Shaft deflection

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:

• As the velocity of a liquid increases, the pressure (measured 90 degrees to the flow) decreases and as the velocity decreases the pressure will increase. This is the same principle we use when we place a venturi in a water hose, so that we can spray chemicals on the lawn.
• Pressure, working against an area, will cause a force. (Pressure x Area = Force)
• For non turbulent liquid flow to occur, the velocity of the liquid times the area it is passing through must remain a constant

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:

• Opening up the mechanical seal faces as the rotating portion of the seal contacts a stationary component.
• Damage to the impeller and volute.
• Excessive wear ring wear and loss of pump efficiency as the wear ring clearance increases. This is a major concern with “vertical in line” designs.
• Excessive shaft fretting (wear) at the bearing seal locations.
• Damage to the bearing seals
• Packing sleeve wear.
• Excessive packing leakage.
• Overheating of the packing.
• Damage to the stuffing box throat bushing.
• Damage to an A.P.I. gland disaster bushing.
• The breaking of a stationary seal face.

Here are some things you can do to help reduce the deflection:

• Shorten the shaft.
• Go to a larger diameter shaft. You can do this by either replacing the present pump power end with a larger diameter shaft, or in some cases you can replace the sleeved shaft with a solid version.
• Remove the packing and substitute a sleeve bearing in its place. The seal can be relocated between the face of the stuffing box and the bearing case. Any time you get the seal closer to the bearings you are better off.
• Install a suction recirculation line between the pump discharge and a low pressure point in the system. This will work for throttled applications if you are prepared to lose some of the pump’s efficiency.
• Go to a double volute pump design. The slight loss in efficiency is worth it.
• If the main head is “system head” a variable speed motor would make sense.
• Tell the operator to operate the pump at its best efficiency point. (Good luck with that one!)
• You will notice that I did not recommend up grading to a different shaft material. Unfortunately all of the common shaft materials have approximately the same modulus of elasticity, so they will all have the same bending problem.

Posted

• On February 17, 2018