Common types of smart pressure transmitters include general pressure transmitters, differential pressure transmitters, single-flange transmitters, double-flange transmitters, and insertion flange transmitters. Smart Pressure Transmitters can not only measure pressure and differential pressure but also indirectly measure other parameters. For example, pressure transmitters can measure liquid levels inside equipment and, when combined with various throttling elements, can measure flow rates. They are widely used in industries such as petroleum, chemical, metallurgy, power, light industry, and building materials.
Proper Selection of Smart Pressure Transmitters
The selection of Smart Pressure Transmitters is primarily based on the measured medium, measurement range, temperature environment, accuracy level, installation, and maintenance costs.
1. Measurement Medium
For relatively clean fluids, a standard pressure transmitter is sufficient.
For high-viscosity, crystallizing, or highly corrosive media, an isolated transmitter must be used. When selecting, consider the corrosion of the diaphragm material by the medium. Choose the appropriate diaphragm material; otherwise, the diaphragm and flange may corrode quickly, leading to equipment failure and safety hazards. Common diaphragm materials include standard stainless steel, 304 stainless steel, 316L stainless steel, and tantalum.
2. Measurement Range
Transmitters typically have an adjustable range. It is best to set the operating range within 1/4 to 3/4 of the transmitter’s full range to ensure accuracy. This is especially important for low differential pressure transmitters.
In some applications (e.g., liquid level measurement), range migration may be required. Calculate the measurement range and migration amount based on the installation position, and then perform positive or negative migration.
3. Wetted Materials
Choose the appropriate diaphragm material based on the medium’s corrosiveness:
Water, wastewater: 316L stainless steel.
Hydrochloric acid, dechlorinated brine: Tantalum diaphragm.
Undechlorinated brine, wet chlorine gas, chlorine water: Titanium diaphragm.
Low-concentration, low-temperature potassium hydroxide, sodium hypochlorite: Hastelloy C.
Sulfuric acid: Hastelloy B.
High-concentration, high-temperature potassium hydroxide: Nickel diaphragm.
4. Medium Temperature
The signal of a pressure transmitter is converted through electronic circuitry. Generally, the measurement medium temperature ranges from -30°C to +100°C. For higher temperatures, a cooling loop is typically used to cool the medium.
5. Output Signal
Pressure transmitters offer various output signals, including 4–20mA, 0–20mA, 0–10V, and 0–5V. The most commonly used are 4–20mA and 0–10V. Only the 4–20mA signal is two-wire; others are three-wire.
6. Other Considerations
After determining the above parameters, confirm the process connection interface and power supply voltage of the transmitter.
For special applications, consider explosion-proof and protection ratings.
Practical Applications and Common Faults of Smart Pressure Transmitters
After completing the selection and installation of a Smart Pressure Transmitter, subsequent calibration is crucial. The focus of this task is to perform zero and span migration on the transmitter. Only by doing so can it accurately reflect changes in process parameters such as liquid level, pressure, and flow rate.
In practical use, for pressure transmitters, if the wetted diaphragm is not corroded, most faults are caused by zero drift. Technicians can restore normal operation by adjusting the zero point using a handheld communicator. For differential pressure transmitters, two common faults during operation are positive migration and negative migration.
a. Positive Migration Fault
To determine whether a differential pressure transmitter with positive migration is measuring accurately during field use, first close the positive and negative pressure measurement chambers of the transmitter’s three-valve manifold, and open the balance valve and the instrument vent plug. At this point, the instrument output should be below 4mA. If the output is not below 4mA, the positive pressure chamber lead or the three-valve manifold may be partially blocked. Next, close the positive pressure tap and open the vent switch. The output should then be 4mA. If the output is below 4mA, the migration amount may have decreased or the zero point may be too low. If isolation fluid is used, it may not have been filled completely or may have leaked. If the output is above 4mA, it indicates that the migration amount has increased or the zero point is too high.
b. Negative Migration Fault
To determine whether a differential pressure transmitter with negative migration is measuring accurately during field use, first close the positive and negative pressure measurement chambers of the transmitter’s three-valve manifold, and open the balance valve and the instrument vent plug. The instrument output should be 20mA. Next, close the positive and negative pressure taps and open the vent switch. At this point, the instrument output should be 4mA. If the output is not 20mA or 4mA, check whether the positive and negative pressure chamber leads are blocked, whether the migration amount has changed, whether the zero point is accurate, and whether the isolation fluid has been lost.
Accurate monitoring with Smart Pressure Transmitters is essential for the stable operation of production systems. By mastering the proper selection and troubleshooting of Smart Pressure Transmitters, they can be effectively utilized in practical applications, enhancing the performance of the entire automation control system.
