Simplicity is in the Eye of the User

Incorporating plant and operator needs is essential to effective automated control and telemetry system design

• By Troy A. Hertog
• Jul 01, 2007

While the demand for automation is on the rise, facility managers interested in simplifying their operations should ensure that such a system actually will make work easier.

Automating control and telemetry in water and wastewater treatment, distribution, and collection systems adds a level of complexity to overall system design. Addressing that complexity on the design end should ensure that an operator can confidently use the system.

Simplifying the system
Many system designs provide basic operating information, but they often omit key troubleshooting diagnostic tools. In general, users need a control and monitoring system that not only provides information about the system it is controlling and monitoring but also about the control and monitoring equipment itself. Knowing how to use these tools can help an operator quickly find and fix a problem that occurs with the process or with the equipment controlling the process. This concept is important in simplifying an automated control system.

To get the optimal balance of information for operating and troubleshooting an automated system, the selected technology or product should be made for the application. A product made for a specific application usually is developed by a manufacturer familiar with the process and the end-user’s needs.

Application-specific control and monitoring devices are usually • easier to operate, • have built-in status and diagnostic features for troubleshooting, and • often cost less than generic devices.

Updating existing systems to include more information and, if feasible, adding an operator interface capable of displaying system and diagnostic information at every site are two other important measures. Operator interfaces are an efficient way of presenting a lot of system data. Having locally available system information, including site alarm and event histories, process trends, processor, communication and I/O status, is key to facilitating troubleshooting. The local operator display without the use of specialized equipment, software, or knowledge is preferred.

Nonapplication-specific operator interface components still can be used. However, system designers should carefully identify and build in required information and diagnostic or troubleshooting functions that will simplify a system’s operation.

Easy accessibility to automation indicators, selector switches, push buttons, and resets are design considerations that will make the control panel more user-friendly. Ideally this equipment should be mounted on the front of the panel/dead front inner door. Obtaining the needed system/troubleshooting information while avoiding exposure to high voltages increases safety. If possible, new panels should be constructed with components mounted to permit easy access and have additional space for future modifications and enhancing serviceability.

Reducing failure risk
Several factors will make an automation system more reliable. Selecting maintenance-free or low-maintenance technologies made for the application is one of the best ways to improve system reliability. This is especially true of sensor selection. Because sensors convert a physical change into an electrical signal to provide vital information, they often are mounted in harsh environments, where damage and fouling can occur. The control and monitoring system depends on the sensor’s reliable operation.

Personal computers (PCs) used in automation present another area of concern. As computers, operating systems and application software become more powerful, and they are used more frequently in automation designs. Some designers try to use a PC as the main controller or telemetry device as well as the graphical operator interface and data historian/report generator. In addition, they will incorporate an alarmdialing software package on top of the process applications software.

Despite the fact that these systems cost less initially, and operating systems and application software have improved the reliability of the PC system, it is still not a good idea to use a PC for control/telemetry functions. PCs used for automation are less reliable than their dedicated controller/telemetry unit counterparts.

Computers used for water and wastewater applications become unreliable due to the high volume of data accumulated and archived. Hard drive failures are the most common problem. A supervisory control and data acquisition (SCADA) system operated 24/7 is continuously updating. The updated data frequently are written to the hard disk, which wears down the hard drive three to four times faster than if it were used as a typical office computer.

Another cause of PC failure is the environment in which it operates. Typically, PCs are installed at a plant site that has large electrical loads that turn on and off, causing electronic spikes and surges. Plants often have high levels of humidity and other suspended contaminants that get inside the machine, cause components to deteriorate, and interfere with the machine’s ability to cool the electronics. PCs should really only be used as a window to the process that is monitored and controlled.

Even really good equipment can perform poorly if not installed properly. Installation practices should include having a good electrical ground and shielding and isolating low-voltage wires from those carrying high currents. All electronic protection systems rely on a good grounding point. Even though the National Electric Code (NEC) requires 25 ohms or less resistance to ground, it is a good idea to design a ground system to 5 ohms or less. By comparison, the U.S. military requires 1 ohm or less for installations that use electronics.

It is important to seal all conduits that are connected to the electrical control panel— especially conduits that extend into areas that have high-moisture content or corrosive/explosive gases. All wire terminations should be made and fastened tightly to suitable terminal blocks. (Loose wires cause intermittent problems and are dangerous.) Poor installation practices account for more than 90 percent of electronic equipment failures.

A design with ample spare capacity will make a system more reliable. Running equipment with load-carrying devices, such as power supplies, transformers, relay contacts, and selector switch contacts, at maximum capacity will lead to excessive heat and eventual premature component failure. Every current-carrying device has associated resistance that causes heat when electrical currents run through the device. Higher currents bring higher heat and a greater potential for damage or shortening equipment life. Motor starter selection can affect system reliability.

Water and wastewater system designs often call for switching large electrical loads associated with starting and stopping motors. Smaller footprints and lower initial costs are making International Electrotechnical Commission (IEC)-rated components more popular to use in switching these loads. The tradeoff is that the IEC rating is not as heavy-duty as the National Electrical Manufacturers Association (NEMA)-rated equipment.

Tests have shown that the extra capacity found in NEMA-rated components can double and sometimes even quadruple a motor starter’s life expectancy and thus make a plant’s system operate more reliably over a longer life.

Electronics often are installed in locations subject to temperature and humidity extremes that are outside of their rated operating environment. The system design must maintain a proper environment for the selected equipment’s operation. High heat and below-freezing temperatures can cause permanent damage or, at a minimum, greatly reduce system component life and reliability. Heaters, sun shields, and ventilation/ air conditioning equipment should be used in applications where temperature extremes exist.

Good panel design also should include a heater for condensation protection in environments with high humidity. Airborne moisture that condenses on electronic circuits will cause electronic shorts and equipment corrosion and oxidation, shortening the system’s operating life.

Most control systems are designed to monitor equipment that is controlling the process (motors and valves) as well as the process itself (levels, pressures, flow rates, and turbidity levels). When a problem is detected, an alarm horn sounds, and the affected equipment or process is usually shut down until manually placed into operation.

Eventually, every piece of automation equipment will fail and disrupt the control and/or monitoring of a process. The control/telemetry system can be less reliable without mitigating the risks of automation failure. The most common way to mitigate automation failure is through good sound system design practices. It is important to determine critical functions that must be maintained as well as which types of automation/equipment failure would cause a disruption. It is equally imperative to evaluate the user’s risk tolerance for specific types of failure.

Ideally, the system should be designed so that in the event of an equipment failure, operators have a contingency plan that includes what needs to happen and when, enough information to troubleshoot the system, an alternative means of operating the system, and use of other available sensors to operate the system in skeleton mode.

Backing up the system
Back-up systems help keep mission-critical functions operating until the primary system is repaired. A back-up system should be based on a different technology, whenever possible. Otherwise, the back-up system may be susceptible to the same conditions that damaged the primary system. For instance, a float switch system can be used to back up a submersible level transducer system. If the sensitive electronics of a submersible level transducer have been damaged from a voltage surge and fail, the float switch system will likely survive the surge, continue to work, and take over basic control.

Where sensing equipment is susceptible to malfunction under certain conditions, using different technologies in the system might improve the system’s overall reliability. As an example, a submersible level sensor can be used as a backup to an ultrasonic level sensor. If foaming occurs in the wet well, the ultrasonic level sensor will fail, but the submersible sensor will continue to work. Operators should routinely check backup system operation to make sure it is still available in the event of primary system failure.

A simple switch can be used to disable the primary system and engage the back-up system. A reset button will re-engage the primary system. System reliability can usually be improved, regardless of a system’s age or design. Operators can monitor any problem, whether it is in or out of their control. There are many types of sensors available for monitoring virtually any kind of condition. Although incoming power levels, high turbidity concentrations, or phone line availability are potential problems that are outside an operator’s control, they can be monitored and properly addressed to minimize an uncontrollable issue. If the problem is controllable, these items can be monitored and usually backed up.

Mission-critical functions should be monitored and include a status indication of operation. Sometimes components that are designed to protect other equipment from damage can themselves be damaged and thus hinder operation of the very mission-critical equipment they are protecting. For example, power monitors are commonly used to protect a motor from high- and low-voltage and phase problems. If the power monitor fails and cuts out the motor, the pump would be prevented from operating even though power was good. In this case, simply adding a bypass switch can prevent a nonessential malfunctioning component from inhibiting operation, with operators ensuring the power is still functioning.

Surge suppression is an important design consideration, especially in environments prone to frequent high-voltage discharges (lightning). Equipment often is designed with surge suppression of the incoming power lines, but as with most processes, the control panel will have several other wires that extend into the field. Each wire can be a pathway for damaging electrical surges into the panel. This includes sensor wires, phone lines, control wires, antennas, and more. It also is important to provide surge protection on these lines. Ideally, every wire that enters or leaves the control panel should be protected against surges.

Note that without good grounding, a surge protector will not protect against surges.

Once the system is designed and onsite, it must be kept serviceable and complete. It is highly recommended that the end-user have a secure and easily accessible place to store system documentation. This will help ensure wiring diagrams are available, complete, and accurate. Operators should have documentation of system summaries, product manuals, and bills of materials. For software-based products, operators should ask the supplier to provide them with a copy of the configured software. Depending on the critical nature of the system, a licensed copy of the programming software may be warranted.

Following these guidelines and considerations will simplify a plant’s automation system, providing many years of reliable service.

This article originally appeared in the 07/01/2007 issue of Environmental Protection.