Horizontal or Vertical?

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
considered objectively.