How should the dry critical speed and wet critical speed be understood in API610? Is it useful in practice?

The dry and wet states here refer to the conditions with and without a medium!

Do you work with pumps? For wet rotation, the critical speed is calculated taking into account the supporting stiffness and damping of the mouth ring, as well as the Lomakin effect

Thanks for sharing. I would like to ask Engineer Wang: Is there any specific explanation or description for the 13780933572 Lomakin effect?

The LOMAKIN effect: The annular seals of a pump (such as friction rings and balance drums) can have a significant impact on its dynamic properties. By altering the stiffness of the rotor support and thus its natural frequency, it is possible to avoid or induce resonances that may occur between the double and quadruple rotor frequencies and a lower natural frequency. A small portion of the stiffness and damping of the annular seal is provided by the squeezed oil film and the hydrodynamic wedge (well-known in sliding bearing design). However, due to the high axial-to-circumferential flow velocity ratio in the annular seal relative to the bearing, changes in the circumferential gap can generate large forces in the annular gap; as the rotor deviates from its proper position, a Bernoulli pressure drop occurs. This phenomenon is known as the Lomakin effect, and it represents the main mechanism for generating stiffness and damping forces in the pump’s annular seal. The Lomakin effect depends directly on the pressure drop across the seal; regarding the flow resistance of a constant system, the Lomakin support stiffness changes approximately as the square of the rotational speed. However, for a roughly constant system head, only a very small Lomakin effect results from changes in rotational speed. Other important parameters are the ring seal length, diameter, and gap ； Fluid properties are secondary unless very high viscosity is involved. However, fluid vortices can lead to a significant reduction in the Lomakin effect; perhaps by adding the associated interpenetrating coupling, it is important to note that when the reaction force of this interpenetrating coupling exceeds the damping reaction force, it may cause rotor dynamics instability (as estimated by properly designed rotor dynamics programs). The gap effect is the strongest geometric scale effect, while the Lomakin effect is approximately inversely proportional to its square. The physical explanation for the significant effect of the gap is that it allows the circumferential pressure distribution (the cause of the Lomakin effect) to be eliminated through circumferential flow. Any annular sealing chamber with grooves serves to a certain extent the same purpose as adding a gap; from this perspective, deep grooves are worse than shallow ones.