Wednesday, April 26, 2017

Operating Principles and Application of Vortex Flowmeters

vortex flow meter flowmeter
Vortex Flowmeter
Courtesy Yokogawa
To an untrained ear, the term “vortex flowmeter” may conjure futuristic, potentially Star Wars inspired images of a hugely advanced machine meant for opening channels in warp-space. In reality, vortex flowmeters are application specific, industrial grade instruments designed to measure an important element of a fluid process control operation: flow rate.

Vortex flowmeters operate based on a scientific principle called the von Kármán effect, which generally states that a fluid flow will alternately shed vortices when passing by a solid body. “Vortices” is the plural form of vortex, which is best described as a whirling mass, notably one in which suction forces operate, such as a whirlpool. Detecting the presence of the vortices and determining the frequency of their occurrence is used to provide an indication of fluid velocity. The velocity value can be combined with temperature, pressure, or density information to develop a mass flow calculation. Vortex flowmeters exhibit high reliability, with no moving parts, serving as a useful tool in the measurement of liquid, gas, and steam flow.

While different fluids present unique challenges when applying flowmeters, steam is considered one of the more difficult to measure due to its pressure, temperature, and potential mixture of liquid and vapor in the same line. Multiple types of steam, including wet steam, saturated steam, and superheated steam, are utilized in process plants and commercial installations, and are often related to power or heat transfer. Several of the currently available flow measurement technologies are not well suited for steam flow applications, leaving vortex flowmeters as something of a keystone in steam flow measurement.

Rangeability, defined as a ratio of maximum to minimum flow, is an important consideration for any measurement instrument, indicating its ability to measure over a range of conditions. Vortex flowmeter instruments generally exhibit wide rangeability, one of the positive aspects of the technology and vortex based instruments.

The advantages of the vortex flowmeter, in addition to the aforementioned rangeability and steam-specific implementation, include available accuracy of 1%, a linear output, and a lack of moving parts. It is necessary for the pipe containing the measured fluid to be completely filled in order to obtain useful measurements.

Applications where the technology may face hurdles include flows of slurries or high viscosity liquids. These can prove unsuitable for measurement by the vortex flowmeter because they may not exhibit a suitable degree of the von Kármán effect to facilitate accurate measurement. Measurements can be adversely impacted by pulsating flow, where differences in pressure from the relationship between two or more compressors or pumps in a system results in irregular fluid flow.

When properly applied, the vortex flowmeter is a reliable and low maintenance tool for measuring fluid flow. Frequently, vortex flow velocity measurement will be incorporated with the measurement of temperature and pressure in an instrument referred to as a multivariable flowmeter, used to develop a complete measurement set for calculating mass flow.

Whatever your flow measurement challenges, share them with a flow instrument specialist, combining your process knowledge with their product and technology expertise to develop effective solutions.


Friday, April 21, 2017

Thermal Mass Flowmeters



Thermal dispersion mass flow meters provide an accurate means of mass flow measurement with no moving parts and little or no encroachment on the media flow path. There are a number of different configurations to be found among various manufacturers, but all function in basically the same manner.

Two sensors are exposed to the heat transferring effect of the flowing media. When the media composition is known, the mass flow can be calculated using the meter reading and the pipe cross sectional area. One of the flow meter sensors is heated, the other is allowed to follow the media temperature as a reference. The heat dispersion from the heated sensor is measured and used to calculate mass flow.

Some positive attributes of thermal dispersion flow meters:
  • In-line and insertion configurations available to accommodate very small to large pipe sizes
  • Rugged Construction
  • No moving parts
  • Measure liquid or gas in a wide range of applications
  • Measurement not adversely impacted by changes in pressure or temperature
  • Wide range of process connections 
  • In-line versions provide unobstructed flow path
  • Wide turndown suitable for extended flow range
  • Flow rate and totalized flow
  • 4-20 mA output interfaces easily with other instruments and equipment
Share all your process measurement challenges and requirements with product application specialists, combining your process knowledge with their product application expertise to develop effective solutions.

Thursday, April 13, 2017

Digital Bar Graph Process Displays Still Have a Place in Your Panel

bargraph digital analog indicators for industrial control
Analog process value indicators are available in a wide
variety of form factors
Courtesy Ametek - Dixson
Analog indicators provide a graphic display of a process value. The value can be a setpoint for a particular operation, or the value returned from a sensor or transmitter. In the current digitally focused environment, we sometimes devalue analog displays. They do, however, have some attributes that set them apart from digital displays. Let's look at bar graph displays.

The ability of an analog display, such as a bar graph, to display useful information depends heavily on its graphical scale. The scale length and resolution should allow significant change in the process value to be displayed in a manner that is readily discernible to an operator. Bar graph displays are composed of illuminated segments, so any significant change in the process value should be sufficient to change the illuminated state of one or more segments.

The scale length and range of the display should extend across the whole of the process, and slightly beyond. An indicating range that far exceeds any possible process value can compromise the display resolution and fail to maximize the use of the instrument. For example, an operation with a maximum process value of 100 should not be paired with an indicating scale that extends to 2500. In this case, the entire range of possible process values indicated will only use four percent of the available indicating scale. A scale range extending to 150 would be more appropriate and deliver better performance.

Analog indicators, especially bar graphs, can provide rapid assessment of the state of a process value. As an illustration, it may not be necessary for an operator to know process temperature with resolution to a tenth of a degree. The key requirement may be to answer the question, "Is it too hot?". Analog displays excel at providing rapid answers to those types of decision-making questions. An onboard digital display of real time process value provides additional information about current process state.

Analog bar graph displays have a proven track record of accuracy and reliability over decades of field use. Modern units include programmable auxiliary functions and take advantage of their microprocessor based design to enable adaptation and setup for almost any application. More information is included in a data sheet below, or your can share your process indication requirements and challenges with instrumentation specialists. The combination of your own process knowledge and experience with their product application expertise will yield an effective solution.


Tuesday, April 4, 2017

Achieving Close Control of Process Temperature

process temperature controller DIN mount digital display
Process Temperature Controller
Courtesy Yokogawa
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 process fluid properties.

  • 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.