The control valve is widely used in oil refining and chemical industry. It is used to control the flow rate, pressure and liquid level of the fluid in the production process.In the application of control valves, calculation and selection is the premise, installation and debugging is the key, use and maintenance is the purpose.If the selection of the control valve is not appropriate, improper installation or debugging is not good, it will not play a regulating role.Therefore, it is very important to be familiar with the control valve working principle and maintenance skills.

Principle of control valve

Pneumatic control valve to compressed air as power source, through the cylinder, film as the actuator, and with the help of electrical valve positioner, converter, solenoid valve, position valve and other accessories, to achieve switching or continuous adjustment, receiving DCS control signals to adjust the medium flow, pressure, temperature and other process parameters.Pneumatic valve is mainly divided into two types of gas open type and gas closed type, characterized by simple control, quick response, essential safety, no need to use explosion-proof measures.

Control valve failure mode and cause

The main failure forms of pneumatic control valve are: the internal leakage valve does not act, the opening degree fluctuates greatly, the jam phenomenon appears, the operation is normal but the flow rate does not change.The main reasons are: valve body fault, cylinder fault, locator fault, DCS fault, control cable fault, maintenance is not in place, valve quality problems, selection or design reasons, pneumatic components fault.

01. Valve body failure and countermeasures

Common faults

    1) The metal valve seat of the butterfly valve is damaged and the valve body is stuck;The butterfly valve fixing bolt falls off or the connecting bolt between the spool and the stem of the half ball valve slides. The main performance is that the spool appears to have acted but the spool does not act

    2) Internal wear of the ball valve body causes the valve to be easily stuck or the valve to act slowly;Damaged ball valve soft seal gasket causes valve to leak or get stuck.

    3) The damage of the shaft sleeve of the half ball valve causes the valve to operate difficult.

    4) steam leakage caused by high temperature aging of steam valve stem seal;The steam valve muffler falls off, causing the valve to often get stuck and the pipe to be unusually noisy.

    5) Internal leakage caused by wear or corrosion of valve stem;Damaged seal at stem packing resulting in leakage.

The reason countermeasures

    1) the cause of the spool valve seat leakage is the surface wear or corrosion of the spool valve seat, the actuator force is too small, and the valve seat thread is worn and loosened.Troubleshooting: improve the grinding joint surface, adjust the actuator and stem connection, tighten bolts or replace the spool seat.

    2) The cause of leakage between the seat ring and the valve body is that the tightening torque is too small, the surface finish of the valve body is poor, the gasket is not suitable, and the valve body has small holes or burrs.Fault treatment: increase the twisting force, clean and treat the valve body again, replace the appropriate gasket, and weld the valve body again.

    3) the cause of packing leakage is that the stem finish is not good or bending, the packing cover is not pressed or the packing type is unreasonable, the packing layer is piled too high, the packing appears corrosion pit or the packing cover deformation and damage.Fault treatment: polish valve stem and press straight, tighten or replace packing;Install spacer ring, reduce height, replace with good performance packing, repair and replace gland and associated flange bolts, etc.

    4) The valve stem and the valve vibration cause the valve stem connection to break off or break, the reason is that the torque is too large, the pin connection is not appropriate.Fault treatment mode: use the valve core stem integral part or welded valve core to connect the connector firmly and eliminate the vibration factor.The cause of valve vibration is the leakage of sealing filler, the bypass is not adjusted well, the positioner gain is too high, and the vibration source is not well supported.Fault treatment: lubricate the packing and adjust, readjust the bypass, select the low-gain positioner, and fix the valve firmly to avoid the vibration source.

02. Causes and countermeasures of cylinder failure

Common faults

    1) The actuator limit bolt falls off, resulting in air leakage of the cylinder, which causes the valve to be unable to act or to act slowly, and can not reach the predetermined opening.

    2) The damage to the O-ring of the cylinder leads to air leakage, which makes the valve difficult to operate or unable to operate.

    3) the deformation of the cylinder piston leads to the increase of friction, and the loosening of the cylinder bolt causes the cylinder to leak, thus causing the valve to operate difficult.

    4) The linkage mechanism of the cylinder driving the valve body action is loose or rusted, resulting in difficult valve action.

    5) Small cylinder capacity or cylinder spring failure leads to difficult valve action.

The reason countermeasures

    1) The cause of the leakage at the piston seal is the poor smoothness of the cylinder;The sealing of piston ring assembly is not enough, the temperature of the sealing ring is too high, and the sealing parts are damaged for too long.Fault treatment: grind the cylinder and readjust the cylinder limit bolt, select the sealing ring again according to the use temperature, and install it correctly when replacing.

    2) The treatment of cylinder limit bolt loosening: tighten the bolt again.

    3) the treatment of large piston friction: re-lubricating oil, re-cleaning and grinding cylinder and piston.

    4) the valve action is difficult to reach the fixed stroke, the actuator spring value is not enough or the spring is damaged, the position of the limit block is not accurate.Treatment: calibrate, reschedule and replace the spring.

03. Locator Fault Causes and Countermeasures

Common faults

    1) The fault of the locator circuit board causes the abnormal action of the valve.

    2) The damage or wear of the small slide valve causes the abnormal action of the valve. The reason is that the air cleanliness of the instrument is not high, or dirt enters the positioner and causes the wear of the small slide valve, which causes the abnormal action of the valve.

    3) The damage of the proportional valve of the positioner causes abnormal valve action.

    4) The damage of sliding rheostat leads to inaccurate positioning of the valve and abnormal operation of the valve. The main reason is that the ambient temperature is too high and water and other liquids enter, which leads to the damage of the rheostat.

    5) The O-sealing ring of the air source interface of the positioner is broken, which causes the broken O-sealing ring to jam the stem of the small slide valve and make it unable to act.

    6) The sealing of the gran head of the locator is not good, which leads to the water damage of the locator circuit board.

    7) due to the pipeline vibration is too large, the surrounding environment temperature is too high, humidity is too large, cause the circuit board failure.

The reason countermeasures

    1) The seal problem at the glen head leads to the water inlet of the locator and causes the circuit board damage.Fault treatment: improve the sealing of the gran head, so as to avoid the damage of the circuit board caused by the water inside the locator.

    2) due to the instrument air humidity and more dust, leading to the small slide valve into the dirt caused by the small slide valve wear, so that the valve action is not normal.Trouble handling: add air filter to improve air quality of instrument.

    3) Frequent damage of sliding rheostat and circuit board is caused by excessive ambient temperature, humidity and vibration.Fault treatment: the circuit board and the small slide valve and slide rheostat separation, made of separated valve positioner.

    4) The O-ring of the positioner often breaks, leading to the stem of the small slide valve being stuck. The main reason is that the air source pressure is too large. Normally, it should be about 0.5Mpa, and higher than 0.5Mpa will lead to faster aging of the sealing ring and easy damage.Fault treatment: keep the air source pressure at about 0.4Mpa, and provide better quality sealing ring.

04. Failures caused by process and design and countermeasures

Common faults

    1) The valve cannot be operated due to the large pressure difference in pipeline caused by the change of process parameters.

    2) Scaling of the valve body caused by the medium causes abnormal valve action.

    3) The valve cannot operate normally due to pipeline blockage.

    4) Due to the design and selection reasons, the actuator is small, the torque is not enough, the valve can not operate normally, and the material of the valve body is not appropriate, leading to the damage of the valve body.

The reason countermeasures

    1) Excessive pressure difference causes the control valve to fail to open normally.Treatment: a bypass valve is installed before and after the control valve.

    2) The control valve can not be opened normally due to scaling of the valve body due to process reasons.Treatment: increase the flushing water pipe, in addition to regularly open the valve to clean the valve body, if necessary, grinding the valve core.

    3) The control valve can not operate due to pipeline blockage.Treatment method: optimize the operation method, as far as possible not to clog the pipeline, in addition to open the pipeline flushing valve.

    4) About the design selection.Treatment method: in the design stage, try to optimize a variety of process parameters, to ensure that selection will not cause problems due to inaccurate process parameters.

05. The fault caused by DCS and countermeasures

Common faults

    1) I/O card damage without signal output, resulting in the control valve can not operate.

    2) DCS output signal is unstable, resulting in the control valve opening fluctuation, so that the control valve action is not normal.

    3) the control cable is damaged or the insulation is not good, the wiring is loose and the signal is not normal, which causes the control valve to act abnormal.

    4) Because of the maintenance is not in place, the locator water, cylinder corrosion and air leakage, air source pipe sleeve fall off and air leakage and other reasons cause abnormal action of the control valve.

    5) Because the inspection is not careful, the hidden danger is not found in time, which leads to further expansion, and finally causes abnormal action of the control valve.

The reason countermeasures

    Daily maintenance and inspection should be done well, and hidden dangers should be dealt with in time.When the opening of the control valve fluctuates, timely check whether the PID parameters are set correctly and adjust the parameters in time.

06. Failure caused by damage of pneumatic components and countermeasures

Common faults

    1) The air supply pressure is low due to the aging and damage of the air supply hose or the leakage of the stainless steel air supply pipe, which makes the operation of the control valve abnormal.

    2) Abnormal action of control valve caused by air filter fracture or dirty filter element.

    3) Abnormal action of control valve caused by low air source pressure or closed air source.

The reason countermeasures

    1) replace the air source hose or air filter with better quality;If the air source hose or air filter is found to be damaged or leak during regular inspection, or the filter element is too dirty, it is necessary to find the opportunity to replace it as soon as possible.

    2) Adjust the air source pressure to meet the minimum air source pressure requirements of pneumatic valves.