The centrifugal pump

It is apparent, therefore, that the required vane angle in the diffuser is dependent on the throughput, the speed of rotation, and the angle of the impeller blades. The pump will therefore operate at maximum efficiency only over a narrow range of conditions, 

If the fluid is travelling with a velocity u and at an angle 6 to the tangential direction, the angular momentum of this mass of fluid 

The torque acting on the fluid dr is equal to the rate of change of angular momentum with time, as it goes through the pumps or  

The volumetric rate of flow of liquid through the pump  

The total torque acting on the liquid in the pump is therefore obtained by integrating dr between the limits denoted by suffix 1 and suffix 2, where suffix 1 refers to the conditions 

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Efforts to develop an artificial heart have resulted in a number of advancements in the assist area. The centrifugal pump for open-heart surgery, the product of such an effort, has frequently been used to support patients after heart surgery (post-cardiotomy), or as a bridge to life prior to transplant. 

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware comoination for adjustable speed pumping. The motor is operated by an electronic-motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command input signal from the process controller. 

Unhke the control valve, the centrifugal pump has poor or nonexistent shutoff capabihty. A flow check valve or an automated on/off valve may be reqiiired to achieve shutoff reqiiirements. This requirement may be met by automating an existing isolation valve in retrofit apphcations. 

The centrifugal pump is the type most widely used in the chemical industiy for transferring liquids of aU types—raw materials, materials in manufacture, and finished produc ts—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc. These pumps are available through avast range of sizes, in capacities from 0.5 mVh to 2 X 10 mVh (2 gal/min to 10 gaJ/min), and for discharge heads (pressures) from a few meters to approximately 48 MPa (7000 Ibf/iu"). The size and type best suited to a particular apphcation can be determined only by an engineering study of the problem. 

Flow rate and pressure behavior for the three types of filtration are shown in Fig. 18-106. Depending on the characteristics of the centrifugal pump, widely differing cuiwes may be encountered, as suggested by the figure. 

In the left upper corner of Figure 3.4.1, the centrifugal pump performance is shown. As ean be seen, the head generated depends on RPM but is independent of the flow, within a 10 % error, up to a eertain limit. The pressure staits to deeline when that point is reaehed at whieh the flow is high enough that the pump itself limits the flow beeause of its eross-section. 

The pumps can be either centrifugal or positive displacement types. The centrifugal pumps should have a head curve continuously rising toward the shut-off point. The standby pump should be piped into the system in a manner that permits checking of the pump while the main pump is in operation. To achieve this, a restriction orifice is required with a test bleeder valve piped to the return oil line or the reservoir. 

The final element of the equipment description is the equipment boundary figure. A boundary figure is included with each data cell to define the components and limits of the equipment associated with that cell. For example, the data cell boundary figure (Data cell 3.3.7.2.1.1) in Figure 3.2 shows that the centrifugal pump, seal system, motor, motor control unit, lube oil system, coupling, and transmission are all components of the equipment in the data cell. The equipment boundary is inherently reflected in the taxonomy number. 

The centrifugal pump is a versatile unit in the process plant, since its ease of control, non-pulsing flow, pressure limiting operation fits many small and large flow systems. 

Generally speaking the centrifugal pump has these characteristics ... 

In the dynamic pump, in particular, the centrifugal pump, the discharge rate Q is determined by the resistance pressure P in the flow system the pump discharges to (assuming some given speed of the pump). This is illustrated in Figure 3-49. 

The centrifugal pump is the most important pump of the dynamic class of pumps for the oil and gas industry. Other pumps in this class are covered in other references [17,18]. 

For the centrifugal pump, two terms are in common use, the Thoma cavitation number cr and the suction specific speed Sn ... 

When the pressure increase across the centrifugal pump is less than 125 psi (9 kg/cm2) and the flow rate is less than 300 GPM (70 m3/hr) the pump specified should be large enough to meet expansion needs.39 This possibility should also be considered for other situations. A smaller impeller can be used to meet the lower initial requirements. Then when expansion occurs this can easily be replaced with the normal-sized one. The engineer must check to see that under both situations enough power is produced and there is an adequate NPSH. 

System total heads should be estimated as accurately as possible. Safety factors should never be added to these estimated total head values. This is illustrated by Figure 4.8. Suppose that OAi is the correct curve and that the centrifugal pump is required to operate at point A. Let a safety factor be added to the total head values to give a system curve OA2. On the basis of curve OA2, the manufacturer will supply a pump to operate at point A2. However, since the true system curve is OA, the pump will operate at point Ai. Not only is the capacity higher than that specified, but the pump motor may be overloaded. 

The process engineer s job is to size both the centrifugal pump and the control valve. The bigger the control valve, the less pressure drop it will take. This means a lower-head pump can be used and energy costs will be tower... 

The higher flow rate might also reduce the head that the centrifugal pump produces if we are out on the pump curve where head is dropping rapidly with throughput. For simplicity, let us assume that the pump curve is flat. This means that the total pressure drop across the heat exchanger and the control valve is constant. Therefore, the pressure drop over the control valve must decrease as the the pressure drop over the heat exchanger increases. 

The designer must specify the maximum flow rate that is required under the worst conditions and the minimum flow rate that is required. Then the valve flow equations for the maximum and minimum conditions give two equations and two unknowns the pressure head of the centrifugal pump APj> and the control valve size C . 

J4. Reactant liquid is pumped into a batch reactor at a variable rate. The reactor pressure also varies during the batch cycle. Specify the control valve size and the centrifugal pump head required. Assume a flat pump curve. 

Pumps that handle liquids more viscous than water shall have their water performance corrected in accordance with the Centrifugal Pump Section of the Hydraulic Institute Standards. 

The centrifugal pump operates over a very wide range of flows and pressures. For low heads but high flows the axial pump is best suited. Both the centrifugal and axial flow pumps impart energy to the fluid Iw the rotational speed of the impeller and the velocity it imparts to the fluid. 

A power take-off arrangement from the internal combustion engine is used to drive the centrifugal pumps. 

Centrifugal compressors must operate on a curve, just like centrifugal pumps. A typical curve is shown in Fig. 28.3. The horizontal axis is actual cubic feet per minute (ACFM). This is analogous to GPM, used on the horizontal (x) axis of centrifugal pump curves. The vertical axis is H (poly tropic feet of head). This is analogous to the feet of head used on the vertical (y) axis of the centrifugal pump curve. 

Feed tank with centrifugal pump the centrifugal pump replaces the gravity, positioning the feed tank, even below the reactor level. Since a centrifugal pump is not volumetric, it is necessary to provide an additional control valve to limit the flow rate. The flow control strategies are the same as described above. 

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