Pumped fluid effect on rotordynamics of API BB3 pump 2

[NGLENGR]

I have six United 4x11 MSN 10-stage pumps running a variety of light hydrocarbon products, ranging from ethane down to gasoline. The most common product is "Y-grade" which has ~45% ethane, ~25% propane, ~18% butanes, and the balance is natural gasolines.

I've found that these pumps are operated across a wide flow range (just below MCSF to beyond published EOC) and a range of product gravity, composition, viscosity, etc. They seem to have difficulty in Y-grade service, particularly when running a higher ethane content (lower product gravity and viscosity). The pumps' vibration has a stronger correlation to the product composition than to total flow; vibration is dominated by vane pass frequency (7x) with significant 1x presence.

Does anyone know enough about pump rotordynamics to explain how to model the damping effect of different products? I would like to explain to the area manager why his pumps have short life other than "They can't pump Y-grade."

 

[1gibson]

Is there a thrust balance line, maybe not sized for the lower SG fluids?

Seal problems?

 

[BigInch]

The wide range of flow probably isn't helping.

Does it correlate with higher product vapor pressures?

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[NGLENGR]

1gibson,

There is a balance line, but it's not sized for any specific product. (Assumption is propane.) The suction pressure can vary from 100 psig to 700 psig, depending on the product.

Typically, the seals don't have problems (~6 month MTBR.) They are Crane 8B1 liquid face design.

BigInch,

I agree, but that isn't changing since the operational needs come before equipment health. It does correlate with increasing VP for the products. I have convinced the manager to sign off on adding a density measurement device on the suction of one of the pumps to try and improve our suction pressure control. I believe some of the problem is that the increasing ethane content can make the pumps run into cavitation even when running inside the curve.

I'm still waiting for the work to get done, but it should help prove out the theory.

 

[1gibson]

High suction pressure and lighter fluid will both reduce the pump thrust. Assume the problem product is closer to the 700 psi range of suction pressure?

I don't think the fluid itself (viscosity, or SG alone) will be to blame. Secondary characteristics like higher vapor pressure (therefore suction pressure) and reduced thrust would be the first things to check.

Maybe there is axial shuttling causing the vibration?
Modifying the balance line may be a solution, would obviously want the OEM's input before playing with it.

"You might just have a duplicate project in the works if the OEM can help you solve this issue." Feed that line to a salesman and you should get some free support.

 

[1gibson]

For some reason I can't see where to edit that last post.

It may not even be shuttling, a large change in thrust could change the rotor position slightly, causing impellers to exit partially onto the diffuser vanes, increasing radial loads and causing vane pass vibration.

 

[JJPellin]

I don’t think that this is rotor-dynamic. A rotor this long is not likely to have any natural frequency modes as high as 7-times run speed.

I have seen this problem several times. The two best examples that come to mind had different causes. The most recent one was a new pump that had chronic vibration at vane pass frequency but only when the product temperature increased above 325 °F. While we were attempting to figure out the problem, we found that we had the same vibration if the product specific gravity dropped below 0.825 even if the temperature was lower. It took a lot of work to prove that the problem was acoustic resonance in the long cross-over channel between the third and fourth stages (six stage pump). The vibration was highest on the outboard bearing housing in the vertical and axial directions. We found that the bearing housing had a natural frequency that was very close to the same vane pass frequency. We remachined the mounting surface for the outboard housing to detune it and changed the number of vanes in the third stage impeller and the problem went away completely.

The other pump that had high vane pass frequency vibration was a big barrel pump (United – 10 stage). In this one, the vibration showed up only at low flow rates. It ended up being a bearing housings resonance (inboard housing in the horizontal direction) that was being excited by suction recirculation cavitation.

Based on your description, I suspect acoustic resonance. The resonance will be excited any time that the speed of sound within the pumped product lines up with the proper wave length multiple in the long cross-over channel. The speed of sound is difficult to quantify for most products. There are very few labs that can test for speed of sound at full pressure and temperature. It can be closely correlated to the bulk modulus of the product at pumping conditions.

But, I would still check for bearing housing resonance. Have the bearing housings tested for natural frequency in both vertical and horizontal direction. If you have a resonance that is close to vane pass frequency, you can detune it by reducing the mounting surface area by adding a couple of thin shims between the housing and the bearing bracket. This might reduce the response enough that you might not even need to fully eliminate the forcing function (acoustic resonance or suction recirculation cavitation).

Sorry I don’t have time to give more details. I have to run. I may be able to elaborate more tomorrow.


Johnny Pellin

 

[NGLENGR]

JJ,

You're probably right. I have looked into this problem on similar units, and the crossover definitely contributes to vibration on the outboard bearing. I had once collected pressure data which indicated approximately 200 psi pk-pk pulsation in the crossover of similar pumps in similar service. This may be the problem with certain mixes of hydrocarbons.

I'll see if we can check the bearing housings, but I doubt they're contributing as much as the fluid itself is.

1gibson,

I hadn't thought too much about axial thrusting, but that's definitely something worth checking. I have seen that pop up on pumps with heavy wear on the rings, so maybe this pump is exhibiting the same behavior.

Thank you gentlemen for your input.

 

[JJPellin]

Don't rule out the bearing housings until you test. We only have four large (1000 HP and larger) United pumps and all of them have had bearing housing resonance problems. And all of them showed up at vane pass frequency. We had one other large multi-stage pump with chronic high vibration at vane pass. It only presented itself when the suction pressure was lower than average. Some assumed it was simple cavitation. But, calculations showed NPSH margins of 70 to 100 feet. It ended up being a double suction first stage impeller with the wear ring on one side failed. The flow imbalance between the two impeller eyes was the cause. In that case, I think it might have showed up axially as 1gibson suggests. Is your first stage impeller single or double suction?

Johnny Pellin

 

[NGLENGR]

JJ,

Will do. I have seen that kind of bearing housing resonance in a 12-stage Sulzer pump, so it's definitely possible to have it in these 10-stage Uniteds.

All of these particular pumps are single suction.

Last night the operators switched to high purity propane and the vibration dropped back to well within acceptable levels. I suspect the fluid acoustics are more at fault.

 

[dabluffrat]

Acoustic resonance in the cross over can be reduced, if not eliminated, by introducing a different vane # impeller that feeds the crossover. i.e. if the pump has all 7 vane impellers, you may have one 5 vane impeller that feeds the crossover. This is the standard for most OEM's on BB3's.

Also look to see if the lighter SG product could be flashing. Operating back on the curve, to left of BEP results in increased radial loading and uneven flow distribution through the eye of the first stage impeller. At low flow conditions, the efficiency of the machine also is greatly reduced. This results in a greater portion of the input energy into the pump contributing to temperature rise of the product. Flashing will significantly affect the thrust loading on the shaft and the radial stability of the rotor.

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