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eccentric reducer

09/08/2008 5:20 AM

why there is need of eccentric reducer at the suction side of the pump?

i want to convince technicaly my purchasing dept. because they are saying we never used before why do u want now

please help me

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#1

Re: eccentric reducer

09/08/2008 5:48 AM

Why need an eccentric reducer?

Always use an eccentric reducer at the pump suction when a pipe size transition is

required. Put the flat on top when the fluid is coming from below or straight and the flat on the bottom when the fluid is coming from the top. This will avoid

an air pocket at the pump suction and allow air, gas or vapors to be evacuated.

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#3
In reply to #1

Re: eccentric reducer

09/08/2008 11:45 AM

I dissagree with Aurel Mares on the orientation of the Ecc reducer when the suction source is above the pump.

This question comes up at least twice a year. It has been answered in this Forum and others. The answer below is from another question on another Forum.

The questions:

This was from Karthik,

I am aware of the use of reducers in vertical & horizontal pump piping.
I have consulted with all my Senior Eng regarding this. but I am not able to get a exact reason. My Question is, do we use a reducer to avoid cavitation in the pump. I want to know in detail the reason of using the reducer & how does it work in both horizontal & vertical piping.
Please, can anyone help me?


My answer:

First of all the main reason there is a reducer in a pump suction line or a pump discharge line is because the pipe line size is different from the pump nozzle size.

The process engineer determines what line size is required for all lines to perform the function of the process. This is true for both the suction and the discharge lines for pumps.

The Pump Engineer specifies and purchases a pump that will perform the function of the process. Due to the nature of the pump, the suction and discharge nozzles almost always (95% of the time) turn out to be one or more sizes smaller than the connecting line sizes. You may ask why don't they buy a bigger pump? Or at least a pump with a bigger nozzle? Well it is pure economics. The bigger pump or the pump with bigger nozzles cost extra money and probably will not perform properly or be cost effective.

So what do we have? We have a pipe line (specified by the process engineer) and a pump (specified by the pump engineer) which needs to be connected. The piping is done by a responsible, well trained, and experienced piping designer. It is the piper's job to know what to do and do it right.

The detailed reason for having the reducer is that good pipers know that to change line size you must use a reducer.

Now, how do you install a reducer?
Vertical lines:
It is okay to use a concentric or an eccentric reducer (flow up or flow down) in vertical lines. You should choose an eccentric reducer when you want to keep one side of the pipe "flat" such as against a wall or group of supports.

Horizontal lines (not at pumps):
It is normal (and recommended) to use eccentric reducers (with the flat side down) for line size changes in horizontal lines on pipe racks. This allows for the different sized piping to have better contact with the pipe supports.

Pump suction piping:
If the pump has a top suction nozzle then you should use a concentric reducer to make the line size change. You should also make the line size change a minimum distance form the pump suction nozzle.

If the pump has an end suction nozzle or a side suction nozzle, then you must arrange the piping so the reducer is top flat regardless of line origin (below, even or above the pump) and a minimum distance from the pump suction nozzle. The reason for the top flat is this. If the reducer is installed with the flat side down then a "vapor trap" is created by the change in diameter. This space allows bubbles to collect and leads to cavitation.

Now back to your questions. No you do not use a reducer to avoid cavitation of the pump. You use the reducer to change line size. You use the correct installation (orientation) of the reducer in a pump suction line to avoid cavitation of the pump.

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#6
In reply to #3

Re: eccentric reducer

09/09/2008 2:22 AM

My view is; this is over-engineering at its best. In 42 years of working w/ these pumps, I have NEVER seen one fail due to the wrong reducer. The small area of the air bubble in the conc. reducer will quickly be pulled through the pump on prime negating any negative effects.

Even though I am one, sometimes I think it's best to just let the engineer 'drive the train'... lol

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#8
In reply to #6

Re: eccentric reducer

09/09/2008 2:34 AM

May be you didn't noticed the problem, but the accumulation of gas/air/vapour will formulate a pocket of bubbles which will resist the flow of fluid in the form of a pressure drop which will cost you more pump head and inturn more power.

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#15
In reply to #8

Re: eccentric reducer

03/30/2009 6:48 PM

I hardly believe that the short run of pipe and reducer will be able to separate gas or air from the fluid! I have calculated and designed more than one deaerator.

The eccentric reducer (or the concentric) allows a larger pipe to be connected to smaller pump nozzles for lower velocities in the piping, that is all.

In the suction line (as in the whole system of course, but more dedicated on NPSH) it is advisable to reduce pressure drop calling for one or two sizes larger pipe, right? But if the angle of reducer is sharp it will certainly help less that desired.

If liquid comes from top, let the the reduccer be flat on bottom for any small solids being carried to the impeller, any bubble will obviously try to reach the upper surface of the tank or container rather than go down to the pump suction, but if it comes within the liquid stream at the velocity it was carried downwards it will also be carried to the impeller eye. Also, from top there is not much difference between the reducer being eccentric or concentric unless anti rotation effect of eccentric is considered which I doubt, if it is needed to prevent rotation use a flat guide.

If liquid is taken "up" I would recommend the flat side on top for a better drainage when the pump is off.

Sorry if my Cuban English prevents clear understanding

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#12
In reply to #3

Re: eccentric reducer

09/28/2008 8:39 AM

Please explain me with some more technical ideas how the vapor trap is created when flat side is down. because change in diameter is also there when flat face is up.and vapors are always top of the water or liquid stream.

please guide me i am confuse.

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#7
In reply to #1

Re: eccentric reducer

09/09/2008 2:23 AM

Good answer AUREL MARES,

and I used to explain that phenomenon through my lecturers for both beginners and experienced engineers/technicians, since they are thinking in Piping Handbooks where you can find that they use concentric reducer or eccentric reducer in suction side with its bottom flat whatever the fluid coming from below or straight which is considered a mistake.

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#2

Re: eccentric reducer

09/08/2008 10:33 AM

Aurel Mares us right but to help you explain it to admin the following.

In a normal suction any air that may enter the system will seek out the high place because air is lighter than water. This can be demonstrated by farting in a bath.

The pump creates a vacuum by trowing away the water in the pump, Atmospheric pressure then have to deliver the water there. if there is a bubble of air in there the lower pressure will only succeed in getting the bubble bigger.

Because of the velocity of the water in the pipe the bubble will also be pushed in the direction of the pump which will result in a deformed bubble actually reducing the area where the water can flow resulting in a even higher velocity and more deformation.

With all these the pressure in the eye of the pump may drop beyond the limits of the pump and cavitation will follow. The impeller of the pump may be damaged quickly. (seconds - minutes)

Sometimes when the bubble accumulated is too big it passes through and the prime may be lost.

So what you tell these guys is that "it is cheaper to fit an eccentric than to replace the impeller or pump on regular intervals and that anyone that wants to change the design will be kept liable.

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#4

Re: eccentric reducer

09/08/2008 12:02 PM

Because the mechanical engineer determined that it was necessary to meet the system and pump requirements for the process.

I have put together copies of the design calculations, pump curves, and cost data and forwarded them to Procurement/purchasing for their use.

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#5

Re: eccentric reducer

09/09/2008 1:28 AM

Thank you to all for your comments

In our plant we have over 1000 of centrifugal pumps. Let say all have the suction nozzle with lower diameter than suction line. Which possibilities we have couple the flange of suction line to suction pump nozzle? Install a reducer, not ? How many types of reducer are? Two, eccentric or concetric. To avoid cavitation it is mandatory touse a eccentric reducer how I explained above. If I am not wrong in pump vendors manual it is stated similar.

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#9

Re: eccentric reducer

09/09/2008 8:17 AM

Hi,

as I understand diferents fluids will need diferents reducers.

Flat on bottom (FOB, excentric reducers), when you will pump slurries (aqueous solutions with more than 10 % of solids in suspension), in order to avoid that the solids settles in the pump suction.

Flat on top (FOT, excentric reducer)), with aqueous solutions with less than 10% of solids in order to void the air pocket (by acumulation) of water vapor or air at the suction pump.

When we work with organics fluids (petrochemical, refineries), we always use concentric ones.

Regards

Ernesto

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#11
In reply to #9

Re: eccentric reducer

09/09/2008 3:16 PM

why for organic fluids concentric one.please see one of our pump for oil is having eccentric reducer flat side top. is it correct?

actually the picture is reverse.

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#10

Re: eccentric reducer

09/09/2008 9:45 AM

Orientation of eccentric reducers depends on the service. Sometime the 'flat' side is down to ensure against build-up of debris and facilitate complete draining of the pipe (saves a valve, nipple, weld-o-let and potential vibration induced failure.)

Sometimes avoiding a vapor pocket in the suction line is the most important objective, and 'flat side up' may avoid same piping,valve etc. as above.

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#13

Re: eccentric reducer

10/05/2008 9:26 PM

Air pockets may form if concentric reducer is used at pump suction, which results inCavitation, and cause damage to Pump. To avoid this problem, Eccentric Reducer with

Flat Side Up (FSU)is used in Pump Suction.

A pump is designed to handle liquid, not vapour. Vapour forms if the pressure in the
pump falls below the liquid's vapour pressure . The vapour pressure occurs right at the
impeller inlet where a sharp pressure drop occurs. The impeller rapidly builds up the
pressure which collapses vapour bubbles causing cavitation and damage . This is
avoided by maintaining sufficient NPSH.
(Cavitation implies cavities or holes in the fluid we are pumping. These holes can also
be described as bubbles, so cavitation is really about the formation of bubbles and their
collapse. Bubbles form when ever liquid boils. It can be avoided by providing sufficient

NPSH.)

NPSH: Net Positive Suction Head. NPSH is the pressure available at the pump suction
after vapour pressure is substarcted.
It is calculated as : Static head + surface pressure head - the vapor pressure of your
product - the friction losses in the piping, valves and fittings.
It thus reflects the amount of head loss that the pump can sustain internally before

vapour pressure is reached.

------------

more detail

www.pipingguide.blogspot.com

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Anonymous Poster
#14

Re: eccentric reducer

10/12/2008 2:21 AM

pump Handbook (IGORJ third edition chapter 12 page:12.9 FIGURE 9).

Put the flat on top when the sorce of supply is below the pump (see figure). and put the flat on the bottom when the fluid is coming from the top(the sorce of supply is above the pump).
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Anonymous Poster #1
#16

Re: eccentric reducer

12/29/2011 12:36 PM

Interesting topic, but as I read through the replies, I see that there is a difference in opinion between whether the top or bottom side should be flat when the suction source is above the centrifugal pump.

Personally, I disagree with the design manual and I think that even if the source is coming from the top, it is still better to put the flat side on the top, especially if it is for services like steam condensates. And I don't really see the disadvantages in putting the flat side top even when the suction source is above the pump.

The worries about accumulations of solids when the flat side is top, in my opinion, does not poses as much threat because of the safe guards in placed (for pump suction reducers only). Reason being that, first of all, as far as I understand, centrifugal pumps are not really designed to pump fluid with solids in the first place, excessive solids create abrasions in the impellers which is why there is usually a strainer upstream of the pump suction.

P.S. this is only for centrifugal pumps. For sludge services, I agree the flat side should be bottom to prevent any accumulations. And as far as I know, cavitation induced from entrapped vapour does not exactly apply to PD pumps.

But that are just my thoughts, any comments on them will be great. It is always good to learn from more experienced people!

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#17

Re: eccentric reducer

12/30/2011 4:18 AM

...........................

At the above drawing, there are 3 assemblies of suction piping of pump:

• Assembly (A), where an eccentric reducer with bottom-flat (incorrect).

• Assembly (B), where an concentric reducer (incorrect).

• Assemby (C), where an eccentric reducer with top-flat (correct).

...........................

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Users who posted comments:

Abdel Halim Galala (3); Anonymous Poster (4); AUREL MARES (2); ernesto1962 (1); Hendrik (1); Keith E Bowers (1); PennPiper (1); sandeep lokhande (2); user-deleted-9 (1); wmanso (1)

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