Subject: Starting the centrifugal pump.13-3

It appears simple. Fill the pump with liquid, crack open the discharge valve and start the motor. But, as you would guess, it is a little more complicated than that.

We'll begin by making sure the pump is filled with liquid. There are several ways to do that:

If you elect to fill the pump with liquid from an outside source you're going to have to vent the pump to remove any air that might be trapped during the filling process.

Keep in mind that you cannot vent a running pump because centrifugal force throws the heavier liquid out and leaves the trapped air in the center. or eye of the impeller where it can cause all sorts of cavitation and pumping problems.

Here is the proper way to vent a centrifugal pump after it has been initially installed, or the system has been opened. I am assuming the pump is empty of liquid and both the suction and discharge valves are shut.

If this is a vertical installation it's a good idea to install a permanent vent from the mechanical seal gland back to the suction side of the pump. The stuffing box is now a high point in the system that can trap air in the stuffing box, and without this vent the seal could be running dry in a pocket of air. Be sure to check that the vent is above both the lapped seal faces and any dynamic elastomer that might be installed close to the seal faces.

So far so good, but now comes the impossible part:

If you have aligned the pump and driver properly, eliminated all pipe strain, dynamically balanced the rotating assembly, checked to see if the shaft is straight, and compensated for thermal growth, then the only shaft displacement you would have to be concerned about would be operating the pump off of its best efficiency point (BEP), and that is exactly what is going to happen at start up.

Those pumps with a low L3/D4 ratio (less than 60 in imperial dimensions and 2 in metric units) can handle operating off the best efficiency point (BEP) for a time and not be concerned about displacing or breaking the rotating shaft., but the high L3/D4 designs bend severely with a high probability of damaging the mechanical seal and possibly both the bearings and the rotating shaft. This is one of the reasons we have so many mechanical seal failures at start up.

This means that there is no sensible way to start a centrifugal pump with a high L3/D4 ratio and explains why we have so much trouble with intermittent service pumps.

If you want to check your pump for evidence of this shaft displacement problem, then the next time the pump is disassembled look for shaft rubbing marks at the end of the stuffing box, with the rub marks located at 60° or 240° from the cut water, measured in the direction of shaft rotation. If you are using closed impeller pumps you will see this same rubbing marks on the stationary portion of the impeller wear ring.

It all goes back to the philosophy I preach at my schools. "If you're going to run a pump twenty four hours a day, seven days a week and not open and close valves, choose a low cost pump and mechanical seal, and size the pump to run at its best efficiency point (BEP)".

If frequent starting and stopping is normal in your process then get the lowest L3/D4 design you can afford and follow those procedures I mentioned in the above paragraphs to insure the pump is full of liquid at start up. The low L3/D4 will insure the shaft will not deflect enough to cause you a mechanical seal or bearing problem .

 

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