Boiler

Hello, I work in Power industry and last week Feed Water conductivity increased (more than 300µs/cm) in four - 4 of our running Boilers, one after the other. The High conductivity was noticed in boiler drum water, as well, while steam was not affected.
Chemical dosing regime was as normal as any other day.
After suspending Chemical dosing and temporarily increasing CBD in each boiler, things returned to normal.
Can anyone explain this phenomena? I am ready to submit the relevant data where required!
Thanks.