Maximum allowable operating speed (MAOS) for centrifugal pumps

By Esteban M. Araza

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There are instances when centrifugal pumps are to meet increases in operating conditions, or are re-purposed for new service applications, that require them to run at higher speed (RPM) based on affinity laws. In such instances the question is often asked: what is the maximum allowable operating speed (MAOS) for a particular centrifugal pump?

There is no quick answer to this question because each service operation is unique; there are many factors that limit the MAOS, and each factor must be evaluated on its technical constraint. Many manufacturers include the MAOS in their published pump technical data, but the data are mostly generic and are based on assumptions that may not be valid.

There are also the questions of what is the pump MAOS based on its “as built” construction, and what would be its MAOS based on upgrading some of its design and construction features.

Here are some major factors that will limit the MAOS for a centrifugal pump:

·???????Maximum allowable working pressure (MAWP) for the casing and nozzle flanges?- Increasing the RPM?will increase the pump differential?pressure?directly by the square of the speed?ratio; in turn, this will increase its shut-off pressure. The maximum RPM will then be limited?by the pump MAWP at its rated temperature. Note that the casing nozzle flanges typically have lower pressure rating at higher temperature, than the casing itself - hence the lower nozzle flange pressure rating may become the prevailing factor in limiting the MAWP.

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·???????B-gap?- The standard volute B-gap, or cutwater clearance, may no longer be adequate for the increase in power or energy density of the pump at higher speed. This is critical if the pumped liquid is difficult to handle – such as those with high gas or vapor content like carbonate solutions, CO2, and similar liquids. The B-gap may have to be increased to minimize the risk of high rotor vibration at impeller vane pass frequency.

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·???????NPSHR?-?Increasing the RPM will increase?the?NPSHR, typically in direct proportion to the square of the speed ratio (assuming suction specific speed remains constant.) The increase in RPM will be?limited to that speed where?the?NPSHA remains higher than the NPSHR?at the higher speed, preferably by about 10%.

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·???????Driver HP?- Increasing the RPM?will increase the required horsepower by the cube of the speed ratio. A decision will have to be made if the increase in pump performance should be limited to loading up the existing driver, or a new driver with higher horsepower rating would be procured.

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·???????Shaft size?-?Increasing the?RPM?will increase?the?required?horsepower (HP). The shaft, coupling, mounting keys, etc., should be good?for the increase in HP. In some instances, where these parts become undersized for the HP, it might be possible to upgrade to a?stronger material, or to increase the size of these parts.

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·???????Bearing size?-?Higher speed will result in?higher axial and radial thrust loads. The existing bearings may?become inadequate to?carry the increase in?thrust loads, and some modifications or upgrades may be needed.

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·???????Ball bearing maximum speed?- The?increase in?RPM should not exceed the allowable?maximum speed of the thrust ball bearings.?Bigger?ball bearings have?lower maximum allowable RPM.?For example, an oil-lubricated 7310 ball?bearing has a maximum?allowable?speed of 5000 RPM; that of 7313 is?4000 RPM.

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·???????Bearing lubrication requirement?-?The?bearing?lubrication and cooling requirement increases?with?RPM. At higher?RPM?oil-ring lubrication may no?longer be sufficient and?forced-feed lubrication?may be required.

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·???????Impeller peripheral speed?-?The impeller tensile strength?will?limit?its maximum?tip peripheral speed. Example, at ambient temperature, the allowable maximum speed for a 15” diameter impeller is:

3130 RPM for Class 40, ASTM A-278 cast iron

4120 RPM for alloy 952, ASTM B-148 aluminum bronze

4425?RPM for ASTM A-216, Grade WCB cast steel

·???????Auxiliary piping may need modification?-?Auxiliary piping - such as mechanical seal flush, cooling?piping, balance line - may require?changes to pipe size,?flange rating, orifice size, etc., to break down the higher differential pressure to a more manageable level.

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·???????Others?– In very high speed operation, the combined effect of high differential pressure and centrifugal force acting on the impeller wear rings may make their mounting on the impeller very difficult. In such instance, the impeller wear rings may be omitted altogether.

Another determining factor to the MAOS is whether a pump will run at the higher speed (RPM) continuously, or intermittently. For example, at higher speed, the liquid velocity at the pump volute throat area and at the discharge nozzle will consequently be higher also; this may be acceptable in intermittent operation, but not in continuous operation where the high velocity may result in rapid erosion of the volute lips, and in elevated vibration and noise levels.

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Because of the many variables that restrict the pump MAOS, a better approach is to identify the intended new operating conditions and ask the vendor if the pump can meet those conditions, or how close it possibly can. This may, or may not, be based on some pump modification and upgrades.

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It is recommended that, if the anticipated MAOS would be significantly higher than its current speed, a rotor dynamic analysis (RDA) be conducted, including a review of the rotor’s critical response, dynamic balance, hydraulic thrust, and bearing selection and lubrication.

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Maximumspeed#, pumpspeed#, RPM#, #bearinglubrication, #criticalspeed, #impeller, #volute

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Disclosure: Author is retired and wrote this article on his personal capacity. To request permission to reprint, republish, or excerpt this article, in whole or in part, please email the author at [email protected].