|
Pump Forces/Impeller Damage
|
|
Source：Internet Author：Unknow Pubdate：2008-04-15
|
|
gamebusta (Mechanical)
5 Dec 07 21:58
What type of thrust forces would develop if a submersible pump that was built to discharge to a 4-8 inch line was suddenly opened up to a 24 inch line? I know the flow would shoot way to the right of the pump curve, but would the force of this new flow slam the impeller into the wear plate? The impeller in the attachment came from a pump wear this occured. The pictures show the suction side. I am trying to learn how thrust forces work depending on flow. Would the huge increase in flow have been able to destroy the the bearings on the shaft?
- http://files.engineering.com/getfile.aspx?folder=21ed08c8-d419-4095-97ee-6f
Artisi (Mechanical)
6 Dec 07 0:52
First up, pumps are designed to pump X flow at Y head, they are not designed to pump on any particular pipe size, the head is a function of the vertical distance and the pipe line friction losses to give you the total head at which the pump will deliver X flow. 字串5
You need to establish the failure mode of this damage as operating out on the curve is unlikely to cause the impeller and wear plate to clash.
A likely failure would be - the pump was running way out on its curve causing cavitation - this lead to high destrucive loads on the bearings which in turned failed - the impeller and wear plate when started to clash giving you the damage as shown in your pic.
A few more pics of the impeller and more of the wear plate might give us a better handle on it as well.
Artisi (Mechanical)
6 Dec 07 0:58
Just looking a little closer at your pic, the cavity shown on the wear plate could very well be cavitation damage.
gamebusta (Mechanical)
6 Dec 07 1:22
Heres another pic- http://files.engineering.com/getfile.aspx?folder=4011deda-905d-4cf2-a784-05
字串2
gamebusta (Mechanical)
6 Dec 07 1:23
one more- http://files.engineering.com/getfile.aspx?folder=fea74647-bb6b-487a-9d9c-8d
Artisi (Mechanical)
6 Dec 07 1:55
The impeller doesn't seem to show any sign of excessive cavitation.
The impeller / shaft fit seems Ok- no signs of any fretting between them.
There is a lot of wear at one point on the impeller - is the shaft bent.
How have the bearings failed - any analysis undertaken as to the mode of failure.
Running out on the curve can result in some very high loads bought about by the mis-match of flow onto the impeller - any signs of vibration while the units were running or any signs signs on vibration on the installation baseplate or hold-down bolts or supports etc.
Was the pump running in the correct direction. 字串2
Have you contacted the manufacturer for any advice as to the failure.
Lots of questions - but necessary to gather as much info as possible.
rcooper (Mechanical)
6 Dec 07 6:35
First impression, the hole shown in the first picture looks like a cavity in the casting which has been exposed by the wear. (I am not involved in manufacturing, so somebody who has seen lots of poor castings could verify this)
I noticed a thread in the last picture. Is this used to stop axial movement of the pump? The pressure on the back of the impeller is higher than the front, so if the pump is allowed to move axially, it will move towards the wear ring.
JJPellin (Mechanical)
6 Dec 07 7:50
I have seen this sort of problem, but only in multi-stage vertical pumps with many stages. But this looks like a very unusual impeller to me. The problem I refer to was also documented in a technical paper presented at the International Pump Users Symposium a few years ago. 字串1
In any pump, as Artisi noted, the designer expected the pump to run within a certain range of flows and pressures relating to an acceptable region on the pump curve. Some designs rely on these design criteria when determining the thrust characteristics of the rotor. If the impeller has suction pressure on one side and discharge pressure on the other side, the design intended for the resulting net thrust to be within the range that the thrust bearing could support. In some designs, the absence of discharge pressure when the pump is allowed to run "off the curve" at very, very high flow rates will result in a net thrust that the pump cannot accommodate. In some vertical turbine pumps, the net thrust in this mode is upward. The upward thrust can be enough to buckle a long thin shaft resulting in catastrophic failure.
Since this is a submersible pump, then the impeller must run directly on the motor shaft. I would check on the configuration of the motor bearings. Some motors have wave spring loaded bearings that allow for a substantial axial movement if a high thrust load is applied. Some motors are only designed to accommodate thrust in one direction. I suspect that the high flow condition applied axial loads to the motor that it could not take and the axial movement resulted in the rub damage to the impeller. But without more details, this is just speculation. 字串2
Johnny Pellin
Artisi (Mechanical)
6 Dec 07 23:18
Did the total head actually decrease when switching to the 24" pipeline or is this only an assumption, is the highest point in the discharge pipeline the same as previously?
It is possible of course that changing pipeline resulted in a different (higher) static head and you could now have a much higher head than previously.
There is something odd regarding this failure, decreasing head (assumed) is unlikely to give this result as shown in your photo's.
Artisi (Mechanical)
6 Dec 07 23:23
Johnny Pellin
The mis-match I was thinking of is actually the angle of attack of the impeller blade to the incoming flow, now much higher than design. As you know this can result in major hydraulic problems but unlikely to cause this type of damage.
字串9
pumpking (Chemical)
9 Dec 07 12:07
From my experiences of pump failures, it looks that there is excessive thrusts resulting in excessive wear and in turn heat generation on the contact faces.
I strongly suspect the pump is running well off of its safe operating curve - when we say "off of its curve", we mean within acceptable operating points - even if pump performance curves show a full spectrum of flow and head's, a safe operating point is normally within certain tolerances of best efficiency point.
Im confident that if you re-calculate the new head, then this will be the cause of the problems. Ash Fenn
www.cdrpumps.co.uk
（Click：）
|
| Previous：axial or centrifugal pump?? Next：Viscosity Coefficients Determination |
|
[ Add TO Favorites]
[TOP] [PRINT]
[CLOCE WINDOWS] |
|
|
|
