Connecting to RTUs to monitor & control water & wastewater systems
Drinking water distribution and wastewater collection systems often are widely distributed over large geographic areas, making automated and remote monitoring essential. These functions typically are implemented with supervisory control and data acquisition (SCADA) systems, which can be used to help operators improve efficiency, detect problems and comply with regulations.
The heart of a SCADA system is usually one or more human machine interface (HMI) software platforms, each running on a PC or an embedded operating system, such as Linux or Windows Embedded. These HMIs typically connect to remote terminal units (RTUs), which act as intermediaries to collect, store and transfer operations data to the HMIs.
Once the operations data are received at the HMI, they can be displayed locally on monitors and distributed to mobile devices, such as laptops, smartphones and tablets. To understand how SCADA systems work, let’s start with a high-level view, explore connectivity options and examine HMI implementation.
Drinking water is distributed through pipe buried underground. Much of the impetus for flow is provided by gravity generated by elevated water storage tanks, but pumping stations also are needed in most drinking water distribution systems to maintain adequate pressure.
Wastewater collection systems do not have the advantage of gravity and must deal with water that has been used and contaminated to some degree. These systems rely heavily on lift stations to generate the pressure needed to transport the water to treatment plants.
Both pump and lift stations contain a number of sensor and instrument field devices to gather operations data. An individual sensor may indicate whether a motor is on or off, a valve is open or closed, if a process variable reaches a limit value, or other simple discrete conditions. Instruments provide more complex measurements of analog variables, such as flow, temperature, pressure and level.
These field devices typically are connected to an RTU. The RTU is usually a programmable logic controller (PLC), but some RTUs are custom-designed for water and wastewater applications. The RTU controls and monitors station operation and is connected to the SCADA system in a variety of ways.
An RTU installed at a pumping or lift station contains a number of discrete and analog input modules to receive data from field devices mounted on pipes, motors and other equipment. It also contains discrete and analog output modules to control equipment like motors and valves.
Larger stations may have a local HMI, typically a panel-mounted component with appropriate NEMA and IP ratings. This local HMI is connected to the RTU, usually via an industrial Ethernet protocol such as EtherNet/IP, Modbus TCP/IP or Profinet.
The local HMI or RTU then is connected to central control room HMIs. In the central control room, one or more HMIs, usually desktop PCs, take operations data and transform them into information usable by control room operators, engineers and technicians. The control room HMIs also can be used to distribute information to mobile devices.
There are many connection options to link SCADA system field devices, RTUs, HMIs and mobile devices—and the cost and complexity to implement these different types of connectivity is dropping rapidly.
Field Device Connectivity
The simplest type of connection, and the one with the longest history, is a simple hardwired link. Hardwiring often is used to connect pump and lift station field devices to an RTU, which makes sense because these devices usually are installed in close proximity to the RTU. Hardwired links can be either simple discrete or analog wiring, or digital via some type of fieldbus protocol, such as DeviceNet, EtherNet/IP or Foundation Fieldbus.
But hardwiring presents a number of problems, and thus is often replaced or supplemented by a wireless mesh network. The main problem is the cost of installing and maintaining wireless infrastructure between the field devices and the RTU. Another issue arises when field devices need to be added, as it is generally more expensive and disruptive to install new wiring in an existing pipe system or station.
The two most popular wireless mesh networks used to connect field devices to RTUs are WirelessHART and ISA 100.11a. Both of these networks use a gateway to gather wireless data from field devices, with the gateway hardwired to the RTU via a standard protocol such as serial Modbus or an industrial Ethernet protocol.
Each field device talks to the gateway either directly if it is so equipped or through a wireless adapter if it is not. A wireless adapter receives a discrete or analog hardwired input from a field device, converts the input data to the proper wireless protocol, and transmits this data to the gateway.
A water distribution or wastewater collection system typically will have a number of RTUs installed, particularly at pumping and lift stations. In a SCADA system, each of these RTUs communicates with the central control room HMI.
As with connections from field devices to RTUs, there are a number of options, including wired, wireless and cellular. Wired connections typically are made by purchasing bandwidth from a service provider’s installed network, similar to home networks connected to a cable or phone company network. These networks may not be available in close proximity to stations, necessitating other options.
When hardwiring is not practical, wireless methods must be used to connect each RTU to the central control room HMIs. Popular connectivity options include satellite, cellular and leased radio frequencies.
All of these options can be used to connect the RTUs directly to the HMIs, or through the cloud. A major advantage of using the cloud is ease of access to data from a wide variety of mobile devices worldwide. If data is sent directly to control room HMIs, then each mobile device must access the data from these HMIs. If data are in the cloud, access becomes much easier.
The heart of the SCADA system is the HMI, with the most common implementation using multiple PC-based HMIs installed in a control room. These HMIs acquire operating data from RTUs, and the data are transformed into information to allow monitoring, control and optimization by operators.
The HMI software must offer all of the required connectivity options, including support for hundreds of popular communication protocols. This allows communication with nearly every type of RTU, via wired and wireless data links, as well as communication with the cloud and upper-level computing platforms, such as enterprise resource planning and asset management systems.
At major pumping and lift stations, an HMI often is installed to provide local monitoring and control. These local HMIs do not need the power, data storage and large screens required by control room HMIs, but they do usually need to be mounted in panels and be able to withstand harsh environments.
For these reasons, it can be more cost-effective to use embedded HMI platforms, as they provide much of the functionality of a PC-based HMI at a lower cost and come packaged in housings able to survive potentially hostile environments at pumping and lift stations.
Increasingly, operators want to make HMI information accessible via mobile devices. The right HMI software can provide multiple options to implement this remote connectivity.
The high-performance option is a thin client, which provides remote viewing and interaction with the HMI at a level similar to local operation. Thin clients can be configured to provide varying levels of access, depending on the user’s requirements.
The other main remote access option is browser-based, in which the control room HMIs act as web servers to provide information to browsers via the Internet. The browser can run on any Internet-connected device, including a laptop, PC, smartphone or tablet. Like thin client access, browser-based access can be configured at the HMI to provide the required level of access and no more.
Virtually all water distribution and wastewater collection systems require some degree of remote access and monitoring, and thus a SCADA system of some type. The HMI software is a critical component of the SCADA system, as it is responsible for acquiring data, transforming them into usable information and making this information available to local and remote users.