Here are a few hints to help you do a better job of preparing your centrifugal pump for a mechanical seal:
All centrifugal pumps -
- When repairing the cutwater area make sure that you retain the proper minimum clearance between the impeller and the cutwater. This minimum is 4% if the impeller diameter on impellers up to 14 inches (255 mm), and 6% of the impeller diameter on impellers above 14" (255 mm).
- If bearings have failed catastrophically or are "locked up" be sure to take a precision measurement of the bores that the bearings fit into and a precision measurement of the shaft journals that the bearings are mounted on.
End suction pumps -
- When fabricating large bore seal chambers make the new chamber as long as possible to place the seal as close to the bearing as possible. This will reduce the amount of shaft deflection that the seal is subjected to.
- Take a close look at how the thrust bearing (usually the bearing closest to the coupling end of the pump) is retained. Make sure that the bearing is held securely in place when you reassemble the pump. When the pump is running, the thrust is normally towards the volute. Some manufactures hold the thrust bearing with a simple snap ring. Try to come up with a better arrangement.
- When eliminating packing sleeves and replacing them with solid shafts be sure to use a shafting material that is as corrosion resistant as the original sleeve.
- One of the bearings on the pump has to be allowed to "float" in order to accommodate thermal growth of the rotating assembly. Make sure that the fit on the "floating" bearing will allow the bearing to move easily. If the bearing cannot easily move it will be subjected to excessive thrust loads and fail prematurely.
Split case double suction pumps:
- Some of these pumps use "sleeve nuts" to lock the sleeves in place on the shaft. When mounting mechanical seals on these sleeve nuts be sure to check the sleeve nut outside diameters for concentricity after assembling the rotating element.
- The impellers on these pumps are usually held centered in the pump case by the location of the sleeves. If the impeller is not properly centered in the pump casing there will be a hydraulic imbalance that could result in excessive thrust loads on the pump bearings.
- When converting these pumps from packing to mechanical seals make sure that the sleeves are sealed either against the shaft or the impeller. Failure to do this could result in a leak path under the sleeve to the atmosphere.
- Most of these pumps use a double volute design. If you are repairing the cutwater areas on these pumps make sure that the cutwaters are exactly 1800 apart. Failure to do this will result in shaft deflection due to uneven hydraulic loads on the impeller.
- These pumps often use diffuser fins in the pump case in the suction path of the pump. These fins are designed to reduce turbulence as the fluid enters the impeller. If these fins have been worn they must be repaired to their original shape in order for the pump to operate smoothly.
- The stuffing box areas on all split case pumps deserve special attention. When cutting or installing casing gaskets make sure that the gasket is at least flush with the stuffing box face. Many OEM gaskets are too short in this area.
- Pay close attention to the condition of the gasket surfaces on the pump casing adjacent to the stuffing box. This area is subject to more corrosion than the rest of the pump casing due to frictional heat caused by packing and the presence of oxygen around the packing outside diameter.
- Impeller wear rings should have their final outside diameter machined after the wear rings have been installed on the impeller if possible. This eliminates any "out of round" conditions due to "set screw distortion", welding or rough handling.
- The fit between the casing wear ring outside diameter and the pump casing should have no clearance. Any clearance in this area will cause internal re-circulation and rapid casing erosion.
- When purchasing casing rings, check them for concentricity before installing them. These parts are often damaged during shipping and handling.
- On February 16, 2018