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Length of straight pipe before the suction of pump
2

Length of straight pipe before the suction of pump

Length of straight pipe before the suction of pump

(OP)
Hi all-

Can anyone advise on what the required length of straight pipe before inlet of centrifugal pump is.

Any link where I can look into for quidelines?





Thanks All

RE: Length of straight pipe before the suction of pump

ciise,
You really need to qualify your question a little more.

Is this a Centrifugal Pump or a Positive Displacement (PD) Pump?

If Centrifugal pump/base plate mounted/horizontal shaft what is the Suction/Discharge nozzle configuration (Top/Top, End/Top, Side/Side)?

If Centrifugal pump is it a Vertical "Inline" pump?

If PD pump what type (Piston, Gear, Diaphram, other)?

What is the suction nozzle size?

What is the suction line size?

RE: Length of straight pipe before the suction of pump

Our specifications require a straight run of at least 5 pipe diameters before the first elbow.  We prefer to have 10 diameters of straight pipe when it is practical to do so.  This is applied to all centrifugal pumps regardless of configuration.  But, it is especially important for pumps with double suction impellers that to avoid elbows that turn the pipe parallel to the pump shaft.  This can result in uneven flow to the two impeller eyes.

Johnny Pellin

RE: Length of straight pipe before the suction of pump

I this length relates to the entrance length for fully developing flow?

RE: Length of straight pipe before the suction of pump

no

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

RE: Length of straight pipe before the suction of pump

Have at least 5 pipe diameters of strait pipe before the suction inlet.  If it is an end suction centrifugal pump, then provide a suction diffuser.  I assume you are talking about a horizontal split case?

RE: Length of straight pipe before the suction of pump

Suction diffuser - Why?
A minimum of 5 pipe diameters is normally sufficient unless the flow is extremely disturbed prior to this point.
johhny's advice re double suction pumps is extremely important "it is especially important for pumps with double suction impellers that to avoid elbows that turn the pipe parallel to the pump shaft.  This can result in uneven flow to the two impeller eyes."

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.)  

RE: Length of straight pipe before the suction of pump

I had a recent design where a colleague suggested the use of a suction difusser for a horiz split case double suction pump.  I checked it out out of respect for his position but couldn't find any benefit from it or any reason to use it.  They seem to be very popular in commercial building pumping systems.

Since my application didn't have the luxury of the required or actually ANY upstream straight length, I piped the inlet from above with a vertical reducing elbow into the suction of the pump perpendicular to the pump shaft centerline.

rmw

RE: Length of straight pipe before the suction of pump

No one in this thread has yet mentioned recommended suction pipe diameter. Suction pipe size is also a factor affecting reliability.

The "Pump Handbook" discusses recommended suction piping schemes.

Suction pipe diameter should be one nominal pipe size greater than the suction nozzle size. An eccentric reducer can be mounted on the pump suction

   

RE: Length of straight pipe before the suction of pump

Artisi, it is common practice in industrial pumping as well when you have END SUCTION centrifugal pumps - he did not specify what type of pump.   

RE: Length of straight pipe before the suction of pump

MechEngNCPE, I would comment that in over 30 years in the pump industry I very once came across nor used a suction diffuser, not saying that there isn't a role for them in certain cases.
Regarding Johnny's comment re HSC pumps and bends etc, I was just re-confirming an important point.    

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.)  

RE: Length of straight pipe before the suction of pump

Artisi,

I have over 40 years in lots of Industries that use lots of pumps and even some time within the pump industry.  Likewise, I have never been involved with a pump that had a suction diffuser on it although I have seen a few in very light what I would call commercial applications - building water systems, hospital boiler rooms, etc.

It is not something that comes to my mind when faced with a pump suction problem.

rmw

RE: Length of straight pipe before the suction of pump

I have seen many large municpal double suction pumps which have eccentric reducer followed by a downward elbow (or a reducing elbow) on the suction side of the pump and there are no problems.  Very often the 5 or 10 pipe diameters cannot be met in the real world.  The incoming piping, as stated above, should be perpendicular to the pump shaft.

 

RE: Length of straight pipe before the suction of pump

I have seen about 30 end suction pumps ranging from 5 to 100hp in industrial settings.  I'm thinking condenser/chilled water, as well as heating hot water pumps.  I prefer end suction over horizontal split case for the reasons given above.  I do like Vertical split case pumps however, no need for suction diffuser there.  

How is this such a rare sight??  Do you all deal with END SUCTION pumps often?    

RE: Length of straight pipe before the suction of pump

MecEngNCPE,

What is your definition of "industrial"?

rmw

RE: Length of straight pipe before the suction of pump

"Industrial" covers building services and manufacturing as opposed to mining, mineral processing, municipal, pharma, petrochem and nuclear.

End suction pumps are very common in all industries.

Municipal tends to use horizontal split case running at 4 pole motor speed because of a historical view tha slow running pumps cost less to maintain because they last longer.

If a full cost benefit analysis is performed it may work out that a 2 pole speed pump, with its smaller frame (& components) works out to have a lower total life cycle cost. Also need to take into account the real estate used for the larger frame size pumps, buildings, aircon, VSDs, cable and pipe lengths etc not just the prime cost of the pumps.

Spares for a smaller frame pump are cheaper than a larger frame pump.

Before posters jump on what I have said I repeat "may be". The calculations need to be done.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

RE: Length of straight pipe before the suction of pump

stainer, I would tend to agree with you for building services and some areas of manufacture, however depends on what area of manufacture and on the application. I also think our 50 Hz environment influences the 2 pole argument to a great extent.

MechEngNCPE, still haven't convinced me or others by the look of it on the use of suction diffusers. Bear in mind there are probably millions of HSC pumps throughtout the world of which I have seem a very small proportion, none of which were fitted with suction diffusers and in some case with  pretty poor inlet conditions and operating fairly well.   

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.)  

RE: Length of straight pipe before the suction of pump

"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be."

Lord Kelvin

Do or measure the numbers whether it be life cycle costing or the design of suction conditions!

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

RE: Length of straight pipe before the suction of pump

Lord Kelvin - probably best known for his "balls"  

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.)  

RE: Length of straight pipe before the suction of pump

MechEngNCPE never responded but maybe stanier did for him/her.  I suspected as much.  It is part of being separated by a common language.

To me Industrial is Pulp & Paper, Chemical, Metallurgical, Refining and Petrochemical, etc.

Commercial is more like buildings and light manufacturing, laundrys and the like.

Municipal is kind of in a world all of its own with other governmental entities.

When I worked for a British company, their use of the word "industrial" meant what "electric utility" does in the states.

In what I define as Industrial, I have never seen the use of suction difussers.  And I would add to that that I have never seen them in Electric Utility applications as well.  I have, however seen them in building systems and municipal applications and maybe the occasional steam laundry.

rmw

RE: Length of straight pipe before the suction of pump

Just to get away a little from the norm, for pulp and paper when pumping 4% or more consistency and especially as you get to 5 - 6% you need anything but smooth flow into the pump inlet, the more agitated the better to ensure the stock is not dewatered at the pipe/pump reducing inlet, but this can be another discussion in another thread if anyone is interested       

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.)  

RE: Length of straight pipe before the suction of pump

The tendency to overthink things is common amongst all of us.  In my many years as a pump mechanic on all types of pumps there are some things I have learned.  The same pump used in any setting be it heavy industry, commercial/municipal is the same pump.  It may get treated very differently in regards to maintenance and so on.

Suction diffusers are handy when one can not meet the recommended pipe diameter and straight run before suction end mating.  However as another poster stated Diameter is as important if not more important then straight pipe length.  Industry norm depends completely on the industry.  Having seen many things some of them very unusual yet they worked.  In Power Generation we like 5 times the diameter for straight run and one and half times suction for diameter with an eccentric reduction.  

All of this can be found in the various handbooks and lets not forget that all of this is a moot point if NPSH is not met.   

RE: Length of straight pipe before the suction of pump

Spud6414, a good reply, many times it is useful to get back to basics and as you stated "The tendency to overthink ......." can be a problem.

Worth your first star.

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.)  

RE: Length of straight pipe before the suction of pump

I have rarely seen a pump fail from overthinking, but I have seen many suffer and/or fail due to not thinking about all that is required to make the installation successful.

rmw

RE: Length of straight pipe before the suction of pump

Depends what you are thinking.   

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.)  

RE: Length of straight pipe before the suction of pump

rmw, I agree completely, that falls under many strange things I have seen pipe

Sometimes one really wonders what someone was or was not thinking.

Spud says; Meticulous effort and willingness to learn are honorable traits.

RE: Length of straight pipe before the suction of pump

I have seen many pumps fail because of overthinking. Generally it is senior engineers or management thinking they need to add 20% to the duty as a safety factor. This leads to pumps being oversized and running well to the right of BEP. This leads to vibration and failure of seals and bearings.

In respect of suction design instead of just using Sank's Pumping Station Design, ANSI/HI 9.8 or the US Army references on the subject engineers try and think of ways of designing pump suctions. If only they stopped "thinking" and tried ways that have worked for decades the problems would not arise.

I have seen many a pump design modelled using FEA only to see the same solution that would have taken 5 minutes of reading.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

RE: Length of straight pipe before the suction of pump

Stanier,
I would suggest that pumps that have been oversized by "others" normally run to the left of BEP as a result of being throttle back to reduce flow to the real system neeeds. Further, being oversized also impacts on pipeline sizes, motor sizes etc.

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.)  

RE: Length of straight pipe before the suction of pump

NPSH...  Oversize pumps.  Bad Idea.

Spud says; Meticulous effort and willingness to learn are honorable traits.

RE: Length of straight pipe before the suction of pump

While I don't disagree with the scenario that stanier presents, I wouldn't classify that as overthinking, rather I would postulate that it is not thinking at all.  Moreover, I'd call it brainless.

And I have been a victim of it, and have been guilty of deliberately undersizing equipment, pumps included, because I knew that some anal superior was going to brainlessly add his/her "fat" no matter what was done in the initial sizing.

The bad scenario is when there are several steps up the approval ladder and everyone up the ladder adds some "fat" because they don't have the ability to think through what the engineer has (or has not) done with respect to doing their job and sizing the pump correctly in the first place.  Because of this fubar, as an example, while not a pump per se but closely related, I have seen actuators with 3" output shafts coupled to driven devices with 1" input shafts.  There is a mismatch.  Someone added too much fat somewhere and I hold that that is brainless.

rmw

RE: Length of straight pipe before the suction of pump

The rule of thumb I've used in the past with good success is:  One pipe diameter of suction straight-run for every fps of suction line velocity.  Usually this is about 5 diameters, but in some cases, velocities may necessarily be higher or lower.

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