Tech Tips August 2006



August 29, 2006

Common valve plug guiding methods.

Accurate guiding of the value plug is necessary for proper alignment with the seat ring and efficient control of the process fluid. The names of the common methods are generally self descriptive, and they include:

  • Cage guiding - The outside of the diameter of the value plug is close to the inside wall surface of the cylindrical cage throughout the travel range. Since bonnet, cage, and seat ring are self-aligning on assembly, correct valve plug/seat ring alignment is assured when the valve closes.

  • Top guiding - Value plug is aligned by a single guide bushing in the bonnet or valve body, or by packing arrangement.

  • Stem guiding - Valve plug is aligned with the seat ring by a guide bushing in the bonnet that acts on the valve plug stem.

  • Top-and-bottom guiding -Valve plug is aligned by guide bushings in the bonnet and bottom flange.

  • Port guiding - Valve plug is aligned by the valve body port. This construction is typical for control valves using small-diameter valve plugs with fluted shirt projections to control low flow rates.

Source: Control Valve Handbook, Third Edition, Fisher Controls International and Emerson Process Management, Marshalltown, IA, 2003, p. 59-60. 52.


August 22, 2006

Measuring moisture accurately

Moisture measurements are critical to the control of many processes. Excessive moisture can result in loss or poor quality of product, significant downtime of a process, or corrosion and damage to critical equipment, resulting in costly repairs or loss of revenue. To minimize process moisture problems, reliable, on-line moisture measurement is needed. Several basics concepts should be considered to achieve accurate moisture measurements.

Sensor/analyzer maintenance
As part of a normal maintenance cycle, moisture sensors should be inspected and calibrated to ensure that the accuracy and performance of the sensor are maximized. Periodic sensor calibration provides information to identify potential problems with the application, such as sensor corrosion.

Some moisture analyzers are provided with on-board calibration systems, which can be complex, expensive, and require further maintenance. Also, due to the polar nature of the water molecule and its ability to adsorb to wetted surfaces, certified moisture standards in gas cylinders are generally unavailable for field calibration. Consequently, most moisture sensors are typically returned to the manufacturer or suitable third-party laboratory for traceable calibration to national standards.

Although some moisture probes can be installed directly in-line, they will not withstand the long-term rigors of the process. For this reason, most installations use sample-conditioning systems to expose the moisture sensor to the process fluid. Sample systems provide the ability to isolate the moisture sensor, filter contaminants, meet hazardous area requirements, provide environmental protection, and control process conditions at the moisture sensor, such a temperature, pressure, and flow rate.

Typically, a sample tap is installed in the process pipeline, and connected to the sample system by a short, continuous piece of tubing. Stainless steel wetted parts and tubing are recommended because stainless steel has excellent adsorption/desorption properties with respect to water molecules, and is not susceptible to permeation of ambient moisture. Sample systems should be periodically leak-tested to prevent leakage of ambient moisture into the system; and filter elements should be cleaned or replaced as part of routine maintenance.

When reviewing a moisture analyzer installation, it is important have a good, practical understanding of moisture measurement units. For gas applications, the two most common units of measure are dew point temperature and parts per million by volume (PPMV). Depending on the application, one measurement unit may be more applicable than the other for interpreting moisture measurements.

For example, in cryogenic natural gas processes, gas flows through a 'cold box' maintained at a very low temperature to recover heavy hydrocarbon products. It would be practical to compare the dew-point temperature to the cold-box temperature to ensure that the dew-point temperature is lower than the cold-box temperature to prevent frost formation within the cold box.

Another example involves comparison of moisture measurements conducted at two pressures. Many moisture analyzers can only measure moisture content at atmospheric pressure, while a few analyzers are capable of measurement at line pressure. Dewpoint temperature is functionally equivalent to water vapor pressure. Since water vapor pressure is one component of the total gas pressure, dew-point temperature will increase or decrease with corresponding changes in total pressure. Consequently, comparing dew-point temperature readings at two different pressures can lead to confusion in understanding the moisture measurement reading. In this situation, it is beneficial to use a PPMV reading, which is not dependent on system pressure, to determine if the analyzers are in agreement. By comparing the PPMV reading, the problems of the dew-point temperature to pressure relationship can be avoided.

John Kerney, hygrometry product manager, Panametrics Inc., Waltham, MA

Source: John Kerney, 'Measure moisture accurately,' Back to Basics, Control Engineering, Aug. '02, p. 12.


August 15, 2006

Avoiding common mistakes in automation purchasing—Part 2

Buying automation equipment can be a complex process. Experience gained from common purchasing mistakes serves as a guide to avoid them in the future. Here are additional points to help along that road. (The first five points appeared in the previous 'Tip of the Week.')

6. Poor communication with the automation vendor —Even with detailed equipment specifications submitted to the vendor, constant, constructive communication must be maintained. 'Constructive communication' is the operating term; simply documenting all conversations and responding to written correspondence to maintain good records is not nearly enough.

7. Accepting automation equipment from the vendor before it's ready —Don't allow shipment of the machine before it is ready. This usually prolongs the automation system from perforating according to plan, and damages vendor/company relationship, leading to extra costs in the long and short run.

8. Failure to supply the vendor with latest drawings and parts within specification —Maintaining up-to-date documentation is an ongoing challenge for most companies. Failure to supply the vendor with sufficient, up-to-date project drawings will cause expensive delays, since nonconformance from parts to prints will not always be detected until it is too late, causing rework.

9. Failure to design for automation —Some products are not designed for automatic manufacturing or assembly methods. When automation is difficult, perhaps a semi-automatic solution makes more sense.

10. Using the wrong technology for your application —Failure of a project engineer to do the required homework could result in less than efficient use of equipment. Is there off-the-shelf equipment available for your application? Should flexible or hard automation be used? These are types of questions to be answered before building machine.

Source: May 2006 Packaging Automation+Controls supplement to Control Engineering, and Packaging Digest, 'Ten common mistakes in purchasing.'

August 8, 2006


Avoiding common mistakes in automation purchasing—Part 1

Buying automation equipment can be a complex process. Experience gained from common purchasing mistakes serves as a guide to avoid them in the future. Here are five points to help along that road:

  1. No equipment specification —Failure to define your company's expectations about performance, esthetics, and hardware preferences will lead to confusion and misunderstandings. A detailed equipment spec will force the project engineer to look at all aspects of the project.

  2. Failure to visit prospective automation houses before the quoting process starts —Quotation requests often go out with little prior knowledge about the automation company. A visit to the supplier early in the process helps ensure that you're looking at viable companies and solutions.

  3. Incorrectly estimating automation project cost —Presenting an underestimated automation project for internal management approval and receiving it leads to the possibility of having to look for the right price rather than the right solution. Requesting a quotation from a couple of automation houses provides a more accurate cost estimate and may prevent a nonviable project startup.

  4. Insufficient in-house technical capability —Companies have been known to buy a piece of automation without fully considering technical expertise needed to maintain the equipment. Be sure to consider all costs associated with new or unfamiliar technology.

  5. Failure to involve production staff in the purchase process —People responsible for ultimately operating the automation system can make the machine look good or bad. Allow production people to be involved early in the project.

Additional guidelines will appear in the next 'Tip of the Week.'

Source: May 2006 Packaging Automation+Controls supplement to Control Engineering, and Packaging Digest, 'Ten common mistakes in purchasing.'

August 1, 2006


Thermistor pros and cons

A thermistor is a common type of sensor for measuring temperature. It offers distinct advantages as well as some tradeoffs compared to other temperature measuring sensors.

Thermistors typically offer higher sensitivities than either RTDs (resistance temperature devices) or thermocouples. They have a negative temperature coefficient (in general)—where device resistance decreases with increasing temperatures. Thermistors are also less linear than RTDs, requiring a correction for linearity.

Though generally nonlinear, much development has gone into making thermistors linear. Traditional methods involve use of external matching resistors to linearize thermistor characteristic curves, but linearization concerns are becoming less important because modern data-acquisition systems offer built-in correction features, eliminating the need for hardware linearization.

Thermistors are simple to set up and operate, using a two-wire measurement scheme. However, here the lead resistance is part of the measurement, which can cause some error. (In the four-wire technique, current is sourced on one set of leads, while voltage is sensed on a different set of leads. Voltage is sensed at a different spot from the source current so that test lead resistance is completely out of the measurement path.) Thermistors also provide fast response to temperature changes because of their small size.

On the down side, nonlinear properties of thermistors require linearization. They also have a limited temperature range and are fragile. Because they are semiconductors, thermistors are more likely to have de-calibration issues at high temperatures.

Thermistors further require a current source and have self-heating characteristics, which must be recognized. Not allowing for self heating, and selecting a device with an inadequate temperature range are common mistakes made when using thermistors. Self-heating effects can be reduced through several approaches, for example, use of the smallest possible test current and using a pulse-current method instead of a continuous current.

Source: Control Engineering, May 2006 Application update, ' Basics of temperature measurement-thermistors '

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