Vertical Pump Repair and Upgrade Considerations


Written by:
Brian Blackman

Typical vertical pump types include turbine, mixed and axial flow. The Hydraulic Institute classifies these pumps as type VS (Vertically Suspended). Unlike horizontal pumps, these pumps typically have the pumping element (impeller, diffuser, column pipe and pump shaft) submerged in the pumpage. Therefore, the metallurgy of all pump components must be compatible with the pumpage to achieve adequate life (MTBF).

The vertical pump is versatile, both in construction styles and hydraulic capabilities from 1,500 through 10,000 specific speed. It is used effectively in many industries such as nuclear power, fossil power, oil and gas, mining, municipal, general industrial and agricultural markets.

To properly consider repair upgrades, one should be familiar with the operational service of the pump in its specific application. Some vertical pump sensitivities include:

  • High Speed-Stator and rotor alignment, cavitation, rotor balance
  • High Specific Speed-Intake design, cavitation
  • Corrosive Service-Materials compatibility, protective coatings

Well Pumps-Lineshaft lubrication, start-up and shut-down coordination with valving, variable frequency drive operation, installation

It may be possible to more than double a standard manufactured vertical pump's life by upgrading the pump repair to a "precision remanufacture." The pump vibration will be reduced and can be verified upon start-up as proof of the upgrade. The repair and upgrade cost is usually a small item in the pump's life cycle cost.

 

Figure 1. (l.) The resultant eccentricity between rotor and stator caused by loose fits between the bowls and columns. (center) The resultant eccentricity caused by a lack of parallelism between the mating faces. (r.) The resultant rotor eccentricities caused by loose fit by threaded couplings.

Vertical Pump Repair Concepts

The overriding repair concept for vertical pumps is two-fold: establish true centerline compatibility between the rotor and stator at the motor-to-discharge head flange location and ensure that the maximum relative radial displacement of the stator to the rotor-at any point for the pump's entire length of the pump-does not exceed the minimum clearance of the closest running fit.

The goal is to insert the rotating assembly into the stationary components with uniform radial clearance maintained through the pump from the lower bearing to the head stuffing box. This can only be achieved through the application of tight tolerancing standards.

Important Considerations When Making Upgrades

Bearing Spacing

If the spacing of the bearings is too far apart, a natural frequency on the shaft can develop and cause unwanted vibration problems. If the pump service provider has an in-house engineering staff, then this factor is usually not an issue when upgrades are proposed in the repair plan.

 

 

Figure 2. Open lineshaft, product-lubricated

Lineshaft Lubrication Methods and Bearing Lubrication for Long Pumps and Below-Base Discharge

The two types of lubrication systems are open lineshaft, which is product-lubricated, and enclosed lineshaft, which is fresh water flush, oil-lubricated (gravity feed or pressurized) and grease-packed. There are many valid reasons to change from one method of bearing lubrication to another, and if doing so, all given considerations should be discussed.

 

Bearing Materials

Bearing materials vary due to product temperature, abrasiveness, lubricity, corrosiveness or any combination of these. Availability and/or reliability of a good lubricant may also be an issue.

Shaft Coupling Design

The shaft coupling design can be threaded, keyed (various types) or neither. Larger shafts typically use the keyed shaft coupling design; the threaded design depends on the type of material used, reverse-rotation issues, etc. Couplings may not be needed at all for short pumps depending on the application and shaft size.

 

Figure 3. Enclosed lineshaft, water flush

 

 

Figure 4. Enclosed lineshaft, oil lube

Leakage

Determining how to handle leakage depends on the product pumped. If the pumpage is flammable, a mechanical seal may be required instead of a packing box to avoid any leakage. If pumping water, a packing box may be more effective than a mechanical seal since decreases in temperature could cause the water to freeze and damage the mechanical seal. If pumping a corrosive material, a packing box that drips would require consistent cleanup. Other leak-free and maintenance-free designs exist that can be considered.

Pump Stiffness and Weight

It is important to evaluate the pump in its whole environment when determining the effectiveness of upgrades. Details to consider include the dimensions of the entire pump, data on the baseplate and mounting. This is crucial when trying to avoid natural frequency and vibration problems. A qualified pump service provider considers this information when determining the root cause analysis of pump failure and factors it into the pump repair plan.

 

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See also:

Upstream Pumping Solutions

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