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BEARINGS:
The function of bearings in a centrifugal pump is to keep the rotating assembly in correct alignment with the stationary parts under the action of radial and axial loads those that give radial positioning to the rotator are known as line bearings and those that locate the rotator axially are called thrust bearings.
All types of bearing are used in centrifugal pumps. Even the same basic design of pumps is often manufactured with two or more different bearings to cover the requirements of varying service conditions. Two external bearings are normally utilized for the double suction single stage general service pump one on either side of the casing.
In horizontal pumps with bearings on each end, the inboard bearing is the one between the casing and the coupling, and the outboard bearing is located on the opposite end.
Pumps with overhung impellers have both bearings on the same side of the casing; the bearing nearest the impeller is the inboard bearing and the one farthest away is the out board bearing.
Ball bearings are the most common antifriction bearing used on centrifugal pumps. Roller bearings are used less often although the spherical roller bearing is frequently used for large shaft sizes. Bearings are normally grease lubricated, although some services use oil lubrication, depending on design loads, speed, and service conditions.
Sleeve bearings are used for large heavy duty pumps with shaft diameters of such proportions that the necessary antifriction bearings are not readily available. Other applications include high pressure multistage pumps operating at speeds of 3,600 to 9,000 rpm and vertical submerged pumps such as vertical turbine pumps in which the bearings are subject to liquid contact. Most sleeve bearings are oil lubricated.
Thrust bearing used in combination with sleeve bearings are generally kingsbury or similar type bearings.
CENTRIFUGAL PUMP PERFORMANCE:
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The performance of a centrifugal pumps is normally described in terms of the following characteristics:
rate of flow or capacity Q, expressed in units of volume per unit of volume per unit of time, most frequently gpm US or cfs (1 cfs = 440 gpm);
increase of energy content in the fluid pumped or head H, expressed in unit mass usually ft per lb, or more simply, ft of liquid pumped;
input power BHP expressed in units of work per unit of time, horsepower;
efficiency E, the ratio of useful work performed to power input;
rotative speed N in rpm.
Because the parameters indicated are all mutually interdependent, performance of a centrifugal pumps is represented by characteristic curves. The section below will introduce the common characteristic curves in everyday use.
PUMPS RATING CURVES
Head-capacity curve
Any centrifugal pump has a rating curve that indicates the relationship between the head developed by the pump and the flow through the pump for a particular speed, and for a particular diameter impeller when handling a liquid of negligible viscosity, usually water (figure 45 1).
FIGURE 46 - 2. Brake-Horsepower-capacity curve
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As the capacity increases, the total head that the pump is capable of developing is reduced. In general, the highest head that a centrifugal pump can generate is developed at a point when there is no flow through the pump.
Brake- horsepower-capacity curve
In order for the centrifugal pump to deliver the capacity that is required, it is necessary to provide the pump with a certain horsepower.
A curve (Figure 46 - 2),which represents the relationship between brake horsepower and capacity, is plotted based on the same constant factors as outlined previously.
Efficiency curve and brake horsepower-capacity curve are determined by testing an actual pump for efficiency.
Efficiency-Capacity curve
The efficiency cannot be measured directly, but must be calculated from the measured information.
The formula for efficiency is a follows.
Ep=H ? gpm ? SP.gr ?100
3960?BHP
Where
EP =pump efficiency (%)
H =head developed by the pump (ft)
gpm=capacity delivered by pump (gpm US)
sp.gr=specific gravity of liquid pumped
BHP = horsepower required by pump
Using the above formula, the efficiency at which the pump is operating at any given capacity can be determined. The efficiency points are then plotted on the graph and the curve is generated (figure 47 - 3).
Net positive suction Head (NPSH)-capacity curve
This curve is an important characteristic of a centrifugal pump and is always part of the pumps performance curves (figure 48 - 4). It shows the relationship between the capacity
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