1. Pressure Interlock Fault
Fault Phenomenon:
Fault Analysis:
Normally, when the heavy oil main pipeline pressure drops to a certain value, the standby pump should start automatically to maintain a certain flow rate and pressure of heavy oil. The failure of the standby pump to start indicates that it did not receive the signal of pressure drop, meaning that the pressure transmitter in this system did not detect the change in main pipeline pressure.
The investigation revealed that the isolation fluid inside the impulse line was drained, allowing heavy oil to enter the impulse line and the positive chamber of the transmitter. Since isolation fluid was used to measure the main pipeline pressure, and no heat tracing was applied to the impulse line and instrument, the heavy oil, which has a relatively low solidification point, solidified inside the impulse line and diaphragm chamber, preventing it from sensing and transmitting changes in main pipeline pressure. Additionally, due to the expansion of solidified heavy oil, the sensor element indicated an excessively high force and maintained this value.
When the main pipeline pressure dropped, this value remained unchanged, causing the standby pump not to start, ultimately leading to a shutdown incident.
Handling Method:
2. Pressure Indicator Returning to Zero
Due to significant pressure fluctuations during normal operation, the system used a method of opening up the primary pressure tap valve and controlling resistance with a needle valve to reduce the fluctuation of the instrument’s indication. However, because the instrumentation technician did not fully understand the specifics of this system, they reduced the primary pressure tap valve size when the instrument indication fluctuated too much. Given the large diameter of the primary pressure tap valve, it was difficult to control precisely. Once the primary pressure tap valve was closed down to a point where the pressure indication fluctuation was within an acceptable range.
the valve was essentially fully closed. This issue was not noticed during regular operations. After turning off the insulation heating, the impulse line cooled down, causing the originally vaporized medium inside to condense into liquid, reducing in volume and causing a sudden drop in pressure (almost to zero). If the pressure tap valve was not fully closed, the condensed liquid from the tower would refill the impulse line, maintaining pressure consistency between the instrument and the tower.
With the valve fully closed, it became a sealed container. During insulation, the medium was in a vaporized state with higher or stable pressure. When the temperature dropped, the medium liquefied, causing the pressure to decrease and the indication to return to zero. The zero signal from the instrument caused the control valve to close completely, leading to a rapid increase in tower level and resulting in a shutdown incident.
Handling Method:
3. Inaccurate Pressure Measurement
Fault Phenomenon:
Fault Analysis:
Handling Method:
4. Capillary Tube Breakage in Single Flange Pressure Measurement Instruments
Fault Phenomenon:
A facility using a single flange for pressure measurement experienced a capillary tube breakage, resulting in a system shutdown.
Fault Analysis:
Mechanical damage: The single flange capillary pressure measurement instrument consists of a flange, capillary tube, and transmitter. Any mishandling during transportation, calibration, or installation can cause damage to the capillary tube. Additionally, activities around the instrument during its use can also lead to accidental damage.
Vibration damage: If the instrument is installed in a part of the high-pressure circuit that experiences significant vibration, the capillary tube may fatigue and break over time due to continuous vibration.
Material issues: If the measured medium is corrosive or prone to crystallization, improper material selection for the capillary tube can lead to problems such as: firstly, if the single flange diaphragm is damaged, the measuring medium can quickly enter the capillary tube, causing internal corrosion and breakage; secondly, the presence of corrosive substances like ammonia in the measuring environment can also cause external corrosion and breakage of the capillary tube.
Quality issues: Inappropriate welding materials or poor welding between the capillary tube, transmitter, and flange can also result in capillary tube breakage.
- Improper selection of filling silicone oil: Insufficient filling of silicone oil can lead to diaphragm puncture, causing the capillary tube to break.
Handling Method:
5. Large Fluctuations in Furnace Negative Pressure During Heavy Wind and Rain
Fault Phenomenon:
Fault Analysis:
Furnace negative pressure is a strictly controlled process parameter in production processes, which should not fluctuate significantly. The reasons for significant fluctuations in negative pressure under conditions of high winds and heavy rain are as follows.
- Rainwater infiltration on the negative pressure side during rainy days.
- As the furnace negative pressure is very low (around -80Pa), differential pressure transmitters are generally used to measure it. Strong winds alter the force acting on the transmitter, especially under irregular wind speeds, causing significant fluctuations in the transmitter’s input signal. This results in fluctuation of the regulator output and the actuator, creating a vicious cycle of negative feedback that leads to significant fluctuations in the instrument indication.
Handling Methods:
- Modify the installation method of the transmitter by adding a short pressure guiding pipe to the atmospheric end on the negative pressure side, pointing downwards, or redirect the outlet of the transmitter’s negative chamber to a sheltered area to prevent rainwater from forming static pressure and avoid the impact of strong winds on the instrument.
- Adding an air capacitor to the output line of the transmitter can also help reduce indication fluctuations.
6.Indication of the negative pressure in the cracking furnace chamber is lower than expected.
Fault Phenomenon:
Fault Analysis:
Where p is the indicated pressure of the differential pressure transmitter.
Handling Methods:
7.Pressure Measurement Indication Fluctuations
Fault Phenomenon:
Fault Analysis:
Handling Methods:
8.Bypass Shut-off Valve Leakage Causes Low Indication
Fault Phenomenon:
Fault Analysis:
Handling Methods:
9.Explanation Using a Steam Pressure Regulation System as an Example:
The steam pipeline pressure recording suddenly drops to zero, and the safety valve trips. When a fault occurs between meters causing sudden changes in the control valve’s opening degree, it results in a sharp increase in steam pressure, but the recording instrument does not respond. In such cases, switch to manual remote control of the control valve before addressing the fault.
If the steam pipeline pressure recorded value does not exceed the setpoint yet the safety valve trips, instrumentation personnel should compare relevant instruments. If all points’ temperatures are normal, it indicates that the safety valve is improperly adjusted. If the temperature values at various points are high, it suggests that the pressure recording value is lower than the actual pressure.
For large but slow pressure fluctuations, the cause should generally be sought from the process side.
For rapid oscillatory pressure fluctuations, investigate parameter tuning and issues intrinsic to the instruments themselves.
Variations in load, reflux, temperature, and improper operation can all lead to changes in internal equipment pressure. It is necessary to look for causes from the perspective of process operation.
- One should be aware of the usual pressure fluctuation patterns of each instrument, distinguishing between abnormal and normal conditions, and use other process parameters as references for judgment.
Safe Replacement and Installation of a Pressure Gauge
Work Process:
Prepare tools and equipment — Record pressure data — Close the root valve of the pressure gauge — Open the pressure relief valve to completely release pressure — Dismantle the old pressure gauge — Clean out deformed old gaskets and debris — Install the new pressure gauge (with gasket) — Close the pressure relief valve — Slowly open the root valve of the pressure gauge — Leak test — Confirm and record the pressure values, serial numbers of the old and new pressure gauges — Clean up the site and recover tools and equipment.
Tools and Equipment Preparation:
Pressure Gauge Inspection:
- Ensure the seal is intact.
- Verify that it is within the valid inspection period.
- Check if the pointer returns to zero.
- Gently tap to ensure there’s no displacement.
- Ensure all screws are tightened.
- Check that dust holes and ventilation holes are unobstructed and that screws are undamaged.
- Confirm that the accuracy class meets requirements.
- Ensure the dial has no cracks and the scales are clear.
Dismantling the Pressure Gauge:
- Slowly close the shut-off valve of the pressure gauge and wait until the pointer returns to zero before dismantling. Use an adjustable wrench to hold the gauge fitting and an open-end wrench to remove the pressure gauge without damaging it.
- After loosening the pressure gauge, support it with your hand while gently swaying it side-to-side to release any remaining pressure.
- Place the removed pressure gauge in a dry, dust-free location for inspection by relevant departments.
- Use cotton cloth and a screwdriver to clean debris inside the gauge fitting, check if the threads are intact, and ensure the pressure tapping hole is not blocked. If the fitting is normal, tighten it without replacement.
- Record pressure data, close the root valve of the pressure gauge, slowly open the pressure relief valve, and once pressure is fully released, use an adjustable wrench to secure the root valve and an open-end wrench to remove the pressure gauge.
- Support the loosened pressure gauge with your hand, gently sway it side-to-side to release any remaining pressure.
- Place the removed pressure gauge on a large cloth (or in a pressure gauge box).
- Use cotton cloth and a hook needle to clean debris inside the gauge cock, check if the threads are intact, and ensure the pressure tapping hole is unobstructed.
Risk Warning:
- Use an adjustable wrench and a fixed wrench to dismantle the old pressure gauge in the correct direction. When the pressure gauge becomes very loose from the fitting, hold the gauge with one hand while gently twisting and slightly shaking it to release any remaining pressure inside the gauge.
- Clear any sealing tape and debris from inside the pressure gauge fitting to prevent blockage of the gauge’s pressure inlet.
- Wrap the qualified pressure gauge’s threads with PTFE tape (Teflon tape) in a counterclockwise direction to avoid blocking the pressure inlet. Pay special attention not to cover the inlet hole with the tape.
- Wind the PTFE tape around the threads in a clockwise direction for 3-5 turns.
- Hold the pressure gauge steady on the fitting with your hand, and initially tighten it by hand for several turns to ensure correct alignment. Then, use a wrench to fully tighten the gauge.
- Ensure that the orientation of the pressure gauge is parallel to the pipeline for easy observation.
- Hold the pressure gauge steady and align it properly, then mount the gauge onto the fitting. Turn it by hand for several threads to ensure it is correctly aligned. Use both a fixed wrench and an adjustable wrench to securely tighten the pressure gauge.
- Close the pressure gauge cock’s pressure relief valve, then slowly open the root valve to allow pressure to rise gradually. This prevents sudden pressure surges that could damage the gauge pointer.
- Once the working pressure is reached, ensure it falls within the 1/3 to 2/3 range of the pressure gauge’s capacity. The system should neither leak nor seep at this pressure level to be considered qualified.
- Record the pressure values along with the serial numbers of the new and old pressure gauges. Clean up the site and recover all tools and equipment used.
Effective pressure instrument management in chemical production requires a proactive approach to troubleshooting industrial pressure systems and addressing root causes like improper calibration, environmental stress, or material incompatibility. By selecting vibration-resistant gauges for high-impact zones, ensuring proper installation with accessories like siphon tubes, and conducting regular maintenance (e.g., draining condensate or clearing clogged impulse lines), facilities can mitigate risks of pressure interlock faults or measurement inaccuracies. For specialized applications—such as hydrogen pipelines or steam systems—always prioritize industry-specific gauges and validate their performance under extreme conditions. Invest in training personnel to recognize early warning signs, such as erratic pointer movements or silicone oil discoloration, to prevent catastrophic failures and maintain operational integrity.
