Factors affecting normal operation of control valve and Countermeasures

Factors affecting normal operation of control valve and Countermeasures

During the use of the control valve, some abnormal operation problems often occur. What measures should we take to deal with these problems? Zhongzhi valve summarizes several common problems and corresponding solutions for your reference!

1、 The control valve is blocked

The frequent problem of the regulating valve is jamming, which often occurs in the new operation system and the initial stage of overhaul and operation. Due to the blockage of welding slag and rust in the pipeline at the throttle and guide parts, the medium flow is not smooth, or the filler is too tight during the maintenance of the regulating valve, resulting in the increase of friction, resulting in the inaction of small signals and excessive action of large signals.

Troubleshooting: quickly open and close the auxiliary line or regulating valve, so that the dirt can be washed away by the medium from the auxiliary line or ball valve. Another way is to clamp the valve stem with a pipe wrench. Under the condition of external signal pressure, rotate the valve stem with positive and negative force to make the valve core flash past the card. If not, increase the air supply pressure, increase the driving power, and move up and down repeatedly for several times to solve the problem. If it still does not act, it needs to be disassembled.

2、 Leakage of control valve

2.1 the valve has internal leakage and the length of the valve stem is inappropriate. When the air valve is opened, the valve stem is too long and the upward (or downward) distance of the valve stem is not enough, resulting in a gap between the valve core and the butterfly valve seat, which can not be fully contacted, resulting in loose closing and internal leakage. Similarly, the valve stem of the air shutoff valve is too short, resulting in a gap between the valve core and the valve seat, which can not be fully contacted, resulting in loose closing and internal leakage.

Solution: shorten (or extend) the valve stem of the regulating valve to make the length of the regulating valve appropriate so that it will no longer leak.

2.2 packing leakage. After the packing is installed in the stuffing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it produces radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts are loose, some parts are tight, and even some parts are not in contact. During the use of gate valve, there is relative movement between valve stem and packing, which is called axial movement. In the process of use, with the influence of high temperature, high pressure and highly permeable fluid medium, the stuffing box of control valve is also a place with more leakage. The main cause of packing leakage is interface leakage, and there will be leakage for textile packing (pressure medium leaks outward along the small gap between packing fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the packing contact pressure and the aging of the packing itself. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.

Solution: in order to make the packing easy to install, chamfer the top of the stuffing box, and place a metal protective ring with small erosion resistance gap at the bottom of the stuffing box (the contact surface between the stop valve and the packing cannot be an inclined plane) to prevent the packing from being pushed out by the medium pressure. The metal surface of each part of the stuffing box in contact with the stuffing shall be finished to improve the surface finish and reduce the wear of the stuffing. Flexible graphite is selected as packing because of its good air tightness, small friction, small change after long-term use, small burning loss of wear, easy maintenance, no change in friction after re tightening of gland bolts, good pressure resistance and heat resistance, no erosion by internal media, and no pitting or corrosion of metal in contact with valve stem and stuffing box. This effectively protects the sealing of the valve stem stuffing box of the pressure reducing valve and ensures the reliability and long-term sealing of the packing.

2.3 deformation and leakage of valve core and valve seat. The main reason for the leakage of core and valve seat is that the casting or forging defects in the production process of regulating valve can lead to the strengthening of corrosion. The passage of corrosive medium and the scouring of fluid medium can also cause the leakage of control valve. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will erode and impact the valve core and valve seat materials, making the marine valve core and valve seat oval or other shapes. With the passage of time, the valve core and valve seat are not matched, there is clearance, and leakage occurs if they are not tightly closed.

Solution: the key is to control the selection and quality of valve core and valve seat materials. Select corrosion-resistant materials and resolutely eliminate products with pitting, trachoma and other defects. If the deformation of valve core and valve seat is not too serious, it can be ground with fine sandpaper to eliminate traces and improve sealing smoothness, so as to improve sealing performance. If the damage is serious, replace the valve with a new one.

3、 Control valve oscillation

The spring stiffness of the regulating valve is insufficient, the output signal of the regulating valve is unstable and the fluorine lined rubber lined valve changes sharply, which is easy to cause the oscillation of the regulating valve. It is also said that the frequency of the selected valve is the same as the system frequency, or the pipeline and base vibrate violently, which makes the regulating valve vibrate accordingly. Improper type selection, there are sharp changes in flow resistance, flow rate and pressure when the regulating valve works at a small opening. When the valve stiffness is exceeded, the stability becomes poor and oscillates seriously.

Solution: as the causes of oscillation are various, the specific problems are analyzed in detail. For slight vibration, the stiffness can be increased to eliminate it. If the ceramic valve uses a large stiffness spring, use a piston actuator. Severe vibration of pipeline and foundation, and eliminate vibration interference by adding support; If the frequency of valve selection is the same as the system frequency, replace valves with different structures; The oscillation caused by working at a small opening is due to improper selection, and the C value of flow capacity is selected to be large. It is necessary to re select the one with small C value of flow capacity, or adopt split range control or sub parent valve to overcome the problem that the regulating valve works at a small opening.