The flow of fluid in vertical pumps and intake systems

Consider these guidelines when selecting a vertical pump type and design to ensure reliability and performance.

A series of large vertical pumps during installation. Image courtesy of Amin Almasi
A series of large vertical pumps during installation. Image courtesy of Amin Almasi

While horizontal pumps are commonly used in many services, other applications can best be served by vertical pumps — with various types and configurations for a wide range of applications. 

Vertical pumps are often used in plants and locations where there are space or weight restrictions. Another advantage of vertical pumps is that the alignment between the driver, usually an electric motor, and pump is easier to maintain than many horizontal pumps. Vertical pumps use a unique set of shaft and bearing support configuration that allows vertical configuration — with the driver at the top and the pump components at the bottom. While horizontal pumps generally require a firm foundation for mounting, vertical pumps need a smaller foundation or, alternatively, may not require such a foundation. For example, inline vertical pumps can be attached to the piping with minimal support beneath the pump. Vertical pumps can also be used in some high pressure or extreme temperature applications.

Configurations and types of vertical pumps include inline vertical pumps, submersible vertical pumps, can-type vertical pumps and others. Vertical pumps in different arrangements and configurations can have many stages or impellers — sometimes up to 40 stages or more. The following discusses vertical pump types and the flow of fluids in them, as well as intake systems, which are important for many vertical pumps. 

Considerations for selecting a vertical pump

There are many models and designs of vertical pumps including inline vertical pumps, high-speed integrally geared vertical pumps, submersible vertical pumps, different models of wet pit suspended vertical pumps and can-type vertical pumps, among others. Can-type vertical pumps have the driver above the liquid surface; submersible vertical pumps are enclosed packaged pumps, operating deep inside the liquid. 

Owing to the diverse types and applications, numerous options of vertical pumps are available. While this provides a high degree of flexibility, less expensive pumps are not always the best choice. It is important to ensure that the vertical pump is accurately specified, carefully selected and purchased to reflect the requirements of the application. The best pump type should be selected for each service.

Another consideration is the quality of the selected pump. Material selection is important because vertical pumps are usually in closer contact with the pumped liquid than other pumps.

The assurance and reliability of part straightness, concentricity and parallelism throughout the life of the pump should also be considered.

The assurance and reliability of part straightness, concentricity and parallelism throughout the life of the pump should also be considered. Particularly, the shafting should be concentric with the pump assembly and driver (electric motor), and these two assemblies should also be fully aligned with each other and with their own individual parts. Although this involves a lot of accuracy needed during both installation and maintenance of a large pump, it will be rewarded with high performance and reliability. 

Can-type vertical pumps

Can-type pumps are used to lift liquids from storage tanks, reservoirs or liquid sources. The pump driver (usually an electric motor) is located at or above the liquid surface, and the shaft extends from the driver to the pump, which may be located some distance below the liquid surface.

This type of pump contains a vertical series of impellers. In other words, a series of impellers or stages are used to take the pump liquid from the lowest impeller and pressurize it to the top. Multiple impellers are together in a series so that an impeller’s discharge is directed into the next impeller’s suction and so on. The same flow rate passes through all the impellers, and each adds a particular amount of head. Therefore, the total pressure gained is theoretically the sum of the pressure added by each impeller. 

Can-type vertical pumps, particularly those with many stages, are long and narrow with a single electric motor driving at the top. The electric motor is accessible for operational inspection and maintenance. 

Submersible vertical pumps

A submersible pump is an integral pump-motor unit in a sealed enclosure. This pump is inserted inside the liquid in a reservoir, sump, bay or tank. An electric cable runs to the electric motor. These pumps are used for lifting liquid from almost any depth. For applications where the pump is installed deep into the liquid, the submersible pump offers many advantages. This is often the best option for such deep liquid services, but there are also disadvantages. The electric motor is not accessible for inspection or maintenance without pulling up the entire pump. Also, there is much less opportunity for operational observations and condition monitoring. 

This type of pump as a package is rather simple in concept but its parts and components should be capable of supporting the pump assembly, electric motor and others in harsh, challenging operating conditions deep inside the liquid. A critical point is the reliable operation of the electric motor and bearings in an enclosed configuration. The reliability and life of a submersible pump depend greatly on the electric motor and quality of the bearings. The pump bearings are lubricated with the pumped liquid because there is no practical way to supply lubricants of any kind from an outside source. The bearings of the electric motor are sealed in the motor and, depending on the motor, are lubricated with the pumped liquid or grease. The hydraulic axial thrust and the weight of the pump and the motor rotor are usually carried by the motor thrust bearing. 

There are different types and models of submersible pumps in many configurations and arrangements. Some are installed on a structure or similar installation; others are installed with a small clearance at the bottom of the reservoir or liquid tank. 

Vibration in vertical pumps

Vertical pumps have intrinsically weak structures for dynamic, vibration and resonance. One reason is their long, slender shape. Another is their relatively weak and thin shell structures, particularly for large vertical pumps, which are prone to dynamic excitations and resonances. Even if best analytical and experimental methods are used, sometimes, unexpected resonances arise after installation. The unexpected resonance can be due to many reasons and root causes, for example, the difference between rigid-foundation test operation and jobsite real flexible-foundation operation. Another reason is the variation in the liquid elevation, which can change natural frequencies of the vertical pump. 

The general shape of a vertical pump usually consists of a long column with a large mass at each end. Too often, this configuration is weak for external or internal vibrations, thus has a tendency for resonance and high vibration. To prevent these vibrations of a vertical pump, any resonance should be avoided, and the natural frequencies of the total pump assembly should be calculated and verified. It is important to predict the natural frequencies and mode shapes of a vertical pump analytically and to confirm them experimentally in a realistic and accurate test — preferably at the jobsite with all final attachments. Accurate simulations, realistic experimental verifications and parametric studies should be used to fully investigate resonance cases. Particularly, parametric studies should be employed to investigate the effects of all operational variables on the dynamic characteristics of vertical pumps and to propose proper operational parameters and strategies for lower vibration and better dynamics.

Centrifugal versus mixed flow

Many vertical pumps use centrifugal impellers in a vertical arrangement. The impellers used in vertical centrifugal pumps are similar to those in horizontal centrifugal pumps. However, some vertical pumps, such as turbine-type vertical pumps, use mixed flow or axial flow impellers. This can develop significant operational differences, particularly in the larger sizes and high flow services. 

There are some disadvantages for these impellers. For example, in the performance curve for a typical mixed flow impeller, the shutoff head is usually high; it may be more than twice the head at the best efficiency point (BEP). An axial flow impeller will produce an even higher difference at the shutoff condition. The power draw is at its maximum at the shutoff for these pumps, and the required power is very high at or near the shutoff. Therefore, a provision such as an automatic bypass will be needed if the discharge is clogged or restricted by the valve. Otherwise, the driver will be overloaded.

Intake of vertical pumps

Some vertical pumps, such as inline vertical pumps, have suction piping, but many vertical pumps take liquid from a kind of intake system. The performance and reliability of a vertical pump is dependent on the intake system and flow of liquid at the intake/suction. This is closely related to where the pump is installed, such as the sump, reservoir, column or tank. 

For horizontal pumps, there should be proper suction piping. Compared to vertical pumps, the design of such suction piping is easier than proper configuration and arrangement of an intake system such as a sump or suction intake area. Proper configuration of an intake system in a vertical pump is challenging, which frequently results in performance and reliability problems. The layout of the intake system and the relative positioning of the pumps, if multiple vertical pumps are used within that intake system, are important to the reliability, performance and operation of vertical pumps. For many vertical pumps, thorough analytical studies and even model testing are needed prior to the design and construction of the intake system.

A poor intake system can result in not only operational problems, but also capacity and efficiency losses and even damages due to vibration and cavitation. The effects of disturbed approach flow on operational behavior increase with the high specific speed and the size of the vertical pump. Therefore, this is more challenging for large vertical pumps that use mixed flow or axial impellers. 

A poor intake system can result in not only operational problems, but also capacity and efficiency losses and even damages due to vibration and cavitation.

The objective of an intake system is minimal hydraulic turbulences and no negative influence from the incoming liquid flow lines. The velocity distribution at the impeller entry should be as uniform and calm as possible. This is achieved by suitable management of the approaching flow to the pump; the entire flow path in the intake should be considered and managed. 

A key element is the velocity of the approaching liquid. Roughly, this velocity should be about 0.25 to 0.4 m/s. The location of each pump to the other(s) and to the side and back walls of the sump, tank or reservoir is also important. These distances are related to the entrance velocity of the pump, which can be a function of the suction bell diameter. In addition, the clearance of the bottom of the pump to the floor (bottom) is critical. Roughly, this clearance should be more than half the suction bell diameter of the pump. Note that bell diameters for the same size and type of pump may vary with different manufacturers. 

Arrangements of pumps and accompanying baffles are used in different situations. Many installations are now using dividing walls between each pump to contribute to the smooth flow of liquid to the suction bell of each pump, as this is usually necessary for optimum operation. It is particularly important to avoid any cascading effect from incoming liquids that might create air/gas entrainment or vortexing that will detrimentally affect pump performance and reliability. 

Formation of vortices in the approaching flow upstream of a vertical pump should be prevented. This applies to all kinds of vortices, for instance, air-entraining or hollow vortices proceeding from the liquid surface, bottom or wall vortices and vortices resulting from flow separation or skewed velocity profiles in the approach flow. Air-entraining vortices should be avoided by suitable configuration of the pump intake area immediately upstream of the entry and adequate immersion depth (or minimum submergence). Vortices due to flow separation in the approach liquid should be prevented.

Asymmetric approaching flow can induce preswirl and vortices. This source of preswirl should be prevented. Every impeller is subject to flow recirculation at the impeller suction. The liquid recirculating from the impeller induces a strong pre-swirl that can lead to the formation of cavitating or noncavitating vortices. To avoid detrimental effects on performance, cavitation and vibrations, this type of preswirl should be suppressed to a large extent by swirl-breaking elements upstream of the impeller such as ribs or floor splitters. 

Typically, the design and configuration of the intake for vertical pumps is a compromise between the structural constraints, hydraulic requirements and pump specific mandates. Only close collaboration between the design team of the purchaser and pump manufacturer can lead to reliable, optimum installation. The requirement for undisturbed approaching flow to the impeller can be met by different provisions, such as with a proper pump bay, suitable suction bell or a good inlet bend. For larger capacity pumps, such as large vertical pumps involving heavy construction costs, the design and final setting of the intake system should be reached after thorough technical and commercial studies and assessments.

Conclusion

Vertical pumps are special classes of pumps for many specific applications. When selecting a vertical pump, take into consideration the environment and requirements for best pump performance and reliability. 

AlmasiAlmasi

 

Amin Almasi is a senior machinery and equipment consultant. He is a chartered professional engineer of Engineers Australia and IMechE. Almasi is an active member of Engineers Australia, IMechE, ASME and SPE, and he has authored more than 150 papers and articles on rotating equipment, condition monitoring, offshore, water treatment, wastewater treatment and reliability.

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