Choosing the Right Communication Protocol - Emerson

1 The Right Communication ProtocolELECTRONICALLYREPRINTED FROMWhen using throttling valves in closed-loop process control, a variety of factors impact the selection of the most appropri-ate communications network. Chief among them are performance and installation cost. Within the chemical process industries (CPI), three networking technologies are commonly used to interface throttling valves with the automation system: 4 20 mA with HART, fieldbus and wireless. This article ex-amines these three options, and also pro-vides information on emerging technologies to improve control affecting control performancePerformance in a closed-loop control appli-cation depends on the dynamic response of the controller, valve, measurement and pro-cess (Figure 1).

2 To achieve a target control objective, it is necessary to consider all of these components (see sidebar, Value and Positioner Technology).Performance and cost discussions should always start with the process, but care should be taken not to focus on one area and exclude others. For example, the valve should be considered along with the rest of the process-loop elements to achieve oper-ating objectives. There are three main criteria that may be used to evaluate closed-loop throttling-valve control: process variability, reliability and control responsiveness.

3 Reduced process variability can provide a competitive advantage in manufacturing and can lead to higher operating profits. As an example, some processes have a maximum allowable operating pressure (MAOP, Figure 2), and the closer to the MAOP that the pro-Terry Blevins and Kurtis JensenEmersonClosed-loop control performance depends on the dynamic response of the controller, valve, measurement and process. Can wireless compete with conventional networks? Communication Technologies for Throttling Valve ControlFIGURE 1. Several factors can affect the performance of the control loop for a throttling valveIN BRIEFFACTORS AFFECTING CONTROL PERFORMANCE4 20 mA WITH HARTFOUNDATION FIELDBUS AND PROFIBUS PAWIRELESSHARTCOMPARING INTERFACE TECHNOLOGIESFUTURE ENHANCEMENTS Set point - change frequency Control macrocyclen Sampling intervaln PID executionn Output interval Resolutionn Accuracy Dead timen Actuation capacity Stoking speedn Mechanical velocity Resolutionn Sensitivity Dead timen Filtering and dampening Sampling interval Dead time (delay)

4 Time constant (lag) Gain (sensitivity) Process disturbanceControllerMeasurementValvePro cesscess operates (without exceeding MAOP), the higher the profit. By reducing process variability, it is possible to operate closer to the reduce variability, the valve and asso-ciated positioning technology must meet current and future needs. For example, the goal may be to achieve a resolution in valve movement of 2% of span today, but future continuous improvement projects could require a resolution of less than of span.

5 This will rule out current-to-pressure (I/P) transducers and electro-pneumatic po-sitioners, because they cannot perform at this level of reduced variability. Digital valve positioners will be required, as they do a much better job of overcoming backlash and static-friction (stiction) a valve experiences vibration and is not performing as needed, examine the position feedback technology and eliminate linkages. For the valve assembly, review the specifica-tions and verify that it can meet your needs today and in the future. Remember that the process sensor and transmitter accuracy must also meet refers to the integrity of the com-ponents involved with the control loop and the fail-safe behavior of the valve.

6 Single points of failure can compromise the avail-ability of the process loop. Redundancies in equipment can be used to eliminate critical concerns and increase mean time between failures. This approach is common in safety integrity level (SIL) design and can be used in general process design, but redundancy does increase cost. Responsiveness relates to the capabilities of all the components in the control loop. These include the sensitivity of the process sensor to the resolution accuracy of the valve. Each component should be exam-ined, with priority assigned to addressing those with the greatest impacts.

7 For ex-ample, a process sensor with a 1%-of-span resolution and a valve assembly that can modulate with a resolution of 5% of span would suggest making valve improvements first, as these would have the greatest posi-tive impact on variability. Networking technologies can also impose limitations, with an example being the fre-quency of measurement update and control-loop execution. This may limit how quickly the control can respond to unmeasured dis-turbances. Delays introduced into the con-trol loop involve the entire processing cycle from process sampling, to calculating, to delivering output.

8 The total delay time is im-portant because it directly influences control FIGURE 2. Operating a pro-cess close to the maximum allowable operating pressure (MAOP) can mean maximizing profitsVALVE AND POSITIONER TECHNOLOGYC ontrol valves may be the most important part of a control loop, but sometimes they are the most neglected. They are a leading cause of pro-cess variability and poor control in loop performance. Improvements aimed at the process or equipment require understanding important aspects of throttling valve control. Variability and responsiveness are the most sought-after improvements, and both demand a digital valve solution with position feedback technologies to deliver better accuracy, and to address stiction and backlash.

9 Travel deviation, drive signal and cycle accumulation alerts indicate when a valve needs attention. One test in particular the step response test will analyze how a valve responds to small step input changes (Figure 3). For example, a valve requiring a 5% input change to move would be a clear target for valve controllers offer several advantages, including the following:Start-up and commissioning. Digital instrumentation includes the ability to perform auto-calibrations. The result is every valve is commis-sioned the same, eliminating differences introduced by personnel, and commissioning is completed faster.

10 Operating mode. If the valve positioner has a problem, it can switch modes (for example, changing from position feedback to pressure feedback). If it does, the benefit is continued operation and corrective actions started sooner via an alert sent to maintenance, preferably before process variability is mode. When placing the control valve in a state that does not act on a signal from the control system, the automation system must be aware. If a service technician performs maintenance on a valve, he or she may place the valve in an out of service state and perform requested actions.