Supervisory control and data acquisition (or SCADA) systems and connected instruments are vital to many water and wastewater treatment facilities.

They perform control functions much faster and respond much quicker than humans. They alert operators to upcoming issues or events that have already happened. They can archive historical data for reports and trending to better gauge what needs to be done in the future.

Plant operations rely upon these systems; failures can cause noncompliance, waste costly chemicals and electricity, and cause bypasses and other operational upsets.

Unfortunately, most SCADA systems are installed and then forgotten until there is a problem. When problems arise, they are addressed on a “get it up and running now” basis. Maintenance is usually limited to calibrating the instrumentation, and often the system itself is not part of any capital or life-cycle programs.

SCADA and control systems are mission critical; it is prudent to keep them in a high state of reliability. Close adherence to basic maintenance practices can ensure these systems function as designed and help keep plants running smoothly and in compliance.

The basic elements

Most SCADA and control systems consist of programmable logic controllers (or PLCs) that are connected to the field devices, such as motors, pumps, variable-frequency drives (or VFDs) and other equipment, and to online instruments and analyzers.

The PLCs are the brains of the system and perform the control work, turning equipment on and off, speeding things up or down, monitoring conditions and collecting data. PLCs are manufactured by a variety of companies including Allen-Bradley (Rockwell Automation), Siemens and Schneider Electric. They have input/output racks or modules that allow connection to the field devices, which have ports used for communication to operator input devices.

PLCs are programmed using manufacturer-supplied proprietary software that produces the control program, which is then downloaded into the PLC. This is what does the control work. Operator interface terminals (or OITs) are also programmed using the manufacturer’s software, which produces the screens needed to access what the PLCs are doing. For a simple control system, there will be PLC programming software, the PLC programs, OIT programming software and the OIT programs.

Control systems may have touch-screen panels (and keyboards), referred to as OITs. A SCADA system has a human machine interface (HMI), which is a software program running on a computer connected to the PLCs. Some commonly used HMIs are VTScada (Trihedral Engineering), Wonderware (AVEVA), RSView (Rockwell Automation), FactoryTalk (Rockwell Automation), iFIX (GE Digital) and Ignition (Inductive Automation).

A SCADA system communicates to one or more control systems or field devices and exchanges operating parameters, setpoints, real-time data and alarms with control systems, PLCs and field devices.

Life-cycle phases

There are four phases to the life cycle of a PLC: development, active sales, after-sale support and obsolescence. Here is a look at each phase.

Development. Here the manufacturer is designing a new product, which may or may not be backward-compatible with existing products. The new product usually includes updates to hardware, communication protocols and security enhancements; it may even require new programming software.

Active sales. In this phase, there are new units on the market. Often, development on the product continues through this phase. The new product is used in new installations and in upgrades and replacements of existing control/SCADA systems.

After-sale support. This phase begins when the manufacturer has discontinued product development. The product may still get critical firmware updates addressing security and operational issues. However, availability of spare parts (such as PLCs and input/output modules) will become tighter as the manufacturer winds down and ceases production.

Obsolescence. This occurs when the manufacturer no longer supports the hardware and no longer has new parts. At that point, the only sources for parts are through secondary market sources including some vendors and online outlets such as eBay. Items purchased through the secondary market do not have manufacturer warranties and support.

A typical life cycle for a PLC is 10 to 15 years. However, if a new SCADA or control system was installed five years ago, that doesn’t mean there are 10 years left in the PLC life cycle. For example, if the system was installed with PLCs that had been on the market for eight years, there may be only four to seven years left in the PLCs’ service life.

Many operators have been around computers for 20 years or more; many remember the systems that came out in the late 1980s and early ’90s and the many changes made since then. In the ’90s, for example, there were operating systems that were disk operating system (or DOS) based, which changed into Windows 3.1 and Windows XP, NT, 7 and 10.

Unfortunately, PLCs often lagged these operating system changes. It is not uncommon for PLCs manufactured in 2000-10 to only have serial or proprietary connections for downloading the control programs. Ethernet ports did not become a common practice on PLCs until later. The older PLCs often required programming software that only runs on Windows XP or requires a laptop with a serial port — and those are no longer being produced.

The HMI and the computers they run on must also be looked at. If the operating system is Windows XP, that is a red flag from a security and product update standpoint. The HMI software needs regular updating as well, especially for security patches.

System maintenance

Water and wastewater treatment operators need to maintain these systems. That maintenance consists of at least these items:

• A complete inventory of the critical system parts. This includes the manufacturer and model of each PLC and each type of input/output module, communication equipment (radios, adapters, network switches) and OIT panels.
• Copies of the latest versions of the PLC and OIT programs and, preferably, copies of the PLC/OIT programming software, as well as a copy of the HMI programs. These must be dated and assigned version numbers.
• Wiring diagrams for all control panels: a complete set in the office and a set in each control panel enclosure. If the drawings are not available electronically, they should be scanned.
• A list of all of the control loops describing their functionality.
• A list of all instrumentation with the scaling used in the programming.
• A list of all the analog outputs and their scaling.
• A map of the network system.
• A copy of all work done on the system by in-house staff or contractors. This information should be in the computerized maintenance management system (or CMMS) as work orders.
• Instrument calibration and cleaning schedules (also available in the CMMS).
• Periodic inspection of the control panels; a function check of the uninterruptible power supply, heaters, lights and other devices in the panel; and good general housekeeping (such as dust and cobweb removal).
• A log of any errors produced by the system and the corrective actions taken.
• Documentation and addressing of nuisance alarms.

Life-cycle planning

Besides these maintenance items, operators need to provide a list of items that need repair or replacement, now and in the near future. Historically, this activity has been limited to major equipment and processes, but it must be applied to SCADA and control systems as well. Communication of needs for repair, replacement and upgrade should be part of the asset management plan or capital plan.

For PLCs that are at or near obsolescence, a proactive approach to upgrade or replacement is highly recommended. A facility without backups of the latest programs in these units is just one lightning strike or power surge away from being down and out with no easy fix to get back up and running. If an HMI is running on Windows XP or has not been updated in more than two years, attention to that is past due as well.

Operators unsure of what type of system, PLCs and HMIs are in place should have a discussion with in-house instrumentation and control personnel, the maintenance staff, or the system integrator. Most important, proactive planning for repair or replacement of SCADA and control systems is critical to keeping a facility operating efficiently and in compliance.

The maintenance and life-cycle planning described here also applies to the control system embedded with process equipment and process operations. Control systems supplied as part of a UV system, sequencing batch reactors, oxidation ditches, dewatering equipment and pumping system need to be an integral part of this planning.

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About the author

Charles Fiero (charles.fiero@inframark.com) is senior process engineer for automation with Inframark Water & Infrastructure Services.