Quick and proper detection methods help prevent flow and surge issues, pump cavitation and clogged inlet and outlets for pump control.
By Bill Bernhardt, senior commercial engineer, Rockwell Automation
Pumping water is an application in which system dynamics play a big part in pump control selection. A pumping application’s ultimate goal is to move the fluid, but it’s just as important to reduce the amount of damaging water hammer while detecting issues quickly within the pumping system.
So what is the most effective way to round up the control for the pumping application?
Water Pumping Scenario
We’ll use a scenario in which the need to pump water from one source to another using a centrifugal pump while reducing water hammer. In this case, the application calls for the pump to start smoothly and run the pump motor at full speed until the application calls for a stop command. This is a good fit for a soft starter as opposed to a variable-frequency drive (VFD), because no speed control is needed between the start and stop.
Three methods commonly are used for the starting portion:
- The soft start method is used in soft power source applications in which current spikes aren’t desired when starting. This is the traditional way to use a reduced voltage to start an application.
- The pump control method is used to follow the natural S-curve of the pump to provide enough control and torque when needed by application, conserving energy. Sometimes the current in this method is higher compared to soft start method, depending upon system dynamics.
- The linear acceleration method is used to provide the smoothest start. Linear acceleration isn’t as load-dependent as the previous two starting methods. Most applications require an external tachometer to achieve this method.
The common stopping methods for pump applications are just as easy:
- Soft stop reduces voltage when stopping, which extends the ramp time beyond that of a coast to rest.
- Pump stop, similar to the pump start, follows the system dynamics when stopping.
- Linear deceleration provides a smooth, controlled stop of the pumping application that isn’t as load-dependent.
The graphs in Figure 1 illustrate the torque and speed of a pump load when stopping by the three methods. By controlling the motor torque, the water hammer is controlled.
Pump Application Diagnostics
Now that the starting and stopping are working and water hammer is under control, other aspects of the pumping application diagnostics, such as pump cavitation, plugged inlet or outlets or low flow, can be examined.
Some standalone pump cavitation detection devices use only current to detect an issue. The problem with this method is current is not linear on a motor as the curve in Figure 2 illustrates for this 150-Hp motor. Current alone doesn’t tell the whole story if a motor is lightly loaded and therefore isn’t a good way to detect pump issues.
However, in unison, power measurements such as power factor, real power and current can indicate what the system is doing. Both power factor and real power go toward a zero value, while motor current doesn’t. Motor magnetizing current always exists even at no load.
Using power factor, real power and current in combination can help to indicate whether a pump issue is taking place.
For example, if real power decreases, this may indicate pump cavitation or a clogging issue on the pump intake, which may result in the pump’s running dry. As real power increases, this may indicate an overload condition such as bad bearings or a rupture in the discharge line.
Monitoring with a Flowmeter
Another way to monitor a pumping application is to use a flowmeter. This device typically is fed back to an analog card in a PLC rack that is monitored in a control program.
Smart Motor Controllers
The question is whether a good way exists to round up the different methods for controlling and monitoring a pumping application and reduce the number of devices. Using a single device such as a smart motor controller might offer some or all of the following features:
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc. Lead photo: Georgia Evans / Shutterstock.com.