Considerations when selecting the elements of automated valve & process control
When considering whether a water processing location is a candidate for automation, it is important to understand why automation is implemented. Automation can reduce labor costs, control devices in remote areas, engage a sequence of operations and increase safety. One of the best ways to add automation to an existing system is to use an actuator instead of a hand-wheel and control the valve with a control cabinet.
Choosing the right level of automation and implementing a control cabinet for water applications begins with an audit of the current system. Making a detailed list of valves and equipment that control the process is essential when planning to upgrade a system. Common types of equipment to consider include pumps, valves, sensors and the presence of a compressor. Other important considerations are whether the location of the facility is indoors or outdoors and if hazardous systems or environments exist. Planning for automation also must take into account the type of media that are flowing through the pipes—media other than water may impact the type of components and control panels to be specified.
Sensors Are Key
Consider the types of sensors that the automated controls will rely on for system feedback. Temperature, turbidity, flow and pressure are common measurements in a water facility. Once an inventory of the number of sensors that could be included in the system is made, make a note of which ones are critical for operation, which ones may be luxuries and which ones are required for safe operation. With this list, a minimum and maximum requirement for the inputs and outputs can be determined. Different types of systems can be configured to allow for expansion.
Sensors can be used as triggers to initiate a sequence of events or a single operation in an automated system. For example, a pressure sensor may indicate to the controller—typically a programmable logic controller (PLC)–that the pressure is too low and that a filter may be clogged. This can cause the system to pause the operation until the filter is replaced. When the sensor indicates an acceptable pressure, the PLC can notify an operator or automatically turn on the flow.
In addition to pressure measurements, other operations can include a sequence of automated valves opening and closing based on time, level, temperature and water condition. A likely scenario begins with a large butterfly valve providing the water to the system. This valve may be operated manually, as it is opened once at the beginning of the day and closed at the end, or that same valve may be automatically opened and closed. The water passing through the butterfly valve then is transported to a filtration skid or preparation equipment before it is diverted to another area of the plant. Depending on the type of filtration the water will undergo, multiple valve actuator assemblies are likely to be installed, and they can be controlled via the PLC in a control cabinet. These valves will divert the water through multiple filtering assemblies, which may be separated based on the particle size that they are rated to filter. These systems often are configured to remove large particles first and smaller particles afterward.
Considering Valve Assemblies
Here we will look at four valve assemblies and the combinations of inputs and outputs that can be used for pneumatically actuated valve control. With many automated valve configurations, including pneumatic and electrical signals, a short run of wires to the control cabinet can prevent costly installations and difficult troubleshooting time if an open circuit becomes an issue. With a cabinet implemented for local control, an Ethernet wire is all that is needed to ensure communication throughout the facility.
Configuration 1 uses a Namur valve connected to the compressed air line, which will direct the air to either the “open” port or “close” port of the actuator. This setup requires an electrical signal to be wired to the Namur valve, a sensor box and a pneumatic connection to the compressed air line. The sensor box detects which position the valve is in and sends the feedback to the controller. The inventory for Configuration 1 includes two electrical wires and one pneumatic line. This configuration is best suited for a local control panel, especially if multiple valves will have a similar configuration. This one-input, one-output setup does not require a valve terminal to be installed in the control cabinet.
Configuration 2 uses a valve terminal in a cabinet and will have two pneumatic lines running to the actuator. One of the two lines will be pressurized, depending on the desired state of the valve. This setup also includes one electrical wire to communicate the state of the valve. The inventory for Configuration 2 includes one electrical wire and two controlled pneumatic lines. This system benefits from short runs of electric and pneumatic lines to a local control panel.
Configuration 3 uses only a Namur valve and does not provide feedback to the controller. This setup requires one electrical line for control and one connection to the supply air. The advantage of this system is that it does not require a local controller in a panel. It does, however, require a potentially long run of electrical wire to communicate with the PLC. Inventory includes one electrical wire and one pneumatic connection to the compressed air line.
Configuration 4 uses a valve terminal to control an actuator and two controlled pneumatic lines from a cabinet. This design requires only two pneumatic lines and may be ideal for a hazardous location or an area where there are concerns of electrical devices that may be exposed to excessive water. Potential disadvantages include the run of long pneumatic lines if a local controller is not included. Inventory includes two controlled pneumatic lines.
In all of these scenarios, consider where the panel will be mounted. Will it be at risk of overheating or require positive ventilation? Different types of enclosures offer different levels of protection. The manufacturer of the cabinet will provide a rating for environmental resistance based on National Electrical Manufacturers Assn. standards.
Once the review and inventory have been completed, plant personnel should work with local distributors, contractors or suppliers for implementation, including cabinet building and PLC programming.
Understanding the basics of automating valves puts plant personnel in a better position to operate and maintain these systems. This knowledge takes some of the mystery out of automation and ultimately leads to higher water quality and consistency.