pump ragging vs speed/flow

[panelman]

We need to limit the flow through a raw sewage centrifugal pump, basically the flow needs to remain constant with suction heads ranging from 1 to 6 m. My solution was to insert flow meter and then use a VSD to slow the pumps down to limit flow. Client has concerns about pump ragging if they are slowed down so wants valve to throttle the pumps which can then run at full speed.

Question: is pump ragging (or lack of) related to rotational speed or flow?

All comments welcome

Cheers

Mike

 

[quark]

What you think is correct.Go ahead. However I would like you to explain me what is pump ragging. (Only thing I knew is ragging juniors when I studied engineering)

Just check for NPSHR at various speeds and suction lift.

 

[panelman]

I'm electrical so don't really understand the process but....

"Rags" are the raggy components of raw sewage

"Ragging" is the build up of this in (on?) the impeller (in the eye?) which eventually will stop the flow and then needs to be removed by hand

I know ragging is more likely to happen when you reduce the flow by slowing the pump down electrically…my question for you experts is…

Is it a function of rotational speed or flow?.....so would reducing the flow by throttling be better than doing the same by slowing the pump down electrically?

Mike

 

[BobPE]

mikemk:

how big is the system we are talking about? what RPM? ragging is sewer people slang for tanpons, handi-wipes, diapers, towels, beer cans, 2x4's and just about anything else that gets into sewers. we do spin pumps slow in wastewater but that is by design usually, not by retrofitting so your client probably has good experience in making their request. There are grinders and other macerators you can retrofit for the system to handle the ragging.

In putting a VFD on the pump, the limiting design factor is not the electrical but rather the hydraulics of the system. These hydraulics will tell you if a VFD will fit the pump and allow the system to work.

Speed of the impeller will matter if it is a cutting type impeller but the most important mechanism is the velocity (flow) of the fluid through the pump to move the solids.

BobPE

 

[dadfap]

mikemk,

Can I suggest that you review thread 407-52493, "What is a Centrifugal Screw Pump". It reviews a pump that is perhaps better suited to this application manufactured by Hidrostal.
Perhaps shutting the door after the horse has bolted, but worth considering as an alternative

All the best

 

[panelman]

Cheers guys…let me tell you more

The suction sump is in the corner of a storm tank which means that the suction pressure varies between 1m (normal) and about 6m (full storm tank)

The flow through the works must not exceed 8 l/s. The existing pumps deliver this when the suction pressure is 1m but when the storm tank is full the flow goes up to 20-30 l/s.

Plan A…Limit the flow by slowing the pumps down using VSD and flow signal

Plan B…Limit flow by using a throttling valve and flow signal

A is my preferred option but client thinks that valve would be better as he feels ragging arises due to reduced impeller speed….I suspect that maybe flow is the major contributing factor to ragging.

Mike

 

[CB2]

mikemk

"Ragging" or solids handling relates first to velocity. Minimum velocity required to "lift" solids in the vertical pipe run.

Generally, the valves used in solids handling applications do not offer much restriction when full open. Using a valve to restrict flow will become blocked readily since the valve will probably be 60% to 80% closed to get the flow control you want.

Variable speed drives are the way to go, BUT, you need to limit the minimum pump speed to match the minimum velocity for moving solids, easy VFD programming. AND, you need to check with the pump manufacturer about their pump's minimum speed for pumping solids. Not all "solids handling" pumps are created equal in impeller design for pumping solids.

Note that the requirments for minimum velocity and pump speed may not be compatible with you system problem, come back for the more bizarre solutions if that happens.

We are having fun.
CB2

 

[JKC]

mikemk,
To maintain pump speed and flow rates why not fit a bypass on the discharge returning to the tankand control this to the required flow rate.

 

[CB2]

JKC,
The bypass control valve will have the same problem.

CB2

 

[panelman]

CB2

Thanks for your thoughts

Thinking about what you said....if the valve is closed 60% and starts to block then the flowmeter will see the reduced flow which will open the valve some more, thus it will tend to be self cleaning

If you slow the pump till it starts to rag then they do not self clear even if you turn it back up to full speed, ie once it has started to rag the only way to clear it is mechanically

mike

 

[CB2]

mikemk,
We have beat on this problem with generalities to the point of needing more specific application info. For the flow rates you have given, the pumps you are using are on the small side in the municipal raw sewage pumping world. It would be helpful to have the following:
1 - Design flow, head, efficiency, and max solid size of the pump?
2 - What type of impeller is used, single port, two port, or vortex (recessed impeller)?
3 - Are these pumps submersible or drypit design?
4 - The motors designed for a flow of 8l/s would seem to be undersized for 30l/s. Are the motors overloading?
5 - What is your discharge header pipe size?
6 - If you were to use a valve to throttle or bypass, what type of valve were you considering?
7 - What kind of actuator would you use on the valve control, electric, pneumatic, or hydraulic?
8 - How much do you pump on an average day? Will help in estimating cost of ownership.

We are having fun.
CB2

 

[Berko]

Panelman,

Try the Hidrostal pump to reduce ragging.

We recently put an 8" Hidrostal submersible up on a 16 week trial in Queensland, Australia.

The existing multi vane sewage pumps (130 kw units) were rated for the semi tropical wet weather flows Queensland experiences, however they normally run at much lower flows via VSD control for dry weather flows (approx. 85% of the pumps operation is at dry weather flows).

When operated at lower flows (down to 25 Hz speed), the multi vane pumps were ragging upto twice a week.

We believe this was caused by a combination of too low internal casing velocities for the rags to escape, coupled with internal fluid (& rag) re-circulation as the pumps operating point at lower speeds veers way to the left of the pumps Best efficiency Point (BEP).

The Hidrostal's (single vane, corkscrew impeller) when operating under the same VSD regime has not had a single blockage in 8 weeks (its predecessor would have blocked some 16 times in the same period).

The pump has proved very successful & will save the end user (largest council in Qld) a lot of money in reduced, unscheduled maintenance costs. It also operates at higher efficiencies at left of BEP, saving heaps on power costs too.

I have seen a case study showing that a similar sized Hidrostal in the USA (8" with 145mm free passage) pumped a 20 ft commercial fishing net which found its way into a storm water system !!

I would be interested to hear of anyone elses experience with this type of pump ?

 

[panelman]

Berko...thanks for that, the pumps that are causing the problem are actually Hidrostal ones like you are talking about.

Any thoughts on whether ragging is linked to speed or flow?

 

[Berko]

Panelman,

You surprise me, if a Hidrostal can't cope, then I can think of no other pump that will perform better.

Can you give me the pump model & its operating point/s (at full speed & lower speed) & I will check it out against the pumps curve (I have the Hidrostal pump selection CD ROM, which contains all the models & their curves).

I look forward to your reply,

Berko

 

[piffer]

Most ragging goes undetected. Conventional sewage pumps lose efficiency over time (per pumping cycle) as soft solids build up on the leading edge of the vanes (causing a drop in efficiency too). Flygt has done considerable research on clogging and developed their N-Pump line from their experience; including considerable field testing in difficult applications.
Hidrostal's are typically fairly resistent to clogging (sacrificing efficiency for clog resistance), but they can also clog (all pumps can be clogged!). Hidrostals have lower hydraulic efficiency compared to a similar sized Flygt N-Pump. Larger Hidrostals may be able to pass a fishnet, but a Flygt N-Pump will pass normal sewage solids more efficiently.
Reduced speed operation and operation away from BEP will lower any pump's clog resistance. Also keep velocities in your piping > 2 ft/sec (> 3 ft/sec. better), otherwise solids start accumulating (wetwell and suction piping) and in some instances can form into a significant mass that will really CLOG a pump.
Adding a submersible mixer to the wetwell can help prevent large masses from forming and periodic pump-downs of the wetwell can also help. Your wetwell design may also be contributing to the formation of a solids mass. Good luck! I know deragging a pump periodically gets expensive and the pump doesn't care what time it is.

 

[panelman]

Thanks for the input, as you guys might have guessed i'm electrical with very little knowledge of pump physics....what i was hoping for was a simple answer to the question....

If you throttle a pump to produce a certain flow, is it any less likely to rag up than if you get the same flow by slowing it down with a drive?

 

[Berko]

Panelman,

A picture (or performance curve) says a 1000 words.

Without looking at the pump curve & plotting your operating duty points (flow vs head) it is difficult to comment further (sorry).

With this info, I can check on my HidroCommerce CD selection program (one of Hidrostals sales reps freebies that they give out) & see where the pump is operating on its curve.

All I need is your pump code & duty points (normal duty + duty where ragging occurs). Then info from the CD should put me in a better position to answer your original question

 

[Berko]

Piffer,

If I recall you actually work for Flygt ?

Maybe you can answer me something. On the Flygt selection CD (Flyps), when choosing an N impeller pump as a selection, the pumps free passage is never stated (unlike all the other impeller types available on the CD) ?

Why is this ??

Regards,

Berko

 

[piffer]

Hello Berko,
Yes, I will admit that I work for Flygt (nothing to be ashamed of!). The N-Impellers up to ~30HP have clear throughlets less than 3". The notion that a sewage pump must pass a 3" non-compressible sphere is absurd to us. We have thousands of N-Pumps (w/smaller than 3" throughlets), efficiently pumping sewage without clogging. A comparable "traditional non-clog" with 3"+ throughlet is inefficient, has higher radial hydraulic load (shorter bearing & seal life)and is more prone to partial clogging. if sewage solids>3" are encountered, the N-Impeller will shear/pass solids with it's self-cleaning technology (impeller design, stationary relief groove combination).
There are only a handful of municipalities that will not budge from a pump w/throughlet<3&quot;. They will eventually see the light...
Who do you work for?
Regards,
Pifff

 

[electricsub]

Have been reading all the threads on this subject with interest. Panelmans problem is he has a submersible pump located in the corner of a Storm Tank, his pump has not to exceed a maximum flow into the treatment plant. Therefore as the level goes down in Storm Tank the flow drops from maximum to minimum.
Ragging is caused by several reasons, not least keeping the rags / rubish is suspension & maintaining a minimum flow to help stop ragging on the pump impeller.
VSD & Acutator Valve will all effect the velocity on the discharge, again leading to ragging. As it is a Storm Tank, everything & its uncle will be inside, the idea of using a agitaor / blowers / submersible mixers to get the rags etc into suspension, is one way. All submersible pumps can only handle a certain percentage of solids, exceed this and raggging will occur, regardless of velocity etc.
In a Storm Tank the submersible is in the worst situation possible, that is decling amounts of liquid increasing solids as the level in the tank drops. Sorry but normally its a no win situation, pumps will block / rag up regardless.

Your client may have to live with it as a operational problem.

One way to help is to limit the flow over say a weir, anything above the maximum flow goes by gravity back into the storm tank, using a fixed speed motor, manufacturer Flygt, Hidrostall, Goulds, ABS, Sarlin etc etc etc

Flygt N Pump, now there we have a big big question mark, thousands in operation, yes, but what happens in Panelmans case: Storm Tank - silt / sand ???? repair costs for the impeller & volute, NI hard impeller ?? still in a storm tank with volumes of sand & silt?


Over a period of 35 years in the industry i've seen this problem & others time after time, lost count of the number of times submersible pump manufacturers now have the magic answer to all our problems, only to find much later ???

Bottom line use experience & you pays your money and takes your pick.

Electricsub

ps.No i don't work for a manufacturer.