Reducer on discharge side of cent. pump 1

[shahyar]

Hi,
I know it is not common to put reducer on discharge side of a centrifugal pump. But I like to know WHAT will happen if I put it? (centrfugal pump/blower).
The similar question could be "What if we put expander on suction side of a centrifugal pump?"

Thanks

 

[pennpiper]

Shahyar,
Here is some advice on "Piping terminology"
-Reducer- A device used to change the size of a pipe line from one line size to another line size. No matter which direction the commodity in the line is flowing (in process plant piping) the device is still called a "Reducer." In piping fittings there is no such thing as an "Increaser"or "Expander".

Now, to addressr what I think is your question. (can you put a reducer in the discharge line of a pump?)
Yes! you can put a reducer in the discharge line of a pump. It is done all the time, however it is normally required to change the discharge line from the (smaller) pump discharge size to a larger pipe line size.

I have seen a few (not many) installations where the pump discharge flange is larger than the required line size. In this case a reducer was included just downstream of the pump nozzle.

 

[katmar]

I agree with what pennpiper has said, but I would go further and say that it is so unusual to have a reducer installed on the discharge of a pump in such a way that the line size is smaller than the pump discharge that I would worry that either the pump has been wrongly selected or that it will run too far away from the BEP to be reliable. There would have to be a very good reason for me to accept such a set up. I'm not saying that it can't happen, but it is so unusual that it should be carefully checked.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rmw]

If an "expander" is put on the outlet of a fan or blower it is called an evase.

It is to convert the high velocity flow out of the fan discharge to static pressure.

rmw

 

[BigInch]

It would be an indication that the system was poorly designed for its present duty, or that no other pump or pipe was available.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

Eccentric reducers attached to the suction flange are arranged with the flat side at the bottom when the supply comes from above the pump, and with the flat portion at the top when feeding the pump from below.

 

[pennpiper]

25362,

The question above did not ask about the style of reducer (eccentric or concentric) used at pumps. It just asked about the use of a reducer in both the suction and discharge.

Eccentric reducers (when required) should be and will be installed with the flat side up no matter where the suction comes from.

 

[25362]

Pennpiper,

Your last paragraph:

Eccentric reducers (when required) should be and will be installed with the flat side up no matter where the suction comes from.

Igor J. Karassik's chapter 12 (Installation, operation, and maintenance), fig 9, in Karassik's et al. Pump Handbook, McGraw-Hill, tells a different story.

Kindly comment.

 

[pennpiper]

25362,

It is the way I do it and that is the way I teach other pipers to do it.

 

[Artisi]

Agreed - the flat side up no matter where the suction (inlet) comes from. This means from below the pump inlet or above the pump inlet or inline with the pump inlet.

 

[katmar]

We are engineers here, and engineers like reasons for their rules. It does not help much to say "flat side up, no matter what". There have been many posts on this subject before, for those wanting to follow up on it.

My own rule is - flat side up if there is entrained air (to prevent air pockets), flat side down if there is silt or solids present (to encourage the solids to be flushed into the pump suction). But if you have a flooded suction with no air and no solids you can install it any way you want, or even use a concentric reducer.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

Artisi and Pennpiper, it is well established that gas pockets (high spots) should be avoided and suction piping should be as short and direct as possible to reduce friction losses, therefore your comments on this subject are understood.

This is a shortened version of what Karassik had to say on the subject (second edition of the Pump Handbook):

Where a static suction head will exist the pump suction piping should slope continuously downward to the pump... If the source of supply is above the pump, the straight side of the reducer should be at the bottom (Fig. 9). Installing eccentric reducers with a change in diameter greater than 4 in (10 cm) could disturb the suction flow. If such a change is necessary, it is advisable to use properly vented concentric reducers.

The paragraph goes on explaining the arrangement of fittings to avoid flow disturbance.

What's wrong with Karassik's statement ?

 

[Artisi]

" -- pump suction piping should slope continuously downward to the pump.." and if it doesn't and there is a horizontal section before the reducer then air could collect at the reducer with the flat on the bottom.


For me the best rule is flat on the top - then you are safe from air collection - however, if there is good reason to change the "rule" to the flat on bottom it becomes an engineering decision where a factors can be reviewed.

 

[katmar]

I don't know the Karassik book, but I would not slope piping down towards the pump if I was concerned about air entrainment. I once came across an installation where this was done and it seemed that the capacity was being restricted by bubbles trying to rise against the flow of liquid. I felt it would be better to have the bouyancy and flow forces pushing the bubbles in the same direction. Unfortunately I was not able to persuade the user to change the piping, so I cannot be sure that this was the problem.

If possible I would prefer to have the vertical section sized to cope with self venting flow, and then the horizontal section either truly horizontal or sloping slightly up to the pump. If there had to be a reducer I would put the flat side at the top. Does Karassik give a reason for wanting it flat side down? Of course, if solids are present then sloping down to the pump would make sense.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

Excluding considerations on air pockets created by pipejoint steps, or by projecting gaskets, it appears that when the size of the suction pipe is larger than the bore of the pump inlet air pockets can be avoided precisely by providing eccentric reducers with the flat side on top.

Sam Yedidiah (Centrifugal Pump User's Guidebook), tells us that when the pipe diameter is more than one size or two larger than the bore of the suction nozzle, a straight pipe section of equal diameter as the pump inlet should be installed between the reducer and the pump because the eccentric reducer may deflect the liquid stream toward one side of the impeller eye, reducing output and efficiency.

From the picture in fig. 9 (in Karassik's manual, chapter 12) one can see that (at the end of the vertical pipe) there is a flanged long-radius elbow, followed by a horizontal eccentric reducer, with its flat portion at the bottom, connecting to the pumps' suction flange (itself an elbow). The arrangement shows a smooth flow profile with no air pocket. Contrarywise, with the flat portion on top, a slope would appear at the bottom, giving rise to more turbulence.

 

[Artisi]

What is interesting with this posting is it started out with eccentric reducers on the discharge side and has now done a complete 180 to eccentric reducers on the inlet side, and we haven't really touched on the second part of the original question of expanders (eccentric or conical) on the inlet side which could prove to be an interesting discussion.

 

[katmar]

Artisi, I don't think there is much more to say about "expanders" in the suction line. It would be an even more extreme case of what I wrote in my post of 22 Mar 07 7:11, or what BigInch wrote on 25 Mar 07 4:54. Except that pump manufacturers might find such a setup useful for determining their products' resiliance in a cavitating environment.

The example given by 25362 explains how Karassik justifies putting the flat side down when the feed is from above, or the flat side up when the feed is from below. Does this mean that if the line feeding the pump suction is horizontal and comes from the east that the flat side should be on the west?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[SeanB]

After reading the posts above I had to throw my 2 cents in here and I pasted in the below reference that I have - which would agree with what 25362 had posted.

GUIDELINES FOR PUMP SYSTEM DESIGNERS
Jacques Chaurette p. eng., Fluide Design Inc.
August 2005

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 (see next
Figure) 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 to be evacuated.

 

[Artisi]

East or west - flat on top

My reading of original posting re expander on the suction side was from small to large against the inlet or close to the pump inlet which could be problematic.

 

[JJPellin]

I have never understood the concern about an air pocket trapped on the suction side of a pump. I have never heard how it can cause any problem. Anyone working in an oil refinery with a jet water pump has seen an example that seems to disprove the problem. Every jet water pump I have ever seen was a barrel style with top suction and top discharge. Every one I have ever seen was pumping from an atmospheric tank sitting on the ground. That means that all of these pumps have a high point in the suction piping where it turns to go into the top suction. In a typical refinery, this jet water pump will be the highest pressure, highest energy pump in the plant. If an air pocket at the suction is such a terrible problem, why do we not have a problem with jet water pumps. The only problem I have seen with the suction to a jet water pump was when someone tried to prevent the air pocket. They installed a suction pot, open to the atmosphere on the top so there would be no air pocket. This suction pot caused several wrecks of the jet pump when the level controller plugged off with coke fines and the pump was allowed to run dry. Simply because someone states that a certain pipe fitting has to be installed a certain way, I will not do so unless there is some basic engineering principle to describe the need or solid test data to prove the benefit. Does anyone have such hard test data?

Johnny Pellin