Pump Suction Piping Design Problem 1

[JJPellin]

I have a piping designer that is proposing a piping change to a set of pumps that concerns me. The existing suction piping from the vessel is as follows:

There is an 8" nozzle off the very bottom of a fractionating column. The 8" line continues out of the skirt and tee's to go to two pumps in parallel. The individual suction lines continue as 8" through suction gate valves and butterfly remote isolation valves. The lines come down vertically and elbow at 8" and then reduce down (eccentric reducer, flat on top) to the 6" end suction nozzle on the pump.

They have a project to increase the flow considerably. The pump will have inadequate NPSH at the higher flow. In order to reduce the flow losses and the velocity in the suction line, the designer is proposing increasing the line size to 16" over the full length. At that point, the piping would have an 8" to 16" concentric reducer off the vessel nozzle. At the other end, the 16" line would continue through the valves and around the elbows. The eccentric reducer right at the pump suction would be a 16" by 6" reducer. I didn't even know that you could buy a reducer with that much difference between the line sizes. I am concerned about the turbulence from the reducer right at the pump suction. Since it is an end suction pump, the velocity profile coming into the eye of the impeller would probably be very non-uniform.

Does anyone have any suggestions of a better way? I don't like the proposed piping arrangement but am have having a hard time coming up with convincing arguments that it is a bad idea. Any help will be greatly appreciated.

 

[abeltio]

consider adding a vortex breaker inside the vessel and also check the NEMA standars for the nozzle loading.

the 16" pipe will create a lot of mechanical concerns... i've seen the pipe lifting the pump from its base and damaging seals and bearings... mechanical design must be very careful... whereas the 8" pipe could have a natural flexibility... the 16" will be so stiff that it will need much bigger spans to absorb the thermal expansion.

from the flow point of view the specific suction speed (Ns) should be below 7000, as recommended by various authors (Karassik, Yedidiah, Stepanoff)calculated in standard (gallon/feet) units.

HTH




saludos.
a.

 

[CRG]

How long is the run between the bottom of the fractionating column and the pump inlet? If it is a long run between the two, there may be justification for the design. You could use straightening vanes upstream of the pump if required. You are correct in that a 16X8 is not a standard fitting.

Are there other considerations for future expansion of your facility that may have resulted in the decision to use a 16 inch line?

You stated that, “I don't like the proposed piping arrangement but am have having a hard time coming up with convincing arguments that it is a bad idea.” Well for engineers there is no better argument than numerical analysis of the system illustrating your concerns. Have you made a hydraulic analysis of the system to justify your concerns? Without knowing a lot of details such as flow, fluid properties at design temp, length of run, number of bends, and other restrictions, it is difficult to make a comment. If you take the time to make a detailed hydraulic analysis of the system with all of the design data, most of your questions should be answered by the inspection of your own calculations.

 

[zdas04]

I'm having difficulty visualizing a scenario where I would need to feed a pair of pumps that have 6-inch inlets with 16-inch pipe. Is it a REALLY long distance? Does the fluid have really nasty flow characteristics in shear? Is it really viscous?

CRG has the right approach. Do the arithmetic. Look at bulk velocities in each segment of pipe. I've never been in a facility that didn't have an upper design-limit on fluid velocity, does the 8-inch approach your facility design limit? If any of it is boarderline then I'd run it into a simulator and see what the high-dollar stuff says to do.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[NozzleTwister]

JJPellin,

You are right on target to be concerned with this arrangement. You don't have a snowball's chance in &*%$ meeting the pump allowable nozzle loads. Swaging up to a 16" line from an 8" nozzle is ridiculous as is a 16" by 6" reducer on a pump nozzle. If your Piping Designer doesn't agree, then you need to replace him with an experienced Piping Designer.

Who's doing the flow calc's? You didn't mention a Process Engineer. I assume that all of your valves are going to be line size and that you WILL NOT have butterfly valves in the new arrangement.

After the Process Engineer sharpens his pencil to determine your real needs, I expect that you will have to replace your fractionator bottoms nozzle with one that is the required line size, the entire nozzle from the bottom head through the skirt penetration.

With these restrictions corrected, hopefully you can use a more reasonable line size. Also, do both pumps operate at the same time? Is the main line sized for the flow of both pumps simultaneously? Talk to your Process Engineer, you may be able to reduce down a line size after the tee where you split to each pump.

I also question whether the existing pumps are adequate under any circumstances.

As far as the great reduction at your reducers, it's common to put two reducers back to back for this type of situation.

BTW, I'm a Pipe Stress Engineer and although yours is the worst that I've encountered, I've seen similar situations on numerous retrofits.

Bottom line; spend the money now to do the job right and you'll just have to do the job once.

Good luck,


NozzleTwister
Houston, Texas

 

[JJPellin]

Thank you all for the valuable responses. I will add a few details that may shed additional light. The pumps in question both run in parallel at all times. After the rerate, both will be running near the end of curve continuously. There is a butterfly valve on the suction. The pumps are very close to the fractionator. In a fire situation, it would be impossible for the operators to reach any of the valves. So they added remote shut-off valves in the suction to each pump so that the entire contents of the fractionator would not continue to feed a fire. The block valves are gates, but because of the tight space, the remote isolation valves are butterfly.

The larger line is required for a few reasons. First, inadequate NPSH at the higher flow. Second, the existing line would exceed maximum allowable flow velocities. The pump Nss is already 11,000 (US units). I don't want to go to a lower NPSH required impeller which would drive the Nss even higher. I can't raise the vessel and I can't lower the pumps. Any way this plays out, it looks like trouble. I am going to push to have them replace the nozzle on the vessel ($100,000+) and replace both pumps with between bearings, double suction impellers ($350,000+). I may get the nozzle, but probably won't get new pumps. I am going to ask to have them move the remote isolation valve to the common line feeding both pumps and then drop the line to about 12 inches after it splits to the two pumps. I will have to redefine the minimum level in the vessel to take advantage of the few feet of extra head there. And in the end, I probably still won't be able to meet the 3 foot NPSH margin that our spec's call for. The project is still in Phase I. They will be performing all the hydraulic modeling and completing all the calculations next. This should provide additional justification to roll in the extra elements I am asking for.

 

[DesignerMike]

I'm not familiar with a fractioning column, but I like to keep it simple if possible. Feeding two pumps from a single suction line is more complicated due to the turbulance of tees and different flow conditions to each pump (and what happens if one pump is down).

Would you be money ahead to simply add another 8" nozzle to the bottom of the vessel and have a streamlined inlet to each pump with a smaller set of valves?

There are a lot of factors involved, and just wanted to throw out another option.

 

[NozzleTwister]

JJPellin,

After your second post I'm confident that you are headed the right direction.

I'm still puzzled about the butterfly valves. I have to think that they will only contribute to the NPSH problem. If you need a quarter-turn valve, you might consider a ball valve.

You mentioned using the butterfly because of being tight on space, with the space that tight do you have the pipe flexibility between the pumps to meet the pump nozzle allowables? With your increased pipe size, I doubt you'll be able to keep the same pipe routing as before. Assuming that you’re operating at 700 deg. F or more, I can see your Stress Engineer wanting the loop those pump lines around the fractionator to get the flexibility he needs.

If you have the real estate you might consider relocating the pumps as an option if the space is two tight, especially if you wind up replacing the pumps. Also, re-consider raising the fractionator elevation. I've been on several jobs where tower have been raised by adding a piece to the skirt or the shell and modifying the piping as required.

Get your Pipe Stress studies started as early as possible, extra pipe and elbows required for flexibility will certainly have an impact on your final line sizes.

Good Luck and Happy Thanksgiving!


NozzleTwister
Houston, Texas

 

[itdepends]

I'd recommend you get a hydraulic calculation performed on the proposed upgrade. With such a large reduction ratio and the "hihg" velocities you mentioned you get significant pressure losses at the vessel exit and the pump reducers.

The fact that you're running the pumps towards the end of the curves doesn't help either. The NPSH requirements start to step up sharply. I've seen a scenario with pumps cavitating where the main problem was that they were operating to the far right of the curve with a high NPSH requirement and something like 50% of the suction piping pressure losses was in the final reducer prior to the pump.

The suction piping didn't need an upgrade in that case- just new pumps (about the same size wrt impellor but larger suction/discharge connections).

Cheers

 

[StressGuy]

You're getting lots of good info from my colleauges. As a stress engineer myself, I cannot over emphasize the serious issues you are looking at in increasing the line size from 8" to 16" in terms of the abuse you are going to heap upon the pump nozzles.

You are also correct to be concerned about having that big reduction at the pump nozzle. You want to take a look at the document from the American Petroleum Institute (API) called RP-686 which governs the design and installation rotating equipment and the piping associated with it. Chapter 6 addresses piping systems.

Paragraph 3.1.2.6 says "The pump suction line shall have a straight run (typically five pipe diameters) between the suction flange and the first elbow, tee, valve, REDUCER, permanent strainer, or other obstruction sufficient to ensure stable and uniform flow at the pump suction nozzle."

Emphasis mine regarding the reducer. Common practice forever has been to put the reducer at the pump nozzle. According to API, this is not correct. The five diameters is based on the pump nozzle size and is a minimum. I've dealt with cases where the pump vendor has specified 10 diameters straight run.

Any company doing design of piping systems should have a copy of this standard in their library.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[rsv]

I would agree with all of above.
Installing butterfly valves in pump suction lines is an absolute no no due to turbulence and pressure drop across valve - to reduce pressure drop in line reduce number of bends & reduce number of valves, try and automate the gate valves & get rid of b/fly valves.

 

[prabir]

JJPELLIN,

I have two suggestions:

1)If hydraulic analysis,piping layout and piping flexibility are all favorable, then you can use 16" size with reducers of 16x10 and 10x6 welded together at pump suction but maintaining a minimum distance of 5D( 30 inch)from pump suction nozzle. To reduce turbulance at suction you can use flow straightner.I presume that pump is capable of handling higher flow rate.
2)Introduce another nozzle of 8 inch size at the bottom of the fractionator column, if possible, and run individual line to each pump.

Prabir