Vibration increase at only a narrow temperature band 1

[Nick22]

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,

 

[Nick22]

Thanks. My feeling is the pumping system design should deliver what the process demands at the greatest efficiency not the other way. The tail should not wag the dog. Little confuse how do you change the specific speed without change the impeller?

 

[GPRnD]

Nick, just for the sake of clarity when you quote an Nss of 12,000 is that in US units (USGPM,ft) or metric (m3/hr, m) ?

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.

 

[Nick22]

Thanks good suggestion. The units are metric m3/hr,m. Yes corret at about 107%. Time for bed now. Get up at 4:30. Talk more later I hope. We have work to do and gather more data.
Thanks,

 

[JJPellin]

Sorry. I misinterpreted the Nss based on assumed units. Even above BEP, you can have recirculation. The example I mentioned runs near BEP. Changing Nss would require a new impeller. At that point, you would be wise to change vane count. But, detuning a bearing bracket might solve your problem.

Johnny Pellin

 

[Nick22]

We did run at BEP for a time in 2013 and still had the problem. The average vibration was lower but the peak still happened in the temperature band. Lots of great suggestion and more to investigate. You are wise and experienced. I appreciate all you have contributed. We will keep in touch and I will let you know our findings for all to share.
Thanks,

 

[Artisi]

Really off-chance comments for consideration and discussion.

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.)

 

[1gibson]

Good call to check the staggered vanes on both sides. Despite what the drawings may say, folks at the foundry may decide that it looks better when it is symmetric, so they "fix" it for you.