STARTING A CENTRIFUGAL PUMP S083
Assuming that the impeller is not frozen in place, the seal faces are not stuck together, the pump is full of liquid, properly vented, and the pump is not wired to run backwards; there really is no good way to start a centrifugal pump.
If you are using a low specific speed impeller the pump will require less horsepower if you start with the discharge valve throttled. If you have a higher specific speed impeller the high power requirement comes at higher head so you would want to start with the discharge valve open.
Some vertical and horizontal pumps require power input at the shut off or zero flow condition that is higher than the power required at the best efficiency point (BEP).
- The motor is usually selected to match the power requirements at the BEP.
- Pumps designed with a specific speed of 5000 or greater exhibit such power curves. These pumps are typically mixed-flow or axial-flow designs and often have power requirements at shut-off that are 200 to 300 percent of BEP power.
All of that is fine if power is your only concern. The fact is that in both cases the pump will initially run off its best efficiency point (BEP) and this will cause high radial forces that could bend the shaft enough to fail the mechanical seal, break the shaft, or cause leakage at start-up.
Your best protection against damage at start-up is a low L3/D4 shaft that will resist these bending forces or a close fitting bushing in the stuffing box of a sealed pump that will support the shaft as it deflects.
In other words it is the intermittent service pump that is going to cause all of the difficulties. Pumps that run twenty-four hours a day are seldom a problem because it is easy to specify a pump that will run close to its BEP. (best efficiency point). The other applications that do not have the problem include:
- Double volute pumps.
- Very low L3/D4 pump shafts.
- If the application is predominately system head and not static or pressure head.
Using a VFD to start a pump
- The VFD applies a low voltage at low frequency to the motor avoiding the high inrush current that occurs when a motor is started by turning on a switch
- This can run 600% of its rated flow.The VFD then increases the applied frequency and voltage at a controlled rate to accelerate the load without drawing excessive current.
- Allows the motor to develop rated torque while drawing rated current
High temperature pumps should be warmed up prior to starting, and the open impeller clearance checked to be sure there is no rubbing that will damage both the impeller and volute.