shaft design vertical pump in CTW service

[phil29]

What is the best shaft design for long life for a two stage vertical pump - single shaft, two piece or three piece? Our experience indicated that a two piece design, a standard bowl shaft with a 10" stickup above the bowl, then only one other shaft through the column and through the motor is the best. The manufacture is indicating this is not the "standard" but a bowl shaft with a 10" stickup, a column shaft which sticks up 10" above the stuffing box and a top shaft which goes through the motor. Manufacture concern is when installing the motor over the shaft we will bend the shaft during the installation. We are using a mechanical split seal on pumps.

 

[ktm5]

phil29,
I have been involved with vertical pumps which had long columns (50 feet+) and at least five sections of shafting. The manufacturer's concern is well founded, there are inherent dangers having a long solid shaft with a relatively small diameter. In these cases, quality and alignment of your steady bearings, driver, and pump is the critical mode. If you take the extra time (and money) during the installation phase, your life cycle costs will show the return.
rgds, ktm5

 

[Kailee]

phil29,
You've listed some loose facts but not enough detail to give you a precise answer.

1. What is the pump length below the discharge head? It is common on what are termed "Close coupled" pumps, the bowl assembly is bolted to the underside of the discharge head. In these cases, depending on the overall length of the bowl assy, a continuous shaft is used.
2.Longer bowl/column assemblies generally require multiple lengths of shafting for the simple reason that quality shafting is limited in length and cannot make the full length required.
3. Shafting should be of sufficient diameter to handle the pump loads and the radial bearings need to be spaced according to these loads and operating speeds.
4. Stickup of 10" is typical of all pumps of this type for a product lubricated design. 14" is common for enclosed shaft designs which use external clean lubrication.
5. The top shaft which goes through the motor seems to indicate that you are using a hollow shaft design motor. This is not recommended for pumps using mechanical seals. It would be prefereable to use a solid shaft motor and an adjustable coupling for a mechanical seal design. Hollow shaft are normally only used on pumps with packing.
6. Split seals are OK, but are a little tricky to install correctly and tend to be limited by the operating speed. They are not bad on 1200 RPM and lower speeds but have probelms above 1800 RPM in particular. A standard design seal is easily installed on a pump with a solid shaft motor and an adjustable spacer coupling. The hub on the pump shaft is easily removable to install the seal. This type of seal is also a lot cheaper than a split type.

Hope this helps! CHEERS!