As before, I find myself asking help from you, a person who really knows pumps. I’m an engineer in a South American refinery. I have an Italian (Nuovo Pignone) overhung refinery pump (P-2022C) with an enclosed impeller. The impeller has four blades, eight balance holes, with front-and-back rotary wear bands. Stationary wear bands are seated into the casing.
We ordered another factory impeller from Italy for an emergency turnaround. The replacement impeller arrived emergency freight. Either we, or the pump factory, made a mistake. The replacement impeller has seven blades and seven balance holes. All dimensions, tolerances and gaps on the wear bands mate with the original four-bladed impeller and the pump internals.
There is no time to return this impeller to Italy and trade messages about what went wrong. I must install the new impeller into my pump this weekend for service next Monday morning. My question is: “How will I affect my pump with the new impeller?” Can you please answer fast?
Replacing a four-bladed impeller with a seven-bladed impeller has modified the profile of your pump’s performance curve. You can easily plot the new performance curve with a flowmeter and differential-pressure gauges.With the instrumentation installed, you have two options:
1. Install the pump into your process line; or
2. Install the pump into a test recirculation loop through a tank.
Turn on the pump with a closed discharge control valve. Record the flow and differential pressure across the pump. Do this quickly because the pump is under high stress while pumping against a closed valve. The flow should be zero. There may be some instability on the suction pressure gauge at zero flow. The differential pressure may also be unstable.
With the pump running, open that control valve two revolutions on the wheel. Record the new flow and differential pressure across the pump. Flow should increase. The differential pressure should begin to fall, but there may still be some instability between the gauges.
Leaving the pump operating, open the control valve two more revolutions (four revolutions total). Record the flow and differential pressure. Flow should increase. The instability between the pressure gauges should disappear.
Open the control valve two more revolutions (six revolutions total), and record the readings on the flowmeter and the differential gauges. The pressure differential should reduce and the flow should increase.
Continue opening the control valve two rounds and recording the flow and differential pressures at each step.
If the pump is on a test stand:
Follow this procedure until flow is at maximum and there is no (or very little) differential pressure. Plot your flows and differential pressures on a graph. You have just plotted your new performance curve.
If the pump is installed into the process line:
You can follow this procedure until the resistance across the control valve is equal to the resistance through the system. At this point, the system will control and limit your pump. You must extrapolate the remainder of your performance curve (the portion that you cannot measure).
I recommend you do two other things to your altered pump within the first few hours of operation:
1. Do a vibration analysis looking for harmonic vibrations.
2. Because your pump is overhung (OH) design, verify and monitor the temperature / stress in the axial bearing. The original impeller has eight balance holes. The new impeller has seven balance holes.
Good luck on start-up next Monday morning.
The Pump Guy
We discuss replacement impellers and many other issues that affect pump performance at the Pump Guy Seminar. We’ll be presenting Pump Guy Seminars in Houston, Las Vegas, Boston, and New Orleans in 2011. Be there!
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at firstname.lastname@example.org or 615 361-7295.