In selecting a pump for a given application there is sometimes the option of choosing either a vertical or a horizontal shaft pump arrangement.
For instance, in a process plant certain product transfer applications might be performed by a can-type vertical pump, or a vertical in-line pump, or a horizontal process pump.
Large capacity cooling water applications are often served by either a vertical mixed-flow or axial flow bowl-type pump or a horizontal between bearings double suction pump.
The choice of a vertical can-type pump is often dictated by the necessity to raise the NPSH available by virtue of the added static liquid head above the first stage impeller. A practical advantage of the vertical can and the vertical open pit pump types is that the submerged pumping element is always primed.
Common to vertical pump types generally is the minimal floor plan area required for the installation. Where available site real estate is scarce such as onboard ship, on an
oil production platform, or in below ground structures, vertical pumps may be an attractive option.
The bearing arrangement for a vertical lineshaft type pump is fundamentally different than that of other vertical or horizontal type pumps. With vertical lineshaft type of pump,
only the upper portion of shafting is guided by precision, close clearance bearings. The balance of bearings below the seal are bushings with a liberal running clearance which are relatively incapable of sustaining continuous, heavy radial loading.
Vertical lineshaft pumps have oil or grease lubricated bearings in the driver, or are fitted with a separate “thrust pot” bearing unit located above the seal or packed stuffingbox.
Vertical diffuser bowl pumps have inherent radial thrust balance and radial bearing loads are minimal. Often the lineshaft bearings are product lubricated. Depending on the known presence of abrasives in the pumped product a shaft enclosing tube may be selected for conveyance of clean water or lubricant to the bearings. Other bearing lubrication options such as clean water or grease injection are possible.
Some users express a strong preference for a horizontal pump arrangement based on unsatisfactory reliability experience with vertical pumps. Vertical lineshaft pumps are, in fact, more prone to bearing wear and vibration problems. These problems are related to the fact that virtually every vertical pump shipped is structurally and mechanically unique. Reliability problems with vertical lineshaft-type pumps can usually be avoided at the selection, specification and engineering stages using appropriate selection of materials and engineering analysis to avoid structural resonance or rotor instability.
Some users prefer a horizontal pump, or dry pit vertical pump installation for complete, full access to the pump and driver. This may be the deciding factor for severe or harsh duty pumping applications or where the consequences of unexpected pump failure can be costly.
In many hydrocarbon and chemical pump applications, ready access to the seal, the coupling, the bearings, or the complete internals of the pump is vitally important for safety, diagnostic and maintenance purposes.
Common to a large segment of both vertical and horizontal pump types is a cantilevered-type shaft with externally lubricated bearings. The majority of these are the horizontal
overhung pump and vertical in-line pump types. Bearings for these pump types are most often precision rolling element type bearings with oil lubrication. The shaft sizing and bearing arrangement is designed to a very low value of deflection at the seal within the preferred operating range of the pump.
Between bearings type pumps, most often horizontal, support the shaft between two externally lubricated bearings. Shaft deflection at the seal or stuffingbox is minimal which is desirable for seal or packing longevity. But shaft mid-span deflection may be a substantial percentage of the internal running clearance. This shaft deflection can increase substantially depending on stator arrangement the operating flow relative to
Best Efficiency Point flow. Many, if not most, of these pumps rely upon a combination of both the external bearing support and internal hydrodynamic support of the rotor to prevent rotor to stator contact.
There are advantages and disadvantages inherent in either horizontal or vertical pump arrangements. Neither type should be dismissed out of hand – each type should be
Saurabh Sharma says
Thanks for the informative post Randy Sir. A few queries:
As you say, vertical pumps are generally always primed. But there is another issue with vertical pumps. Large pump-turbines are started in the pumping mode by depressing the water in the draft tube (using compressed air) below the impeller level so that the intial power drawn is smaller. For this, the adjustable Guide vanes are closed and then compressed air is pushed into the pump, thus depressing the water below the runner. The pump is then started and then the compressed air is withdrawn and gradually the guide vanes are opened.
In case, a conventional pump of a very large capacity (of the order of 100 MW input power) with a fixed diffuser (and not adjustable guide vanes) is being designed, what would be the best start-up method? Is water depression really required? If yes, then what should be the scheme of start-up operations since there are no guide vanes to be closed?
Randal Ferman says
Sharma, Good to hear from you again.
I do not have experience with large pump-turbine start-up procedures. It will serve you best to find someone who has direct experience with this class of machine.
Conventional submerged start procedures are used for all other types of pumps that are normally submerged at start-up, as far as I’m aware. I can only imagine that a submerged start-up arrangement could also be applied to the subject large scale pump.
Best regards, Randy
Visitar Pagina says
It’s really a nice and helpful piece of info. I am glad that you shared this useful info with us. Please keep us up to date like this. Thank you for sharing.
Randal what is the advantage of mixed flow vertical pump over axial flow pump.
Randal Ferman says
Nallathambi, Vertical mixed flow pumps are in the middle range of specific speeds and axial flow pumps are in the higher range of specific speeds. Higher specific speed represents higher flow and lower head for a given pump rotational speed. The two design types overlap in specific speed. The reason for selecting a mixed flow versus axial flow pump has to do with the required hydraulic performance characteristics. There is no definite advantage of one type over the other. Hope this helps, Randal
mohd zuraury says
Dear Sir Randy,
I have inquiry regarding the radial deflection tolerance for vertical pump. Radial deflection here means that the tolerance for shaft and bushing because this type of pump is not using bearing. Where can I find the tolerance for this radial deflection?
Randal Ferman says
Dear Mohd Zuraury,
Radial deflection is covered in subsection 126.96.36.199 Shaft deflection, standard ANSI/HI 1.3-2009, American National Standard for Rotodynamic (Centrifugal) Pumps for Design and Application. That subsection references three other standards that also address the topic for overhung impeller pumps: ASME B73.1 and ISO 5199 for dimensional standard pumps, and API 610 9th and 10th Edition.
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