DYNAMIC BALANCE D035
Everyone agrees that balancing the rotating components of a centrifugal pump is a good idea, but it is seldom done. Evidently it doesn’t appear to be too important or it would be receiving some sort of priority when the pump is being overhauled or rebuilt.
To understand the importance of dynamic balance visualize yourself going down the highway in your automobile at sixty miles an hour, and you throw off a small, lead wheel weight; suddenly you notice a severe vibration in the steering wheel that makes you feel very uncomfortable. Do you have any idea how many rpms the wheels were making at sixty miles an hour or 100 kilometer per hour? Do you think it was slower or faster than the rpm of your centrifugal pump? Let’s figure it out in the inch size and then we will do it in metric:
A fourteen inch automobile wheel has a tire that is approximately twenty five inches in diameter. This means that the circumference of that tire is 25 inches times 3.14 (pi) or 78.5 inches. Divide the 78.5 inches by 12 and you get 6.5 feet for the circumference of the tire.
At sixty miles an hour you car is going a mile a minute or 5280 feet a minute. Since the 6.5 feet represents one revolution of the wheel we divide that into the 5280 feet and we get 812 rpm at sixty miles an hour.
A common metric tire would have a diameter of 635 mm. Multiply that by 3.14 and you would get just a little bit less than two meters for the circumference. At 100 Km/hr you would be going 1.7 Km or 1700 meters/ minute.
1700 divided by two meters for one revolution of the wheel is 850 rpm.
This means that if a small lead weight can become that significant at 812 rpm or 850 rpm what does an out of balance shaft in your pump do at electric motor speeds?
- The bearings will experience higher loading that will translate to premature failure.
- The mechanical seal faces can separate because of the induced vibration and shaft run out.
- The seal faces can become damaged as the vibration causes the carbon to bounce against the hard face. This bouncing can cause a chipping of the carbon outside diameter.
- Seal drive lugs will experience premature wear.
- Shaft fretting will increase dramatically under bearing grease seals and original equipment mechanical seal designs.
What cause a pump shaft to go out of dynamic balance assuming it was balanced at pump assembly?
- Product attaches to the impeller.
- Impeller wear and damage caused by erosion and corrosion.
- A seal or sleeve that is not concentric to the shaft. Set-screws guarantee that the seal will not be concentric to the shaft or sleeve.
- The coupling, impeller, mechanical seal, bearings, sleeve, keys, etc. were not balanced as an assembly.
- The impeller diameter was reduced and not rebalanced. Impeller material is not homogeneous.
Balancing the impeller
- Be sure to balance the impeller after repair, or coating,
- A two-plane balance is preferred.
- Balance impellers to Quality grade G 6.3 based on the ISI 1040 and ANSI S2.19; 1975 specification.
- Use quality grade G 2.5 for turbine driven pumps and large armatures. Precision bearing replacement
- On a multi stage pump, balance each impeller separately and then balance them as an assembly
You can purchase two plane balancing equipment to do in house dynamic balancing, or you can send the assembly to an outside vendor. The newer electronic vibration analysis equipment has a balancing program available for an additional charge over the vibration package. Check with your vendor for its availability.
If you send the rotating assembly to an outside vendor for balancing be aware of several potential problems:
- Unlike automobile tire balancing, it is not simple to calculate how much weight to remove and exactly where to remove it.
- Many of these people are experienced in only balancing electric motor armatures where it is common to remove weight by removing portions of the fan blades.
- Outside vendors frequently do not know how to remove weight from an impeller. I have seen instances where weld bead was added to the impeller, or holes drilled through the shroud that interfered with the pump hydraulics.
See: Pump repair P045