01 April 2004
Applying impact flowmeters
Solids flowmeters are a convenient, accurate, and dust-tight alternative to continuous in-line weighing.
By Sheldon Shepherd and Richard Calciano
Dry solids flowmeters monitor the rate of bulk material flow in a process. These devices constantly measure the impact force of the material under gravity feed conditions and convert this signal into a flow rate.
In process plants, dry solids flowmeters can work in many locations to provide rate and totals for many types of bulk solids such as cereals, seeds, grains, and minerals. Information from the flowmeter feeds back to control the rate into a process or blending operation.
A careful evaluation of the application, proper installation and calibration, and regular maintenance are essential if operators hope to enjoy continuous and reliable service. The choice and setup of an appropriate prefeed device is critical in conditioning the material for reliable measurement.
Solids flowmeters can function in stand-alone measuring operations or they can interface to a facility's process control system using industry standard protocols. They may also connect to and operate in conjunction with other equipment such as weigh feeders, belt scales, liquid additive pumps, and other process sensors.
Reducing material costs and improving product quality are of growing concern in today's global marketplace. Using solids flowmeters to accurately monitor the rate of material during processing can help companies assure their long-term survival by improving quality and lowering costs. Solids flowmeters have proven to be a convenient, accurate, and dust-tight alternative for continuous in-line weighing. By measuring the impact of material flow, these alternatives to conventional weighing devices provide accurate, continuous measurement of free-flowing solids. To accomplish this it is important to understand how to properly select, install, calibrate, and interface a solids flowmeter for a particular application.
Horizontal force dependent
Solids flowmeters are impact-weighing devices that measure the mass flow rate of a free-flowing, continuous stream of dry solids material. The principle of operation is simple: The material produces a mechanical deflection as it strikes the flowmeter's sensing plate. The flowmeter converts the deflection into an electrical signal that feeds into an accompanying integrator, which instantaneously displays the flow rate and integrated total weight.
The flowmeter measures only the horizontal component of the force of the material flow striking the plate. It is critical these following factors remain constant for reliable and repeatable measurement. The horizontal force is dependent on particle mass, velocity of the particle, angle of impact of the particle against the plate, and the energy absorbing characteristics of the particle.
Thus, with these factors held constant, the flowmeter responds to the mass or weight of the material striking the plate. Because the flowmeter measures only the horizontal force, it is unaffected by vertical force changes caused by material buildup on the nonimpingement area of the sensing plate. Consequently, there is no zero drift, which in turn eliminates the need for frequent recalibration.
The current generation of solids flowmeters available on the market provides several advantages over earlier models, including lower cost and improved material flow for more accurate measurement. As weighing devices, solids flowmeters offer several advantages over other systems. They can work in situations where other weighing devices are inappropriate. Their compact size allows you to install them without major modification. They easily and accurately calibrate by comparing the weight of a measured material sample to the weight totalized by the flowmeter and then correcting the span of the integrator. They generally do not require maintenance or recalibration after the initial installation and material tests. Finally, most solids flowmeters have dust-tight construction, making them suitable for use in hazardous areas or in applications that require minimal cleanup and reduced housekeeping, such as milling and food processing.
Solids flowmeters continuously weigh a variety of materials, from powders to granules larger than 1 inch in diameter. Suitable material densities range from puffed wheat to iron ore. Flow ability suitable for solids flowmeters covers the spectrum from fluidized powder, such as fly ash, to lathe turnings. Successful applications include grain tailings and screenings, cement, soybean and rice hulls, gravel, unshelled peanuts, wood chips, starch, coke, sugar, potato flakes, coal, and plastic pellets.
Models are available in flow rate ranges from 500 pounds per hour to 900 tons per hour. By using material samples, solids flowmeters calibrate to ±1% of full scale and provide ±0.2% repeatability. Many applications have accuracies of ±0.5% or better. Users can commonly expect a 3 to 1 flow rate turn-down ratio from most suppliers.
There are two basic types of impact flowmeters: the linear variable differential transformer (LVDT) and the strain gauge load cell. Each type uses a different sensor to convert the horizontal force applied to the sensing plate. In general, selecting the appropriate flowmeter depends upon both the flow rate and the temperature of the material weighing.
LVDT models use a range spring, which offers an opposing force to the material flow, and a viscous fluid damper, which eliminates pulses in the movement of the sensing plate. The weighing mechanics are externally located, so they are unaffected by the material being processed. A mechanical stop serves for overload protection. These flowmeters measure materials with temperatures up to 450°F when silicon gaskets are used. Capacities range from 40 tons per hour to 300 tons per hour, depending on the model.
Inherent opposing force
Strain gauge load cell models support, or assist in supporting, the sensing plate and have an inherent opposing force and mechanical damping. A mechanical stop protects the load cell during overload conditions. These flowmeters have either single-point or four-point suspensions. Capacities range from 1 ton per hour to 900 tons per hour, depending on the model.
In addition to the sensing element and mechanical installation, it is important to select an appropriate integrator. Typical integrator functions include flow rate, totaling, and analog output as well as pulsed output for remote totaling. Additional integrator features, including functions for process control, ratio control, batching control, and flow rate alarms, are available as options from most manufacturers.
Although analog control systems have been used extensively in the past, digital communication networks are replacing them at a fast pace. Many users are now requiring solids flowmeter integrator/controllers that are compatible with industry standard communication protocols such as Allen-Bradley Remote I/O, DeviceNet, Profibus, or Modbus Plus. These protocols offer a wide range of options for transmitting and receiving data between the solids flowmeter and the plant's control system.
Typical integrator/controller interfaces:
- 4-20 mA outputs programmable for flow rate
- One or more 4-20 mA proportional, integral, derivative (PID) control outputs
- One or more 4-20 mA set-point inputs
- At least one pulsed output for a remote totalizer
- Discreet inputs to initiate auxiliary functions such as batch control, routine calibration, PID modes, or external alarms
- Several alarm relay outputs programmable for rate, load, PID deviation, and batch control
- Industry standard communication protocols
Air flow past sensing plate
The manufacturer will typically request a completed application data sheet from the customer to determine the suitability of the application and select the most appropriate flowmeter. It is important that this information be accurate and reliable so that the performance of the unit may be likewise. Be sure to directly contact one of the factory application engineers if there are any concerns or questions. Many potential problems may be resolved at this stage.
Information considered by the factory in applying the most appropriate solids flowmeter includes:
- Flow rate
- Type of preferred device
- Bulk density
- Particle size
- Abrasiveness, particularly if it is a specialty material
- Accuracy required
- Possibility for a material test
- Material aeration
- Moisture content
- Hazardous area classification
Application and installation are critical to reliable and accurate solids flowmeter operation. It is certain that the performance of a solids flowmeter is only as good as the application and installation of the device.
Factors affecting accuracy include material consistency, air flow past the sensing plate, vibration, and installation. The material consistency must remain constant, particularly velocity of the material, impact angle on the sensing plate, energy absorbing characteristics, and coefficient of friction. Solids flowmeters are relatively immune to vibration. The rules for application when vibration exists are as follows: If you can only feel the vibration, don't be concerned. If you can see the vibration, take measures to reduce it. The same guideline may apply to air flow past the sensing plate. If the air flow is great enough to feel directly on the skin then it will likely affect accuracy.
You should install a solids flowmeter at level and securely. Wiring should be in accordance with local laws as well as generally accepted principles for instrumentation and continuous weighing equipment. The wiring should have protection from damage and electrical noise.
One of the most critical aspects of solids flowmeter performance is the use of an appropriate prefeed device. A prefeed device conditions the material prior to weighing. A user may also control the flow rate by controlling the speed of the prefeed device.
You should calibrate the solids flowmeter in accordance with the manufacturer's guidelines and timetable. Like any type of continuous weighing device, it is important that consistency is of primary importance and then accuracy. Generally calibration procedures include the following:
- Ensure that the integrity of flowmeter installation.
- Determine calibration weight value in accordance with factory specifications.
- Perform zero calibration.
- Perform test weight span calibration.
- Recheck to verify the zero calibration.
- Perform material tests if possible. Perform at least three tests before the final adjustment happens.
- If the device is repeatable, perform manual span adjustment and factor the test weight to the material test. DT
Behind the byline
Sheldon Shepherd and Richard Calciano are managers with Siemens Process Automation in Grand Prairie, Texas and Spring House, Pa., respectively. Write them at Sheldon_shepherd@siemens.com and Richard_calciano@siemens.com.