sewage screw pumps

I'm a student in Environmental Science who recently visited a sewage treatment plant in Indianapolis.

There were 16 huge screw pumps: 6 open, 10 enclosed (tubed). All were rusting. We were told they were made in Germany at almost a million bucks a pop and that the bottom bearings were always wearing out.

I don't get why these things were made out of a ferrous alloy and not maybe aluminum or even a plastic. Further, I don't understand why they couldn't be made in segments so that they might be maintainable without having to use a crane and a crew of riggers.

Is aluminum too brittle, or does it have a toxic reaction with hydrogen sulphides? Too much expansion/contraction at those temps? I just don't get it. I do understand how it's a better type pump in that it doesn't mix up the influent as much as another type would. But, otherwise, they look like just a monstrous expense waiting to happen...

 

[tstead]

No doubt an Archimedes screw pump. Ever try to weld aluminum?

Cast iron is the standard material of cosntruction for sewage and wastewater. Virtually all of your sewage and wastewater pumps will be cast iron with a good two-part epoxy coating.

I dont think Aluminum would hold up too well with all the caustic chemicals that people put down their drains (Drano, et al). Plastic is nice on paper, but a screw pump that size is impracticle. In addition, plastic pumps can get quite expensive.

Can't comment too well on how well the pumps are holding up, but the bearings wearing out is probably more of an issue with the bearing design itself as opposed to the materials of cosntruction of the pump.

Regards,
Tim

 

[arunmrao]

Large pumps are commonly manufactured in cast iron due to cost considerations and the ease of manufacture. Aluminium is rarely used as a pump material for such applications. What perhaps could be considered is stainless steel or wear resistant cast iron like NIHard. Normally the castings are manufactured as an integral piece and not in segments as mentioned by you. This could be due to pump characteristics. Plastics are neve talked about for such applications.
As regards corrosion near the bearing surface this could be handled well by proper maintenance.Leakages and sealings will have to be considered. If some portion has worn out perhaps rebuilding will have to be considered.
Hope this helps

 

Sounds like a case of zero safety factor built into the design. Makes for an inexpensive product that will last through the warranty period then fail. Shades of planned obsolescence and continuous spares replacement. The Germans are notorious for this.

If these are large scale Archimedes style pumps, that bearing at the bottom has got to be subjected to some incredible forces that will result is resulting in its self destruction. This sounds like a case where a specially designed bearing is required. Taking into account all the forces required, plus the environment in which the bearing is operating, plus the operational characteristics of the equipment.

Sounds like something right up Timken Roller Bearings alley. But mind you it won't be cheap or off the shelf.

 

[TBP]

I wouldn't necessarily blame the German supplier. It could be his fault, of course, but I've found most often, problems like this can be traced back to the end-user who's main criteria is "lowest bid". They get the lowest bid engineers, the lowest bid suppliers, and the lowest bid conractors. After the mess is installed, they then kick the maintenance people around for "not being able to repair" things, and being "over their maintenance budget".

There are exceptions, but most of the time, end-users very much tend to get EXACTLY what they've paid for.