Pumps, centrifugal process pumps

CENTRIFUGAL PUMP PROCESS DATA SHEET STUN ITS... 

The demands of this hydraulic system are best satisfied by the use of a single-stage centrifugal pump. These pumps are widely used throughout the chemical processing industry and offer several key advantages over other types as listed below ... 

Specification for Hori2ontal End Suction Centrifugal Pumps for Chemical Process," ASME B73.1M, Washington, D.C., 1991. 

Impervious graphite centrifugal pumps, pipe fittings, and valves were developed because most chemical processes require the movement of Hquids. Graphite pipe and fittings in sizes ranging from 25 to 635 mm ID are used to convey corrosive fluids. 

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. 

Development The following discussion relates specifically to the use of what could be called radial-inflow, centrifugal-pump power-recovery turbines. It does not apply to the type of unit nurtured by the hydroelecti ic industry for the 1 ge-horsepower, large-flow, low- to medium-pressure differential area of hydraulic water turbines of the Felton or Francis runner type. There seems to have been little direct transfer of design concepts between these two fields the major manufacturers in the hydroelectric field have thus far made no effort to sell to the process industries, and the physical arrangement of their units, developed from the requirements of the hydroelectric field, is not suitable to most process-plant applications. 

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

Positive pumps are normally fitted with relief valves. These are not usually fitted to centrifugal pumps unless the process material is likely to explode if it gets too hot. As an alternative to a relief valve, such pumps may be fitted with a high-temperature trip. This isolates the power supply. Or a kick-back, a small-diameter line (or a line with a restrietion orifice plate) leading from the delivery line baek to the suction vessel, may be used. The line or orifice plate is sized so that it will pass just enough liquid to prevent the pump from overheating. Small-diameter tines are better than restriction orifice plates as they are less easily removed. 

Centrifugal pumps are widely used in the process industry because of simplicity of design, low initial cost, low maintenance, and fle.xibility of application. Centrifugal pumps have been built to move as little as a few... 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

This chapter presents information on rating, sizing, and specifying process pumps. The emphasis will be on centrifugal pumps, which are by far the most widely used in the process industries however, applications of other types of pumps will also be discussed (see Table 3-1). 

The primary pump types for the chemical industry for horizontal and vertical inline applications have been standardized in ANSI B-123, ANSI Std B73.1M for horizontal end suction centrifugal pumps, and ANSI B73.2M for vertical inline centrifugal pumps. The standards are in a continuous process of upgrading to suit requirements of industry and the manufacturers. The API-610 standard is primarily a heavy duty application, such as is used for the refinery and chemical industry requirements. This is the only true world pump [21] standard, although the International Organization for Standardization (ISO) is studying such an improved design [20]. 

Figure 3-12. Single-stage, high speed (>5000 ft) centrifugal process pump. Pumps are high speed, gear driven, and especially suited for applications in light hydrocarbon liquid service. (Courtesy Sundstrand Fluid Handling, Inc.)...

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. 

The effects of impeller shape for the usual centrifugal process pump performance are given in Figure 3-34. The only part the process designer can play is in the selection... 

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. 

Hallam,J. L., Centrifugal Pumps Which Suction Specific Speeds are Acceptable , Hydrocarbon. Processing, 61, No. 4, 1982. 

Karassik, 1., Fact or Fiction Some Misconceptions About Desirable Construction Features of Centrifugal Pumps, Chemical Processing Mid-March 1989. 

Karassik, I.J., When to Maintain Centrifugal Pumps, Hydrocarbon Processing, V. 72. No. 4, April 1993, p. lOL Nasr, A., Extending the Life of Magnetic-Drive Pumps, Chemical Engineering, V. 98, No. 10, October 1991, p. 159. 

The reason for speed variations in constant-speed machinery is variation of process load. For example, a centrifugal pump s load will vary due to the viscosity of the fluid being pumped or changes in suction or discharge pressure. The pump speed will change because of these load changes. 

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