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Commentator

Join Date: Dec 2008
Location: Young NSW Aust
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Pumping Problem

12/08/2008 9:45 PM

I have a problem at our abbatoir where fat is coating the inside of a 100mm polyproylene pipe. The fat is carried by 60 oC wastewater in suspension. I have been told that as the pipe is larger than the pump outlet dia. (50mm) that the flow rate of the pump is therefore lower and this may be causing the problem. Two questions:

1. Is the flow rate lowered because of the larger pipe size compared to the pump outlet size?

2. Would the low velocity of the water through the larger pipe allow the fat to stick to the walls of the pipe?

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#1

Re: Pumping Problem

12/08/2008 11:06 PM
  1. Your pump volumetric flow rate isn't affected by the size of your pipe; rather, your water velocity is reduced in the larger pipe.
  2. Yes.

Try adding some caustic soda to the wastewater flowing through your pipe. It will help dissolve away the fat deposits.

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#2

Re: Pumping Problem

12/09/2008 1:16 AM

Dear Whezmabeer

1- The answer of your first question is NO .

You have to speak here about velocity not the flow rate

Since you have the same head , same shaft rotation i.e same outlet velocity ,same motor i.e same power , then you have constant flow rate :

we have the flow rate Q= vxA then we have v1xA1=v2xA2

i.e v2=v1xA1/A2

So the the velocity is lower .

By the way, the normal choice of the pipes for pumps is as follow :

-suction pipe dia.is larger compared to the inlet pump dia.

-discharge pipe dia. is equal or smaller compared to pump outlet dia.

2-The answer of the second question is YES .

3-I suggest to solve your problem,please,you have to check :

a-The max safe velocity for your fluid .( from tables,sorry because I do not have it for the time being,I shall send it later)

b-Choose the required safe dia. of the pipe relating to this velocity ( sure you will change the outlet pipe into smaller one to increase the liquid velocity,this will help in scavening your sticked fats.

c- You may insulate your outlet pipe if you are in cold weather .

Hoping that will achieve your goal .

greetings

Aly

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#3

Re: Pumping Problem

12/09/2008 3:24 AM

I have not had any personal experience with this problem, but I go along with the above suggestion that the pipe should be insulated, a larger pipe has a larger cooling surface. This is similar to wax build up in fuel lines, placing two large change over filters in line, with differential gauge (alarmed if necessary) , can be regularly changed over and cleaned, this may keep it under control?

Regards JD.

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#4

Re: Pumping Problem

12/09/2008 12:05 PM

I don't have any experience with your application but believe "fat" can be quite tenacious and maybe particularly at low velocity. I had previously heard also of affinity of certain plastics for grease as Mr. Sobisch indicated in the thread at http://www.waterandwastewater.com/cgi-bin/yabb/YaBB.pl?board=general;action=display;num=1173720606 , e.g. pvc pipe as discussed on page 63 of the document at http://www.epa.state.oh.us/dsw/permits/sso%20solutions%20asce%20epa%20guidance%2..., but I have not yet heard of this issue specifically with regard to polypropylene pipe. This cited state epa reference apparently uses the term "oleophilic" to describe this affinity

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#5

Re: Pumping Problem

12/10/2008 12:30 AM

Accumulation of suspended solids in piping is usually caused by an improper flow rate in fpm in the piping. Usually the highest deposition is when the fluid is moving in plug flow. If you can bring this up to laminar flow, or better yet turbulent flow, deposition decreases. Figure the Reynolds number of your plumbing. The down-side is turbulent flow takes horsepower and may erode plumbing.

In short, reduce the pipe size, or increase the pump output, if line friction does not prohibit

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Commentator

Join Date: Dec 2008
Location: Young NSW Aust
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#6
In reply to #5

Re: Pumping Problem

12/10/2008 12:51 AM

Thanks guys.

Pipe friction is not a problem so we can increase the flow. Caustic is out as well, as all our wastewater is irrigated to land and the soil is already naturally sodic. I now know what do, just have to get it done.

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#7

Re: Pumping Problem

12/10/2008 2:59 AM

Reply to your Q1

It seems that your are talking about flow velocity in the pipe line instead of flow rate. It is simple terminology matter. Flow rate will not be affected by discharge pipe dimension. (Actually there could be, but it seems not be your case) Having bigger discharge piping than discharge nozzle of pump is the normal practice of piping design. There is nothing wrong.

Reply to your Q2

Yes, flow velocity sometimes directly influences the fouling of piping. Someone who advised you that low pump flow rate due to large discharge piping, might indicate this point. What you are talking about is fouling. Not all the material occurs the fouling.

Fat is one of the most fouling occuring materials. It is normal practice to design the one step larger discharge piping than discharge nozzle of pump. For instance, if the discharge nozzle of pump is 4 inch, than discharge piping is 6 inch.

If your piping is still larger than one step, changing discharge piping line will be helpful. Remember one thing, if your discharge piping is already fouled severly, it causes much discharge back pressure which will REALLY REDUCE PUMP FLOW RATE!!!. It is a vicious cycle.

CHECK YOUR PUMP RATED CAPACITY AND OPERATIONAL CAPACITY BY FULLY OPEN THE DISCHARGE VALVE. YOU WILL FEEL WHETHER PIPING IS ALREADY FOULED OR NOT. IF FOULED, CLEAN IT OR CHANGE IT.

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#8

Re: Pumping Problem

12/10/2008 4:22 AM

I would head up stream of the problems and make users install grease traps that are large enough to control the grease before spending a lot of money on a problem that will only get worse if you don't keep the fat out.

The waste water treatment plant can't handel all the grease anyway !!!!!

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#9

Re: Pumping Problem

12/10/2008 8:27 AM

If I may add my 3ยข worth (inflation) let me say that you have been given some good advice and some bad assumptions. You should analyze your whole application from front to back. Being an abbatoir, you may be able to eliminate more of the fats and grease before it enters the pump. This may be simply a matter of "housekeeping".

You must then analyze the discharge piping system. What is the length of the forcemain? What is the flow you must maintain to accomplish the task at hand? This means, how fast must you empty the vessel from which you are pumping? What is the elevation difference from the pump to the final delivery point?

The misinformation that you have been given is the contention that the flow rate is not affected by the size of the discharge line. It is, indeed, affected. The larger line will reduce the velocity, but the flow rate will increase. The smaller pipe will increase the velocity in the pipe, but the flow rate will decrease. If the pipe is long or non accessible, it may not be cost effective to remove and replace the discharge pipe.

Another option might be to increase the speed of the pump, or otherwise increase the output of the pump. That is, if the increase in flow can be accommodated by your needs and the discharge location. This action will have a corresponding increase in the horsepower required to effect that change since the pump will now be delivering more flow and, as a side benefit, an increase in velocity.

As someone eluded to earlier, the way the flow is managed is an added component to the solution. If the flow is somewhat constant, the fats would have less opportunity to adhere to the sides of the pipe. This could be helped by using a VFD with a transducer or bubbler or some other type of variable signal producing primary sensor, and match the incoming flow to the pump discharge. If the flow is stop-start with long intervals of downtime between the starts and stops, the fats would be allowed to 1) - cool off to the point where they would become "sticky" and adhere to the pipe walls and 2) - stop their forward motion which would also allow them to adhere to the walls of the pipe.

I am sure there is a solution to your problem, but it may take some "engineering" to make that happen. Have your in-house people look at it as if it were a new installation and see where improvements can be made, or hire an outside engineer to look at it for you. The benefits can be enough to offset the fee the engineer may charge you.

Good luck.

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#10

Re: Pumping Problem

12/10/2008 8:42 AM

I have a few further suggestions for you:-

1) Increase the water temperature say once a week to as near to boiling as possible

2) Insulate pipe to reduce cooling effect

3) Increase pump pressure if possible - the faster the fluid is pumped, the less cooling will happen and the less fat will be precipitated out

4) I clean pipes on paint machines with a ball of plastic foam, slightly larger than the pipe to be cleaned. I copied it over 30 years ago from a brewery (they clean the beer pipes once a week in this manner to get the yeast off that builds up in the pipes.....)

Maybe a hot rinse, with high pressure and a foam ball...........together!

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#11

Re: Pumping Problem

12/10/2008 9:37 AM

1) flow rate is the same, the velocity is lower. (Flow=Cross_Area x Velocity)

2) Use a grease trap. It's cheaper than anything else.

M.

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Aly Badawy (1); Andy Germany (1); Anonymous Poster (2); dadw5boys (1); DVader1000 (1); jdretired (1); rockbit (1); SardMan (1); The Commoner (1); whezmabeer (1)

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