Straight length suction pipe 4

[SENGUTTUVAN]

The suction piping straight length ( after the first elbow) for one of the Between bearing pump ( suction and discharge nozzles - Top),is only 2D. We are not able to increase further . The pumpflow is 1060 m3/hr , head - 320 metres and the temperature of the hydrocarbon fluid is 338 deg C. My query is , whether the straight length requirement is important for horizontal suction piping or applicable for vertical suction also? Whether this minimum straight length requirement of 2D will affect the pump performance badly and lead to any other problems ? Any other way to overcome the problem? The size of the suction nozzle is 16 Inch.

 

[BigInch]

The recommended minimum is 10 diameters. Anything less involves some risk of turbulence entering the pump, especially when less than 5 Diameters. Straightening vanes might be a solution, but you can't fit those in 2 diameters. Straignt length is important for any orientation of suction piping.

With those temperatures you may have NPSH problems already and the 2D certainly won't help.

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"Pumping accounts for 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/

 

[pennpiper]

With a top suction-top discharge pump I do not think the 10D rule (or any other "D" rule) applies or is effective. So stick with what you have.

 

[JJPellin]

I would add one comment to the above. If the pump is a single stage pump with a double suction impeller then the direction of that first elbow may be more important that the straight run upstream of the elbow. If the elbow turns the line to be parallel to the pump shaft, then it can direct uneven flow to the two impeller eyes. If the elbow turns the flow perpendicular to the pump shaft, then it may introduce turbulence, but at least the turbulence is largely symmetrical to both impeller eyes. Uneven flow to the two impeller eyes can result in high thrust loads, thrust shuttling or suction recirculation cavitation in the eye with the lower flow. A straight run of piping is always desirable. But there are no certain rules about how long is necessary to avoid any problem. If you have a higher NPSH margin, you might be able to run with less of a problem. If you have a very high suction energy, you might be more sensitive to suction piping configuration.

Overall, I agree with BigInch. You should always try to allow 5 diameters of straight run. And if you can get 10 diameters without too much trouble or expense, take it.


Johnny Pellin

 

[SENGUTTUVAN]

Thanks to all of you for keep replying. This pump is for the Fractionator bottom pump to be used in the Refinery plantThe pump is a two stage pump , with first stage as Double suction impeller and the second stage is single. JJ , I will look into the direction of the elbow. I can have max of 2.5D. Since this pump is a turbine driven , and lube oil console to be kept in the floor level I cannot bring it down further to increase the straight length.
JJ & Biginch, The NPSH margin avl is only 0.6 metres. As pennpiper was indicating , does this vertical suction have an advantageous role to play?

 

[Artisi]

My view on the top suction is that pipe length before the impeller / inlet flange is still important as is flow straightening. Do a search on flow straightening methods - a long past memory is a flow straightening cone arrangement prior to the inlet - all too hazy though to give any definite info. on this.

 

[JJPellin]

There is a particular flow conditioning vane arrangement that might help in your situation. I have never used one but am looking at a coupling of possible installations. It is called a Cheng Rotation Vane and serves the purpose of directing the flow around an elbow without the pressure drop, turbulence and uneven velocity distribution that would normally occur. I have attached a link.

Johnny Pellin

 

[pennpiper]

I see no logical reason to spending a lot of money and extra effort trying to keep the flow line straight before the pump suction flange and then after the pump suction flange the route to the eye of the impeller is akin to a cork screw.
Makes no sense to me.

 

[BigInch]

The pump assumes its not getting any prerotation of the fluid, esp in the opposite direction, which the pump would have to reverse, and if energy is being expended on rotation, or forcing other changes in direction, its not available in the form of suction head. It also would need to have a uniform bite across the impeller, so if fluid tends to load up on one side of the impeller first and lightly load the other side, efficiency is lost.

**********************
"Pumping accounts for 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]

pennpiper, that is an onerous assumption as highly disturbed and uneven flow at the pump inlet can only get worse, never better prior at the impeller eye. Having a smooth and even flow pattern at the inlet means that the impeller should see reasonable flow onto the blades - as it is hoped that the pump designer knew what he was doing during the design stage.
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Johnny Pellin's comments re bends prior to a pump should be carefully read and understood as it is important point and the root cause of many pump inlet problems in the field.

So far as the number of pipe diameters before a pump inlet flange for the pipe length there are no rules, in some cases 2 is OK in others 8 is insufficient - a lot of the problems are introduced well before this point and are carried thru into the pump.

 

[turbodare]

You need at least 2D of straight pipeline (for nq<50). To be on safe side instal one or two ribs in the elbow. The end of the rib has to be sharpened in non symmetrically way at the outlet in order to prevent creation of any vortices (so called »Karman vortex street«).