The great majority of centrifuges used in drilling are decanting devices, as described previously. The rotary mud separator (RMS), also known as a perforated rotor centrifugal separator, was developed by Mobil in the 1960s. Although it is not, strictly speaking, a centrifuge because the outer barrel is not rotated, it serves the same function that decanting centrifuges do with weighted drilling fluids by discarding ultra-fine and colloidal solids while salvaging silt-size barite, and is often spoken of as another type of centrifuge.

The usual RMS configuration utilizes a perforated rotor 40 inches long and 6 inches in diameter in a nonrotating horizontal housing with an internal diameter of 8 inches. The fluid being processed is diluted and fed into the annular space between the tubes. The centrifugal force created by the rotation of the inner cylinder concentrates the larger particles against the outer wall of the annulus. The larger particles and part of the fluid exit the annulus at the underflow discharge port at the downstream end of the annulus, while the remainder of the fluid and the finer solids, having passed through the perforations, exit via the overflow discharge port at the downstream end of the rotor. The division of the flow between the two exit ports is controlled by a choke in the underflow line.

Both of the discharge streams include enough liquid to permit them to flow or be pumped freely without clogging the lines or hoses carrying the flow. This permits flexibility in choosing a location for the RMS, which does not have to be mounted over a mud tank, as decanting centrifugesdo. This has proven to be a very useful feature. Another beneficialcharacteristic is that the capacity of the RMS is greater than that of thedecanting centrifuge.

The factor that has limited the use of the RMS is that it requires 70% dilution of the processed fluid. Each barrel of processed drilling fluid requires the addition 0.7 bbl of diluent. While this left many applications in the 1960s and 1970s with the growing need to reduce the volume of drilling waste, it now severely limits drilling applications, although it may still be a useful tool in mud plants.