Gene Vogel
EASA Pump & Vibration Specialist

Vertical turbine pumps (VTP) are driven by vertical motors with large thrust bearings that support the weight of the motor rotor, the pump rotor and the dynamic down-thrust generated by the pump impellers as they lift the liquid. The weight of the motor rotor and pump rotor are easily determined from simple engineering data. The amount of thrust generated by the impellers as they interact with the moving liquid is much more complicated, and the thrust is not always generated in the downward direction.

During certain stages of pump operation, the flow of the liquid through the impellers can generate an upward thrust that can lift the pump rotating assembly. The resulting upward force can cause havoc with thrust bearings in the motor which may be designed to handle only down-thrust. 

This article will briefly review how pump up-thrust occurs and the provisions of VTP motors designed to control it.

Up-thrust conditions

 The idea that a vertical pump lifting water up a column can produce up-thrust on the pump rotor is somewhat counterintuitive. But this does occur on multistage VTPs under certain flow conditions. Generally, up-thrust can occur during shutdown or when the pump is operating at flow rates greater than the allowable operating range. Whether up-thrust occurs and to what extent depends on the hydrodynamic design of the particular pump. The pump manufacturer may specify up-thrust conditions. See Figure 1.

Flow and reaction forces
Figure 2A illustrates liquid flow through the impeller and bowl of a VTP, and Figure 2B illustrates possible reaction forces on the impeller and volute resulting from that flow. Notice that the direction of discharge from the impeller is mostly radial and only partially vertical. (This is typical of mixed flow impellers.) Most of the upward force on the liquid is imparted by the stationary diffuser vanes in the pump bowl. Thus, most of the lifting force is borne by the stationary column supporting the bowls; only a fraction of the lifting force is imparted to the pump rotor. Another force factor acting on the pump rotor is the momentum of the liquid as it impacts the impeller suction eye and is redirected radially. Liquid has mass and is moving in an upward direction as it enters the impeller suction eye and therefore it has upward momentum.

In normal operation, the downward force of impeller suction and discharge head, along with the weight of the pump rotor, exceed the upward force of the liquid momentum entering the impeller. The result is the down-thrust imparted to the pump rotor and supported by the thrust bearing(s) in the motor.

During shutdown, the suction head drops faster than the flow of liquid through the pump, and the upward momentum of the liquid flow can result in an up-thrust condition. During shutdown, the up-thrust is usually momentary (less than 10 seconds). Up-thrust can also occur during high flow conditions. Momentum forces increase with the velocity of the liquid, so at high flow the upward momentum of the liquid increases significantly. The up-thrust in high flow conditions is more than momentary and provisions in the motor bearings must be designed to handle the continuous up-thrust.

Bearing arrangements

The thrust bearings in VTP motors can be angular contact ball bearings, spherical roller bearings or hydrodynamic “Kingsbury” type bearings. Also, VTP motors will have a guide bearing which may be a deep groove radial ball bearing, a cylindrical roller bearing or a sleeve bearing. Various bearing combinations and provisions for handling up-thrust exist. The most common bearing arrangements use angular contact ball bearings (7xxx bearings). If a single 7xxx bearing is used, it will be oriented for down-thrust only. In this case, a deep groove radial ball bearing is used for the guide bearing, and the guide bearing may be designed to handle momentary up-thrust. If a second 7xxx bearing is included, it may be oriented for down-thrust (tandem bearing) to increase the total down-thrust capacity, or it can be oriented for up-thrust to control more than momentary up-thrust conditions. In this case, the two 7xxx bearing may be arranged back-to-back or face-to-face. See Figure 3. In either case, the bearing journal and housing must be designed to provide proper inner and outer race retention and bearing preload. In some instances, three 7xxx bearings are used. Usually two are oriented for down-thrust and one for up-thrust.

Spherical roller bearings
For greater down-thrust capacity, spherical roller bearings are used. Spherical roller bearings have a significant minimum load requirement and are easily damaged by any up-thrust motion. When up-thrust is possible from a VTP spherical roller, bearings are mounted on springs; the springs maintain full contact between the rollers and races during up-thrust conditions and absorb the impact when the normal down-thrust is restored.

Still greater down-thrust capacity can be achieved with hydrodynamic sliding element thrust bearings (Kingsbury type bearings). For these very high down-thrust applications, concerns for up-thrust conditions are uncommon, and when needed, special up-thrust provisions are designed per the specific application.

In all cases where a deep groove radial ball bearing is used for a guide bearing, the endplay adjustment for the motor is critical. Various techniques for setting endplay on VTP motors are described in “Section 8: Bearings” of the EASA Technical Manual. With the array of possible bearing configurations and the potential for up-thrust conditions, it is important that VTP motor repairers have a thorough understanding of how an individual machine is designed to handle both down-thrust and up-thrust. A more extensive discussion of the topic is included in the “EASA Fundamentals of Pump Repair” seminar and other related EASA seminars.

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Categories: Bearings, Pumps

Tags: Vertical turbine pumps

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