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The introduction of Distributed Control Systems (DCS) with universal process I/O has proven to be a vast improvement in process automation system infrastructure. The ability for true soft configuration of each channel as either an analog input (AI), analog output (AO), Digital Input (DI), or Digital Output (DO) with HART communication protocol support for AI’s and AO’s has allowed faster project execution, late project changes, reduced marshalling and fewer spares. The Honeywell Experion® PKS system provides this capability with Series C I/O and, ControlEdge™ UOC/PLC Series 900 I/O with no additional hardware modules required. Though not universal I/O, the ControlEdge™ PLC/RTU I/O includes HART functionality.
While dedicated HART AI & AO channel hardware has existed for several years prior to soft configurable I/O, the release of universal process I/O has further incentivized the move to 24VDC powered field devices. Low power DI and DO discrete field devices, such as pneumatic solenoids and valve position switches, are specified when universal I/O is implemented and required when powering the loop direct from the card.
The Honeywell UIO solution allows up to 0.5A per channel but with a 9A limitation per 32 channel card. The power limitation is inconsequential if Universal Marshalling is used and allows for broader input and signal conditioning options.
With greater channel selection flexibility of UIO, the cost variances between analog and discrete I/O disappear. This means that from a control system standpoint, there can be less incentive for field devices to remain discrete if an analog (4-20 mA/HART) option is available.
Digital bus protocols are used in process automation system applications and many of these such as Fieldbus, Profibus, DeviceNet and ASi have been used in lieu of traditional analog (4-20mADC) and discrete signal transmission. The implications of using these protocols is beyond the scope of this publication.
An analog signal, such as 4-20mADC, uses electrical current proportionately to represent measurements or command signals. “Discrete” control system signals, in the context herein, refer to the presence or absence of electricity. When using UIO solutions, DO’s are typically limited to 1.4W with 24VDC power.
How does this affect automated on-off valves?
What is not as well known is that discrete valve controllers for automated on-off valves, utilizing 4-20mA/HART position feedback, can be cost-comparable with the traditional, individually specified, pneumatic solenoid and actuator mounted switch box (discrete valve position indication). In addition, moving to an automated on-off device with 4-20mA/HART valve position signal can provide valuable diagnostic data to a HART I/O-capable DCS; especially those systems equipped with open, standardized technologies such as enhanced EDDL (Electronic Device Description Language) or FDT/DTM (Field Device Tool/Device Type Manager). Local set up of the devices is also simpler with local pushbuttons or, in some cases, a wireless interface.
Consider the two assemblies shown above. Both are typical 4” CL150 full port, soft seat ball valve assemblies with spring return rack & pinion actuators. The assembly on the left has a bracket mounted 3-way single coil solenoid with StoneL® Quartz™ series proximity switch. The assembly on the right replaces the function of the switch and solenoid with a StoneL® Axiom™ discrete valve controller. The Axiom™ equipped assembly only has a modest 3.5% initial cost premium over the conventional assembly and the Axiom™ provides significant advantages.
While there are many variations in how automated valves can be engineered, specified and constructed (details of which are beyond the scope of this publication), the most common and simplistic definition is used here for purposes of illustration. Automated On-Off Valves are simply defined in their most common arrangement as air actuated and discretely controlled to be open or closed. A conventionally constructed spring return actuated valve assembly includes, at minimum, a 3-way solenoid, which when de-energized by the control system via the Digital Output (DO), will block the air supply while venting air from the actuator, thereby causing actuator spring action to move the valve to close when assembled as a spring-to-close valve and vice versa for an assembled spring-to-open valve. When the solenoid is energized, instrument air flows into the actuator while blocking the vent, compressing the actuator spring, moving the actuator to open the valve that has been assembled as spring to close and vice-versa for the spring to open assembly. An illustration is shown below for a rack & pinion actuator.
Many Automated On-Off valves include Position Switches which provide discrete indication of valve position (typically open and close) via a Digital Input (DI) at the control system. Position Transmitters typically provide continuous feedback via an AI to the control system of exact valve position.
Control Valve Positioners receive an instrument signal that indicates desired valve position and then regulates the supply of air pressure to the actuator based upon feedback of valve position. It is essentially a local valve controller with a setpoint provided by the control signal, a process variable which is valve position and then control output of air pressure to the actuator to meet the position setpoint. HART smart positioner/controllers can provide diagnostic data via the HART signal and this may include actual valve position.
A HART smart positioner receives a 4-20mA signal which indicates the desired position of the valve. A control valve positioner regulates the supply air pressure to the actuator based on the feedback via the positioner feedback arm. Pressure to the actuator ranges from zero to full supply pressure. It is not unusual to see control valve positioners used for automated on-off control. DCS systems can be configured to drive an analog output full scale for full open/close operation of a valve. While using a HART smart control valve positioner is always an option for controlling a two-position valve (automated on-off function), the average cost of a typical HART Smart positioner/controller is over 2X that of the discrete valve controller. Even considering the advantage of valve position control using one 4-20mA to the positioner and valve position indication accomplished via HART input at the DCS, the incremental I/O savings is less than $400 per device, and this assumes that latency with the slower HART valve position feedback can be tolerated.
Axiom Discrete Valve Controller with HART – DCS with HART I/O
With or without HART communications, Axiom™ has tremendous advantages of intuitive local interface, universal power, can be used for double acting or spring return actuators (with spring side rebreather capability), allowing consolidation of plant spares. The basic construction of the Axiom and connectivity to the HART I/O equipped DCS is shown below.
Advantages of using the Axiom Discrete Valve Controller with HART
The reasons why traditionally constructed Automated On/Off Valves fail are shown in the bar graph below. Axiom™ equipped automated on-off valves have continuous position sensing, pressure sensing, current/voltage sensing and alert on low supply pressure, malfunctioning solenoid or stuck spool/pilot valve.
Trend data for open/closed “dead” time, stroke time, position offset and pressure levels (inferring required torque changes) are included. Alarms for air pressure, circuit board temperature are also included with all data stored for over 1MM cycles.
Axiom™ HART Diagnostics promise to improve uptime and reduce maintenance costs, allowing for:
- Optimal maintenance scheduling
- Quicker problem determination
- Operating health acceptable at the DCS
- Detection of problems before unplanned downtime occurs
- Ability to perform remote settings and calibration.
These tasks are accomplished through:
Remote calibration and monitoring
- Open and closed switch settings
- Identification winking
- Pneumatic pressure low and high alarm settings
- Monitor exact position, pressure, and electronics temperature
Identifying root causes quickly
- Low and high pressure alarms
- Malfunctioning solenoid
- Stuck pneumatic spool or pilot valve
- Stuck process valve or actuator
Predictive diagnostics
- Pneumatic pressure alarms
- Open and closed pressure thresholds
- Critical parameter trending and baseline comparison
- Historical trending of key parameters
HART I/O and Asset Management System Equipped Process Automation Systems
The graphical interface for calibration, commissioning and diagnostic data access (examples shown above and below) can be achieved via process automation systems that are equipped with EDDL (Electronic Device Description Language) or FDT (Field Device Tool) compatible asset management systems (Many systems handle both EDDL and FDT today). EDD files for the EDDL capable systems and DTM (Device Type Manager) files for the Axiom™ can be downloaded from the StoneL website.
The Emerson AMS, Siemens PDM, and Honeywell FDM asset management systems have EDDL capability. FDT compatible systems include Honeywell FDM, Yokagawa PRM and ABB AVM.
Conclusion
Given all the demonstrated advantages, there is considerable justification for moving to the Axiom™ discrete valve controller over conventional solenoid/switch arrangements when specifying automated on-off valves. If a customer’s conventional or approved solenoid must be integrated in an automated solution then the StoneL Quartz solution with standalone discrete or 4-20mA position feedback is an attractive option for a variety of reasons, not the least of which is local pushbutton setup. The case for the Axiom™ HART becomes even greater for process control engineers attempting to take full advantage of HART integration at the process automation system with remote monitoring, greater field device dependability and improved process uptime. Both the StoneL Quartz and StoneL Axiom™ have been in service all over the world in the harshest environments for many years with dependable results.
Additional Contributions
- Jeff Peshoff – AWC, Inc.
- Brett Cook – AWC, Inc.
