Instruments: [One Question per Week] Fifth Week of 2011

Instrument air blockage, air line valves not open, throttle orifice blocked, regulator leaking air, debris between nozzle baffles

The process reaction control valve cannot be automated. Upon on-site inspection by the instrumentation team, it was found that the actuator of the pneumatic diaphragm control valve was not generating any output pressure. What could be the reasons for this? The amplifier of the locator is clogged, or the nozzle section is blocked.

A similar fault occurred here recently. The possible causes are as follows: 1. The nozzle baffle of the valve positioner was blocked by dirt. 2. The air supply is faulty; there is no air supply. 3 No input 4-20MA signal

1. First, determine whether it is a gas supply fault by checking whether the inlet pressure of the locator is within the specified range (the pressure regulator also needs to be checked)； Check whether the air supply for the positioner’s output is blocked somewhere in the actuator diaphragm section, etc. 2. Determine whether there is power and signal input. 3. Under conditions permitted by the process, for certain positioners switched to manual mode, check whether all air supply is delivered (this step can be used to determine whether it is a problem with the diaphragm mechanism) ； 4. If none of the above issues exist, then it indicates that there is a problem with the locator. 5. It’s best to perform a self-tuning process. If the positioner still doesn’t produce any output, it’s possible that the positioner is damaged; try using another one

1. Blockage in the instrument air line. Since the ball valve acts as a throttle at the end of the instrument branch air line, dirt in the air line tends to accumulate and cause blockages there. This results in too low instrument air pressure, preventing the control valve from opening or closing fully, or even causing the control valve to fail to function at all. 2. Fault in the air filter pressure reducing valve. After long-term use, too much dirt accumulates in the air filtration pressure reducing valve, causing it to leak air. Additionally, the set output pressure of the valve is too low, resulting in the output pressure of the instrument air being below the specified value. This causes the control valve to operate slowly, failing to open or close fully or even not operating at all. 3. Copper pipe connection failure. Ageing of copper pipes leads to air leakage; loose joint connections or debris blocking the pipes result in low signal air pressure, preventing the control valve from operating properly and preventing it from opening or closing fully. In manual mode, the valve position becomes unstable, causing regulation oscillations. 4. Failure of the instrument air system. There is an abnormality in the air compressor station, and the purification air tank of the device is faulty. Inadequate water removal leads to icing in the air lines, while leaks in the instrument air lines or blockages caused by debris result in too low pressure of the instrument air in the device, or even a complete lack of air supply. 5. The valve on the instrument air branch line was not opened, resulting in the control valve failing to operate. It often occurs during major equipment overhauls and during commissioning after modifications.

1. Fault in the power supply circuit (including the safety barrier section), resulting in an open circuit; 2. Water entering the electrical components, causing a short circuit in the circuit； 3. Improper adjustment of the locator, resulting in a minimum output ； 4. The range spring of the positioner is not secured, resulting in a gradual decrease in output ； 5. Torque motor failure; cannot be reset ； 6. Water enters the developed device or there are foreign objects present ； 7. The baffle is broken and cannot make contact with the nozzle, resulting in minimal output.

1. No signal, no air supply. 1. The air supply is not turned on, 2. The air duct is blocked or the filter and pressure regulator are clogged and malfunctioning, 4. There is a leak in the main air supply pipe. 2. There is air supply, but no signal. 1. Positioner failure, 2. Signal tube leakage ； 3. Leak in the locator bellows ； 4. Damage to the regulating retinal membrane. 3. The locator has no air supply. 1. Filter clogged ； 2. Faulty pressure relief valve, 3. Pipeline leakage or blockage. 4. The locator has an air supply but no output. The throttle orifice of the locator is blocked. 5. There is a signal, but no action. 1. The valve core has fallen off; 2. The valve core is stuck to the surrounding structure or to the valve seat ； 3. Valve stem bent or broken ； 4. Frozen valve seat and spool, or blockage by debris ； 5. The actuator spring has rusted due to long-term disuse.

Gas supply failure； The lever inside the locator is stuck.

The main reasons why the locator does not generate output pressure are: 1. The throttle hole is blocked； 2. The positions of the nozzles and baffles are inappropriate ； 3. There is a problem with the actuator ； 4. The cam is not installed in the correct position ； 5. The baffle is not perpendicular to the nozzle end face, and a large amount of exhaust gas is still released after the baffle makes contact with the nozzle end face. 6. There are pits on the end face of the nozzle orifice. 7. The locator is leaking, etc~~~

1. No input signal to the locator. 2. No air supply to the locator. 3. Abnormal operation of the baffle