SOUTHERN ENERGY’S Canal power plant in Cape Cod, Mass., uses a holding tank for waste boiler water prior to its discharge into the Cape Cod Canal. Although the water is usually clean, its pH is too high for safe discharge, so first it must be neutralized to a pH level of 7 to 8.
When full, the head pressure of the holding tank was sufficient for a flow of 300 gpm out of the tank, which is the minimum flow rate specified in the plant’s EPA permit. When the water level in the tank dropped, however, there was a corresponding drop in the flow rate.
To maintain the tank’s flow rate to meet the EPA permit requirements, an operator had to manually open or close a gate valve. This entailed a trip to the “doghouse,” where the gate valve was housed. The operator had to contend with working in a limited space, and the doghouse was frequently wet and slippery, especially in the winter. Manual control of the wastewater process was not only non-productive, it was also very costly. For these reasons, Southern Energy decided to automate the process.
Trips to the doghouse have been eliminated with the installation of a Masonelian Camflex valve and smart valve interface (SVI) digital controller supplied by Dresser Valve Division. The SVI is an intelligent digital valve positioner and proportional integral derivative (PID) process controller. Placing the control function closer to the process means more efficient control of the process.
Positioning the PID control at the valve reduces installation costs because it eliminates the need to connect it directly to the control room. With the SVI, the transmitter for indicating the level of the tank is the only input into the valve.
With the new setup, the flow rate of the tank is maintained at 300 gpm and the process no longer requires an operator’s constant attention. Should there be a need to open and close the gate valve, it can now be initiated remotely. The days of going to the doghouse are now only a memory.