Bearing load is defined as the force in pounds or newtons that is felt on the bearing during operation. Radial load is the force on a bearing radially from the shaft. Axial load or thrust is the force on a bearing axially along the shaft.
Radial Loads
Some single-volute pump casings are designed for uniform or near uniform pressures (specific speed range between 500 and 3500 English units) around the volute casing at the BEP. The uniform pressures act equally on all sides of the impeller when the pump is operated at design capacity.
Figure 42A illustrates the equal pressure that surrounds a pump impeller during operation at design capacity. At capacities other than design capacity, the pressures around the impeller are not uniform and there is a resultant radial load (thrust). Figure 42B illustrates the unequal pressure that surrounds the pump
impeller and the resultant force (F) when the pump is operating at capacities other than the design capacity.
Figure 42. Radial Force
The magnitude of the resultant radial force on the impeller during operation at capacities other than the design capacity can be calculated through use of the following equation:
) )(b )(SG)(H)(D K k( Fr = r 2 2 Where: Fr = Radial force k = 0.433 (9790 metric)
Kr = Radial force factor coefficient
SG = The specific gravity of the fluid pumped H = Pump head in feet or meters
D2 = Outside diameter of the impeller
b2 = Width of the impeller at the discharge,
The radial force factor coefficient has been determined experimentally as a function of specific speed and capacity. Figure 43 shows a graph of specific speed (English units) versus radial force coefficient. The three plots are for when the pump is operating at or near the capacity at BEP, when the capacity is one half the capacity at BEP, and when the pump is operating at shutoff head.
The resultant radial thrust can be calculated through use of the following equation: ) )(b )(SG)(H)(D K k( Fr = r 2 2 pounds 1279 = 2.5) )(15.125)( (1.00)(252 .433(0.31) Fr =
Radial load can be lowered throughout the entire capacity range through the use of a double-volute or a diffuser-type casing. The use of a double-volute or diffuser-type casing should be
considered when a pump is normally operated at variable capacities, especially at shutoff head.
As described in the casing designs section in MEX 211.01, radial load is minimized in multistage centrifugal pumps by staggering the volutes and by canceling out opposing radial thrusts.
Axial Loads
Axial load is the sum of the unbalanced forces that act on the impeller in the axial direction (axial thrust) and, in the case of vertical pumps, the force and the mass force of the pump rotor assembly. Axial pump loads vary with the type of pump and impeller. Figure 44 shows the resultant axial thrust for a horizontal, single-stage, single-suction, closed-impeller pump.
Figure 44. Hydraulic Axial Thrust Produced by a Horizontal, Single-Stage, Single- Suction, Closed-Impeller Pump
For overhung, single-stage pumps, an additional axial force, which results from the difference between atmospheric pressure and suction pressure, is felt on the shaft area. Figure 45
illustrates this additional axial force. This force acts towards the impeller when the suction pressure is less than atmospheric, and it acts in the opposite direction when suction pressure is higher than atmospheric.
The difference in pressure between the suction and the
discharge of the pump acting on the area of the pump impeller suction provides an axial thrust on the impeller. Double-suction pumps are often used to minimize the effect of axial thrust from an impeller. Figure 46 shows the resultant hydraulic force for a horizontal, single-stage, double suction, closed-impeller pump.
Figure 46. Hydraulic Axial Thrust Produced by a Horizontal, Single-Stage, Double-Suction, Closed-Impeller Pump
In practice, the hydraulic balance on a double-suction pump may not be achieved for the following reasons:
• The suction passages to the two suction eyes may not provide equal or uniform flows to the two sides.
• External piping conditions, such as an elbow located too close to the pump suction nozzle, may cause unequal flow to the two suction eyes. Proper piping arrangements to double suction impellers are of utmost importance. As a rule, three to five straight pipe diameters must be present downstream of an elbow to ensure that equal flow enters each side of the impeller, and the suction pipe should be perpendicular to the pump axis.
• The two sides of the discharge casing waterways may not be symmetrical, or the impeller may be located off-center.
These conditions will alter the flow characteristics between the impeller shrouds and the casing, and thereby cause unequal pressures on the shrouds.
• Unequal leaking through the wear rings on either side of the impeller can result in differential pressure across the impeller and upset the balance.
Axial thrust in horizontal, single-suction, semi-open, radial flow impellers is illustrated in Figure 47. The pressure on the open side of the impeller varies from essentially the discharge
pressure at the periphery (diameter D2) to the suction pressure
at the impeller eye (diameter D1). The pressure distribution at
the back of the impeller shroud varies from discharge pressure at the periphery to a slightly lower pressure at the impeller hub. The unbalanced portion of the axial thrust on the impeller is represented by the crosshatched area in Figure 47.
Figure 47. Axial Thrust in Horizontal, Single-Suction, Semi-Open, Radial Flow Impellers
Axial loading for vertical pumps must take the weight of the rotor assembly (shafting, couplings, and impellers) into consideration when determining the axial load.
Thrust Direction
The sum of the axial loads in one direction is balanced against the sum of the axial load in the opposite direction, and this situation results in a net active thrust in one direction. Active thrust is defined as the normal thrust direction when a pump is operating. The axial direction opposite the direction of active thrust is called the direction of inactive thrust. Depending on the pump design, the direction of active thrust can change with changes in pump capacity.
The direction of active thrust can cause tension or compression in the shaft. Shaft tensile stresses can occur when the direction of active thrust is away from the thrust bearing. Compressive stresses can occur when the direction of active thrust is towards the thrust bearing. The direction of axial thrust varies with flow rate, which results in a compressive or tensile stress on the pump shaft. The magnitude of thrust varies with the pump design. Figure 48 shows examples of compressive and tensile stresses on pump shafts for horizontal and vertical pumps. If a pump is operated in the discharge recirculation zone, the
stresses on the pump shaft can cycle between compressive and tensile. Repeated cycling between compressive and tensile stresses can cause pump thrust bearing damage and shaft damage from high axial loads and from fatigue cracking corrosion.
Thrust Balancing Designs
The amount of active axial thrust in single-stage and multiple- stage pumps can be minimized through the use of the following different methods:
• Back and front wear rings with impeller balance holes • Pumpout vanes
• Double suction impellers • Stacked impeller design • Opposed impeller design • Balance drum
Back and Front Wear Rings with Balance Holes - The ordinary, single-suction, closed, radial impeller with the shaft passing through the impeller eye is subject to axial thrust because a portion of the front impeller wall is exposed to the suction pressure while the area in back of the impeller wall is exposed to the discharge pressure. If the discharge chamber pressure was uniform over the entire impeller surface, the axial force acting toward the suction would be equal to the product of the net pressure generated by the impeller and the unbalanced annular area. In actual use, the pressure on the two single- suction closed impeller walls is not uniform. The liquid trapped between the impeller shrouds and the casing walls is in rotation, and the pressure at the impeller periphery is higher than at the impeller hub. Figure 49 illustrates the actual pressure
To minimize the axial thrust of a single-suction impeller, the pump impeller can be equipped with both front and back wear rings. The front and back wear rings effectively isolate the high pressure and low pressure areas of the impeller. The thrust areas are equalized through the use of the same inner diameter of both the front and back wear rings. Pressure that is
approximately equal to the suction pressure is maintained in a chamber located on the impeller side of the back wear ring by the drilling of balance holes through the impeller. Figure 50 shows an example of a single-suction impeller equipped with front and back wear rings and balance holes.
Figure 50. Front and Back Wear Rings and Balance Holes
Leakage past the back wear ring is recirculated back to the pump suction through the balance holes. Large (greater than 10” suction), single-stage, single-suction pumps do not commonly use balance holes because the leakage from the back wear rings through the balance holes opposes fluid flow through the suction of the impeller and creates disturbances that can affect the pump capacity. Large, single-stage, single-suction pumps commonly use a piped connection from the area behind the impeller to the pump suction piping.
Pump-Out Vanes - The primary function of pump-out vanes is to minimize packing or seal leakage by reducing the fluid pressure on the seal chamber. Pump-out vanes also prevent foreign material that can be suspended in the pumped fluid from lodging in the clearance space between the shroud and the adjacent wall of the casing. Reducing pressure behind the impeller shroud with pump-out will also reduce axial thrust. Figure 51 illustrates the effect of pump-out vanes on the pressure differential across an impeller.
Stacked Impeller Design - The stacked impeller design is used on multi-stage pumps. The stacked design consists of several single-suction impellers mounted on one shaft, each having its suction inlet facing in the same direction and its stages following one another in ascending order of pressure. Thrust increases with the increasing number of impellers in the stacked impeller; however, the stacked impeller design axial thrust is balanced by a single hydraulic balancing device (a balance drum, which is discussed later in this section). Figure 52 shows an example of a multi-stage pump using the stacked impeller design and a hydraulic balancing device.
Opposed Impeller Design - The opposed impeller design is used on multi-stage pumps. The opposed impeller design consists of single-suction impellers mounted on a single shaft, with a portion of the impellers facing one direction and the other impellers facing the opposite direction. With this arrangement, axial hydraulic thrust is minimized by balancing the thrust of one group of impellers against the opposite group of impellers. When an even number of impellers is used, typically one-half of the impellers face one direction, and the other half of the
impellers face the opposite direction. When an odd number of impellers is used, the pump shaft diameter and the interstage bushing diameters are varied to provide the effect of a hydraulic balancing device that will compensate for the hydraulic thrust on one of the stages. Figure 53 shows an example of a multistage pump that uses the opposed impeller design.
Balance Drum - A balance drum, which is also known as a balance piston, is a hydraulic thrust-balancing device used to reduce the axial thrust in a pump. There are two types of devices that are commonly used to balance axial thrust in centrifugal pumps: a balance drum and a balance disk.
A balance drum is shown in Figure 54. The balancing drum is either keyed or screwed to the pump shaft and separates the balancing chamber at the back of the impeller (or if multi-staged, an end-stage impeller) and the interior of the pump. A balancing drumhead is fixed to the pump casing, and it allows for a small radial clearance that separates the drum and the stationary portion of the balancing device.
Figure 54. Balancing Drum
The area on the seal chamber side of the balance drum is subjected to the pump suction pressure. The area on the impeller side of the balance drum is exposed to the high- pressure fluid in the pump. The difference in fluid pressure across the balance drum provides a force on the balance drum that is opposite to the direction of axial hydraulic thrust from the impellers. The typical balance design is 90 to 95 percent of total axial impeller thrust. Any residual thrust that is not balanced by
the balance drum is absorbed by the thrust bearing on the end of the shaft. The amount of residual thrust that must absorbed by the thrust bearing changes as a function of the differential pressure from the suction pressure and the internal pump pressure. The use of a balance drum enables the selection of a smaller thrust bearing, which results in lower horsepower losses.
Another form of balancing device is called a balancing disk. Similar to the balancing drum, the balancing disk also uses a balancing chamber. The balancing disk is secured to the shaft, and the balancing disk head is fixed to the casing, as shown in Figure 55. The leakage to the balancing chamber flows through a small axial clearance between the balancing disk and the balancing disk head. The liquid, depending on system design, would then flow to either the pump suction or back to a tank. A restricting orifice is typically placed in the leakage return line. The orifice provides backpressure in the balance chamber by restricting fluid flow out of the balance chamber. The balance chamber backpressure is required for the proper operation of the balancing disk.
The balance disk automatically compensates for changes in axial impeller thrust by varying the amount of axial clearance between the balancing disk and the balancing disk head. For example, if the impeller thrust increases, the disk moves towards the disk head and reduces the clearance between the disk and the disk head. The reduction in clearance reduces the amount of leakage from the impeller side of the disk to the balance chamber. The reduction of leakage to the balance chamber reduces the backpressure in the balancing chamber. This drop in pressure provides a higher differential pressure, from the discharge pressure side to the balance chamber, across the balance disk. The higher pressure on the discharge pressure side of the balance disk provides the force to oppose the axial hydraulic thrust from the impeller(s), and it allows the disk to move away from the disk head until a balanced axial thrust equilibrium is achieved.