Minimum allowable operating speed (MiAOS) of centrifugal pumps

Minimum allowable operating speed (MiAOS)

By Esteban M. Araza

In a separate article by this author the maximum allowable operating speed (MAOS) of centrifugal pumps is discussed. This article discusses the other end of the spectrum – their minimum allowable operating speed (MiAOS).

It is easily understood why one should be aware of the maximum allowable operating speed - this has to do with keeping the working pressure, HP, shaft stress, NPSHR, bearing thrust load, lubrication, and other parameters within the pump design limits.

But why should one be concerned with minimum allowable operating speed (MiAOS) since those parameters are decreasing anyway and should be well below the limits?

Indeed, there are several factors that would put a downward limit to a pump’s MiAOS. Among these factors, and the reasons for checking them out, are:

·At MiAOS there should still be adequate flow and differential pressure across the wear parts (such as rings, bushings, sleeves, etc.) to provide adequate lubrication and cooling of these parts. Typical ballpark minimum values are 1 GPM flow, and 5 PSIG differential pressure; insufficient lubrication and cooling can cause seizing or galling of these parts. Where these minimum values are not obtainable, it may be necessary to increase the running clearances of these wear parts, or to use non-metallic materials that are resistant to dry-running.

·There should be sufficient injection pressure to the seal flush piping to provide adequate vapor suppression at the mechanical seal, unless the seal flush comes from an external source. There should be enough hydrodynamic forces at the seal face to provide seal face separation. Inadequate  pressure can cause vapor lock and cavitation at the seal faces. Resizing of the restrictive orifice/s in the seal piping may be needed. Consult with the mechanical seal vendor, if needed.

· In multistage pumps there should still be adequate flow to any bleed-off, or balance line, to keep the hydraulic axial thrust within acceptable range relative to the thrust capacity of the bearings.

·For bearings with oil ring lubrication, the MIAOS should allow the oil rings to rotate and not stall; otherwise, the bearings can be damaged due to insufficient lubrication.

·For sleeve bearings, there should be a minimum oil film thickness to provide the stiffness and dampening to the rotor. One method to calculate the MiAOS needed to meet the minimum oil film thickness is to calculate it empirically using the Sommerfeld Number.

·For tilting-pad bearings, consult the manufacturer for their recommended flow vs. RPM data to ensure proper bearing lubrication at MiAOS.

·In forced-feed lube system where the auxiliary lube pump is mounted integral to the pump shaft, the lube pump capacity and pressure at the reduced RPM should still be adequate for the requirements of the main pump.

·Although a rarity, a pump running at lower RPM may encroach into its first critical speed; this should be looked into.

Whichever RPM calculates higher based on the above factors dictates what should be the MiAOS for that pump.

The above checks apply to a pump running continuously at MiAOS, not to slow rolling pump that is on hot standby. The pump should not be deadheaded, i.e., the liquid passes through the pump and is not recirculated back to its suction.

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