Selection criteria for centrifugal pumps

Selection criteria for centrifugal pumps and installation layouts

Centrifugal pumps are often considered as simple components to be included in more complex circuits. In reality great care must be taken with regard to their configuration, which must always be considered in relation to the system's characteristics, pumping requirements and the user's individual needs.

Selecting an impeller pump,in fact, requires in-depth knowledge of the specific operating conditions: whoever manufactures centrifugal pumps must know how to evaluate all information useful achieving the best possible hydrodynamic design.

To guarantee the proper use of the plastic anti-corrosion pumps, the user must provide the manufacturer thorough details regarding the specific application and in particular the liquid that the pump is intended handle. Furthermore, to ensure that similar pumps work at full capacity and efficiently demonstrate their special characteristics, the pump itself must be installed carefully to evaluate the effects of operating conditions with the aggressive liquid.

Selecting a pump for chemically aggressive liquids requires careful review of several fundamentally important data values to offer a product which can meet the needs of the handling system A reasonable safety margin and possible friction head losses should be considered, though without proposing unnecessary oversizing: only in this way can unsatisfactory performance, unexpected failures or unjustified increases in purchasing and operating costs be avoided.

For this reason, Savino Barbera has prepared a detailed data sheet (See the Downloads) section for the analysis of all elements necessary for the correct pump sizing, where, among others, the following items are considered:

• the liquid's properties (name, composition, concentration, temperature, specific gravity)
• the liquid flow rate (volume to be transported per unit time)
• the specific characteristics of the hydraulic circuit (head, friction head losses, ancillary plant equipment)

The evaluation of such information (not the only information, but nevertheless that which is most important) will allow proper selection of the pump (in terms of material and type), determination the correct impeller and the precise motor power (as a function of the characteristic curve).

Analysis of certain elements to be evaluated when selecting a pump

Liquid composition: this is of fundamental importance for correctly choosing the materials of construction for the various pump parts which come into contact with the liquid. The more precise the information is regarding the composition of the liquid pumped, the more precise materials selection will be for pump components, such as gaskets and any mechanical seals. For example: different concentrations of the same acid may require materials of construction with different characteristics.

Dry operation
it's always advisable to protect the pump from dry operation. One of the most widely used methods still is however the employment of a minimum tank level that interrupts pump operation as soon as a dangerous level for the pump is reached.
Operating temperatures
knowledge regarding the maximum and minimum temperature is important (in addition to the normal operating temperature) for the reasons discussed in the preceding paragraph. The temperature of the fluid to be handled has important effects on the materials: very low temperatures can make particular plastic material become fragile whereas high temperatures may result in softening and deformation of the components.
Solids in suspension
in this case also, knowing the nature and quantity of the suspended particles is crucial. In fact there are materials which have different abrasion resistance but identical resistance against chemical aggression. The nature of the suspended solids can also influence the same selection of the pump type: in certain cases a vertical pump might be needed without guide bushings or it might be necessary to avoid pumps with magnetic coupling
Installation of the pump
ensure that the pump can draw from a zone of the basin or tank where the liquid has favorable characteristics. For example if the liquid has the tendency to form sludge sediments on the bottom of the tank, then the liquid must be kept continuously agitated to prevent the formation of said sludges. Or also the pump intake must be positioned high enough to avoid pumping slurries that are excessively concentrated or which might obstruct suction.

Best Efficiency Point (BEP)

All pumps (both the centrifugal pumps and the positive-displacement pumps) have operational limits. Specifically, centrifugal pumps have certain limitations which, if not properly evaluated can drastically reduce their working life. The BEP (Best Efficiency Point) is not only the maximum operating point but it is also the point where the speed and pressure at the impeller and the spiral stator are equal. As the operating point diverges from the Best Efficiency Point, the speed changes which in turn modifies the pressure acting on one of the sides of the impeller. This irregular pressure on the impeller manifests itself as a radial thrust which deflects the pump shaft causing, among other things:

• an excessive load on the bearings
• an excessive deflection of the mechanical seal
• irregular wear on the shaft bearing

The damages that might occur then consist of a shorter bearing life or a broken shaft. Radial loading is maximized when everything closed. If operating outside of the recommended operating range, damage to the pump could be caused by excessive speed and turbulence. Vortexes can create cavitation damage which very quickly can destroy the pump and impeller casing.
When choosing a pump, it's very important that unrealistic safety margins not be calculated or that inappropriate information not be included in the evaluation. The true curve could exceed the recommended operating range resulting in series consequences.
It is best to verify the actual operating conditions for the pump the during while it is in operation (using a flowmeter and/or a pressure gauge) in order to make the adjustments needed to ensure the proper working conditions and a long service life.

Materials

Carefully selecting the materials of construction, whether for the structure, or for the seals and for the wear components, made as a function of plant type and the fluids to be handled, allows the problems related to pumping of liquids "at risk" to be solved safely and efficiently; even in the most complex working processes.

PP (Polypropylene)
Characterized by good mechanical properties, broad resistance against chemical agents and a high operating temperature. Not suitable for strongly oxidizing concentrated acids and halogens (fluorine, chlorine, bromine, iodine). Maximum operating temperature: 90°C

PVC (Polyvinyl chloride)
Material widely used in plant engineering applications. Resistance to acids, bases, saline solutions and organic compounds. Not suitable for aromatic and chlorinated hydrocarbons. Maximum operating temperature: 45°C

PVDF (Polyvinylidene fluoride)
Features an exceptional resistance to acids, saline solutions, aliphatic, aromatic and chlorinated hydrocarbons, alcohols and halogens. Not suitable for organic bases, alkaline solutions, ketones, esters and ethers. Indicated for the semiconductor industry and for liquids to be protected from contamination. Maximum operating temperature: 100°C

PE-HD (High-density polyethylene)
Chemical resistance similar to polypropylene. Valued for its abrasion resistance. Maximum operating temperature: 60°C

EPDM (Ethylene-propylene rubber)
Elastomer with high modulus of elasticity, strong resistance to acids, bases, alcohols ketones. Not recommended for hydrocarbons, oils and greases. Maximum operating temperature: 90°C

FPM (Fluoroelastomer rubber)
Features high thermal stability, low permanent deformation and high chemical resistance against acids, solvents, lubricants or fuels.

Sintered Ceramic (Al203 99%)
Base material for seal rings and parts subject to wear. Superior hardness, wear resistance and resistance against chemical agents. Not recommended in the presence of hydrogen fluoride, sodium carbonateand sodium hypochlorite at high concentrations and temperatures.

Sintered silicon carbide (SiC, without free silicon)
Material with characteristics more resistant than aluminum oxide: greater resistance to thermal shock and chemically inert at the operating temperatures of the plastic pumps.

PTFE (Polytetrafluorethylene)
Chemically inert up to 250°C in the presence of any chemical agent, the only substances for which it is not recommended are cast alkaline metals, chlorine trifluoride and gaseous fluorine at high temperatures. It may be 'loaded' with materials such as glass or ceramic or with graphite and carbon. Excellent for seal components and wear bushings.

FEP – PFA
Fluoropolymer with chemical resistance characteristics similar to PTFE but with less permeability. Suitable for welding processes: it is used for covering mechanical seal and O-ring springs.

Posted in: Chemical Engineering Processing

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