Storage tanks play a pivotal role in the supply chain from upstream oil & gas production through downstream petrochemical-based products. Separation of water from the hydrocarbon liquids during storage create the need for operations to remove and treat this water as it builds up over time.Emerson’s Ryo Hashimoto shared a story of an Asian petrochemical manufacturer who had numerous naphtha and pyrolysis gasoline storage tanks. During the process of storing naphtha in the tanks, water would collect and accumulate at the bottom of the tanks due to its specific gravity. The water at the tank bottom would need to be drained over time to prevent it from flowing into the process.
The water draining process, performed on a quarterly basis, takes up to two to three hours and the operations staff must monitor for any hydrocarbon leakage to prevent oil loss, leakage of naphtha into the drain sump located at the tank area, and additional treatment work needed to remove hydrocarbons from the drained water.
During the draining process, the water is pumped through a drain pipe. If an operator sees the hydrocarbon start to flow through site glass of the pipe, they need to quickly inform the control room to close the drain valve. Traditionally, this was a manual monitoring operation with an operator present during the whole 2-3 hour process. If hydrocarbons begin to drain and the operation is not stopped quickly, these hydrocarbons can flow into the sumps, which increase the load on the downstream water treatment unit. This processing then becomes costlier to clean the water to a level to satisfy environmental regulations. This naphtha or pyrolysis gasoline is also lost from the main production stream.
To automate this time-consuming and potentially expensive water draining operation, the plant staff worked with the local Emerson team to identify conductivity measurement—due to the difference in the conductivity of naphtha and water—as a solution to monitor for hydrocarbon leakage during the water draining process.
The conductivity of mixing water and naphtha or pyrolysis gasoline is very low microS/cm. Typically water conductivity is around 650 – 1,000uS/cm. When there is a leakage of naphtha, the conductivity level will go down to 100 to 500uS/cm.A Rosemount 1066 single-channel transmitter and Rosemount 402 retractable contacting conductivity sensor with retractable ball valve mounting assembly were installed to measure these conductivity changes. This combination of sensor and analyzer provides early detection of hydrocarbon leakage during water draining operations.
With this online continuous measurement used during the water draining process, a downstream drain valve could be automatically closed based on the setpoint. Not only does this solution reduce the time operators spent in hazardous locations, it minimizes downstream water processing costs and loss of production naphtha and pyrolysis gasoline.