A reminder of a few things you should know about installing pumps 14-8
- The pump should be located as close to the liquid source as practical so that a short, direct suction pipe can be used to reduce friction on the suction side of the pump. If you are forced to use a long length of pipe consider going to a larger size.
- The mass of the pump concrete foundation must be five (5) times the mass of the pump, base plate, and other equipment that is being supported or vibration will occur.
- Foundation bolts of the proper size should be embedded in the concrete.
- Up to 500 horsepower (375 KW), the foundation must be 3 inches (76 mm.) wider than the base plate all around. Above 500 horsepower (375 KW) the foundation should be a minimum of 6 inches (150 mm.) wider.
- Imaginary lines extended downward 30 degrees to either side of a vertical through the pump shaft should pass through the bottom of the foundation and not the sides.
- Be sure the pump is leveled before it is aligned or the bearing oil level will be incorrect.
- If you are not going to use a “C” or “D” frame adapter that aligns the pump and electric motor with a registered fit, you are going to have to do a physical alignment using either a laser or reverse dial indicators. Be sure to compensate for thermal expansion and then check the alignment after the grouting has set and the foundation bolts tightened.
- If you are going to be pumping hot liquid you should specify a centerline pump to eliminate the pipe strain problems caused by the wet end of the pump expanding in only one direction. A good rule of thumb says that each inch of stainless steel metal will expand 0.001 inches with each 100°F of temperature increase, The metric expansion is each millimeter will expand 0.001 millimeters with each 50°C increase in temperature.
- A flexible coupling should never be used to compensate for misalignment between the pump and its driver. The purpose of the flexible coupling is to allow for axial expansion of the shaft and to transmit the torque from the driver to the pump.
- After the pump and motor have been aligned, dowel both the pump and the motor to the base plate. Be sure to dowel only the feet closest to the coupling allowing the outboard ends to expand with temperature changes. This is especially important with large pumps handling hot liquids.
- With the coupling disconnected, check impeller rotation after installing the pump. Generally an arrow cast into most pump volutes will show you the proper direction. Even then do not assume it will turn in the correct direction. I have heard about two speed pumps with the second speed wired backwards. They will drive you crazy because the pump will often meet its head requirement but not the capacity when the second speed cuts in. You will also notice excessive noise when it cuts in.
- Discharge recirculation lines should not be piped to the pump suction because they can heat the incoming fluid and possibly cause a cavitation problem. Pipe them to the source or some other logical place if possible.
- Remember that you cannot vent a running pump because centrifugal force throws the liquid out leaving the trapped air in the center or eye of the impeller.
- Vent valves can be installed at one or more points in the pump casing waterways.
- Vertical pumps that are running with a mechanical seal need a vent installed above the seal faces and any dynamic elastomers in the seal to prevent the seal faces from running dry and overheating of the dynamic elastomer. In this instance you can pipe the vent back to the pump suction.
- Positive displacement pumps can develop excessive discharge pressures so a vent should be installed in the discharge piping and piped back to the source.
- To prevent piping vibration problems:
- Eliminate all unnecessary bends because they provide a strong coupling point between pulsation excitation forces and the mechanical system.
- If you must use bends use the largest enclosed angles as possible and provide restraints near each bend.
- Supports should be installed near all reducers and large masses such as valves, filters, flanges, etc.
- Small, auxiliary-piping connections should be tied back to the main piping to reduce vibration problems.
- Install vibration damping devices where ever practical.
- Here is the proper way to vent a centrifugal pump after it has been 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.
- Open the suction valve. The pump fills part way.
- Close the suction valve.
- Open the discharge valve part way. Once the pressure equalizes the air will rise through the pump in the discharge piping. To insure that the pump stuffing box is full of liquid you can drill a small vent hole between the end or bottom of the stuffing box and the impeller. Drill the hole in the corner at the top.
- Open the suction valve.
- Start the pump
- On February 18, 2018