SUBJECT : Solving a major cause of shaft deflection in volute type pumps 6-5
To understand the following paragraphs, you must understand three rules about fluids:
The following illustration describes a volute pump. It is called a volute pump because the impeller is mounted off center. The impeller vane clearance is closest at the cut water and increases as you move towards the discharge.
For this pump to operate properly the pumped liquid must move at a constant velocity around the impeller, even though the volute area is increasing. Since the impeller area (at the outside diameter) is a constant, the pressure generated by the constant velocity of the liquid will not cause any radial forces on the impeller (rule #1). We control this liquid velocity by the design and speed of the pump.
Three possible conditions can be present:
Condition #1- The liquid is fed between the impeller vanes in just the right proportions, and there is just the right amount of resistance, or head at the discharge of the pump to keep the liquid moving at a constant velocity around the impeller causing a constant pressure at the impeller outside diameter (rule #1). We call this "operating at the best efficiency point" (B.E.P.) and there is no unbalanced radial force acting on the impeller, thrusting it in a radial direction
Now we will investigate two other common operating conditions
Condition #2 - The pump is operating to the right hand (high capacity) side of the pump curve with little or no resistance, or head at the discharge side of the pump.
As the liquid travels 180 degrees from the cutwater location (in the direction of shaft rotation) it increase in velocity due to the lack of resistance at the pump discharge. As the velocity of the liquid increases the pressure will decrease at approximately 240 degrees from the cut water, causing a radial force (rule #2) to be generated 60 degrees from the cut water (in the direction of shaft rotation).
Condition #3 - The discharge valve is shut. No flow is entering or leaving the pump casing.
For steady flow to occur, the velocity of the trapped liquid times the area of the volute casing must remain a constant (rule #3). Since the area immediately following the cutwater is very small, the liquid must increase in velocity causing the pressure to decrease, with a resultant force being generated at 240 degrees from the cut water. You will note that this is exactly 180 degrees from the previous force.
The exact points at which the forces will be generated is determined by the Specific Speed (shape) of the impeller. Francis vane impellers (the most popular shape) deflect at approximately 60 and 240 degrees measured from the cutwater, in the direction of shaft rotation. Radial vane impellers deflect at close to 90 and 270 degrees. Axial flow impellers deflect close to 180 and zero degrees from the cut water.
Any time a centrifugal pump operates away from its best efficiency point a radial force is generated that will attempt to bend the shaft. This can cause a rotating component, such as a wear ring or mechanical seal to contact a stationary component causing damage to either or both of them.
You can recognize the problem when you inspect the damage at the point of contact. There will be a mark all around the rotary unit and a mark at either 60 or 240 degrees on the stationary component.
The excessive deflection can cause a lot of other problems including:
Here are some things you can do to help reduce the deflection:
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