vapor pressure and dissolved gasses
vapor pressure and dissolved gasses
(OP)
Does anyone know if it is standard practice to include the effect that dissolved gasses has on the vapor pressure of a fluid? All that I have been able to find is a specification sheet for potassium carbonate at one concentration and temperature. I know that we have been having problems with not enough NPSH, but the cavitation damage on the impellars has significantly accelerated over the past failure rate and we are trying to track down why.
I am currently having a failed impeller metallurgially analyzed and am going to have a brand new impeller checked for casting imperfections. Does anyone know if impeller surface roughness may contribute to increased cavitation damage (more nucleation sites?)?
I am leaning towards thinking that we are either getting bad impeller castings, or the amount of dissolved gasses in the potassium carbonate is higher than in the past. Let me know if anyone has any thoughts.
thanks!
I am currently having a failed impeller metallurgially analyzed and am going to have a brand new impeller checked for casting imperfections. Does anyone know if impeller surface roughness may contribute to increased cavitation damage (more nucleation sites?)?
I am leaning towards thinking that we are either getting bad impeller castings, or the amount of dissolved gasses in the potassium carbonate is higher than in the past. Let me know if anyone has any thoughts.
thanks!





RE: vapor pressure and dissolved gasses
I'd think dissolved gas would not be responsible for pushing you over the cavitation limits and on into the severe area. You've got other problems that I'll just bet you can attribute to typical causes, small diameters, low tank pressures, pump elevations too high in relation to fluid surface levels and hot fluids. I don't think potassium carbonate has an overly high vapor pressure at normal temperatures.
"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization
RE: vapor pressure and dissolved gasses
We are out on the right side of the curve and to reduce discharge recirculation we have tightened the clearance on the wear rings and changed the sacraficial ring to PEEK. We are still seeing the classical signs of cavitation from the failed impellers and also on the vibration data. The only thing that is strange with the vibration data are some sidebands at turning speed that we cannot identify.
This has been a problem pump for the last 40 years with the impellers being replaced approximately every 1-1.5 years. Over the past year we have been having impeller failures about once every 3 months and I am trying to find out what has changed. I am also pursuing replacing the pump with a slower rpm multistage pump, but I don't know if the Life Cycle cost will work out. There will not be an opportunity for a couple years anyway.
RE: vapor pressure and dissolved gasses
"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization
RE: vapor pressure and dissolved gasses
You say that impellers are changed 1 - 5 years, what happens in the 1 year period compared to the 5 year period -must be a story there somewhere?
Are flow rates, power inputs etc monitored - this can also give you a lead into what is happening. Is pump thru-put the same now as over the past 40 years.
The unbalanced cavitation on the inboard side of the impeller also needs investigation- has this always been the case or has something been charged recently with the pipe work close to the pump inlet, ie, new valve fitted etc.
RE: vapor pressure and dissolved gasses
RE: vapor pressure and dissolved gasses
RE: vapor pressure and dissolved gasses
RE: vapor pressure and dissolved gasses
RE: vapor pressure and dissolved gasses
RE: vapor pressure and dissolved gasses
http://www.discflo.com/autoslct.htm
RE: vapor pressure and dissolved gasses
To me, the damage looks to be erosion and could ever be erosion/corrosion - and repeating myself, I do not think it is predominately cavitation.
What material are the impellers, are they OEM or pirate parts, if pirate parts -- have you changed supplier recently.
In my first reply I suggested not to have the impellers metallurgically analysed - seems I will now change my mind and suggest that this could be a good idea to see if the material is to spec. etc.
A closeup pic. of the damage would be helpful - this will allow us to zoom in and not lose definition.
RE: vapor pressure and dissolved gasses
There appears to be damage localized to one side of the rotor on the inboard side, does the damage on the outboard side happen in the same clocking location (same side of the rotor)? Dunno why that would happen, but I can envision sludge buildup when pump is stopped...or a gas bubble...either case adding another element to the corrosion/erosion concerns noted by Artisi. Or, it could just be a red herring caused by variation in the material of the rotor. In any case, the uneven damage could certainly cause unbalance on the rotor, and would likely explain the vibration numbers you say you are seeing.
RE: vapor pressure and dissolved gasses
btrueblood, the damage is focused more on the inboard end, but the location on both the inboard and outboard end is the same. This has caused us unbalance and that is what is making us change the impellers right now, instead of the lack of a lack of flow a little later. The sludge buildup is not a problem because this appears on the pump during continuous operation.
RE: vapor pressure and dissolved gasses
As for gas entrainment, if not excessive and causing vapour locking within the pump can in fact be benifical with regards to cavition as it tends to soften the implosive damage on the impeller surface. An old "field-trick" for a cavitating pump is to introduce some air into the inlet to reduce the noise/damage.
I miss read your first posting re the time span for impeller changes, disregard my comment of 5 years.
RE: vapor pressure and dissolved gasses
RE: vapor pressure and dissolved gasses
The problem might be the result of either under or over capacity resulting in mismatch of flow into the impeller which can result in damage similar to insufficient NPSHa..
Question for consideration - the lastest pic you posted certainly looks like cavitation -but is it the result of flow mis-match.
As for erosion. I have cut a section of one of your pics - if you look closely at the left hand of the pic you can see the signs of erosion on the outer area of the impeller eye.
Lets assume that it is typical suction cavitation - the question is then - why? As stated above you need to investigate the pump performance in service compared to the original design / pump capabilty.
RE: vapor pressure and dissolved gasses
We know that this pump is not in a suitable application, but the reason why we have all of a sudden seen decreased impeller life is what we are trying to figure out and correct. We are sending out the fluid to get analyzed. Since we are so close on the NPSHr and NPSHa it can be something as simple as the specific gravity has changed slightly. Also, there may be a slight increase in suspended solids. We measured that yesterday and got something like 2mg/L on a 1.5micron filter.