Selecting the right centrifugal pump for specific service can be challenging to a pump application engineer, or rotating equipment engineer, more so of one who is in an entry-level position in the pump industry. This article will explain the process to simplify the selection.
First, determine the total system capacity and head for the specific service. Two things need close attention when doing this:
1. It is not uncommon for pump engineers to determine not only the present normal capacity but also to anticipate future pump capacity requirement in anticipation of increased demand or production. To ensure that the pump and its driver are sized to meet the future capacity some engineers specify the future capacity as the pump rated capacity. As a result the pump would be operating at its normal capacity with less optimum efficiency resulting in substantial energy loss and costing the end-user. This practice is acceptable provided it is also specified that the pump BEP shall be at, or closer to, to the normal flow.
2. There is a difference between the terms total dynamic head (TDH) and differential head. TDH includes velocity head which is a head spent to increase the liquid velocity when a pump has different sizes of suction and discharge nozzles; velocity head is not measurable with a discharge pressure gauge. If a pump has the same size of suction and discharge sizes the velocity head is zero and TDH is numerically equal to differential head. Differential head is the actual head needed to overcome the site system head.
3. Most pump standards such as Hydraulic Institute, ANSI, and API 610, specify an acceptable pump head tolerance – typically in the range of +3% / -3%.
Then determine the number of pumps needed to meet those conditions.
Avoid customization. It is preferable to select a pump in a commercially available size. Selecting a pump that requires a custom size will be very costly and will have long lead times. The high cost will to extend to the price of driver and controls that are likely to require size customization also.
In some locations serviced by local power company, the sub-station may be limited in HP due to the power demand of others adjacent industries, and may require another pump station powered by a different sub-station. Similarly, such as in pipeline services, several pump stations located far apart may be needed to avoid over-pressuring the pipeline.
In services requiring more than one pump a decision has to be made if the pumps are to run in series, or in parallel.
It is preferable to select a pump to run at the highest advisable speed. Pumps running at higher speed are generally more efficient and less expensive than comparable pumps running at slower RPM. They are also smaller in size and will require less real estate for installation. There are, of course, some exceptions such as in slurry pumps where high speed is to be avoided to prevent accelerated erosion damage on the impeller and casing.
For example, if a pump were to be driven by an electric motor at 60 hertz power-supply determine if the pump can be selected to run at 2-pole speed, or 3600 RPM.
It is preferable to select the pump with the highest specific speed within the same type of rotodynamic pump, example, within the radial flow type pumps.
Essentially this is going through the process of elimination. If, for instance, the pump were intended for use in the oil and gas industry that requires an API pump, then that would automatically excludes pumps that are non-API compliant. If the requirement were for fire pumps, then it would automatically exclude non-UL listed, non-FM approved pumps.
The liquid characteristics also determine the pump type or construction by default. Typically, foot-mounted pumps can be used for pumped liquid up to 200 degree F, but near-centerline mounted pumps are needed for up to 400 degrees F. The pump casing can either be axially-split, or radially split. But for above 400 degrees F radially-split casing is almost always required.
Pumps of similar hydraulics are available in end-suction, top-discharge configuration; top-top configuration, or side-side configuration. The selection of nozzle configuration is dictated by the pump service, or piping arrangement in a plant. End suction, top discharge, or top-top pumps are popular in refinery service where pumps typically discharge into elevated pipes and columns, but in pipeline service side-side nozzle pumps are always used.
The use of end-suction pumps is to be avoided in services with high suction pressure because of the resultant high axial thrust and potentially high shaft deflection – in such application between-bearing pumps are almost always use.
the pump and driver have separate shafts connected by a flexible coupling. Usually a spacer coupling is used to allow the removal of seals without disturbing the driver. It is also known as flexible-coupled pumps.
Horizontal vs. Vertical Design
A horizontal pump is one whose shaft is in horizontal position. It can be:
- an overhang, or between-bearing design- a radially split, or horizontally split case design
A vertical pump is one whose shaft is in vertical position; it is always an overhang and of radial-split case type design.
The advantages and disadvantages of a horizontal over a vertical design, and vice-versa, are:
Advantages of horizontal design:It is easier to install, inspect, maintain, and service because its rotor and internals are easily accessible.It can be coupled directly to a variety of drivers including electric motor, engine, and turbine (steam, gas or power recovery hydraulic turbine.)It can be supplied in either overhang design for low suction pressure service, or in between-bearing design for high suction pressure service.It can be supplied in various nozzle configurations to simplify, or match the external site piping. The nozzle configuration can be of end suction top discharge, top suction top discharge, or side suction side discharge.Its low headroom requirement makes it suitable for most indoor installations.
Disadvantages of horizontal design:It has limited applications where the NPSHR exceeds the site NPSHA; usually an auxiliary booster pump is required. (With a vertical line shaft pump, the NPSHA can be increased by lowering the setting of its impeller.Its maximum allowable operating temperature and working pressure are generally lower than that of vertical design.It requires a bigger footprint.Advantages of vertical design:It requires a smaller footprint and is suitable for installation where the ground surface area is limited, or is at a premium.With a vertical line shaft pump the impeller setting below the ground can be lowered to increase the site NPSHA.It is suitable for higher temperature service. Because of its radial-split case design it can be centerline-mounted for even thermal expansion.It is suitable for high-pressure service because of its simplified bolting and confined-gasket design.
Disadvantages of vertical design:Most vertical design, particularly the multistage units, or the so called "vertical lineshaft turbine pumps", require large headroom for installation, servicing, and maintenance.It is usually suitable for direct coupling to electric motor. Using other type of driver, such as engine or turbine, will require a right angle gear drive and, possibly, a universal shaft joint and a clutch.Being of an overhang design, its hydraulic axial thrust is difficult to balance in high suction pressure service.A multistage pump usually requires expensive sump pit and barrel.It is prone to mechanical seal problems when pumping liquids with high dissolved or entrained gas which accumulates at the top of the stuffing box or seal chamber where venting can be difficult or less effective.Axial-split vs. Radial-split units
An axial split case pump is a pump whose casing is split axially, or whose principal joint is parallel to the shaft centerline; the casing is split into an upper half and a lower half. An axially split case pumps is always a horizontal pump.
A radial split case pump is a pump whose casing split, or whose principal joint, is perpendicular to the shaft centerline. The complete casing assembly is usually referred to as the case and cover. A radially split case pump can be either a horizontal or a vertical pump.
Axial split case:
Advantages:The pump internal can be inspected by removing the top case; there is no need to remove its rotor.It is relatively inexpensive for three stages, or more, than a radial split case pump.
Disadvantages:It is typically limited to 400 degrees Fahrenheit operating temperature due to thermal expansion consideration because:There is significant difference in the case metal thickness between the upper half and lower half of the pump resulting in differences in thermal expansion;The pump cannot be supported at the shaft centerline where the pump casing is split in half (at best, it can be supported at near-centerline only.) This results in asymmetrical thermal expansion between the upper half and lower half casing.It is typically limited to 3600 PSIG maximum working pressure due to the difficulty in bolting with a flat, unconfined, and irregular case gasket, and due to the non-symmetrical volute and suction areas between the upper half and lower half casing.
Radial split case:
Advantages:It can be of the inexpensive overhang design for low suction pressure application. Or, it can be of the more costly between-bearing design for high suction pressure application.It can be of centerline support design suitable for operating at very high temperature of up to 800 degrees Fahrenheit. The centerline support design ensures equal case thermal expansion in the radial direction.The case and cover design is suitable for higher working pressure than an axial split case pump due to its smaller bolting area, symmetrical bolting pattern, and confined gasket design.
Disadvantages:The casing internal cannot be inspected without removing its rotor assembly from inside the casing.In some multistage pumps the rotor assembly cannot be removed from the casing without removing the driver to clear the way for the rotor assembly.It is very expensive for three, or more, stages because the pump will have to be of double barrel construction.