Pump suction pipeline size less than pump suction nozzle 2

[66851230797]

Is it acceptable if the sizing of pipeline size at pump suction less than pump suction nozzle size?My case is the designer selected to use suction pipe of 2 inch,but the pump suction nozzle is 3 inch.The pump is normal centrifugal type,suction size on the front and dischrage on the top.Please advise the reason if reject or accept.

 

[chief]

Not acceptable. Suction starvation will cause rapid cavitation of your pump casing and impeller.

Offshore Engineering&Design

 

[katmar]

That would be most unusual, but without all the details I cannot say whether you should accept or reject the design. Get the designer to justify his design. If he cannot do that, then reject it.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zeusfaber]

Agree with Chief and Katmar - especially if the reason for the 2" pipework is to make it easier to get the pipe through a series of 90 degree bends to snake round all the rubbish somebody else has already designed into the space (I can picture the HPSW system on a ship that had better remain nameless even as we speak).

A.

 

[Artisi]

There is no rule to say that you can't have a smaller suction line than the inlet flange. Although unusual in many cases it could well be quiet acceptable. What will govern the acceptability or rejection is the NPSHa at the inlet - so long as NPSHa is above NPSHr by a normal acceptable margin the installation would be ok.

Bear in mind that quite often the inlet flange size has nothing to do with the impeller eye diamater - if the impeller eye is lets say 2.375" diameter it stands to reason you would have a 3" flange.

 

[Artisi]

Just to add a further note, I have seen on small pumps the transition from the flange to the impeller eye as tapered - from large on the flange to smaller to suit the impeller eye diameter.

 

[brokate]

In the Pump Handbook, 2nd edition, by Karassik, et al, when he discusses suction piping he states that the "suciton piping should be at least as large as the pump suction". He goes on to recommend that the piping should be one size larger than the pump nozzle. Of course there are also recommendation for the amount of straight pipe that should be in front of the pump suction and we all know that don't happen very often.

 

[Artisi]

Of course it is preferable to have the inlet pipe as large as possible to reduce friction losses which impact on NPSHa - but that's in a perfect world and in the pump industry things are very often not perfect.

 

[BigInch]

Artisi is correct. Textbook recommendations can only cover the general most common cases and, while somewhat unusual to have suction pipe diameters less than discharge diameter, it could be perfectly acceptable, given the appropriate system curve and NPSHr characteristics. At relatively low flow rates and low pump NPSHr, such a design could be reasonably expected. Pumps must be designed for the general case to be commercially viable products, but system curves can fall anywhere within the pump's full range of capabilities.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[TenPenny]

I think that a lot of this will depend on flow rates.

Consider that a pump with a 3" suction could (in a 60HZ world) be operating at 3600 rpm, or 1800, or 1200....

While a 4" pipe would probably be required while running at 3600 rpm, if the pump is running at 1200 rpm, the flow rates are possibly low enough that a 2" pipe would not cause problems.

But that requires input on flow rates, pressures, viscosity, pipe runs, etc etc. One would hope that the piping designer did actually check this, but that's certainly not a given.

 

[nickelkid]

My case is the designer selected to use suction pipe of

Why did the Designer deviate from textbook recommendations? Many good points have been made on why it may be ok to deviate, but what was the rational of the Designer?

 

[BigInch]

Textbook recommendations DO NOT have to be followed if a proper engineering design shows otherwise. If that was the case there would be a lot of textbooks and very few engineers, or perhaps a lot of textbook engineers. Scratch that, there are a lot.

Perhaps he actually made the system curve and it showed that the operation point and NPSHr was satisfied with a smaller suction pipe. What's the problem with that?

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[nickelkid]

BigInch, agreed, it is conceivable that the Designer has done his/her due diligence, but how probable? A quick look at my Goulds Pump Manual states that reducing the suction line size “should never be smaller than the suction connection”; I would think that would be a typical requirement of any OEM. The OP should have his bases covered well if he is going to lift OEM liability.

 

[katmar]

The OP has never been back to Eng-Tips since posting their question. Let's not waste any more time on this thread. Without knowing the real situation we are just shooting in the dark.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

Its not a requirement of the OEMs, just a general recommendation, primarily since they do not know where the operating point(s) will fall within the entire range that their pump casing will function, what else could they possibly write in their installation manual? As long as NPSHa > NPSHr, the OEM has no valid beef.

Even if it works, its not something I would want to do anyway, unless GREAT advantage were to be had, as process systems always tend to increase their flowrates eventually. Why make expansion at a later date so difficult, unless you could really save a tonne of money today. Possible, but probably not much is going to be saved by changing an 8" suction to a 6" inch, etc. But stranger things have happened, right? All I'm saying is that there could be a very real and valid reason as to why it was made that way.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Katmar -
I agree re shooting in the dark - but before we lay this to rest the point regarding suction lines should be clarified for those others who might be interested now or in the future.

The sizing of a pumps inlet pipework is solely a function on NPSHa / Npshr, OEM and text books don't ensure that the margin is correct.

However, common sense usually prevails and the same size or the next size line is normally used and in most cases sufficient- although in a case of a long inlet line even the next size up could be a problematic, NSPHa always needs to be checked.

 

[pumpking]

Most of what needs to be said has been, but there are a couple of orther issues that it could effect: The fluid 'could' be running at excessive velocity, so potentially causing excessive corrosion issues if handling chemical, or indeed, and more importantly increasing the static electrical charge generated which could lead to flashing if not properly engineered out.

Like BigInch, Artisi et al have said, it could well be that the system has been designed adequately to accept the smaller pipe, so we cannot comment too much, but the above are potential resultant factors...

...as well as that, you will give the manufacturers ample opportunity to reject any potential warranty claims that may come out of what they would deem to be "not a recomended installation" - Most IOM manuls would state pipework being ideally 1 size larger than the flange size !!

Ash Fenn

http://www.cdrpumps.co.uk