Pump Rebuilding and Balancing (Part Two)

Pump Rebuilding and Balancing (Part Two)

Written by Dawn Hines and Robin DeRousse, Hines Industries, Inc. and Jerzy Moszynski, MMS ANSIFLO Pump

Pumps are sent to rebuilding and remanufacturing companies because buying new can be expensive and require long lead times. For instance, a 39 in diameter, 1,000 lb bronze impeller may cost $28,000 and take many weeks to deliver. In these cases, the impellers involved in an overhaul are often built back up to the required specifications and rebalanced.

In one particular instance, a pump rebuilder received an urgent request to repair a damaged boiler feed water pump. Boiler feed water pumps are expensive, complex, precision machines with long lead times. The customer's operation was down due to the pump failure, and it was costing thousands per day.

The first step upon receiving the damaged pump was to analyze the condition of the components, considering both costs and lead times for replacement parts versus remanufacturing or reusing existing components. The machine was cleaned and its parts sandblasted before it was inspected. The damaged pump's system was also analyzed.

During initial inspection, the pump shop discovered that the impeller was worn, and the walls on one side were thin from previous repairs and balancing. Because of the long lead time for impeller replacement and the urgent nature of the case, the pump rebuilder carefully analyzed which parts were salvageable and could be safely reused or repaired to acceptable standards.

Pump shafts and impellers can often be reused, but this pump impeller had thin cross-sections in areas that had been balanced in the past. Spray welding was used to build up the material. During the last five to 10 years, welding has become a solution, even for severely damaged impellers. Methods include nickel welding for iron impellers and spray welding technique.

After questionable areas had been built back up, the impeller was rebalanced by removing weight, according to a complete set of procedures that the pump company follows to ensure a satisfactory overhaul:

  1. Ensure absolute cleanliness for the power ends of pumps
  2. Measure shaft runout to check if runout is within allowable limits (i.e., normally 0.002 in at stuffing boxes)
  3. Provide a new shaft if necessary
  4. Balance the shaft first
  5. Build a mandrel for each individual impeller-the rebuilder balances all components on mandrels to ensure true running
  6. Balance each impeller individually for multistage machinery, then stack and balance the entire assembly
  7. Make certain the user has examined the possibility of problems with the motor since a large amount of pump failures can be attributed to the driver
  8. Complete balance certification documentation

Every pump that passes through the rebuilder's operation is balanced because pumps are rotating machines. Balancing prevents vibration, improves performance, increases the mean time between maintenance, avoids deflection and prevents shaft breakage and bearing seal problems. The smoother a piece of machinery runs, along with correct lubrication practices and other factors, the longer its useful life will be. Bearings and mechanical seals are typically the weak links in the chain. The less vibration, the longer these parts will perform at acceptable levels.

The assemblies must be as closely balanced as is practical to offset other problems that may develop in a pumping system. Standards developed by various organizations for pump/motor repairs form the basic guidelines for acceptable machine overhauls.

Standards boards in the pump industry include ANSI (American National Standards Institute), Hydraulic Institute, ASME (American Society of Mechanical Engineers) and the American Petroleum Institute. They have created and maintain standards for the remanufacture and repair of both electric motors and pumps.

Important Features in Balancing Machines for Pumps

  • Pure unbalance reading. Tooling and balancing mandrels always have some amount of inherent eccentricity and unbalance. Be sure balancing machine software automatically zeroes out the error caused by tooling unbalance and eccentricity, to deliver a true reading of impeller unbalance.
  • Software for Pump Balancing. Software should include features to:
    • Account for keys and keyways
    • Determine the proper ISO balance tolerance based on the part weight and usage RPM
    • Vector the unbalance into two veins, blades or segments
  • Accurate reading of amount and angle of unbalance in both single plane and two planes is critical for fast, efficient correction. If the operator removes the amount of material at the angle specified in the first reading, the part should be balanced on the first spin.

Pump Replication to Reduce Downtime

The rebuilder proposed that the customer have one machine running and one spare for the future, along with the required parts on the shelf, to avoid downtime when a pump shuts down or requires periodic maintenance. When a catastrophic failure occurs and there is a need for parts that fall into the long lead time items category, it is then necessary to repair or remanufacture the used parts available on hand.

"All pumps are theoretically engineered products, so pump and part lead times tend to be long. This requires rebuilders to have accurate knowledge as to the application of pumping machinery in order to duplicate the performance of a specific pump during any overhauls, since any inaccuracies can dramatically affect the performance of a machine," states Jerzy Moszynski, president of MMS ANSIFLO Pump.

Conclusion

Pump failures are costly. Balancing and a straight shaft are essential to increasing the time between failures, both for new and rebuilt pump systems. After the system is installed, proper maintenance and attention to resonance levels is the key to long pump life.

Dawn Hines is CEO and Robin DeRousse is a sales engineer at Hines Industries, Inc.
Phone: 734-769-2300

Jerzy Moszynski is president of MMS ANSIFLO Pump
Phone: 435-843-4256.

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