Third in a series. Advances in wireless technologies can help address many common failure modes in mid- to low-level criticality assets and eliminate wiring costs for a range of asset types.
Scanning Systems and Wireless Scanning Systems
Where appropriate, wireless solutions can replace wired scanning systems, primarily where the failure mode detection methods match the technological capabilities of the wireless systems, such as casing vibration (seismic) and temperature data. Wireless scanning systems are only appropriate for essential measurements with half P-F cycles of greater than two hours, while scanning systems can detect failure modes with P-F failure modes of less than 30 minutes. A careful review of asset failure modes, failure cycles, detectability, consequence of an undetected failure and work process upon detection of a failure all need to be considered when deploying a particular technology solution. Failure modes with little warning time or that require automated relays for shutdown are not suitable for wireless applications.
Wireless Mesh Network Components
Wireless mesh topology can complement existing condition monitoring platforms on rotating and reciprocating machines in power generation, oil and gas (upstream and downstream), chemical and other process industries. Various components are used in the wireless mesh topology for condition monitoring.
Low powered casing vibration and temperature sensors are often used on these type assets, although other process variable measurements can also be captured using wireless technology. The sensors typically connect to the wireless sensor node (WSN) via a cable. The WSN is installed on or near to the monitored machine, and can connect to multiple sensors. The WSN powers the sensor, collects the data and may do some minimal signal conditioning and processing. It also contains a radio transmitter/receiver that communicates on the wireless network. Each WSN must be in 50 meters line of sight distance of at least two or three other nodes to form a robust wireless network.
Repeater nodes are available for locations where the WSN density is insufficient or where there are long distances between groups of WSNs. Repeater nodes contain the radio-transmitter device, but do not contain the electronics needed to work with sensors. As such, they do not need to be near machines and are ideally placed in accessible locations where they can maximize the line of sight between nodes, such as in areas with high elevation (eliminating ground-based line of sight interference) or near doorways or building corners.
The wireless mesh network uses a combination of spread spectrum and frequency hopping technologies to provide signal reliability, including Direct-Sequence Spread Spectrum (DSSS) and/or Frequency Hopping Spread Spectrum (FHSS) technologies. Each transmission can transmit among 16 different available frequencies, which allows the network to limit any interference from wireless transmitters that are not part of the mesh network.
Mesh networks are simple to organize and install compared to wired scanning systems. After assigning the nodes to a network gateway, the network is self-organizing. The WSNs will form their own primary and alternate communications routes, depending on other nodes in their radio line of sight. Each node is automatically assigned an IP address. After installation, it may take a couple of hours for the network to fully organize, which is still less time than to install and configure a wired scanning system.
Because of the number of interlinking possible communications paths between nodes, mesh networks are self-healing; if one node no longer works or becomes temporarily blocked, the nodes will automatically switch to alternate nodes and continue to operate, providing a high degree of reliability. Each node only turns on and gathers data or communicates as necessary, reducing the power consumption per node to a fraction of that for continuously powered networks. They are scalable and can be changed to suit the plant needs.
Mesh networks can change their scalability and have tremendous flexibility. If a plant wants to expand its wireless network, it only needs to commission the new nodes and position them as needed in the plant. Similarly, a plant can move WSNs from one location to another. This would only cause a temporary loss in communications, as the WSN would identify that it lost communication with neighboring nodes and would automatically detect new neighboring nodes to rejoin the network.
The network gateway links the wireless network to the Ethernet and manages the network configuration and timing. While each network gateway can manage more than 200 wireless mesh nodes, additional gateways may be required for extensive wireless networks or to manage networks in separate areas of the plant. Each network gateway manages its network with unique join or authentication keys to ensure that only its nodes can access the network, with encrypted session keys to ensure that all transmitted messages are confidential. Additional gateways (to run separate networks) may be necessary if the distance of any node requires more than five "hops" to other nodes to send a message to the gateway.
Most gateways can connect to external antennas, which can then be placed in a location more desirable to maximize connectivity to nearby nodes. The best practice is to have redundant network gateways for each mesh network to enable the backup to continue to run the WSN in the event of a network gateway failure.
The mesh network system will ideally be compatible with the plant's CM software platform. This requires less training, and the similar capabilities between wireless and other platforms make it easier to configure similar alarm setpoints and align maintenance strategies between similar assets. It enables the plant to easily upgrade from a wireless scanning system to a continuous monitoring platform.
The CM platform must be able to collect data through a number of separate network gateways and correlate this data with other continuous, scanning and portable data collection systems. The CM software should be capable of displaying process variable data, temperatures and static and dynamic vibration data. A static value is a measurement that can be depicted by a discrete value, such as direct acceleration, rotor region (1X) acceleration, prime spike acceleration, direct velocity and direct enveloped acceleration. Enveloped acceleration
