Leakage and solution at the level control valve packing

Leakage and solution at the level control valve packing

April 18, 2019

External leakage of pneumatic control valve

The external leakage of the pneumatic regulating valve is easy to find. For example, the packing gland is not pressed, the PTFE packing is deteriorated, and the sealing gasket is damaged. The fastening nut between the valve body and the upper and lower valve covers is loose. The packing gland is not pressed except that the bolt itself is loose. Most of the packing is not enough. The packing should be added before tightening. The double or multi-layer mixed packing method is better. The PTFE filler is easily aged and deteriorated by the influence of temperature. It can be replaced by flexible graphite filler. Without the valve using sealing grease, the sealing grease can be tried to improve the sealing performance of the valve stem. When replacing the new packing, take out the old packing. The steps are: separate the actuator from the valve body; remove the upper bonnet and remove the valve stem and the valve core; use a tube slightly thicker than the valve stem. Insert the bottom of the packing and force the old packing out from the top of the upper bonnet. Separate packing does not require disassembly of the actuator, picking or hooking the old packing with a pointed tool. For level control valve valves with high pressure and large pressure difference, the valve stem is easy to leak. After replacing the packing or pressing the packing gland, it will take a long time to leak. The flow of the regulating valve should be verified. The type of medium flows from the large end to the small end of the spool, and the "flow-through" medium flows from the small end of the spool to the big end. The "flow-through type" can be changed to "flow-open type" to improve the stem sealing effect. The method actually replaces the front and rear pressures P1 and P2 of the valve core. When P2 is at the valve stem end, the sealing performance of the valve stem is good. As shown in Figure 10-4.

Figure 10-4 Schematic diagram of the level control valve spool and medium flow direction

the internal leakage of the pneumatic control valve

The most obvious phenomenon of internal leakage of the pneumatic regulating valve is that the valve is not dead. The failure of the valve to cause the valve to close does not occur. Most of the failures occur in valves that are newly installed. For example, when the process piping is purged, the shut-off valve before and after the regulating valve is not closed, or there is no bypass. Many times it is necessary to remove the valve for inspection, cleaning and removal of the blockage. In the case of easy crystallization or easy clogging, the structure of the valve can be changed to reduce the phenomenon of blockage. The straight-through single and double-seat valves can be changed to sleeve valves, and angled valves with self-cleaning performance can be used.
The pressure difference between the front and the rear of the pneumatic regulating valve is large, and the output force of the actuator is not enough, and the internal leakage fault occurs. The reason why the pressure difference should be checked is the process factor, or the selection is wrong. Increasing the output force of the pneumatic actuator is a common method to improve the sealing performance, such as adjusting the working range of the spring and switching to a spring with a small stiffness; Increase the positioner to increase the pressure of the air source; if there is still no change, you can only use a pneumatic actuator with a larger thrust.
Corrosion and wear of the valve plug or seat are the main causes of leakage inside the valve. The damaged valve core and valve seat need to be repaired or replaced during maintenance. The pneumatic control valve is due to cavitation wear. The flow-closed valve can be used instead, and the orifice plate is installed at the outlet of the regulating valve to maintain the regulating valve. The downstream side pressure exceeds the vapor pressure of the liquid to prevent bubbles from forming in the high flow rate low pressure zone; for large differential pressures, two regulator valves can be used in series to distribute and define the pressure drop of the valve.

pneumatic control valve can not reach the rated stroke

This fault occurs frequently when the pneumatic control valve is used for a period of time. The stroke of the valve is adjusted first. Failure to meet the requirements, usually the actuator or accessories leak, the rigidity of the actuator spring can not meet the requirements, the push rod or valve stem is bent, deformed, the valve core is damaged or the valve core, there is debris in the valve seat, the medium flow direction is not Correct, the friction of the filler is too large, etc. caused by the original. Damaged springs, push rods, stems, spools, and seats should be replaced.
Check if the air supply pressure meets the requirements; check the actuator attachment for leaks and check with soapy water. The media flow is not correct and can be reversed. When the friction of the packing is too large, the packing gland can be loosened to lubricate, and the valve stem can be rotated to reduce the resistance. The correct adjustment of the valve positioner or electrical converter cannot be ignored. Also check that the manual mechanism or travel stop is in the correct position and adjust if necessary.