High Pockets on Pump Suction Line 2

[tkdhwjd]

Dear Collegues,

Will there be any issues for allowing high pockets on pump suction line provided that:
1) fluid is heavy hydrocarbon - no flashing at high points
2) NPSH_available is greater than NPSH_required

Your feedbacks will be greatly appreciated.

 

[JJPellin]

You will probably get some widely different opinions on this one. Many engineers believe that high points are a problem. In 1984, we built a coker unit. The 3500 HP jet water pump had a suction line that went up about 15 feet from the pump, over about 15 feet and then down to an underground route back to the water tank. The pump had immediate problems with cavitation. The first stage impeller was destroyed within 6 months even though the NPSH margin was quite good (40+ feet available, 27 feet required). An outside consultant came in and declared that the high point in the suction line was the problem. At their advice, we added a suction vessel in the pump house with a tall stack open to the atmosphere. But the cavitation problem continued. Eventually, we replaced the first stage impeller with one that had a higher NPSH required (31 feet). This reduced the NPSH margin, but also reduced the suction specific speed. The cavitation problem was not occurring at full flow. It was occurring at low flow when the pump was running in spill-back to the tank. With the new impeller, we have not seen any cavitation damage in the first stage impeller and the pump has run well for 20 years.

To make a long story short (too late), a high point vapor trap is a common villain. Many engineers believe it is always a bad idea and will always be a problem. But that is not necessary the case. I believe that if you have adequate NPSH to keep from vaporizing product, and if you vent the high point well when the pump is first flooded, you may have no negative affects at all.

 

[BigInch]

That's two opinions right there.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Or is it really one?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JJPellin]

BigInch,

I am not sure if I understand your comments. Perhaps I did not make my point clearly. Eliminating the high point vapor trap did nothing to solve our problem. Reducing the Nss was the correct solution. Since that time, we actually eliminated the suction vessel and went back to the original piping configuration and have seen no negative consequences.

If a high point can be easily avoided, it is a good practice to do so. But the presence of a high point will not necessarily cause any problems. Without futher details, I cannot be more specific than that.

I will try to be more clear in the future.

JJ Pellin

 

[pennpiper]

tkdhwjd,
You did not indicate whether this is an existing installation or one that is being planned.
My opinion:
1. If existing, then it sounds like you are stuck with what you have got.

2. If it is still in the planning/design stage, I give this advice. "WHEN IN DOUBT, DON'T". Don't allow any high pockets. Either make the pump suction line step down from the source to the pump or make the pump suction line step up from the source to the pump. This goes for any pump and any commodity.

 

[BigInch]

JJ, sorry for being so obtuse.

I understand there are +/-s and its good to discuss them both. I was trying to agree with the bulk of your comment, that there seems to be a heck of a lota' problems that crop up whenever you try to do this, and even though they might be solved, there's no guarantee the system will operate successfully. So, even though it can be done, it sure doesn't sound like a good idea.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

JJPellin an interesting post.
It is interesting that the problem of “cavitation” you experienced was at low flow and not at the designed operating point and that you are now back to the original pipework design which seems to indicate that there wasn't an NPSHa/NPSHr problem at the design flow or poor pipework design.

My reading of this is that at part flow condition you were experiencing poor entry flow into the impeller which can sound like cavitation and could well be as damaging. The change of impeller to one of different NPSHr would be in fact a different design, - maybe with a different number or different vane angle or both which was more tolerant to operating at the part flow condition.
However, it overcame your problem which was a good result irrespective of the hydraulics involved.


Phitsanulok
Thailand

 

[JJPellin]

This is one of my favorite troubleshooting examples for a few reasons. First it shows that simply focusing on NPSH margin alone can get you in trouble. We are purchasing some pumps right now that concern me in this same regard. The engineering company subtracts a two foot margin from all NPSH(a) numbers. The pumps manufacturer allows another two to five foot margin. The NPSH(a) is based on a liquid level that is at the bottom nozzle of the vessel which may be 4 to 6 feet below the actual working liquid level. We may end up with a pump with an NPSH margin of 10 to 15 feet. But we also end up with a much higher suction specific speed that we would have gotten otherwise. If similar allowances were made on the required flow rate, we could end up with a high Nss pump running well below Best Efficiency Point flow which can lead to suction recirculation cavitation.

The other reason I like this example is that it slows the problems of using a high dollar consultant to solve a problem. In this and other instances, the consultant came up with a complex and expensive solution that not only did not solve our problem, but it created several other very serious problems.

 

[BigInch]

That sounds like it was a poor configuration for a poorly specified operation range. More thought should probably have gone into the actual required ranges of operation and developing an optimum configuration that would have meet the full extent of the required ranges of duty.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JRLAKE]

As a pump sales engineer I love to see suction lines like the one you describe. It leads to a lot of mechanical seal business.

With that said, I agree with Pennpiper. Avoid complicated suction lines. You run the risk of dry running and cavitation. There are ways to make it work but it is always best to keep it simple.

 

[Artisi]

The example you are describing is fairly typical of an engineered system where everyone is isolated from each other without any consultation between each phase of the design /selection. Everyone adds a margin "just in case", this starts with the process guys, then the plant engineer, then the sales engineer, the applications engineer, and the safety margin usually inherent in the manufacturers data - and finally - a bit extra on the impeller diameter just in case.

NPSH - having a large NPSHa/r to me does not represent a problem excepting as you have pointed out it might very well put you into a much larger pump than necessary with associated problems.

Phitsanulok
Thailand

 

[insult2injury]

JJPellin's problems could have been avoided with a properly written spec when it went out for bids. If the Nss requirements are properly defined, you should be able to get a pump to meet them. Pump manufacturers will always try to push the limit on the Nss requirements. They almost always try to offer a high rpm pump. Higher rpm means a smaller pump and hence less money making them the low bidder. However, more often than not, if a proper evaluation of the maintenance and operating costs is performed, the larger, higher initial cost, slower speed pump almost always ends up being the less total dollar figure. There are always exceptions like when the manufacturer tries to jack up his price but thats what the procurement/commercial guys are for.



I2I

 

[Artisi]

A properly written spec. doesn't necessarily relate to having the right pump/s duty specified especially if everyone has added their own little safety factor "just in case".

Having been on both sides of the fence as well as in the middle, I have found the only way to really get down to what is needed is to meet with all parties who had input into the system design and define exactly what is needed and what margin of "just in case" needs to be applied. Only then can the manufacturer make an informed decision and demonstrate why they are making that decision - this also overcomes the second guessing and helps the supplier and purchaser ensure a correct outcome for the project.

Phitsanulok
Thailand

 

[ccfowler]

Artisi,

Well stated! The cumulative effects of multiple "just in case" margins often seems to be a major factor in operating and maintenance problems in pump and piping systems.

Reliance on specifications can provide a very false sense of security. If the specifications are not very simply and clearly written, they can cause more problems than they avoid due to misundertandings or variations in interpretations.

A "star" for you!

tkdhwjd,

Regarding the high pockets in a suction line, unless there is some compelling physical constraint that forces their existence, I can't imagine why all possible efforts would not be made to avoid them. Why invite trouble?

Regarding NPSHa being greater than NPSHr, what problem? Within bounds of practicality, the greater NPSHa vs. NPSHr the better. Where NPSHa = NPSHr, the pump is already suffering performance deficiencies due to cavitation!

 

[insult2injury]

It shouldn't matter how many "just in case margins" are added for the NPSH as long as the Nss of the pump is held to an appropriate level which is what a well written spec should always convey. What is the point in writing a spec if you aren't going to hold the manufacturer to it?

I2I

 

[BigInch]

ccfowler, I think you can pickup a star for bring up what may be the key to this entire thread.

Where NPSHa = NPSHr, the pump is already suffering performance deficiencies due to cavitation!

Much recent evidence has pointed to the fact that there can be significant impreceptible cavitation that begins well above the normal NPSHR figures. NPSHR is not one of the variables you want to peg.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[pennpiper]

tkdhwjd,
I am suprised at the responses you have gotten to a very simple question.

"Will there be any issues for allowing high pockets on pump suction line provided that:
1) fluid is heavy hydrocarbon - no flashing at high points
2) NPSH_available is greater than NPSH_required"

The way I see it you are asking first about the pump suction piping. You did give us important data that relates to that specific question. That was good.
My suprise is at the responses that go so far afield even to suggesting that writing a better pump spec will prevent high-point problems in a pump suction line.

I stand with my original recommendation:
Don't allow high points in pump suction lines.

 

[BigInch]

I do believe that IS rule #1 for pump piping. We even turn the reducers "flat side up" to prevent them.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[MikeHalloran]

One time I worked on a system that suffered from safety factor stackup.

A pump on the ground floor was supposed to pump paper stock up and into the bottom of an open tank on the third floor.

The plant's engineer kind of overdid it, and bought a pump that was too big, by a big factor.

On startup, the paper stock squirted out of the tank entry with enough velocity to reach the third floor's high ceiling, and splatter, everywhere but into the tank.

We had to throttle the pump's outlet rather severely, which wastes energy, and doesn't do paper stock any good either.



Mike Halloran
Pembroke Pines, FL, USA