Dec. 23, 2024
One of the first steps in pump-station design is to determine what type of pump station best suits the system&#;s needs.
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The type of pump station sets the stage for the design of the entire facility. Some specific design items the designer will need to consider when selecting a type of pump station include: system curve, initial demand and expansion requirements, pump availability, storage requirements, NPSH available, maintenance and cost.
Vertical turbine and horizontal split case centrifugal pumps are the most common types of pumps utilized in water applications.
Typical system curves for horizontal and vertical pumps lend themselves to certain applications based on system coverage. Vertical Turbine pump curves are typically rather steep and can cover a wide range of heads while horizontal pump curves are typically flatter and can cover a wide range of flows. Certain criteria are also critical for an Owner to consider when selecting a type of pump station: capital and life cycle cost, potential failure modes, and maintenance activities.
Here are how horizontal split case pumps and vertical turbine pumps stack up.
Frequently an Owner may have a very strong opinion about the type of pump station design preference, but it is incumbent on the designer to bring other ideas that will be well-suited to the application.
Vertical pumps are a special class of pumps used in many different applications from water and utility services to process and exotic applications. They can be used in a range of operating temperatures from low to high, with varying pressures and with many liquids ranging from ordinary water services to corrosive, flammable and even difficult process liquids and chemicals.
While there are services in which horizontal pumps are a better option, such as in refineries or petrochemical plants or in boiler feedwater systems, in many applications, vertical pumps can offer an inexpensive, lighter and more compact option. However, many times vertical pumps are specified and used based on purchase costs only. In short, vertical pumps should be used in applications where advantages outweigh disadvantages. This has been the case for a wide range of pumping services.
This article discusses the types and designs of vertical pumps with special focus on two widely used pumps: vertically suspended (VS) pumps and vertical inline pumps (overhung, OH). It then discusses the most common types and models and explains critical issues related to them.
Vertically suspended (VS) pumps include the subgroups wet-pit and vertically suspended diffuser pump (VS1), which discharges liquid through the pump column. Another type is a volute version of the vertically suspended pump (VS2). Other designs for VS pumps are axial-flow, vertical versions (VS3); volute, line-shaft-driven sump (VS4), which has a separate piping column (parallel to main pump column) to discharge liquid; cantilever sump pumps (VS5), double-casing, diffuser, vertically suspended (VS6); and double-casing, volute, vertically suspended (VS7). For these pumps, discharge liquid should pass through a long column or piping vertically to reach the actual pump package discharge flange. Column static and friction head losses should be carefully considered to avoid underrating the discharge pressure.
Vertical inline pumps have been used in small pumping systems. They are known as overhung type 3 or OH3 pumps. OH3 pumps are single-stage overhung pumps with suction and discharge connections that have a common centerline and a bearing housing integral with the pump to absorb pump nozzle loads. The pump&#;s driver is usually mounted on a support integral to the pump, and the pump and its drivers are usually flexibly coupled. This type of pump is tall, so for stability and good operation, the ratio of the unit&#;s center of gravity height to the contact surface width is usually limited to 2.5 or 3 (height/width). Generally, stability can be achieved through a good design of the casing (low ratio) or by a permanent external stand.
These pumps are offered in two classes. The first class contains those that can float with the suction and discharge piping, which are used in very small applications and are sometimes treated as piping inlines. The second class contains pumps that are bolted to a pad or foundation. Note that flange loading on the pump can increase if it is elected to bolt the unit down. This option is usually selected when the pump is treated more as equipment rather than piping inline. The close tie to piping may cause difficulties in maintenance; often, a device that allows direct rigging or lifting of the back pullout assembly from outside the motor support with the driver in place is provided for ease of maintenance. Some small pumps in this class are often provided with grease lubrication rather than oil lubrication.
The temperature in bearings can be a major concern. As a rough indication, bearing housing temperature should not exceed 80°C, and monitoring should be provided. For these small pumps, the driver is usually installed and aligned in the manufacturer&#;s shop, and the pump is delivered as a complete package to the site.
Other types of vertical inline pumps are rigidly coupled, vertical, inline, single-stage overhung pumps, known as OH4, and close-coupled, vertical, in-line, overhung pumps (OH5). In OH5 pumps, impellers are mounted directly on the driver shaft, which makes them simple and inexpensive to use.
High-speed inline pumps are usually integral-geared, overhung pumps referred to as OH6. This pump design has a speed-increasing gearbox integral with the pump. The impeller is mounted directly to the gearbox shaft; the gearbox is flexibly coupled to the electric motor driver. Lateral vibration can be a concern with these high-speed pumps. Normally, pumps of this type are thoroughly checked and verified regarding all dynamic situations including lateral and torsional vibrations. Careful dynamic balance is needed. As a rough indication, rotating parts are balanced to a residual unbalance of about 4 to 7 gram × mm or as commonly known in the pump industry, grade 1 (G1) or grade 2.5 (G2.5) (ISO -1). Hydrodynamic radial bearings may be used for such high-speed pumps since a lubrication oil system is needed to feed properly selected oil to the bearings and gear unit.
Vertical pumps are commonly poorly selected or undersized. For any centrifugal pump, rated capacity of the pump should usually be within 80 to 110 percent of the best efficiency point (BEP). Generally, pump manufacturers should provide complete performance curves, including differential head, efficiency, Net Positive Suction Head required (NPSHr), and power, expressed as functions of flow rate. Except for some special pumps such as low specific-speed models in which it is not feasible, curves should ideally be extended to at least 135 percent of flow rate at BEP.
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Usually vertical pumps, particularly those with long shafts, have relatively large inertias in the driver and pump stages and are susceptible to some torsional excitations. Often careful torsional evaluation is needed. Thrust load and thrust bearing configuration need great care because of the weight of components added to operational load in the thrust direction to more stress-fragile and sensitive thrust bearings. Vertical pumps without integral thrust bearings require rigid adjustable-type couplings.
Maintenance of vertical pumps can pose challenges. As a rule, except for VS pumps and integrally geared pumps (such as OH6), vertical pumps should permit removal of the rotor and inner element without disconnecting the suction or discharge piping or moving the electric motor driver.
Reports of high vibration including frequent failures are common, especially for medium and large VS pumps. These pumps are susceptible to resonant vibration if their separation margins are not verified properly due to their long and large bodies, which are flexible structures. Detailed dynamic evaluation is needed, and as an indication, a 15 to 20 percent margin of separation should be maintained between the natural frequencies and the operating speeds and the first few harmonics.
A medium or large vertical pump package should be provided with a suitable set of monitoring sensors such as vibration probes, accelerometers, electric motor winding temperature and resistance temperature detectors (RTDs). Ideally, each bearing (pump and electric motor) should have X-Y vibration probes and a key phaser with each shaft. RTDs are required for each bearing. Axial displacement probes, ideally two per shaft, are important for reliability and condition monitoring. For small vertical pumps or those in less critical services, some compromise should be reached as many of these sensors would be assessed as too expensive or even unnecessary for such small vertical pumps.
&#;Vertical pumps are commonly poorly selected or undersized. For any centrifugal pump, rated capacity of the pump should usually be within 80 to 110 percent of the BEP.&#;
Seal selection and flushing seal arrangement are important concerns for any pump including vertical pumps. Dual-flush seals have been used in many pumps including critical vertical pumps. They are commonly used for process pumps or when the pumped liquid is corrosive, dirty, difficult or problematic. As the first option, mechanical seal flushing should be specified for the flushing with the pumped liquid. However, since flushing with such a liquid might reduce the life of the seal over long operating periods, an alternative clean liquid from an external source is usually specified and used for the normal flushing. If the external supply of this clean liquid is interrupted, the pump should be able to continue operation. In other words, the pump should be provided with the automatic switchover, dual-flush system (from clean liquid to pumped liquid) in the event of loss of the clean flushing liquid. Single, simple flushing, which only uses external clean liquid injection, is not a suitable option since in the case of loss of this clean liquid, pumps should be tripped and, consequently, the whole unit, plant or facility would be affected.
An example is critical vertical pumps in seawater services for many processing plants and facilities. The operation of the plant or facility usually depends on these seawater pumps. Dual-flush (utility water and seawater) connection for mechanical seal flushing should be specified for such a seawater pump. Such a pump system is provided with an automatic switchover dual-flush system (utility and seawater) in the event of loss of utility water.
The same method can be used for services in liquid hydrocarbon, chemical liquids and others. For example, instead of the flushing with a difficult pumped liquid, a clean liquid will be used for normal flushing with the capability of switchover to the pumped liquid if the external source of clean liquid is interrupted.
Pump bearings and their lubrication have usually been the source of problems. A sophisticated lubrication system should be provided. Bushings in vertical pumps are usually lubricated by the liquid pumped. Alternative methods of lubrication should be used if the pumped liquid is not suitable for this application.
Bearing details, such as type, sizing and life calculation, and lubrication system details should be reviewed carefully to ensure the reliability and long, trouble-free operation of vertical pumps. Rolling bearings are still widely used in vertical pumps and vertical motors, as it is common to see vertical motors as large as 600 kilowatts (kW), or even larger, equipped with spherical or taper roller bearings. Therefore, bearing life expectation and calculation are usually topics for discussion and challenges. Any bearing life expectation less than four years (say 32,000 hours) at worst conditions is often discouraged; although, unfortunately, many manufacturers still use traditional life values of 16,000 hours or 20,000 hours for the bearing sizing and selection.
In a series of identical vertical seawater pumps at a large processing plant, pumps are vertically suspended (type VS1). The pump capacity and discharge pressure is 8,300 m3/h and 4.4 Barg, respectively. Three pumps operate in parallel to provide the seawater requirement for the plant. The rated hydraulic power of each pump is around 1 megawatt (MW); the rated electric motor power is about 1.4 MW. The electric motor is sized for the end of curve; therefore, suitable margins and factors on power rating are provided.
The BEP flow of the pump is around 8,750 m3/h; the rated point flow is about 95 percent of the BEP flow. The pump efficiency at rated point is estimated around 85 percent, which is relatively high for a vertical pump. The head rise to shutoff is estimated at around 26 percent, which is assessed as suitable for this service.
The pump speed is 750 rpm, and a direct-drive vertical electric motor is used (no gear unit). The pump is wet-pit, a vertically suspended, single-casing diffuser pump with discharge through the column. The discharge nozzle of each pump is 40-inch ASME B16.5 150# flange. The overall vertical pump train is more than 14 meters (m) long. The column section is furnished in parts not exceeding nominal length of 3.2 m to ease transportation and installation. Sections are connected by flanges, and each pump train weight is more than 27 tons (T).
The seawater was considered dirty and polluted for this location; therefore, all materials of construction in contact with seawater are considered super-duplex stainless steel. Line shaft bearings, thrust bearings and mechanical seal are provided with dual-flush system (utility water and seawater); the automatic switchover capability from utility water flushing to seawater flushing and vice versa are provided. In normal operation, clean utility water is used for flushing, and it can switch over to seawater automatically if the clean utility water is interrupted. In addition, an operator can switch over from one to another.
The pump package is provided with two accelerometers (X-Y) for the electric motor radial bearings (NDE and DE), three accelerometers (X-Y-Z) for pump thrust bearing, one key phaser, and one duplex RTD per bearing for an effective vibration condition monitoring.
A sophisticated seawater intake package is provided for each pump to filter the seawater and protect each pump. The seawater intake system is a self-cleaning, vertical traveling type. This is a complex and multistage seawater filtration package including Bar screen &#; 50-millimeter (mm) filtration level &#;, band-screen system and screen wash system to wash accumulated trash and solids. The screen washing is accomplished by using filtered and treated seawater.
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