When pneumatic diaphragm control valves are running, there will be various kinds of process medium flow through the automatic valve, including the large blocks of raw materials, sand, stones, iron scales and iron sheets washed over by steam, and the slags cut off into the pipe in the process of instruments installation and technical improvement of welding and gas cutting. These foreign bodies and choke materials have great influences on the normal operation and accuracy of automatic valves. As time passes, they will cause automatic valves blocked and struck. Take the following measures when this happens:
1.
Blocking
(1)Cleaning method. This method is suitable for the overhauled and repaired control valves. Disassemble the valves to clear them up and then remove dirt and foreign bodies from them. It is worth noting that when disassembling, be sure to turn off the compressed air to avoid any unnecessary injuries. It is better to change the gaskets and seals of the instruments at the same time.
(2)Flushing method. This method is used more often on the scene. When the valves blocked, you can firstly ask the processing manager or OCC personnel if there is any possibility to switch the valve repeatedly, using the flowing pressure of the existing process media to achieve the purpose of flushing. If existing process media and working condition do not the meet the requirements, flushing with outside steam and water with certain pressures can also work when it meets certain conditions. Pay attention to keep the opening of valve in appropriate position when flushing.
(3)Installing filters. Installing filters for pipes is also the common measure for instruments and automatic valves maintenance. This method is especially used in control valves with high requirements on processing and small diameter to achieve the purpose of the appropriate opening and stable working condition of automatic valves.
2.
Internal leakage
Small iron slags and hard objects staying long and undischarged in the valve seats will make gasket, seal and spool of control valves damaged from different degrees and finally affect the leakage ability of the control valves. A little bit of damage even scratches will cause internal leakage and affect the production safety especially for automatic valves of high pressure pipeline of reaction kettle hydrogen generation process.
(1)Grinding. Select the corresponding coarse and fine sand to grind the valve gaskets and spools of automatic valves to improve the smoothness and engagement of sealing face between the
valve gasket and the spool. If the control valves are used in high pressure system, it's necessary to have a bulge test before putting them into operation.
(2)Enhance actuator sealing capacity. By this way, the closing and sealing of the control valve can be guaranteed. This purpose can be achieved by the working scope of mobile spring, big powered actuator and spring with small stiffness.
3.
Outleakage
The packing of the control valve not being replaced regularly, the loose screws of the locking device of automatic valve due to the vibration, even the screws at the bottom of the valve seat not being tightened or the force on the screws being not uniform will cause the leakage of the control valve. Take the following methods when this happens:
(1)Replace the new filler to enhance the seal capacity of the valve stem or select the sealing grease to seal the valve stem.
(2)Appropriately use high-sealing metal winding gasket or PTFE packing. You must use the metal winding gasket with graphite, especially for the steam control valve.
(3)Use lens gaskets to seal the upper and lower covers. If the seal of valve seat and valve body seal is flat, for achieving remarkable effects, it is suggested using lens gaskets to seal.
4.
The Instability of Control Valve
There are various reasons may lead to the instability such as the unreasonable selection and design, the too small or too large valve diameter, the improper use OCC process operators, unreasonable setting of instrument and process parameters; configuration errors without debugging; increased pressure of processing media, instability of flow, etc. Sometimes it is the consequence of the combined reasons. When this happen, take the following specific methods:
(1)Select the appropriate valve caliber. Taking the economy and time into account, you can also consider to use the appropriate adjustment to increase or reduce the diameter between the control valve and the process pipe.
(2)Set a reasonable PID value and put it into use after practically qualified debugging.
(3)Strengthen the stiffness of the movement mechanism to ease the vibration of the control valve.
(4)Adjust the types of control valve to avoid the phenomenon of resonance.
