A Reader Writes:
Save the Pumps
We build and support skid-mount liquid-dispensing systems for specialty chemical batch plants. We've had too many problems with customers running the system pumps dry. The current monitors we've installed don't provide enough advance warning. We need some recommendations about what upgrade path we should take.
-from May Control Design
Current Sensor vs. Power Sensor
There are two main ways to sense motor load: current and power (kilowatt or horsepower). The problem with using current sensors is two fold. Most motors and starters on pumps are sized for the initial current in-rush to start a static load. After start up, most motors may run at less than 65% of full-rated load.
...Here's where your problem comes in. Current is not linear and around 65% or less of rated load becomes ineffective as a signal sensor. However, power is linear throughout the complete load range of any motor. See the load curves:
Power is linear.
Equal sensitivity at
both low and high loads
...As these curves show, most pumps using current sensors will have no effect on showing the small loss of load that occurs when a pump runs dry.
...In sensing motor power, we sense current, voltage, and power factor. Using all three values results in a linear, accurate, and repeatable signal that yields a real-time view of work being produced by the motor driving the pump.
...The graph shows what happens in a typical pump installation:
...The trick to electronically protect pumps from dry running is to sense small changes on load created by low-viscosity material anywhere in the load range of the motor. Not only is power valuable for dry pump conditions but also for overload protection. Using the analog signal is a great way to analyze what is happening in your process, too.
Ron Waltz, Partner
Benchmark Drives LLC, Lake Zurich, Ill.
A simple float switch should solve your problem at little cost. This can be set to open when the tank level approaches a dangerous level, thus stopping the pump well ahead of trouble. It could also set off a loud alarm to let people know of the pending low-level problem.
Terry Baird, Sr. Engineering Technologist
Unison Industries, Jacksonville Fla.
Monitor Flow Through the Pump
Not knowing the consistency, volume, or head pressure involved, I don't know which of the following actions I would take, but I would try either a flow switch, head pressure switch, or a proximity switch to monitor material flow through the pump. When a set minimum is sensed, it would shut the pump down. Simple, but effective and has worked for us many times.
Gregory Burris, Project Engineer
Digital Systems Technology and Robotics, Ashtabula, Ohio
This isn't really a controls problem. If you're using centrifugal pumps, the time that such a pump can run dry is extremely short. The pump might give you 20-30 seconds before failure.
...You have three choices: 1. Instrument the level in the tank or reservoir so that you can signal or alarm when the reservoir is getting close to empty. 2. Have customers who can hire and train operators to watch things a lot more closely. 3. Go to a pump that is insensitive to running dry for at least minutes at a time.
...For the third option, consider an externally cooled/lubricated mechanical seal or, if the process can tolerate pulsations, a diaphragm pump. You can't always instrument around a lack of proper design.
Sebesta Blomberg & Associates, Roseville, Minn.
Not knowing the details of the materials involved, but having had some experience in the adhesives field, I would be conservative and suggest an ultrasonic level sensing system.
...A non-contact system bypasses the problems with materials compatability with the sensors. With an ultrasonic sensing system, you only have to consider installing the sensor on the tank(s) in question. A weight-based solution may not work if the application is mobile, and you need to have the supply tank(s) fastened down.
Gary Sprang, Production Engineer
U. S. Ceramic Tile Co., East Sparta, Ohio
Consider using a volumetric liquid filling system without any moving parts. All fluids are dispensed gravimetrically through a disposable fluid pathway (which eliminates cleaning and cross-contamination concerns). There are no pump components to wear or motors to burn out. The system has proven accuracy to +/- 0.5% and is currently used in the pharmaceutical industry.
Chris Coulter, Business Development Analyst
Filvek LLC, North Springfield, Vt.
Keep It Simple
This maybe a rather simple solution, but you could use a flow meter/switch on the suction side of the pump or discharge, for that matter. This would be a more direct measure of the variable you are trying to control. The feasibility, or more appropriately the cost, depends on the chemicals you are pumping and how small a flow measurement is required.
Gerald White, PE, Maintenance Manager
Berk-Tek, Fuquay-Varina, NC
Watch the Pipe Fluid Level
Since the current monitors are not giving enough warning, it seems they produce alarms only and not shut-downs/trips. If this is the case, the problem is not really the current monitors, it is the time it takes human operators to react. The solution is then either to rewire/reprogram the monitors into a pump-motor trip circuit or to furnish sensing at some location that gives adequate warning, such as level sensing in the suction reservoir/vessel, if one exists.
...If the current monitors are still not acceptable, and you do not wish simply to sense differences between pump suction and discharge pressures, then (without drilling into the pump casings) install two connections into the pump discharge piping at two points in the horizontal section of the pipe that is still level with the pump discharge. One of the points should be on the lower surface of the discharge piping (underneath) and the other high up on the side of the piping or on the top.
...Connect to these two points a small reservoir level with the discharge pipe and fitted with a level switch. The level switch should be wired to trip the pump motor. As soon as the discharge piping is not running full, the level in the reservoir will follow the level in the pipe and will activate the shutdown. A small volume of vapor, gas, or air may continuously reside in the upper reservoir piping but that will be unimportant since it will ensure that the level in the reservoir is always representative of the level in the discharge pipe when the discharge pipe is not full.
W. Brown, C.Eng, retired
Detect Empty Pipe
The perfect solution is an empty-pipe detector, which can protect expensive pumps by shutting them down before they go dry. Newer models are small enough to be installed in the supply tank, feed line (down to 3/4-in. pipe), or in many cases can be installed directly in the pump housing.
Jerry Spindler, Product Manager
Endress+Hauser Inc., Greenwood, Ind.
An inline sensor will mount into the piping prior to the pump. The sensor indicates absence or presence of material in the pipe by using RF admittance technology. When material is absent, the relay in the electronic unit drops out, which can stop the pump and/or activate audible/visual alarms.
Bill Sholette, Product Manager
Ametek Drexelbrook, Horsham, Pa.
Trust the Torque
Use a real-time torque monitoring system that installs in the drive train. The signal is transmitted out of a monitor ring to a receiver that can display the torque on a readout or into a signal conditioner, which mounts to the DIN rail of a controller or PLC in either 0-10 V or 4-20 mA.
John Lerczak, Regional Sales Manager
American Autogard, Rockford, Ill.
We've begun to sell and install our machines in cold climates with limited environmental protection for the equipment. How can we be sure our PLCs will keep working at temperatures as low as -40 degrees F/C? We hope there's something better than heaters or light bulbs, and we don't think insulation alone will cut it. Advice?
Send us your comments, suggestions, or solutions for these problems. We'll include them in the October issue. Send visuals, too,-a sketch is fine. Have a problem you'd like to pose to the readers? E-mail us at CDTheAnswer@putman.net or mail to
The Answer to Your Problems, CONTROL DESIGN, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. You can also fax to 630/467-1124. Please include your title, company name, and location with your response.