Process Valve Networking 101: What, Why and How Much?

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Laura Fleming
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1 Process Valve Networking 101: What, Why and How Much?

2 Manufacturers have quickly come to realize that the only way to see a return on investment is to optimize enterprise and plant operations at every step. In order to do that, they look to automation strategies that use data collection to gather information on plant equipment, devices and processes. Information collected from technology that connects valves to enterprise-wide open control networks enables manufacturers to not only make better informed decisions, but also to find ways to lower operation and maintenance costs. An Actuator Sensor Interface (AS-I or AS-Interface) Bus is one of those networks. Below is a tutorial about how an AS-I Bus can reduce installation costs by up to 40% from conventional point to point wiring. Process Valve Networking 101 There are a number of standardized protocols to select from when choosing the optimal network for your plant installation. In most field-based applications, a two-tiered bus network system is preferred - a higher level bus (such as Profibus PA or Foundation Fieldbus) for the analog dominated control loop, and a device bus (such as AS-I or DeviceNet) for discrete batching applications such as on/off valves. The higher level protocols such as Profibus PA and Foundation Fieldbus have been designed to replace the use of conventional 4-20mA analog instruments in a process control loop. Protocols such as DeviceNet and AS-I are used for discrete applications depending on the specific application and control system interface. For the purposes of this article, we will focus on AS-I, discussing its features and benefits, network construction and how to get started. Catching the ASI Bus By utilizing an Actuator Sensor Interface (ASI or AS-Interface) Bus, plants can network discrete automated valves and plant instruments. Bus refers to the shared communications line with which a set of devices are networked. ASI is a two-wire interface; one wire for power and one for data. In a traditional plant installation, each instrument/device is independently wired and connected directly to a central control location and power supply. With a valve network, like ASI, all of the valves and their accessories (limit switches, transmitters, transducers, etc.) share communication and power on those two wires. This greatly simplifies the installation and maintenance of the devices. So companies using an ASI (and there are thousands of installations in the process industries) are finding a reduction in installation costs and plant infrastructure requirements while improving their process flexibility and maintaining instrument reliability. AS-I has become the preferred choice in discrete applications for a number of very good reasons: Exceptional Reliability AS-I has extremely high tolerance to electromagnetic interference and utilizes simple robust electronics in field devices. Also, short circuit-protected drop connectors may be easily installed for device fault protection. Easy Installation With flexible wiring topology, no restrictions other than total length are needed on wiring. Standard, unshielded, two-wire (power and data) cable suitable for general purpose and hazardous process environments may be used. Seamless Interface with Most Control Systems A wide variety of gateways 1 are available to tie AS-I into most commercially available DCSs and PLCs as well as higher level protocol networks. Low Cost Installation AS-I field instruments and valve communication terminals (VCTs) with related solenoid valves are economical and readily available. Significant Delivery over the Network Conventional, low power solenoid valves for on/off valves may be powered directly over the network. 1 A gateway is an internetworking system that joins together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both. 2 Process Valve Networking 101: What, Why and How Much?

Proven Technology Millions of AS-I devices have been installed by numerous instrument manufacturers in both process and manufacturing industries throughout the world.

Conventional Systems Batching valves with conventional wiring networks means attaching each of the inputs and outputs to a central location.

3 Proven Technology Millions of AS-I devices have been installed by numerous instrument manufacturers in both process and manufacturing industries throughout the world. This standardized IEC protocol has been in existence since 1994 and has operated reliably in many mission-critical applications. AS-Interface vs. Conventional Systems Batching valves with conventional wiring networks means attaching each of the inputs and outputs to a central location. This can result in over 5 individual wires to each field device. (See figure 1) Large expenditures are needed for cabling, conduit, installation labor and computer input/output systems. Also, a large amount of space and infrastructure support must be allocated for a relatively small number of automated valve systems. With AS-I, using the same number of automated valves, one gateway/master may be used with a single two-wire cable being connected to up to 62 field devices. (See figure 2) Data and power are transmitted over the cable to each of the AS-I field devices. The AS-I valve communication terminal (VCT) gathers the open and closed inputs and powers up one or two solenoid valves. Ancillary devices may also be fed into the valve communication terminal from other remote, discrete devices. Figure 2. AS-Interface Network When comparing conventional wiring to AS-I wiring on a network with 16 automated valves located an average distance of 70 meters from the control room, the savings are compelling to say the least. With conventional wiring methods, each automated valve s total installed cost averages $2,435 vs. an AS-I network with a cost average of $1,230 each. This represents a net savings of $1,205 per installation, or a 49% reduction in costs. (See figure 3) And to take it one step further, this savings doesn t even include additional cost reductions acquired through less space consumption for wiring, conduit and input/output racks. In addition to costs savings, other benefits include greater flexibility in adding field devices and/or reconfiguring the network later. Figure 1. Conventional Wiring Process Valve Networking 101: What, Why and How Much? 3

4 Figure 3. Installation Cost Comparison (Per each automated valve system) Conventional AS-Interface Computer I/O; /Gateway $70 $50 Conduit, Cable Tray, Wiring and Fittings $1,400 $200 Valve Monitor/VCT and Solenoid Valve $315 $450 Switched Protected Drop Connector NA $160 Installation and Commissioning Labor $600 $250 $50 $30 Total Installation Cost $2,435 $ 1,230 Total Installation Savings - $1,205 per automated valve system Basic Assumptions: One segment consists of 16 automated valve systems. Automated valves are located in a cluster an average distance of 70 meters. Total bus network length is 90 meters. Network drop legs are short circuit-protected and may be de-energized in the field. Costs may vary depending on specific applications. Figure 4. AS-Interface Network Components / Gateway Protected Drop Connector Field Device Constructing the AS-I Network Three essential components are needed for an AS-I network: The master/gateway, an AS-I power supply, and an AS-I field device module. Another component, which is recommended for missioncritical process applications, is a short circuitprotected drop connector. (See figure 4) Interconnection of the devices is done with standard 16 gauge two-wire unshielded, untwisted cable (Lamp cord). 4 Process Valve Networking 101: What, Why and How Much?

5 Each of these components has the following functionality: and/or Gateway A master acts as the control for the network and accumulates all I/O data from each of the devices. A master/gateway, in addition to the master function, acts as a protocol converter transmitting I/O data from the AS-I network to a higher level protocol such as Profibus DP or Modbus. In an AS-I network of 30 devices, (with 2 inputs and one output per device) all 90 I/O points are scanned by the master in less than 4 milliseconds. s may read both discrete and/or analog information depending on the field devices connected to it. Both power and signal are transmitted at 30 VDC over the two-wire bus cable, thus a special power conditioner must be tied to the power supply decoupling the data. Up to 8 amps may be carried on the AS-I network, but typically less than 2 amps are required for most applications. Field Devices In order to directly connect to the network, the field device must integrate an AS-I application-specific integrated circuit (ASIC) which transmits the I/O messages to the master/gateway. Each device has a specific address on the segment and will communicate input and output information with the master once during every scan. Protected Drop Connectors A connection tee with short circuit protection is recommended at each drop off of the main trunk to a field device. Should a short circuit or fault occur on the drop or field device, the protected drop connector will prevent any interruption of the signal over the network. Wiring the Network Each AS-I segment may be connected with up to 100 meters of cable using any type of topology. (See figure 5) Additional cable lengths may be used with repeater/power conditioner modules. Several repeater/power conditioners may be used in parallel and up to two repeater/power conditioners in series resulting in several hundred meters of effective cable distance. A typical application may consist of several gateway/masters connected to one backbone bus (Ethernet, Profibus DP or Modbus) and an AS-I segment of up to 31 instruments (Up to 62 instruments with extended addressing) running off of each master/gateway. The master/gateway file appears on the computer as a node on the backbone bus with associated data from the AS-I network or as a block of I/O which is mapped into the process software similar to conventional I/O. Process Valve Networking 101: What, Why and How Much? 5

6 Figure 5. AS-Interface Topologies Linear with Spur Topology Getting Started Contact a local distributor for a complete array of AS-I components specifically designed for your process. Have your distributor work with you to select a manufacturer that s right for your needs be sure to specifically enquire about the breadth of their product line and any service packages available. RING Topology TREE Topology LINEAR WITH REPEATER TOPOLOGY Repeater 6 Process Valve Networking 101: What, Why and How Much?

7 Europe Vanha Porvoontie 229, P.O.Box 304 FI Vantaa Finland tel , fax North America 44 Bowditch Drive, P.O.Box 8044 Shrewsbury, MA 01545, USA Tel , fax South America Av. Independência, Iporanga , Sorocaba-São Paulo, Brazil Tel , fax /49 Asia Pacific 20 Kallang Avenue, Lobby B, #06-00 PICO Creative Centre Singapore , Singapore Tel , fax China 19/F, the Exchange Beijing, No. 118 Jianguo Lu Yi, Chaoyang Dist Beijing, China Tel , fax Middle East Roundabout 8 Unit AB-07, P.O.Box Jebel Ali Freezone, Dubai, United Arab Emirates Tel , fax For further information please contact one of our regional offices or visit our web site P591, Issue 12/2011. Product names in this brochure are all trademarks of Metso Automation Inc.

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