The following is a brief narrative describing the intended control strategy between each a Reverse Osmosis (RO) train and the water treatment plant SCADA system.
It describes the control system we will supply and outlines the basic operation of a Reverse Osmosis skid and the required handshaking with the water treatment plant SCADA system. This narrative in no way encompasses all of the responsibilities of the water plant SCADA as we are not familiar with all of the requirements of the end user.
When multiple Reverse Osmosis trains are provided then we will supply identical control systems for each RO train. Typically, each of these control packages will utilize an Allen-Bradley PLC for R/O train control, and communication with the water treatment plant SCADA. Each R/O control panel will also have a color touch screen operator interface (HMI). This will give the operator full access to monitor and control all aspects of each respective Reverse Osmosis train.
Each of the PLCs will communicate over an Allen-Bradley based Ethernet network with the water treatment plant SCADA. The plant SCADA will then be able to communicate with all pre and post ancillary equipment as well as each RO train independently or collectively using message block transfers of data.
Many water treatment plant SCADA systems monitor the finished water level, control the well(s), the well flush valve, the plant inlet valve, the scale inhibitor, the feed pH monitoring, and any other pretreatment equipment or chemicals that may be required. Once again, without full knowledge of the scope of work at a particular facility, we make no statement that this will be all of the equipment at the “front end” of the plant. Assuming these basic components and requirements, the RO control would proceed according to the following series of events.
When either of the Reverse Osmosis trains is in automatic mode and contain no alarms they will send a “ready” status bit to the main PLC over the network. The water treatment plant SCADA will then monitor the finished water level or another RO production starting point. When RO production is required, the main water plant PLC will open the well flush valve and confirm that the plant inlet valve is closed. The plant SCADA will then start one or more wells as required by hydraulic considerations and desired level of water production based on available RO trains.
The well water will be diverted to waste until the turbidity reaches an acceptable level for the RO train. At this point, the main plant PLC will notify the RO train(s) that the well flush is complete via a command over the network. The RO train(s) will open their respective inlet valve(s) and begin to accept the well water as the plant SCADA begins to open the plant inlet valve and close the well flush valve. At this time the scale inhibitor will begin at a preset rate and that rate will be confirmed by the scale inhibitor flow meter at the plant SCADA.
The level in the scale inhibitor day tank and the scale inhibitor flow will be monitored anytime scale inhibitor is being injected. If at any time the day tank indicates a low level or there is a loss of flow, the water treatment plant SCADA system will alert the RO PLCs which will begin a normal shutdown with an RO train flush. This sequence will be described later. The RO train(s) will then go through a pre-flush sequence followed by entering run mode. When the RO train(s) enter run mode the RO PLC will direct the water plant SCADA PLC to call on the RO HPP as well as the RO interstage pump via the Ethernet connection between the plant SCADA PLC and the MCC based VFD’s.
We will read the running and or fault status from both of these VFD’s over the Ethernet connection from the plant PLC. The desired speed of each VFD will be sent to the water plant PLC continuously throughout the run sequence for VFD control. The RO train(s) mode will be shared with the water treatment plant SCADA. Each RO train can be off, in pre-flush, running, or in post-flush due to an alarm or level.
The RO train will now be producing water and will continue until an alarm occurs or the system is directed to end water production by the main water treatment plant SCADA based on finished water level or other events. While the RO train is running all status and monitoring points will be shared with the plant SCADA. This will include all pressures and flows as well as valve statuses.
The water treatment plant SCADA will have control over all post treatment systems common to the RO trains as well. This could include chemical additions, degasifiers, transfer pumps and/or high service pumps. Again, we are not totally familiar with the balance of plant operations outside the RO trains and therefore cannot advise as to control schemes or provide any type of comprehensive I/O lists. However, typically we would see the water treatment plant SCADA start any post treatment chemicals and/or degasifiers based on a running feedback confirmation received from any of the RO trains by way of the communication network. The distribution system controls are typically completely independent based on system demands and in no way affect the Reverse Osmosis water production.
As the RO trains are in production they will be constantly monitoring numerous alarm parameters. Some of these include but are not limited to low RO pump suction pressure, high pump discharge pressure, low concentrate flow rate, high differential pressure across the cartridge filter, low or high feed pH, and high pressure pump VFD fault. The water treatment plant SCADA will monitor for scale inhibitor low day tank level as well as loss of scale inhibitor flow. If the water plant SCADA senses any of these conditions for a time they will communicate the alarm to both of the RO trains. If an RO train is running and receives one of the above mentioned alarms from the main water treatment plant PLC or one of the train’s individual alarms occurs, the train will go into a shutdown mode. If the level in the finished water tank reaches a level where the plant no longer requires water production this will also be communicated to all RO trains thereby also initiating a shutdown.
When the RO train goes into shutdown for an alarm it will immediately communicate the specific alarm to the water treatment plant SCADA. The train will then begin an organized shutdown sequence based on the type of shutdown. This shutdown sequence requires the well to continue to run. When the train is done with its shutdown sequence it will communicate this with the water plant SCADA and begin to close its respective train inlet valve. At this time, it will be communicated to the plant SCADA that certain train(s) are no longer require a well and it will be the responsibility of the water treatment plant SCADA to reduce the number of wells or stop all wells based on just one or both RO trains stopping.
The plant SCADA may need to open the well flush valve at this point to divert the well water if there is latency in stopping the wells due to a telemetry system or other. When the RO train notifies the main water plant PLC of a shutdown, appropriate action should be taken by the water plant SCADA to reduce or stop all pretreatment chemicals and equipment as required.
Finally, we typically see the water treatment plant SCADA with some sort of historical data logging and/or trending of important RO variables. These items will be shared with the main water plant PLC via the Ethernet network for just this reason. They will include all pressures and flows as well as permeate conductivities. It will be the responsibility of the plant SCADA to handle this data as desired by the end user.
This blog was written in conjunction with our control integrator kW Controls.