An embodiment of a method of operating at least two pressure vessels to inject a particulate slurry into a high-pressure line, comprises a first operating cycle comprising: isolating a pressure vessel from the high-pressure line, introducing, under low-pressure conditions, particulate solids into the pressure vessel through a particulate solids inlet aperture, a second operating cycle comprising: providing high-pressure flow into the pressure vessel, and providing a high-pressure slurry flow from the pressure vessel into the high-pressure line. The method further comprises causing a pressure vessel to operate in the first operating cycle while operating a pressure vessel in the second operating cycle, and synchronizing switching a first vessel from the first operating to a second operating cycle and switching a second vessel from the second operating cycle to the first operating cycle such that at least one of the vessels is operating in the second operating cycle.