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Introduction to Centrifugal Pumps - Part 1

By E. M. Araza

Pumps are mechanical equipment that impart energy to a fluid to increase its pressure and move it from one point to another in a controlled and safe environment. They are among the most widely used machineries in various industries. Thus, their worldwide use creates an increasing demand for students, employees, professionals, suppliers, and end-users to gain technical knowledge and keep pace with advances in pump technology. This article is written in response to the need for accurate and reliable information about the industry, with emphasis on centrifugal pumps.

Among other classifications, and for the purpose of this article, pumps may be broadly grouped into rotodynamic type, or positive displacement type.

Rotodynamic pumps

Rotodynamic pumps impart energy by means of rotating impeller or propeller. They convert mechanical energy into velocity energy to develop differential pressure in a continuous state. They can be sub-grouped into centrifugal, mixed-flow, or axial flow pumps. 

Centrifugal pumps operate based on the centrifugal action of their impellers, mixed flow pumps use a combination of centrifugal force and the lifting action of the impellers, and axial pumps impart energy solely by the lifting action of the impellers. In rotodynamic pumps the flow rate will change if their differential pressure is changed under constant speed condition.

The distinctions among centrifugal, mixed flow, and axial flow pumps have become somewhat blurred because some people erroneously lump them together as centrifugal pumps. Even industry standards such as API 610 and ISO 13709 that are specifically written for centrifugal pumps contain provisions that pertain to axial flow pumps. An example is their reference to VS3 class – axial flow pumps.

Positive displacement pumps

On the other hand, positive displacement pumps (PD) deliver fixed flow rate even if the differential pressure is changed at constant speed. PD pumps include reciprocating pumps (piston, plunger, and diaphragm) and rotary pumps (gear, lobe, screw, and sliding vane). They are suitable for low specific speed application, for high pressure service, and for handling highly viscous liquids. A big disadvantage is that their flow is pulsating, and they may require auxiliary flow dampener, such as surge tank, to steady the flow.

Centrifugal pumps

Among rotodynamic pumps, centrifugal pumps are very popularly because they have wider range of service and are available commercially in various sizes and materials of construction.  

One can differentiate a centrifugal pump from a mixed flow pump, or axial flow, by examining its physical construction. However, there are parameters that will clearly identify the pump type. Among these parameters are the pump specific speed (Ns), the ratio of impeller eye diameter to impeller maximum diameter (D1/D2), the direction of the impeller discharge flow, the allowable minimum impeller cut diameter, the shape of the pump performance curve, the slope of the horsepower curve, its speed-torque curve, manner of pump start-up, etc. These parameters are discussed in more details, including tabulated comparisons (Figure 1) in a separate article. 

By common usage, the term pump is generally used to refer to an equipment that handles liquid, whereas one that handles air, gas, or vapor (collectively called gas in this article for simplicity) is aptly referred to as air pump, vacuum pump, compressor, blower, or fan. This distinction is important because a centrifugal pump is not intended to handle gas – the presence of gas in the liquid, even in small volumetric amount, impairs the pump performance and operation.

Although many liquids, such as boiler feedwater, go through deaeration or degasification to remove gas from the liquid, pumps will invariably be put into service to handle liquids with some amount of dissolved or entrained gas. Typical examples are self-priming, slurry, and froth pumps. In recent years modern centrifugal pumps have been designed to handle dense carbon dioxide (CO2) - defined as CO2 in gaseous state but is exhibiting the properties or behavior of a liquid. One of the challenges facing pump engineers is to develop design innovations to mitigate the adverse effects of entrained or dissolve gas in pumped liquids. Some of the adverse effects are reduced flow and head, lower efficiency, higher NPSHR, severe cavitation, higher noise and vibration, and unstable flow.

Centrifugal pumps running in reverse rotation perform the reverse function of recovering energy from a liquid by reducing its pressure. They are called hydraulic power recovery turbines (HPRT), or pump as turbine (PAT); their design, selection, performance curve, and operation are different from those of normally running pumps. Centrifugal pumps acting as HPRTs are discussed in a separate article.

Centrifugal pumps are highly efficient but their performance is impaired when handling viscous liquids; their performance curve has to be adjusted for its equivalent viscous performance when handling viscous liquids. The most commonly used method of adjusting the performance is per Hydraulic Institute. Typically, a viscous performance will show a reduction in flow, head, and efficiency compared to the original water performance. At  some very high viscosity values, centrifugal pumps are no longer practical to use and PD pumps should be used instead.

The hydraulic performance of centrifugal pumps is affected by several factors but is dependent, to a larger extent, on its impeller and volute design. The impeller has a more significant effect on the differential head, whereas the volute has a more significant effect on capacity.

The best efficiency point (BEP) of a centrifugal pump does not necessarily reflect the BEP of the impeller, or of the volute, taken separately; rather, it represents the BEP of the particular impeller and volute combination. This is important to note because it has been an industry practice to combine different impellers with different volutes to expand the hydraulic coverage of the pumps. To some extent this affects the pump allowable operating region (AOR), the preferred operating region (POR), and the recommended minimum continuous stable flow (MCSF). More on this on the topic of pump hydraulics.

Centrifugal pumps may have a combination of the following design features:

§ single stage, two-stage, or multistage construction 

§ single volute, or double volute design

§ radially split, or axially split casing 

§ foot-mounting, near centerline mounting, or centerline mounting of casing

§ horizontal, or vertical shaft mounting

§ single suction, or double suction impeller

§ overhang (OH), or between-bearing (BB) rotor

§ single casing, or double casing (barrel pumps)

§ flexibly coupled, rigidly coupled, or close coupled

These design features are discussed in the full version of this article.

The design of modern centrifugal pumps goes beyond the basics of moving and adding pressure to the pumped fluid - it also need to address the increasing demand for higher efficiency to conserve energy and the need for controlling emission and mitigating noise propagation to protect the environment. The need to control costs means that pumps have to be designed to run at higher speeds, in wider performance envelope, and at optimum efficiency. Advances in pump metallurgy result in materials that are more resistant to corrosion, cavitation, fitting and erosion damage. Remote monitoring and troubleshooting ensure that pumps will have longer MTBF. These modern design considerations will result in more efficient and optimum life cycle for the pumps.

One of the factors that contribute to the popular use of centrifugal pumps is the ease of hydraulically rerating them to meet changes in service demands, such as changes in flow rates, differential head, or both. Technical advances from recent R&Ds allows for more drastic changes in pump hydraulics that one can design a “new pump” within an existing pump. Now, it is not unusual to rerate a pump to increase its flow rate up to 125%, or decrease it up to 60%, of its original rated flow. The ability to rerate centrifugal pumps with more drastic hydraulic changes, involving changes to both the impeller and volute, provides end-users with the attractive option to repurpose their pumps for services that may otherwise require new pumps of different size that will likely require expensive baseplate and nozzle piping modifications. See separate article on hydraulic rerates.

There are various types of centrifugal pumps some of which are overlapping; it is likely that a pump will belong to more than one group. Pumps may be classified according to their hydraulic, mechanical, or functional designs, and their compliance with specific industry standards.

Examples of centrifugal pump groupings according to their compliance with industry standards are:

ANSI pumps - ASME B73.1 specifications
API pumps - API 610 specifications
DIN pumps - DIN 24256 specifications
ISO pumps - ISO 2858, 5199, 13709 specifications
Nuclear pumps - ASME specifications
UL/FM fire pumps - NFPA 20 specifications
 

Compliance with the applicable standards is mandated by governmental regulations in many jurisdictions. Others are dictated by industry practice – for example, insurance underwriters require the use of UL-listed, FM-approved fire pumps in many jurisdictions if they were to provide property insurance coverage, or coverage with reduced insurance premium.

These industry-specific pumps are similar to commonly available commercial pumps, but are designed with added functionality and may have other testing requirements specified in the standard. 

Among the widely used pump designations are based on both API Standard 610 and ISO Standard 13709. These pump standards designate overhung pumps as OH (OH1, OH2, OH3, OH4, OH5 & OH6), between-bearing pumps as BB (BB1, BB2, BB3, BB4, & BB5), and vertically suspended pumps as VS (VS1, VS2, VS3, VS4, VS5, VS6 & VS7). 

OH1 & OH2 are flexibly-coupled single stage, horizontal overhung pumps. They are compact, light-weight, have fewer parts, have small footprint, and inexpensive; they are easy to install, operate, and maintain. OH1 are foot-mounted for use in temperature below 300 degrees Fahrenheit, whereas OH2 are centerline-mounted for use in temperature 300 degrees Fahrenheit, or above.

A foot-mounted casing expands mostly upward. If the upward expansion is excessive, as in high temperature service, the casing would be subjected to harmful thermal stress and excessive nozzle load. The shaft can also get misaligned. To avoid these, centerline-mounted pumps should be used. Centerline-mounted pump have their mounting feet located at the casing horizontal centerline (in line with shaft centerline) to distribute the casing expansion in both upward and downward directions.
 

OH1 and OH2 pumps are typically available in sizes up to 25” maximum impeller diameter, and up to 12” discharge nozzle. Impeller diameter up to 15” can run at 3560 RPM (2-pole speed) and those above are usually limited to 1780 RPM (4-pole speed) maximum. Above these sizes and speeds, between-bearing (BB) pumps are used.

Despite its limited service temperature, OH1 pumps have advantages over OH2; they have smaller foot-print and lower center of gravity. They do not need mounting pedestals and can be mounted directly on top of the baseplate, or skid. They are easier to handle and transport on standalone basis having some feet to stand on.

Selection criteria for OH3, OH4, and OH5
 

OH3, OH4, and OH5 are overhung, single stage, vertical in-line pumps. OH3 is flexibly-coupled, OH4 is rigidly coupled, and OH5 is close-coupled. 

OH3 pump has integral bearing bracket. The pump and driver are flexibly coupled. This design is preferred when the pump is exposed to high axial thrust load. This design first gained wide use in Europe where vertical motors have limited thrust capacity unlike in the U.S. where vertical motors have high thrust capacity bearings. But the separate bearing bracket is a significant cost adder and increases the pump height.

OH4 pump does not have a separate bearing bracket and is rigidly coupled to the driver. Its axial thrust is carried by the motor bearings. In services where high suction pressure the axial thrust can be reduced by unbalancing the impeller wear rings, omitting, the back wear rings, and/or reducing the size of the mechanical seal so that the need for a separate bearing bracket is avoided.
 

OH5 pump is close-coupled to the driver. The term close-coupled means that the motor has an extended shaft that doubles as the pump shaft. The impeller is mounted directly on the motor shaft, there is no coupling. The axial thrust is carried by the bearings on the motor. Because of its compact, lightweight, and inexpensive design, the OH5 is very popular in light service where, in some cases, the pump is mounted directly on the piping similar to a valve installation and may be simply supported with mounting brackets.

OH pumps have significant hydraulic advantage over BB pumps in that they are capable to operate with lower NPSHR and higher suction specific speed (Nss). This is because for a given suction eye diameter, OH impellers have bigger annular eye area than BB impellers because of the shaft blockage in BB pumps.

Selection criteria for BB pumps

The BB designation applies to between-bearing pumps – pumps that have inboard and outboard bearing housings and whose impellers are located between those bearing housings. By this definition, only horizontal pumps can have the BB designation. The advantages of BB pumps, over that of OH pumps, are that they are considered to be of more robust design; they are capable of handling high suction pressure, high axial thrust loads, and can be designed to run at high speeds well above the speed of 2-pole motors, or at turbine speeds.

BB1 – are one-stage, or two-stage pumps of axially split casing design. These pumps are limited in their design operating pressure and temperature. Because of their horizontal split casing design, it can only have near-centerline case mounting thereby limiting its maximum operating temperature because of uneven thermal growth between the upper and the lower halves of the casing. Their flat gasket design at the casing split-line limits their allowable working pressure.

BB2 – are one-stage, or two-stage pumps of radially split casing design. Their design operating pressure and temperature can be much higher than BB1. The ability to design them for centerline mounting allows them to operate at higher temperatures. Their confined gasket design allows them to operate at higher pressure.

BB3 – are similar to BB1 except that they apply to multistage pumps - those that are of three stages, or more. Note that only pumps with three stages or more are considered multistage pumps. Pumps with two stages are called two-stage pump, not multistage.

BB4 – are similar to BB2 except that they apply to multistage pumps.

BB5 – are similar to BB4 except that they are of double casing design; they are also called barrel pumps. They are used in very high pressure and temperature service. BB5 pumps are the most complex and expensive among the BB pump groups.