Fieldbus Control Systems for Valve Actuators

There are now over 100,000 Rotork actuators installed with a fieldbus connection of one sort or another, being used in over 2000 systems around the world. Many of these use popular industry standard open protocols such as Profibus, DeviceNet and Foundation Fieldbus, but the majority use Rotork’s proprietary Pakscan system.

Pakscan has been continually developed to meet the growing demands of valve automation over the last 20 years and is the system of choice for many users.

Why use a network?

Traditional control systems use individual cables for each signal to be gathered or supplied to the field equipment. These cables are taken to interface panels and thence back to the control computer or PLC. As the actuators have become more important this has led to more and more cables being required for the collection of the actuator data. Today this can result in more than 20 individual terminations being required at each actuator.

The proliferation of cables, cable trays and interface racks etc, with the associated installation costs, increased in proportion the size of the plant.

Imagine the impact of digital network technology! Suddenly, large numbers of parallel wires can be replaced be single two, three or four core cables interconnecting scores of individually addressable actuators and carrying all – if not more – information between the plant and the control room.

Installation costs are dramatically reduced, plant efficiency is enhanced and reliability is improved. Furthermore, digital communications can fully exploit the information gathering potential inherent in intelligent actuators to facilitate improve diagnostics and predictive maintenance.

These new systems allow for rapid plant upgrades and simplify the work involved in altering plant functionality. This allows the owner to more readily adapt their plant to changing market requirements. Engineering work is greatly simplified and commissioning times reduced. Finally the cost of ownership will come down despite a higher initial cost for some of the hardware elements.

Actuator Network Capabilities

Proprietary and Open Systems

Rotork was one of the first manufacturers to introduce a network capability to the world of actuators and valve control. The Pakscan product is unique to Rotork and uses a proprietary (or closed) protocol for the network communication. The advantage of a Proprietary system is that the network can be optimised to exactly match the needs of the elements connected to it.

For example Pakscan can cope with extremely long data highways (up to 20km) without the need for any repeating devices, it is able to cope with the removal of power from any actuators on the system at any time and also protects against cable failures by providing dual communication paths at all times.

There are on open systems that can do this unless they are extensively modified from their original design. Almost all cable systems, other than Pakscan, will be compromised by the need to cover such long distances. Pakscan has been continually developed over the last 20 years to meet the needs of its intended market.

The same period time has also witnessed the development and introduction of open systems from Modican (Modbus), Siemens (Profibus), Allen Bradley (DeviceNet), Emerson (Foundation Fieldbus) and many more. These various standards have been passed into the public domain, documented and controlled by IEC and other authorities in the USA and Europe as well as other parts of the world.

Once released for public use the new open protocols have been adopted by many different types of instrumentation and control systems, though their own origins are often easy to see.

The advantage promoted by open systems is the ability to mix and match equipment from several different manufacturers on the same bus highway. Thus a Profibus network can support actuators, flow sensors and motor drives on the same data highway. This can be a particular advantage where there are only a few devices of each type: in these cases the use of independent, proprietary buses for each manufacturer is uneconomic.

Choosing between a Proprietary and Open System

Making the choice between proprietary or open system is an important decision when selecting a network system for valve actuators.

Unlike virtually every other item of plant equipment, the valve actuator is generally inactive for over 90% of the time. However, when it is required to operate it is usually for an important reason, therefore knowing that the actuator is available for the task and that it is successfully completed are essential requirements.

Any of the open systems is capable of reporting the actuator status and carrying the command to move the valve. The choice between them depends on a number of factors such as distance from the control room, number of devices included on the network, the availability of compatible equipment for other tasks, the integrity of the control required and so on. So the advantage of an open system to enable multiple devices from several manufacturers to share a network may make one of them the preferred choice. However, there is a penalty for this choice.

All open systems have a relatively high data overhead to allow for message transactions, configuration and other network maintenance functions. This overhead has the effect of slowing the data refresh so that they must run at high communication data rates to restore a suitable refresh time.

So why consider a proprietary system? Well, it must offer some advantages and Pakscan can claim to have many. It is often the case that the valve actuator network only carries Rotork actuators. When that is the case then the use of the Pakscan system will immediately identify many other advantages.

Because Pakscan has been specifically designed for actuator control it is geared for use on sites where they are found. Pakscan can achieve much longer highway lengths than any open system, it can offer impressive data refresh times despite using apparently slow communication rates and, in addition, it offers increased safety by including a high degree of fault protection.

Pakscan also lends itself ideally to the practice of isolating the valves by powering down the actuators and this action does not affect the communications highway at all.

Pakscan will often prove to be better than an open system once all the operational parameters are examined. This, coupled with the latest developments involving Ethernet technology, makes Pakscan the network of choice for Rotork actuators.

Rotork Pakscan

Case Study:

Camden County (NJ) Municipal Utilities Authority realises significant cost savings with IQ actuators and Pakscan.

The Camden County (New Jersey) Municipal Utilities Authority (CCMUA) has installed a sophisticated system of Rotork valve actuators, two-wire communication loops, and software in the primary sedimentation tank area of its main Delaware No. 1 Water Pollution Control Facility.

According to Herman B. Engelbert, Executive Director of the CCMUA, the installation has resulted in major operational, maintenance, environmental, and cost-saving benefits.

Specifically, the automated sedimentation tank system that CCMUA installed has been able to significantly reduce operational and maintenance costs, improve plant efficiency, help the facility exceed EPA environmental permit parameters and greatly reduce odour problems in the residential areas near the plant.

The Problem

John Connelly, CCMUA’s assistant director of operations and maintenance, said that automating the processes associated with the sludge and scum operation in the primary settling tank area became a very high priority for CCMUA. In 1999, the CCMUA focused on what, if any, improvements could be made to its ten primary sedimentation tanks.

Connelly said, “Every time we had heavy rain, an operator would have to manually open as many as 30 influent weir gates. In addition, we had to do a lot of maintenance on 30 existing scum gate actuators from another manufacturer. All in all, the system was outdated and labour intensive."

He said, “When we looked for a new method of control, we wanted a system that would provide a very high level of automation, and one that would require a minimum of maintenance. Also, it needed to interface seamlessly with our Bristol Babcock DCS.”

The Solution

Robert Cornforth, director of operations and maintenance at CCMUA, said, “Because we are a public agency, we have a special commitment to our constituents to make sure that we have extremely reliable systems and software in place to meet our permits from the EPA and to exceed those parameters.”

“Also,” Cornforth said, “we have been diligent in keeping our rates down. We have a goal of looking for the most reliable and cost-effective operational solutions.”

After a thorough search of available technology, CCMUA decided to install 30 Rotork IQ actuators on a Pakscan two-wire communication loop to operate the influent gates at the ten primary sedimentation tanks. In addition, they retrofitted 30 influent weir gates with IQ actuators.

The Edwin Elliot & Co., a Rotork sales representative located in Lafayette Hill, PA, worked with CCMUA to help make the project a success.

Caption: There are ten primary sedimentation tanks at the Delaware No. 1 plant. Wastewater takes about 12 to 15 hours to flow through the tanks. During primary treatment, solids settle to the bottom of the tanks and form into sludge. The sludge is scraped off the bottom and held for further processing. Also, oil and grease scum is skimmed off the top and delivered to scum wells. A total of 60 Rotork IQ actuators on two Pakscan two-wire communication loops automatically operate the influent sluice gates as well as scum slide gates.

Rotork Product News

Rotork adds Ethernet connection to Pakscan two-wire valve actuator control

The latest version of the Rotork Pakscan two-wire digital control system for valve actuators incorporates an Ethernet connection from the loop master station. Designed specifically for actuator control, Pakscan has been continuously developed throughout two decades and, with hundreds of worldwide installations, is the system of choice with many users. The introduction of a Pakscan Ethernet master station now provides significant additional user benefits.

The Pakscan Ethernet master station, which is capable of controlling up to 240 actuators on a single loop, is connected to a PLC or DCS over an Ethernet LAN using Modbus–TCP protocol, itself one of the easiest Ethernet protocols to use. The connection enables up to ten coincident users to access data from the master station and operates on 10BaseT and 100BaseT networks.

One of the master station’s strongest features is the inclusion of an embedded web server. Operators can simply browse to the Pakscan control loop using Internet Explorer or similar software. Once the password is entered a comprehensive and impressive level of data is available. Screens show the master station configuration, the order of the valve actuators on the Pakscan loop and the condition of the loop communications.

For each actuator there are pages showing current status, alarms present, tag data and, for Rotork’s IQ and IQT intelligent actuators, operating torque profiles. With the right access the valve actuators can also be controlled from these web pages. If an alarm is generated on the system, from either an actuator or the master station, the system can send an email to a specified address indicating the nature of the fault and the time that it occurred.

The Rotork Pakscan Ethernet master station integrates seamlessly with PLC and DCS equipment and can be combined with Rotork’s In-Vision SCADA software for multiple user access with customised plant specific graphics. In-Vision also adds alarm and event logging to the system. For existing Pakscan installations, master station hardware upgrades to include Ethernet connectivity are available for retrofitting on site.

Caption: Pakscan Ethernet web server screens for the Pakscan Masterstation and IQ/IQT valve actuators.