Vibration increase at only a narrow temperature band
Vibration increase at only a narrow temperature band
(OP)
I have a very unusual pumping system. It is a closed loop hot oil system, (Syltherm 800) being pumped by a Gould's 3620 API 610 between the bearing pump at 2975 rpm, 2250 m3/hr. The pump starts from ambient and is heated to 300 deg C. The flow is maintained constant. Is part of a heating/cooling system for a reactor so the manual valve are set and the flow is stable. The pump runs very well, with in API guidelines. The vibrations increase from 2 mm/s to 3.5 mm/s as the pump heat up to operating temperature. The problem is that as the temperature approaches 250 deg C the vibration starts increasing dramatically/exponentially peaking at over 10 mm/s at 255 deg C. As we move away from 255 degrees to a higher temperature to 265 deg the vibration falls back to the 3 to 4 mm/s range and stays lower up to 300 degC actually even improving as getting closer to 300 deg C. This is the first case I have ever seen a pump having such a dramatic increase in vibration at a narrow temperature band within the operating temperature range. I and many colleagues have been analyzing this problem for years and have eliminated may possible root causes. I am interested in others feedback, theories or similar experiences. The vibration is at vane pass frequency and the impeller is a six vane impeller. This being a double volute pumps with the slitter and cut water being 180 degrees apart can be problematic but not the root cause of this phenomenon.
Thanks,
Thanks,





RE: Vibration increase at only a narrow temperature band
1. Changes in support stiffness as the structure heats up. It may be that the thermal expansion provisions on the pump/baseplate and/or pipe loadings is changing with temperature. This might cause the overall structural stiffness to change such that the modal response coincides with vane pass frequency. You could verify this either through bump testing or through an ODS.
2. The fluid properties may be changing sufficiently such that the speed of sound changes and allows an acoustic resonance to occur. This can be checked by measuring the pressure pulsation profile in the suction and discharge with a high frequency (piezo) pressure transducer.
Perhaps you could list some of the root causes you've examined and discounted ?
RE: Vibration increase at only a narrow temperature band
1. NPSHa too low. Calculation work fine and the vibration improves even as the temperature and vapor temperatuer increases.
2. Impeller six vanes and cut water splitter. Have modifiec the cutwater splitter profile and reduced impeller size to 410mm from 416 mm. Max impeller is 432 mm.
3. Piping changes. Increased the piping inlet straight diameters to 5 diameters from 2. Outlet is 1.5 diameters.
4. We have done some analysis of the support and have plans to "beef this up" if you will.
5. A bump test for natural frequency was done early on and eliminated.
6 A CFD anlaysis on the piping flow was done.
It is interesting that your number 2 is still on our list of possible root causes but very difficult to analyze. Are you aware of anyone who does such an analysis? I would think this is envasive and would require connections/nozzles to be made in the piping? What can be done to remedy if proven so? Antinodes or pulsation dampeners? Who designs these? As you can understand this is why this still remains but is looking more like the reason.
Thank you very much for your response.
Regards,
RE: Vibration increase at only a narrow temperature band
If acoustic resonance is found to be the cause, the solution generally involves either changing piping and/or putting in a pulsation damper. Because you have high temperatures a pulsation damper wouldn't be my first choice. Changing pipework typically involves playing with the locations of pipe section changes which act as a reflective surface.
As a last resort you could always change the excitation magnitude and frequency. I'm sure the pump supplier could provide a split and staggered impeller with 5 or 7 vanes (most big pump suppliers have alternate impeller designs in their back pocket). You could also consider cutting back one volute lip to give you a 168 deg volute angle. I've found this can work in some instances.
RE: Vibration increase at only a narrow temperature band
I am currently in the USA but the problem still resides elsewhere.
Thanks,
RE: Vibration increase at only a narrow temperature band
I suppose one (expensive, and almost certainly impractical) way to check would be to change the heat transfer fluid to a different grade, and see if the problem tracks temperature, viscosity, or specific gravity. You'd need a lot of before and after data to pin that down.
RE: Vibration increase at only a narrow temperature band
Thanks,
http://turbolab.tamu.edu/proc/pumpproc/P24/Case%20...
RE: Vibration increase at only a narrow temperature band
Basically it is a checklist of what not to do...
http://turbolab.tamu.edu/proc/pumpproc/P27/Rotor%20Retrofits%20Improve%20Pump%20Station%20Vibration.pdf
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
The second time it happened the temperature range was narrower (175 to 185 F). We have not resolved this second instance. We are getting proposals for different vane counts. The first time taught us that it is much easier to implememnt the solution than it was to try and prove it.
Our accoustic resonance was ocurring inside the pump and was not detected in pressure pulsations within the piping.
Johnny Pellin
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
I'd presume that that the "inside the pump" acoustic resonance was for a BB3 long crossover ? These can be problematic on high stage counts with fluids other than water as the length is sufficient to allow standing waves in the vane pass frequency.
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
Johnny Pellin
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
Johnny Pellin
RE: Vibration increase at only a narrow temperature band
Thanks,
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
We have changed vane count many times, but have never stumbled upon another resonance. It always solved the problem. You could be unlucky. But it is not likely.
Johnny Pellin
RE: Vibration increase at only a narrow temperature band
RE: Vibration increase at only a narrow temperature band
Additionally I'd recommend you go direct to the Goulds aftermarket central engineering office in Glasgow UK. They might be more responsive to the request for a different vane count impeller.
JJPellin, suction recirculation would be unlikely to exhibit as a problem just in a narrow band temperature range. In addition according to the Goulds online selection tool, the 16x18-17SQ is operating at >107% of BEP at 2250 m3/hr.
RE: Vibration increase at only a narrow temperature band
Thanks,
RE: Vibration increase at only a narrow temperature band
Johnny Pellin
RE: Vibration increase at only a narrow temperature band
Thanks,
RE: Vibration increase at only a narrow temperature band
Normally when an impeller is machined or reduced in diameter, the outer diameter is machined parallel with the shaft, an alternate is to machine the outer diameter at an angle to the shaft - this can in effect change the blade pass frequency - not sure if there is an optimum angle and in this case it probably means starting with a new impeller as they have already reduced and you need recalculate the min - max diameter with the max being a larger diameter than probably what you already have.
The other thing to look at and consider if not already done is, as double suction impellers, I assume the impeller have a central shroud separating the flows from the 2 impeller eyes (inlets) and if so are the "blades" on each side staggered in relation to each other, and if not why not?
To be honest, I would have thought these points would have been looked at years ago - but you never know.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Vibration increase at only a narrow temperature band