Showing posts with label pressure. Show all posts
Showing posts with label pressure. Show all posts

Alliance Technical Sales Wraps Up a Good Year

automated process control room
Alliance Technical Sales, like every other business, is finishing out 2017. Everyone at ATS is thankful for the support and opportunity provided by our customers and suppliers.

The company made some changes to its product lineup during the past year, expanding the range of application solutions available.

We also continued our program of product training and education to maintain top flight competence in recommending solutions to our customers' process measurement and control challenges.

Thank you, again. We look forward to the upcoming year and providing service to our customers at all levels.

Calibration of Process Instrumentation

sanitary rtd temperature transmitter
Industrial temperature transmitter requires
periodic calibration to assure reliable performance
Image courtesy of Smart Sensors
Calibration is an essential part of keeping process measurement instrumentation delivering reliable and actionable information. All instruments utilized in process control are dependent on variables which translate from input to output. Calibration ensures the instrument is properly detecting and processing the input so that the output accurately represents a process condition. Typically, calibration involves the technician simulating an environmental condition and applying it to the measurement instrument. An input with a known quantity is introduced to the instrument, at which point the technician observes how the instrument responds, comparing instrument output to the known input signal.

Even if instruments are designed to withstand harsh physical conditions and last for long periods of time, routine calibration as defined by manufacturer, industry, and operator standards is necessary to periodically validate measurement performance. Information provided by measurement instruments is used for process control and decision making, so a difference between an instrument’s output signal and the actual process condition can impact process output or facility overall performance and safety.

In all cases, the operation of a measurement instrument should be referenced, or traceable, to a universally recognized and verified measurement standard. Maintaining the reference path between a field instrument and a recognized physical standard requires careful attention to detail and uncompromising adherence to procedure.

Instrument ranging is where a certain range of simulated input conditions are applied to an instrument and verifying that the relationship between input and output stays within a specified tolerance across the entire range of input values. Calibration and ranging differ in that calibration focuses more on whether or not the instrument is sensing the input variable accurately, whereas ranging focuses more on the instrument’s input and output. The difference is important to note because re-ranging and re-calibration are distinct procedures.

In order to calibrate an instrument correctly, a reference point is necessary. In some cases, the reference point can be produced by a portable instrument, allowing in-place calibration of a transmitter or sensor. In other cases, precisely manufactured or engineered standards exist that can be used for bench calibration. Documentation of each operation, verifying that proper procedure was followed and calibration values recorded, should be maintained on file for inspection.

As measurement instruments age, they are more susceptible to declination in stability. Any time maintenance is performed, calibration should be a required step since the calibration parameters are sourced from pre-set calibration data which allows for all the instruments in a system to function as a process control unit.

Typical calibration timetables vary depending on specifics related to equipment and use. Generally, calibration is performed at predetermined time intervals, with notable changes in instrument performance also being a reliable indicator for when an instrument may need a tune-up. A typical type of recalibration regarding the use of analog and smart instruments is the zero and span adjustment, where the zero and span values define the instrument’s specific range. Accuracy at specific input value points may also be included, if deemed significant.

The management of calibration and maintenance operations for process measurement instrumentation is a significant factor in facility and process operation. It can be performed with properly trained and equipped in-house personnel, or with the engagement of subcontractors. Calibration operations can be a significant cost center, with benefits accruing from increases in efficiency gained through the use of better calibration instrumentation that reduces task time.

Heated Impulse Lines on Pressure Gauges and Transmitters

self regulating heat trace cble
Successive cutaway view of self-regulating heat trace
cable showing various layers of material
Courtesy BriskHeat
Temperature of the environment surrounding process equipment and instruments can sometimes have a deleterious impact on its function. A common example is cold weather impact on the impulse lines connecting pressure gauges or transmitters to process piping in outdoor or unheated locations. While the process lines may be large, with sufficient mass flow and insulation to prevent freezing, this may not be the case for small diameter impulse lines. Liquid freezing in cold weather conditions can be a threat to process operation, depending on the type of liquid being used. A safeguard exists for impulse lines where the lines can be traced with a heat source, allowing for counteraction of the environmental conditions and maintenance of proper operation.

There are a number of ways to deliver heat to an impulse line. Recognize two essential goals, with the first being to prevent freezing or other changes to the fluid in the line that would impact the response or accuracy of the instrument reading. The second goal is related to the heat tracing itself. The delivered heat must not be great enough to impact the fluid in the impulse line and generate a false pressure reading. Optimally, delivering heat in a fashion that is limited to what is necessary to maintain the impulse line fluid in an ideal working state is best.

One example of heat tracing an impulse line is through the placement of a tube or small diameter pipe, located in close proximity to line, through which low pressure steam flows. Insulation is applied to the bundle and the steam line serves as a heat source. The tube transfers heat to the impulse line when steam flows. After the steam heats the impulse line, a steam trap accompanying the system collects condensate for return to the boiler. It is also conceivable that the steam line could ultimately vent to atmosphere, with no condensate return. There are a number of concerns that must be addressed in the design of the steam portion of this scenario, since it would be necessary to keep any condensate from freezing under all anticipated operating conditions, including process shutdown.

A second common solution for freeze protection of impulse lines is through the installation of electric heat tracing. Two-wire cable serves as protection against the cold. When powered, the heat from the cable keeps the line warm. Electric heat tracing is available in a broad range of physical configurations, including cables, tape, blankets, and other flexible and solid shapes. Control of the electric heat system can be accomplished with an external controller and sensor, or a self-regulating heat trace cable can be used. As with a steam heating system, there are some specific considerations for electric heat tracing. Thermal insulation is still considered a best practice. Electric power must be delivered to the installation, and a means of monitoring heat trace performance for faults or failure should be included in the design.

Share your heat tracing requirements and challenges for process piping and other industrial applications with a product specialist. There are many options and product variants from which to choose. A consultation can help direct you to the best solution.

PID Controller Action Simply Explained

industrial process PID controller
PID Process Controller
Courtesy Precision Digital
In the industrial control sphere, PID stands for "proportional plus integral and derivative control", three actions used together in managing a control loop. Process loop controllers use one, two or all three of these to regulate a process by responding in a prescribed fashion to disturbances in the process variable. PID control is used in a wide variety of applications in industrial control and process system management.

Many types of PID controllers exist on the market and are used for controlling temperature, pressure, flow, and just about every other process variable. Here is a brief explanation of the three actions that make up the PID algorithm, without the math.
PID control algorithm diagram
PID Control Loop Diagram
Proportional Control Action (P): The controller output responds in proportion to an error signal. Think of error as simply the distance between where you are and where you want to be. A larger error value will generate a larger output response from the controller. When the process value (the measured value of what is being controlled) is close to the setpoint, output response is reduced.

Integral Control Action (I): The control system will increase the output if the error is present over a period of time. This is called integral control action. The integral portion of the algorithm helps drive the process value to the setpoint if the process reaches some equilibrium point that is not the setpoint. The purpose of integral action is to provide adequate control response to varying demands of the process. Integral action does not function independently, requiring the inclusion of proportional action too.

Derivative Control Action (D): To achieve a stable process, wide proportional band and low integral action are often needed. Due to these settings, the control system can exhibit too slow a response pattern. If large system disturbances occur over a wide range, additional elements are needed in the control algorithm to provide suitable response. Derivative control action, added to the effect of proportional and integral, provides response to not only the magnitude of deviation, but also the rate of change of the error.

Modern PID loop controllers are often provided with a function that will automatically select the proper constants for the PID parameters. What used to be a very time consuming and tedious job can now be done with the push of a button and allowing the controller to "learn" the process dynamics. PID controllers minimize error and optimize the accuracy of any process.

Share your control challenges and requirements with product specialists and combine your process knowledge with their product expertise to produce the most effective results.