Pumps fail for a host of reasons, but as the chart below depicts, more than two-thirds of the failures relate to sealing device issues. With the movement to conserve water and minimize the quantity of waste liquids requiring treatment, the industry is relying more and more on mechanical seals. As the installed population grows so does the percentage of pump breakdowns resulting directly from mechanical seals.
I, like many pump guys, try and separate pump problems from mechanical seal problems. However it’s a package, and the pump supplier has to support the customer in any way that he can. With that in mind, I thought I should delve into some of the general pump requirements for a successful mechanical seal installation.
There are six distinct parameters that must be correct for a seal to provide maximum performance. Some seal designs are more able to accept dimensional inconsistencies than other designs, so specific tolerances must be obtained from the seal supplier prior to installation.
Just to be clear, the tolerances referenced in the diagrams below are for general reference only. You must confirm the tolerances required for specific seals with the manufacturer of that seal.
1) Shaft run-out
Shaft run-out should typically not exceed 0,05 mm (.002”) TIR (Total Indicator Reading) at any point along the shaft for ball or roller type bearings. (1000 to 3600 RPM)
2) Radial shaft movement
Radial shaft movement is generally limited to 0,05 – 0,10 mm (.002” -.004”) for ball or roller type bearings. For sleeve or journal type bearings, values will generally be in the order of 0,10 – 0,15 mm (.004” – .006”)
3) Concentricity, shaft to bore
Concentricity of the shaft to the seal chamber bore should normally be within 0,025 mm per 25 mm shaft diameter (.001″ per 1″ shaft diameter) to a maximum of 0,125 mm (.005″) TIR.
4) Seal chamber squareness
Seal chamber squareness to the shaft centreline should be approximately 0,015 mm per 25 mm seal chamber bore (.0005″ per 1″ seal chamber bore). Note: make sure that shaft endplay does not affect the reading.
5) Shaft endplay
Shaft endplay should generally not exceed 0,25 mm (.010”) TIR, regardless of thrust bearing type.
6) Seal preload
All seals have some type of spring mechanism to provide an initial compression of the seal faces, commonly referred to as a preload. Cartridge seals have a built-in preload that only requires the installer to remove set tabs prior to start-up.
The specification for the preload varies greatly dependent on the specific seal design. You must see the manufactures installation instructions for specifics.
I hope the information in this blog helps some readers confirm that their pumps are ready for a successful mechanical seal install. Next blog I will try and touch on some operational tips on avoiding premature seal failure.