Vertical Turbine Pumps - Advice on buying and installing
Buying a vertical turbine pump without a detailed description of the offering – or simply based on price – is a recipe for disaster. The pumps may only have four components, but any one of those pieces can become a maintenance nightmare if not properly specified and purchased.
Doing a little homework before buying and installing a vertical turbine pump can prevent a lot of headache. After all, how many pumps do you have that require a tall crane to remove and install?
Here’s some advice to consider when shopping for and installing each of the pump’s four components.
The first thing you should demand in a new installation is a soleplate to mount the pump. That soleplate should be shipped ahead of time so that your contractor can get it in place before the pump arrives.
If your pump is less than 20 feet long, it can be shipped full assembled – without the motor. A crane can set the pump in place. A level soleplate is the only alignment that a vertical turbine can have – trying to shim one of these heavy pumps on a concrete pad doesn’t provide accurate results.
And even if you can get it right the first time, there’s no guarantee you’ll be able to do it again during a rushed plant turnaround. The soleplate provides alignment each time.
Buy a hollowshaft motor with a lower steady bearing and a non-reverse ratchet. The ratchet will keep the pump from spinning in reverse during shutdown. The pump will be destroyed if it’s started while spinning in reverse.
I recommend a discharge head that is tall enough let you get to the packing gland without having to get on your knees and elbows. I also like one that can accommodate a two piece headshaft, without using a motorstand.
For mechanical seal installations, think about using a fabricated discharge head, spacer coupling and a solid shaft motor. Remember that on a hollow shaft motor, it is a long way from the mechanical seal to the top of the motor – that’s the location of the first bearing support.
The bowl assembly is determined by the conditions of service (GPM and TDH in feet of head). The size of the bowl must accommodate the flow (GPM) and bowls are added (like pumps in series) until you achieve the required head.
I recommend staying at 1750 rpm or lower. Don't use 3550 rpm if the pump is going to be in continuous service – or the pump will be down for repairs on a regular basis.
Remember that bowl head is at the discharge of the bowl assembly and you must add the column length and friction losses through the column and pump discharge head to your total head calculation. For a condensate pump, consider the NPSHR of the first stage impeller. The pump must be made long enough for NPSHA to be NPSHR plus 2 feet at the first stage impeller.
On bowls 20 inches and smaller, collets are used to connect the impeller to the bowl shaft. Larger pumps use keys to connect the impellers. A collett is simply a tapered cone slid on the bowl shaft and hammered into the back of the impeller.
There are as many bowl choices as there are manufacturers. In some cases a bowl assembly can cover the conditions of service in fewer stages than another can. Usually it’s to your benefit because lesser stages means lower price. Any one manufacturer pump can be "rebowled" by another and match the same hydraulic conditions.
Before you choose this option, match the curves of the new bowl assembly with any others that may operate in parallel with it and tell your repair facility that the pumps need to match the original setting in your sump. Bowl assemblies are not always the same length.
Some manufacturers are trying to reduce the price by omitting the discharge case. But without it, the last bearing in the bowl assembly is not there and you have a pump that is prone to failure.
Column and Lineshaft
Once you’ve decided on a bowl assembly, the off bottom distance is determined. These two lengths are totaled and then subtracted from the sump depth. This remainder is the distance, which must be made up by the column and lineshaft assembly.
You have two choices of column pipe: threaded or flanged. While threaded is the cheaper, a flanged column will extend the time between failures by a conservative factor of two. Lineshaft bearings can be locked and bolted square between two pieces of flanged column pipe.
There’s is no way to be certain the bearing is not cocked where the two threaded pipes are screwed together. If the pump is bumped during installation, a threaded retainer may become cocked and eccentric bearing wear -- causing early pump failure -- will begin when you start the pump.
You can reuse a flanged column as long as the structural integrity of the pipe wall is intact. The threaded column will probably have to be replaced after the second repair because of inability to separate corroded threads.
Your best choice for a lineshaft bearing is an open lineshaft. Enclosed lineshaft bearings only have the support of the enclosing tubes and it’s not enough. If cutlass rubber cannot handle the solids in the pumpage, consider a Thordon bearing.
If you isolate the line-shaft bearings from the pumpage, run individual lines to each line-shaft bearing. An individual cage can be built around the bearing and packed with grease or injected with freshwater flush.
Too many inexperienced salesmen sell manufacturers enclosed lineshaft as a means of isolating the bearings from dirty pumpage. In actual pump operation, the dirty pumpage comes out of the bowl assembly at high velocity and impinges upon the "O" ring sealing the enclosing tube.
In short time – from hours to weeks -- the high-velocity dirty water is inside the enclosing tube and the solids are grinding away between the weakly supported bronze bearing and lineshaft. I have seen many repaired vertical turbines where the enclosed tube has been removed. Unfortunately the tube retainer is often not replaced with a packed stuffing box. The tube retainer has only two rings of packing. The pump is acting like a lawn sprinkler.
Vertical turbine pumps were originally used for irrigation wells. On a setting over 50 feet, it was found that the top lineshaft bearing would run dry and be destroye before the bowl assembly could pump water up the column.
The enclosed lineshaft was invented as a way to drip oil down to the top bearings. Though it was never developed for short set industrial pumps, the isolation idea sold a lot of pumps. And unfortunately, only pump users discover the truth.
Bearing spacing for industrial vertical turbine pumps comes in ten foot, five foot and in some cases, three foot for 3550 rpm. Five foot spacing with flanged column is your best bet for the longest life. MTBF will at least be doubled.
So to summarize: If you are unfamiliar with vertical turbine pumps, you may end up all too familiar with your repair shop.