Sewege Transfer Pump - Rotary Lobe Type 2

[hhmh2004]

In one of the jobs I have to specify a pump for domestic sewage transfer to waste water treatment plant. The Pump Flow is 290 USGPM and Required Head is 230 ft. The pump will take suction from an above ground Sewage Tank.

Here in Saudi Arabia, typically centrifugal non clog type impeller pumps are used for this service.

But due the required duty point specified above the centrifugal type of pump is not available in the market.

Second choice is to go for rotary lobe type of pump.

Is anybody familiar with use of rotary lobe pump in domestic sewege transfer?

Is it good in this service?

Does it require some kind of inlet protection i-e grinding or screening?

What is the maximum particle size it can pass?

Thanks and Regards

 

[micalbrch]

A rotary lobe pump is - like a progressive cavity pump - very sensitive to fibres (hair) and hard particles. The sewage might not contain many hard particles but for sure fibres. So, you will definitely need grinder/macerator.

 

[bimr]

A rotary lobe in this application will probably be a maintenance headache. A grinder/macerator will be an additional headache.

Wet pit submersible sewage pumps also tend to be expensive to operate and maintain. Dry pit pumps are preferred by many lift station operators.

Why don't you look at another centrifugal pump? For example, KSB makes a pump for this application. KSB calls it the Sewatec:

https://shop.ksb.com/ims_docs/0E/0EE4AB4C236E9F4DE1000000CEA40CB4.pdf

https://shop.ksb.com/ims_docs/F3/F359774383FA674DE1000000C3D94CB9.pdf

 

[bimr]

Attached is the pump selection.

 

[Artisi]

bimr,
I would think that the 0.59" solids handling without a grinder/macerator would preclude it as a raw sewage pump.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[hhmh2004]

Thanks to all for replying...

Bimr,

The pump selection that you have provided shows pump is running close to its runout condition OR we can say end of curve operation.

Centrifugal Pump operation at runout conditions should be avoided.

Same is true for pump operation near shutoff conditions.

Actually this is the problem in selection of centrifugal non-clog impeller type pump for this service (290 USGPM, 230 FT). Some of the big names in sewage transfer has refused to offer a centrifugal pump for this service. These include Grundfos, Flowserve, Saudi Pump Factory etc. And if the Vendor do offer a pump, the pump is operating near the shutoff or at the runout condition.

On the other hand there are some pump manufacturing companies like BORGER Germany and NETZSCH USA who offer rotary lobe pump for this service. They claim that solid content of upto 50mm dia can be passed. Please look at the following links for Rotary Lobe Type Pumps is Sewage Transfer service.

http://www.boerger.com/contero/boerger_535_.html

http://www.netzschusa.com/tornado.htm

I am in contact with these Vendors and hopefully will get answer.

My questions are:

Is anybody familiar with use of rotary lobe pump in domestic sewege transfer?

Is it good in this service?

Does it require some kind of inlet protection i-e grinding or screening?

Regards...

 

[Artisi]

haroonhassan2004

First up, I don't think that bimr needs a lesson in pump hydraulics, especially when he is only passing on a pump selection made by KSB.
However,I wouldn't use this pump for reasons other than its operating point;
1: 2 pole speed
2: limited solids handling

What ever pump you use I think that grinding will be needed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[hhmh2004]

Thanks Artisi

@Bimr

I sincerely apologize for any unintentional teaching as pointed out by Artisi. I was only describing the difficulties I am facing in selection of centrifugal type of pumps.

Regards

 

[Artisi]

Welcome to the art of pump selection.

What you are facing is normal for such a duty, low flow / high head dicates a narrow but large diameter impeller not suitable for sewage / solids handling. Therefore you need to make an informed decision of operating at other than the best hydraulic point on the curve. For solids handling it is usually to the left so that larger solids can be handled (larger pump) or else use a different pump than centrifugal.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Artisi]

The pump you need for this application is Goulds type CWX model 5x4x17. It will operate well left on its curve but it is built like a battle-ship and would run on this duty for ever. It is big, heavy and ugly and probably expensive now-a-days but will do what you want of it. It is from the old Allis Chalmers pump range and uses the "Shearpeller" especially designed for solids, entrained air and stringy materials, from memory it will pass 4 inch solids.

http://www.gouldspumps.com/download_files/CW/BCW_CWX.pdf

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[micalbrch]

haroonhassan: I still recommend to refrain from rotary lobe or progressive cavity pumps for raw sewage. The max. particle size pump manufacturer's specify is a theoretical value. It depends upon the kind and upon the shape. Ask Borger and Netzsch if their pumps can handle 50 mm stones. Ask them what the max. length of any hair or fibre can be without clogging the pumps. Hairs tend to wrap around the lobes/rotor. I think that the 50 mm will suddenly no longer be the allowable particle size.

If you can't find a centrifugal pump, look for a peristaltic or a diaphragm pump. At least the diaphragm pump can handle long and big particles.

 

[hhmh2004]

Thanks to everyone for interest and replies. Really appreciate it.

 

[bimr]

I do have experience with a rotary lobe pump on dilute municipal waste sludge. The supplier substituted a rotary lobe for the specified plunger pump. The rotary lobe pump has exhibited excessive wear on the lobe wearing surfaces. If there is any grit in the waste stream, the grit will probably chew the pump up. The application also did not have high discharge pressure requirements.

One option which I failed to mention is to use two pumps in series. Use one submersible and one vertical centrifugal.

However, the best solution is probably the Gorman Rupp Ultramate pump. It has two pump in one assembly, with a centrifugal pump mounted above a suction lift pump and driven by one motor.

http://www.grpumps.com/upload/AV-05796.pdf

Regarding Artisi's slurry pump selection; it is an elegant solution looking for a problem. You probably can not afford it. It will probably require a 3600 rpm as well.

Regarding the KSB pump selection above; the pump selection was made using the online pump selector. The pump selection should be made by the pump manufacturer. The online selector proposed a sewage pump, however, the online pump selector proposed a poor fit for the application.

I agree that it is best to avoid 3600 RPM motors and pumps.

 

[bimr]

Here is a case study for the Gorman Rupp.

 

[Artisi]

bimr,
The Gorman Rupp, now that is an elegant solution.
The Goulds CWX will run 1800RPM - I no longer have curves for these pumps but maybe it will hit the duty with 1500rpm. But you are correct the up-front costs it will be expensive however, keep the bearings oiled and the gland packing in good order and it will last for 50+ years on this application.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

You are correct that the Goulds Model CW pump will run about 1500 rpm. However, the pump will be operating so far off from the BEP that one would doubt that the Model CW pump would be able to operate efficiently.

Operating so far to the left of the BEP imposes force loads on the shaft. These unbalanced loads cause shaft deflection, vibration and premature bearing and seal failure leading to pump failure.

 

[bimr]

Another pump curve.

 

[Artisi]

bimr,
A couple of comments just for the records.
The 5x4x17 CW pump is on the No2 bearing frame with is capable of handling a many hundreds of HP and is used on all 17" impeller units upto and including 12x10x17, the load/shaft deflection from the 5x4x17 duty would be meaningless in comparison to the shaft capabilities and the bearing life would be into the millions of hours.
Having been involved with A-C pumps (now Goulds) for over 20 years I have never seem a bearing or shaft failure (on the CW pump and its sister PWO) due to poor application, discounting things like lack of lubrication, water ingress etc. I've seen the casing pushed from the bearing housing and burst Fenner Dodge rigid couplings due to wrong rotation, the shafts and bearings usually survived.

Yes, in hydraulical terms not very efficient however, in terms of reliablity and life etc it is 100% efficient. Having an 85% efficient pump constantly down for maintainance, repairs and replacement parts, clean out of blockages etc, is really not all that efficient.

If I was the engineer on this project I would have confidence in putting this unit forward for serious consideration.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[DubMac]

Look at a Moyno or any other reputable Progressive cavity pump with a grinder/macerator/chopper in front of it. Perfect for this service, thousands used worldwide. They will provide the package with the grinder included.

The CW and CWX pumps will work for a hundred years but it will take that long for payout as they are EXTREMELY expensive, and with the breakup of ITT, very questionable manufacturing point. This is an industrial slurry pump using hardened chrome internals and priced accordingly.

My son works for Goulds and says the CW/CWX will be now coming out of Seneca Falls; not the old AC plant. That is, once they get their crap together; which may be a year or two. With patterns, drawings, and engineering files scattered all over the place, this will be priced as a "replacement pump"; which in the pump world means "triple the price".

 

[bimr]

A 300 gpm progressive cavity pump will work but they are also extremely expensive with extremely expensive replacement parts (when they wear out).

Progressive cavity pump also have a long barrel which make this type of pump very difficult to fit into a lift station. The cost of the lift station structure will be expensive for a progressive cavity pump.

I work in the environmental field and have never seen a progressive cavity pump in service in a lift station pumping application.