Centrifugal pumps suction head

Suction and discharge heads are determined the same as for centrifugal pumps. Total head and capacity are used in selecting the proper rotary pump from a manufacturer s data or curves. Since viscosity is quite important in the... 

Pump Suction. The net positive suction head required (NPSHR) affects the resistance on the suction side of the pump. If it drops to or near the vapor pressure of the fluid being handled, cavitation and loss of performance occurs (13). The NPSHR is affected by temperature and barometric pressure and is of most concern on evaporator CIP units where high cleaning temperatures might be used. A centrifugal booster pump may be installed on a homogenizer or on the intake of a timing pump to prevent low suction pressures. 

D. J. MMethodfor Estimating the Net Positive Suction Head Required by Centrifugal Pumps, ASME 81-WA/EE-32, Washington, D.C., 1981. 

This system requires a pump with a best effieieney point (BFP) of 94 feet at 300 gallons per minute. If this is a eonventional industrial centrifugal pump with a BFiP of 94 feet, the shut-off head should be approximately 110 feet. And if the motor is a standard NFMA four pole motor spinning at about 1800 rpm, the diameter of the impeller should be approximately 10.5 inehes. If this pump were bought off the shelf from local distributor stock, it would probably be a 3 x 4 x 12 model end-suction centrifugal back pullout pump with the impeller machined to about 10.5 inches before installing the pump into the. system. And that s the way it is done. 

Figure 3-40D. Pressure head, positive suction. (Adapted by permission, Centrifugal Pumps Fundamentals, Ingersoll-Rand Co., Washington, N.J. 07882.)...

For general service the average centrifugal pump should lift about 15 feet of water on its suction side. However, since each process situation is different, it is not sufficient to assume that a particular pump wll perform the needed suction lift. Actually, certain styles or models of a manufacturer s pumps are often specially adapted to high lift conditions. On the other hand it is unnecessary to select a high lift pump when pressure head or flooded suction conditions prevail. Proper evaluation of suction lift conditions cannot be over emphasized. 

The pressure at any point in the suction line must never be reduced to the vapor pressure of the liquid (see Equation 3-6). Both the suction head and the vapor pressure must be expressed in feet of the liquid, and must both be expressed as gauge pressure or absolute pressure. Centrifugal pumps cannot pump any quantity of vapor, except possibly some vapor entrained or absorbed in the liquid, but do not count cm it. The liquid or its gases must not vaporize in the eye/entrance of the impeller. (This is the lowest pressure location in the impeller.)... 

The hydraulic horsepower produced by mud pumps depends mainly on the geometric and mechanical arrangement of the suction piping. If suction-charging centrifugal pumps (e.g., auxiliary pumps that help move the mud to the mud pump) are not used, the pump cylinders have to be filled by the hydrostatic head. 

It is important to recognize that a centrifugal pump will operate only along its performance curve [10, 11]. External conditions will adjust themselves, or must be adjusted in order to obtain stable operation. Each pump operates within a system, and the conditions can be anticipated if each component part is properly examined. The system consists of the friction losses of the suction and the discharge piping plus the total static head from suction to final discharge point. Figure 3-51 represents a typical system head curve superimposed on the characteristic curve for a 10 by 8-inch pump with a 12-inch diameter impeller. 

Centrifugal pumps, 181 Discharge systems, 187 Example calculation, 186 Flow friction losses, 185. 186 Friction losses, pipe, see Chapter 2 Friction, 188 Pressure head, 184—186 Static head, 184-186 Suction head, 184, 185 Suction lift, 184, 185 Suction systems, 186 Hvdroclones, 265—267 Application system, 267 Ignition, flammable mixtures, 493 Impellers, centrifugal, reducing diameter, 203 Impellers,... 

Total head, centrifugal pumps, 180, 183 Discharge, 205 Head curve, 198 Suction head, 184, 186 Suction lift, 184, 186 Type, 184 Tubing, 63, 64 Two-phase flow, 124 Calculations, 125-127 Flow patterns, chart, 124 System pressure drop, 125 Types of flow, 124, 125 Utilities check list, process design, 34 Vacuum,... 

A centrifugal pump is required to circulate a liquid of density 800 kg/m2 and viscosity 0.5 x 10 3 Ns/m" from the reboiler of a distillation column through a vaporisor at the rate of 0.004 m3/s, and to introduce the superheated vapour above the vapour space in the reboiler which contains a 0.07 m depth of liquid. If smooth-bore 25 mm diameter pipe is to be used, the pressure of vapour in the reboiler is 1 kN/m2 and the Net Positive Suction Head required by the pump is 2 m of liquid, what is the minimum height required between the liquid level in the reboiler and the pump ... 

A centrifugal pump is to be used to extract water from a condenser in which the vacuum is 640 mm of mercury. At the rated discharge the net positive suction head must be at least 3 m above the cavitation vapour pressure of 710 mm mercury vacuum. If losses in the suction pipe account for a head of 1.5 m. what must be the least height of the liquid level in the condenser above the pump inlet ... 

A centrifugal pump is used to pump a liquid in steady turbulent flow through a smooth pipe from one tank to another. Develop an expression for the system total head A/t in terms of the static heads on the discharge and suction sides zd and zs respectively, the gas pressures above the tanks on the discharge and suction sides Pd and Ps respectively, the liquid density p, the liquid dynamic viscosity p, the gravitational acceleration g, the total equivalent lengths on... 

Horizontal centrifugal pumps should provide 150% of the rated capacity at 65% of the rated pressure, with a shutoff head of not more than 120% of the rated pressure. This pump should be used only when suction supply is under a positive head. Suction pipes should be designed to preclude the formation of air bubbles. A characteristic curve for a rated fire water pump is shown in Figure 7-15. 

The impeller is the working part of a centrifugal pump. The function of the impeller is to increase the velocity or kinetic energy of the liquid. The liquid flows into the impeller, and leaves the impeller, at the same pressure. The black dot shown at the top of the impeller in Fig. 23.6 is called the vane tip. The pressure at the vane tip is the same as the pump s suction pressure. However, as the high-velocity liquid escapes from the impeller and flows into the volute, its velocity decreases. The volute (which is also called the diffuser) is shaped like a cone. It widens out in the manner illustrated in Fig. 23.7. As the liquid flows into the wider section of the volute, its velocity is reduced, and the lost velocity is converted—well, not into pressure, but into feet of head. 

Lack of available NPSH may also be caused by high frictional loss in the suction piping. If this is the case, a small reduction in flow will not noticeably increase the pressure at the suction of the pump. A properly designed suction line to a centrifugal pump should have a frictional head loss of only a few feet of liquid. However, having a large-diameter suction line, and a relatively small draw-off nozzle, usually will lead to excessive loss of available NPSH. 

Figure 7.8. Topical capacity-head ranges of some centrifugal pumps, their 1978 costs and power requirements. Suction and discharge are in inches (Evans, 1979, Vol. 1).

FIGURE 10 Typical centrifugal pump characteristic curves showing efficiency curves and NPSH (net positive suction head) for several impeller diameters. 

What is the upper limit of specific speed and capacity of a 1750-r/min single-stage double-suction centrifugal pump having a shaft that passes through the impeller eye if it handles clear water at 85°F (302 K) at sea level at a total head of 280 ft with a 10-ft suction lift What is the efficiency of the pump and its approximate impeller shape ... 

A single-suction centrifugal pump is driven by a 60-Hz ac motor. The pump delivers 10,000 gal/min (0.63 m3/s) of water at a 100-ft (30.5-m) head. The available net positive suction head is 32 ft (9.75 m) of water. What is the best operating speed for this pump if the pump operates at its best efficiency point ... 

As a general guide, single-suction centrifugal pumps handle up to 50 gal/min (0.0032 m3/s) at total heads up to 50 ft (15 m) either single- or double-suction pumps are used for the flow rates to 1000 gal/min (0.063 m3/s) and total heads to 300 ft (91 m) beyond these capacities and heads, double-suction or multistage pumps are generally used. 

Peripheral pumps are particularly useful for pumping low-flow-rate, low-viscosity liquids at high pressures than are normally available with centrifugal pumps. Close clearances limit their use to clean liquids. Also, because of the close clearances between the impeller and casing, a peripheral pump has excellent suction lift-up to 8.5 m(128 ft) of head. 

Chapter 5 considered pump types and their evaluation and selection. After selecting a pump type, the next step is to size the pump. This requires calculating the flow rate and the pressure rise across the pump or the pump head. The net positive suction head (NPSH), is also important, particularly for centrifugal pumps. NPSH is the difference between the total pressure and the vapor pressure of the fluid at the pump inlet. NPSH will be discussed later. 

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