When choosing the range for a differential pressure (DP) transmitter, it’s essential to consider both process conditions and the transmitter’s technical specifications. Below is a detailed guide outlining the key steps and considerations for selecting an appropriate DP transmitter range.
1. Determine the Measurement Range
- Identify the Maximum Range of Pressure Variation: Start by evaluating the maximum differential pressure the transmitter needs to measure under normal operating conditions. For example, if the level measurement’s maximum differential pressure is 500 kPa, select a transmitter with a range that can at least accommodate this pressure.
- Allow for a Safety Margin: To ensure accurate readings and reduce the need for frequent recalibration or replacement, it’s generally advisable to choose a transmitter range slightly higher than the maximum operating range. A good rule of thumb is to set the upper range limit to about 120% of the maximum required measurement range. For instance, if the process requires a 500 kPa measurement, consider a range up to 600 kPa.
2. Account for the Properties of the Medium
- Corrosive or Contaminated Mediums: For media that are corrosive, highly viscous, or contain impurities, select a DP transmitter made from corrosion-resistant materials or opt for a remote diaphragm seal configuration. This ensures longer operational life and reduces maintenance.
- Hazardous Media: If dealing with flammable, explosive, or toxic media, it’s advisable to use a capillary remote differential pressure transmitter with double diaphragm seals. Additionally, ensure both capillary tubes are of equal length to maintain accuracy.
3. Evaluate Environmental Conditions
- Temperature and Pressure Constraints: Ensure the transmitter’s temperature, humidity, and working pressure ratings align with the operational environment. For high-temperature or high-pressure environments, certain DP transmitters may require special containment, such as isolating or balancing chambers.
- High-Pressure or Extreme Environments: For processes involving extreme temperatures, pressures, or other environmental challenges, select a DP transmitter with proven durability and reliability under such conditions.
4. Determine the Required Accuracy
- Assess the Process Requirements: Select a DP transmitter that meets the accuracy requirements of the specific process. For example, in energy flow metering, accuracy standards are typically governed by standards like GB 17167-2006. Following the requirements of such standards ensures compliance and optimal measurement accuracy.
- Linear vs. Square Root Ranging: If measuring liquid flow, a linear range setting is generally more suitable. For gas or steam flow, a square root range setting is preferable due to the flow dynamics of these media.
5. Check Output Signal and Explosion-Proof Requirements
- Output Compatibility: Ensure the transmitter’s output signal type meets system requirements. Common output types include 4-20mA or HART protocol signals. For compatibility with existing systems, confirm the output signal type prior to selection.
- Explosion-Proof Requirements: In hazardous areas with explosive gases, select explosion-proof DP transmitters. Also, verify the specific explosion-proof type to ensure compatibility with the safety requirements of the installation site.
6. Comprehensive Considerations
- Additional Factors: Beyond technical specifications, consider factors such as cost, stability, reliability, and maintenance demands. Opting for a reliable and stable DP transmitter can reduce operational interruptions and maintenance efforts in the long term.
Practical Example: Selection Guide for a Process Plant
Imagine a process plant where a DP transmitter is required to measure the pressure of a corrosive liquid with a maximum differential of 800 kPa. The plant operates under high humidity conditions with a potential for explosive gases.
- Range Selection: The required range is 800 kPa. Applying a safety margin, a transmitter with a range up to 1000 kPa is selected.
- Material Requirements: Due to the corrosive nature of the medium, a transmitter with a corrosion-resistant remote diaphragm seal is chosen.
- Environmental Suitability: Given the high humidity and explosive potential, an explosion-proof transmitter with compatible humidity tolerance is selected.
By following these considerations, the process plant can ensure accurate, stable, and safe pressure measurements.
Summary and Recommendations
Selecting the right DP transmitter range involves evaluating process conditions, medium characteristics, and environmental constraints. Keep these key points in mind:
- Choose a range slightly higher than the maximum measurement requirement to maintain accuracy and durability.
- Select materials and configurations suitable for the medium, especially if corrosive or hazardous.
- Ensure that environmental factors like temperature, humidity, and pressure are within the transmitter’s specifications.
- For safety, choose explosion-proof transmitters when necessary and ensure signal compatibility with the existing system.
By thoroughly evaluating these factors, you can make a more informed selection, resulting in better measurement accuracy, improved safety, and reduced maintenance in your application.