Pressure Transducer Troubleshooting
Generally, DCS system treats the current higher than 20.2mA or less than 3.8mA as a fault, and the pressure transducer also has fault current settings. For example, it can set the fault current of pressure transducer to 3.6mA or 22.8mA, or setting as needed and related to failure alarm setting. When the output current of pressure transducer is less than 4mA or greater than 20mA, the fault is analyzed and judged in excess of the range. When checking the over-range fault of pressure transducer, firstly check the components related to the process, whether the process parameters have significantly changed, whether the instrument sampling valve and connecting pipes are normal, whether the connecting line or safety grille is faulty or damaged. Then check the pressure transducer itself, whether its range is selected correctly, whether the measuring diaphragm and circuit board break.
1. Output current ≤4mA
When the differential pressure transducer and orifice plate cooperate to measure flow rate, blockage or severe leakage of high pressure connecting pipe in differential pressure transducer makes the pressure in negative pressure chamber higher than that of positive pressure chamber, the output current of pressure transducer is smaller than 4mA. The output of the differential pressure transducer can return normal through the drainage, dredging the connecting pies or sealing the leakage points. Reverse connection of the differential pressure transducer's high pressure and low pressure connecting pipes will make the output current smaller than 4mA, and smart pressure transducer can solve this problem only by selecting reverse output in parameter configuration.
In the production process, the output current of some pressure transducers suddenly does not change, and even if the process parameters change, the output current keeps unchanged at 4mA, please check whether the supply voltage is standard. If there's still no effect after discharging pollution in the connecting pipes, it proves to be that these faults are mostly electronic conversion faults. Long-term operation of diaphragm pressure transducer would change the elastic modulus of diaphragm or isolation diaphragm, thus causing above fault. In most cases, the fault can be repaired, which only needs to disassemble the cover of the pressure transducer's measuring room. When disassembling, please do not damage the sensor, and the elastic modulus of diaphragm or isolation diaphragm can be returned through disassembling, then install the cover. When installing the cover, please properly place the sealing ring between the sensor and the cover, ensure that the measuring room would not leak. The transducer can be used after calibration.
The over-range output fault or zero-display fault of pressure sensor generally occurs in rainy season, and the phenomenon is that the output current is too small, and it returns normal after a while. This fault occurs because water enters into the junction box of the pressure transducer and thus leading to short circuit. When it dries, it returns normally.
2. Output current ≥20mA
The most common reasons for maximum display is an improper selection of the pressure transducer's range or wrong setting of range, or zero shift is not made in liquid level measurement, thus making the output current of the pressure transducer to exceed 20mA. When the circuit of pressure transducer is suspected to be faulty, the circuit board can be replaced to judge the faults. Damage of the pressure transducer's diaphragm will also lead to over-range failure. For example, the output current of the diaphragm pressure transducer used for measuring vapor is greater than 20mA, and it's found after disassembling that some diaphragms have a lot of bumps, some diaphragms deform after freezing or overheated. The failure causes may be abnormal steam tracing which leads to condensate freezing, or long-term contact between the steam and diaphragm box caused by leakage. It also indirectly illustrates the importance of on-site inspection.
Lightning would also lead to over-range of pressure transducer's output current. For example, several monocrystalline silicon transducers in a company break after a thunderstorm day, the head LCD display is normal and the current is also normal after connecting a HART device, but the control room always shows faults. Output current of the transducer is detected to be 40mA with a multimeter, and the circuit boards are found to be damaged.
3. No output
No output of pressure sensor and differential pressure transducer is a common failure phenomenon that instrument maintenance workers often encounter. Combined with the practical maintenance experience of transducers, this paper shares the inspection and treatment methods for no output fault of pressure transducer and differential pressure transducer. The main method is to test pressure transducer with a multimeter step by step.
Maintenance Examples for Pressure Transducer
1. The pressure transducer displays below zero
Fault inspection: In the field inspection, the gauge outfit displays below zero, and the transducer's power supply voltage is only 1V when detecting with a multimeter. Measure the distributor on the instrument panel and the DC24V power supply is normal, so it's judged that the signal line gets faulty. Disassemble the distribution box and measure the voltage, it's still 1V. After powering off, a signal line is to be short-circuited onto the distribution box, and it's found that the main terminal screw was loose, and can't be tightened.
Fault treatment: After replacing the terminal block, it displays to return normal.
Maintenance summary: When the distributor is normally powered but the power supply of field transducer is very low, it's generally caused by the increase of resistance in the signal circuit. And this is mostly caused by poor contact fault. Poor contact has greater resistance, resulting in a huge voltage and poor contact of transducer's power supply. When the voltage can't be measured at the end of pressure transducer, pay more attention to check the signal circuits.
2. When measuring the liquid level with a differential pressure transducer, there's no change for the display.
Fault inspection: Observe the history curve of DCS, and the liquid level curve has not changed. The differential pressure transducer may be blocked, so the transducer flange is disassembled and clean the diaphragm capsule, the diaphragm is found to damage.
Fault treatment: replace the differential pressure transducer, and the liquid level displays normally after zero correction migration.
Maintenance summary: In this case, the measurement medium contains solid particles, which can easily block the liquid level transducer, so it's firstly judged that the fault is caused by crystals on the diaphragm capsule. Actually, the diaphragm is also corroded and damaged.
3. Power on the differential pressure transducer out of the box, it displays 'Er.01'.
Fault inspection: According to the instructions, 'Er.01' means damage to the diaphragm box. However, it's impossible for the new diaphragm box to damage. Open the case and it's found that the connection plug of the driving board connecting amplifier board is not plugged in.
Fault treatment: Plug in and the differential pressure transducer returns normal.
Maintenance summary: It's impossible for not plugging in the wire when completing the product, the plug may drop for vibration during transportation. The transducer consists of four circuit boards, namely the drive board, power board, amplifier board and LCD board. All circuit boards are universal, and only the range needs to be changed according to actual needs.
Of course, the best way to reduce maintenance is to choose a high-quality pressure transducer. ATO Automation can provide various kinds of pressure transducers/sensors and guarantee for twelve months. Choose ATO pressure transducer by click pressure transducer for air/water/oil, high temperature pressure transducer, digital pressure sensor, diaphragm pressure sensor.