on the smart valve’s position, binary output and limit switch output.

Available with various electrical interfaces, top-down valve controllers support major bus protocols, including AS-i, Profibus, DeviceNet and Foundation Fieldbus. However, no single architecture is best. It’s simply a matter of determining what networking protocol your plant is using.

In the early stages of automation, collecting data and using it for process control revolutionized industry. However, only a tiny fraction of data collected was put to use in real-time or near real-time decision-making, and virtually none of it was used to make business decisions. New plant floor networks provide the ability to connect measurement and control devices. Now that valves (and sensors) have become programmable—with the ability to capture data from individual local control loops over a network—the process can ultimately be tied into the enterprise asset management for significant price/performance improvements.

Are smart valves a good investment for your plant?

Whether or not your plant has a control architecture in place, smart valves can be used as isolated control packages. In fact, isolated control loops and higher level control architecture can even exist together, whether or not the valve is integrated into a larger system.

Smart valves are not only limited to traditional proportional control loops. There are, of course, situations where ON/OFF control is suitable. For example, if you’re controlling a process that can be within five to 10 percent of the setpoint and a valve cycling occurs less than once every three seconds (no more than 20 cycles in one minute), then you probably don’t need a proportional control valve. A standard ON/OFF valve will meet your technical requirements to control and feed back on the open/closed position based on desired process setpoint.

Depending on your plant’s control architecture, criticality of process loop control, safety shutdown requirements and philosophy regarding decentralized control, smart valves can play a significant role in reducing costs, minimizing maintenance and simplifying installation. Because of their many benefits, smart valves are finding greater acceptance in process industries.

Photo:  Burkert, USA

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