Measuring Flow in Open Channels

Parshall flume at a water treatment facility
Courtesy Tracom Fiberglass Products

Industrial, municipal, and commercial processing operations can employ open channels as a means to direct and transport liquids. Open channel flow is technically fluid passing through a conduit with a free surface. The mechanics of this type of flow are well characterized, allowing volumetric flow rate to be determined using a single measurement of liquid depth as it passes through a channel of known shape. These shaped portions of the fluid transport system are known as flumes.

There are a number of different flume types used for flow measurement, each with its own name, shape, and application characteristics. One of the most common is the Parshall flume, named after its inventor. In its simplest application, the Parshall flume directs liquid flow through a narrowed throat. The depth of the liquid is measured at a designated point along the flume. Using known flow characteristics for the flume shape and size, volumetric flow rate can be calculated using the depth measurement.

Flumes are widely used in wastewater treatment plants, irrigation, and other applications where flow measurement is needed in an open channel. The flume can be constructed of almost any suitable material, but must be dimensionally correct and stable. Fiberglass is often a material of choice because of its weight, corrosion resistance, cost, and ease of installation. A fiberglass flume can be prefabricated with dimensional precision, shipped to the installation site and essentially dropped in place as a complete unit. Numerous options are available with fiberglass flumes to accommodate every installation requirement.

• Ultrasonic level sensor mounting brackets
• Bubble tubes
• Sample tubes
• Submerged probe cavities
• Stilling wells (attached and detached)
• Staff gauges
• Removable probe holders
• Inlet and outlet end adapters
• Pipe stubs
• Flanged end connections
• Flow straighteners
• Fiberglass grating
• Inlet and outlet wingwalls
• Multi-piece construction
• Nesting
• Chemically resistant gel coal

Share your open channel flow measurement challenges and requirements with an application specialist for recommendations on complete flow measurement solutions.

Combating Cavitation in Industrial Process Control Valves

Cavitation in liquid processes produces bubbles which can
damage valves.

In process control valves, cavitation results from a rapid drop in pressure as liquid passes through the valve. It results in the formation of vapor spaces or bubbles within the valve cavity. When the bubbles move downstream into a larger cross-sectional area, velocity decreases and pressure increases. The higher pressure now surrounding the bubbles causes them to implode, producing shockwaves which propagate through the liquid. These shockwaves can cause metal fatigue and excessive wear on the internals of the valve. The collapsing bubbles also make a discernible sound with accompanying vibration. The cumulative effects of cavitation can cause rapid deterioration of a valve, resulting in reduced control function, frequent need for service, or premature failure.

There are ways to mitigate cavitation. Some involve changes in the process, others, incorporating a properly designed and selected valve with trim that reduces or prevents the conditions that cause cavitation. The paper below, authored by Flowserve, provides an in depth examination of the causes of cavitation, then continues with explanation of how their specialty valves are designed to overcome the conditions that promote it.

There are detailed illustrations showing the specific valve trim features that impede cavitation. Share your process control valve challenges with application experts, combining your process knowledge with their product application expertise to develop effective solutions.

Cavitation Control in Industrial Valves from Instrument Specialties, Inc.

Computational Fluid Dynamics Applied to Effective In-Tank Mixing

Jacoby Tarbox uses computational fluid dynamics software to reliably and predictably model the performance of their eductors used to accomplish mixing in tanks, open vessels, and other containers. The video provides an overview of how the company determines the arrangement of eductors that will provide the mixing performance required by each customer.

Share your interest or application challenges with a product application specialist, combining your process knowledge with their product application expertise to develop effective solutions.

Achieving Close Control of Process Temperature

Temperature sensor type, construction, and
location contribute to system performance
Courtesy Krohne

Temperature control is a common operation in the industrial arena. Its application can range across solids, liquids, and gases. The dynamics of a particular operation will influence the selection of instruments and equipment to meet the project requirements. In addition to general performance requirements, safety should always be a consideration in the design of a temperature control system involving enough energy to damage the system or create a hazardous condition.

Let's narrow the application range to non-flammable flowing fluids that require elevated temperatures. In the interest of clarity, this illustration is presented without any complicating factors that may be encountered in actual practice. Much of what is presented here, however, will apply universally to other scenarios.

What are the considerations for specifying the right equipment?

KNOW YOUR FLOW

First and foremost, you must have complete understanding of certain characteristics of the fluid.

• Specific Heat - The amount of heat input required to increase the temperature of a mass unit of the media by one degree.
• Minimum Inlet Temperature - The lowest media temperature entering the process and requiring heating to a setpoint. Use the worst (coldest) case anticipated.
• Mass Flow Rate - An element in the calculation for total heat requirement. If the flow rate will vary, use the maximum anticipated flow.
• Maximum Required Outlet Temperature - Used with minimum inlet temperature in the calculation of the maximum heat input required.

MATCH SYSTEM COMPONENT PERFORMANCE WITH APPLICATION

• Heat Source - If temperature control with little deviation from a setpoint is your goal, electric heat will likely be your heating source of choice. It responds quickly to changes in a control signal and the output can be adjusted in very small increments to achieve a close balance between process heat requirement and actual heat input.
• Sensor - Sensor selection is critical to attaining close temperature control. There are many factors to consider, well beyond the scope of this article, but the ability of the sensor to rapidly detect small changes in media temperature is a key element of a successful project. Attention should be given to the sensor containment, or sheath, the mass of the materials surrounding the sensor that are part of the assembly, along with the accuracy of the sensor.
• Sensor Location - The location of the temperature sensor will be a key factor in control system performance. The sensing element should be placed where it will be exposed to the genuine process condition, avoiding effects of recently heated fluid that may have not completely mixed with the balance of the media. Locate too close to the heater and there may be anomalies caused by the heater. A sensor installed too distant from the heater may respond too slowly. Remember that the heating assembly, in whatever form it may take, is a source of disturbance to the process. It is important to detect the impact of the disturbance as early and accurately as possible.
• Controller - The controller should provide an output that is compatible with the heater power controller and have the capability to provide a continuously varying signal or one that can be very rapidly cycled. There are many other features that can be incorporated into the controller for alarms, display, and other useful functions. These have little bearing on the actual control of the process, but can provide useful information to the opeartor.
• Power Controller - A great advantage of electric heaters is their compatibility with very rapid cycling or other adjustments to their input power. A power controller that varies the total power to the heater in very small increments will allow for fine tuning the heat input to the process.
• Performance Monitoring - Depending upon the critical nature of the heating activity to overall process performance, it may be useful to monitor not only the media temperature, but aspects of heater or controller performance that indicate the devices are working. Knowing something is not working sooner, rather than later, is generally beneficial. Controllers usually have some sort of sensor failure notification built in. Heater operation can be monitored my measurement of the circuit current.

SAFETY CONSIDERATIONS

Any industrial heater assembly is capable of producing surface temperatures hot enough to cause trouble. Monitoring process and heater performance and operation, providing backup safety controls, is necessary to reduce the probability of damage or catastrophe.

• High Fluid Temperature - An independent sensor can monitor process fluid temperature, with instrumentation providing an alert and limit controllers taking action if unexpected limits are reached.
• Heater Temperature - Monitoring the heater sheath temperature can provide warning of a number of failure conditions, such as low fluid flow, no fluid present, or power controller failure. A proper response activity should be automatically executed when unsafe or unanticipated conditions occur.
• Media Present - There are a number of ways to directly or indirectly determine whether media is present. The media, whether gaseous or liquid, is necessary to maintain an operational connection between the heater assembly and the sensor.
• Flow Present - Whether gaseous or liquid media, flow is necessary to keep most industrial heaters from burning out. Understand the limitations and operating requirements of the heating assembly employed and make sure those conditions are maintained.
• Heater Immersion - Heaters intended for immersion in liquid may have watt density ratings that will produce excessive or damaging element temperatures if operated in air. Strategic location of a temperature sensor may be sufficient to detect whether a portion of the heater assembly is operating in air. An automatic protective response should be provided in the control scheme for this condition.

Each of the items mentioned above is due careful consideration for an industrial fluid heating application. Your particular process will present its own set of specific temperature sensing challenges with respect to performance and safety. Share your requirements with temperature measurement and control experts, combining your process knowledge with their expertise to develop safe and effective solutions.

Video Shows How to Install a Portable Ultrasonic Clamp-on Flowmeter

Krohne Optisonic 6300 P
Portable Ultrasonic Flowmeter

Ultrasonic flowmeters, being capable of operating from outside a pipe, are well suited to field measurements and other portable applications in industrial process measurement. Krohne, a globally recognized manufacturer of flow, pressure, temperature, level, and other instruments for process measurement and control, has produced video instructions for field installation and operation of their portable ultrasonic clamp-on flowmeter. The video is included below. Though the presentation is based upon the Krohne product, there is good general knowledge about portable ultrasonic flowmeters to be had from the video.

Watch the video and build your knowledge. Share your process measurement challenges with application specialists, combining your process knowledge with their product application expertise to formulate effective solutions.