Double Suction Single Stage Pumps

These pumps are used for general water supply and circulating service and for chemical service when liquids that are non corrosive to iron or bronze are being handled. They are available for capacities for about 5.7 m3/hr (25 gal/min) up to as high as 1.136 x 104 m3/h (50,000 gal/min) and heads up to 304 m (1000 ft). Such units are available in iron, bronze and iron with bronze fittings. Other materials increase the cost, when they are required, a standard chemical are usually more economical.

Closed Coupled Pumps
Pumps equipped with a built in electric motor or sometimes steam turbine driven (i.e. with pump impeller and driver on the same shaft) are known as close coupled pumps. Such units are extremely compact and are suitable for a variety of services for which standard iron and bronze materials are satisfactory. They are available in capacities up to about 450 m3/h (2000 gal/min) for heads up to about 73 m (240 ft). Two stage units in the smaller sizes are available for heads to around 150 m (500 ft).

Caned Motor Pumps
These pumps command considerable attention in the chemical Industry. They are closed couple unit in which the cavity housing the motor-rotor and the pump casing are interconnected. As a result, the motor bearing run in the process liquid and all seals are eliminated. Because the process liquid is the bearing lubricant, abrasive solids cannot be tolerated. Standard single stage canned-motor pumps are available for flows up to 100 m3/h (700 gal/min) and heads up to 76 m (250 ft). Two stage units are available for heads up to 183 m (600 ft). Canned-motor pumps are being widely used for handling organic solvents, heat transfer liquids, and light coils as well as many clean toxic or hazardous liquids or for installations in which leakage is an economic problem.

Chemical Pump and Sealing Pump

The vertical in line pumps, although relatively new additions, are finding considerable use in chemical and petrochemical plants in the United States. An inspection of the two designs will make clear the relative advantages and disadvantages of each.

Chemical pumps are available in a variety of materials. Metal pumps are the most widely used. Although they may be obtained in iron, bronze and iron with bronze fittings, an increasing number of pumps of ductile iron, steel, and nickel alloys are being used. Pumps are also available in glass, glass lined iron, carbon, rubber, rubber lined metal, ceramics and a variety of plastics, such units usually being employed for special purpose.

Sealing the Centrifugal Chemical Pump
Although detailed treatment of shaft seals is presented in the Sealing of Rotating Shafts, it is appropriate to mention here the special problems of sealing centrifugal chemical pumps. Current practice demands that packing boxes be designed to accommodate both packing and mechanical seals. With either type of seal, one consideration is of paramount importance in chemical service, the liquid present at the sealing surface must be free of solids. Consequently, it is necessary to provide a secondary compatible liquid to flush the seal or packing wherever the process liquid is not absolutely clean.

The use of packing requires the continuous escape of liquid past the seal to minimize and to carry away the frictional heat developed. If the effluent is toxic or corrosive, quench glands or catch pans are usually employed. Although packing can be adjusted with the pump operating, leaking mechanical seals require shutting down the pump to correct the leak. Properly applied and maintained mechanical seals usually show no visible leakage. In general, owing to the more effective performance of mechanical seals, they have gained almost universal acceptance.

Centrifugal Pump Characteristic

It is important to note that at any fixed speed the pump well operate along this certain curve and no other point for instance on the curve below. On pump with variable speed driver such as steam turbine, it is possible to change the characteristic curve. It is important to remember that the head produced will be the same for any clean liquid of the same viscosity. The pressure rise, however, will in proportion to the specific gravity. Viscosities of less than 50 kPa's (50 cP) do not affect the head materially.

Single Stage Centrifugal Pump
This pump are available to capacities up to and over 1.136 x 104 m3/h (50,000 gal/min) for heads (pressures) up to 488 m (1600 ft). They are available in a variety of design for particular services.

Process Pumps
This term is usually applied to single stage pedestal mounted units with single suction overhung impellers and with a single packing bos. These pumps are ruggedly designed for ease in arrangements, and are built especially to handle corrosive or other wise difficult to handle liquids.

Specially but not exclusively for the chemical industry, most pump manufactures now build to national standards horizontal and vertical process pumps. American National Standards Institute (ANSI) Standard B73 1 - 1977 and B73 2 - 1975 apply to be horizontal and vertical in line pumps respectively.

The horizontal pumps are available in line pumps for capacities up to 900 m3/h (4000 gal/min), the vertical in line pumps, for capacities up to 320 m3/h (1400 gal/min). Both horizontal and vertical in lime pumps are available for heads up to 120 m (400 ft). The intent of each ANSI specification in that pumps from all vendors for a given nominal capacity and total dynamic head at a given rotative speed shall be dimensionally interchangeable with respect to mounting, size, and location of suction and discharge nozzles, input shaft, base plate, and foundation bolts.

Pump Casings

There are three general types of casings, but each consist of chamber in which the impeller rotates, provided with inlet and exist for the liquid being pumped. The simplest form is the circular casing, consisting of an annular chamber around the impeller; no attempt is made to overcome the losses that will arise from eddies and shock when the liquid leaving the impeller at relatives high velocities enter this chamber. Such casing is seldom used.

Volute Casings take the form of spiral increasing uniformly in cross-sectional area as the outlet is approached. The volute efficiently converts the velocity energy imparted to the liquid by the impeller into pressure energy.

A third type of casings is used to diffuser type or turbine pumps. In this type, guide vanes or diffuser are interposed between the impeller discharge and the casing chamber. Losses are kept to a minimum in a well designed of this type, and improved efficiency is obtained over a wider range of capacities. This construction is often used in multistage high head pumps.

Action of Centrifugal Pump

Briefly the action of centrifugal pump may be shown by drawing. Power from outside source is applied to shaft A, rotating the impeller B, within the stationary casing C. The blade of the impeller in revolving produces a reduction in pressure at the entrance or eye of the impeller. This causes liquid to flow into the impeller from the motion pipe D. This liquid is forced outward along the blades at increasing tangential velocity.

Centrifugal Pump

The centrifugal pump is the type most widely used in the chemical industry for transferring liquids of all types, raw material, material in manufacture, and finished product, as well as for general services of water supply, boiler feed, condenser circulation, condenser return, etc. This pump are available through fast range of sizes in capacities from 0.5 m3/hour to 20,000 m3/hr (2 gal/min to 100,000 gal/min) and for discharge head (pressure) from a few meters to approximately 48 MPa. The size and type best suited to particular applications can be determined only by an engineering study of the problem.

The primary advantages of a centrifugal pump are simplicity, low first cost, uniform (nonpulsating) flow, small flow space, low maintenance expense, quite operation, and adaptability for use with a motor or turbine drive.

Centrifugal pump in a simplest form, consists of an impeller rotating within a casing. The impeller consists of a number of blades, either open or shrouded, mounted on a shaft that projects outside the casing. Its axis of rotation may be either horizontal or vertical, to suit the work to be done. Closed-type, or shrouded, impellers are generally the most efficient. Open or semi open type impellers are used for viscose liquids or liquids containing solid materials and on many small pumps for general service. Impeller may be of the single suction or double suction type, single if the liquid enter from one side, double if it enter from both side.

Pump Selection

When selecting pumps for any service, it is necessary to know the liquid to be handled, the total dynamic head, the suction and discharge heads, and in most cases, the temperature, viscosity, vapor pressure, and specific gravity. In the chemical industry the task of pump selection in frequently further complicated by the presence of solids in the liquid and liquid corrosion characteristics requiring special materials of construction. Solid may accelerate, or require delicate handling to prevent undesirable degradation.

To manage on selection should intense to this main point:

1. Range of Operation
2. Pump material of Construction
3. Pressure of Liquid or Solids
4. Pump Cost

Range of Operation will determine pump head from the required capacity must be operated. This value usually gets from table that correlates the capacity and the head pump should be choose.

Pump Materials of Construction; in the chemical industry, the selection of pump materials of construction is dictated by considerations of corrosion, erosion, personnel safety, and liquid contamination. The experience of pump manufactures of often valuable is selecting materials.

Presence of Solid, when pump is required to pump a liquid containing suspended solid, there are unique requirements which must be considered. If the liquid to pump is not contain solid materials, the selection is just considering capacity and the type of solid to be pumped, that will determine the pump material should be used.

Pump cost is vary from pump supplier, so the choice in have to be accommodated by the budget of your company to chosen the certain pump brand. Usually the price of certain pump type is not too far from many brands.

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