The Maintenance of Valve Pneumatic Actuators in Offshore Oil Fields

1.Overview

After the pneumatic actuator is used for a long time in the offshore oil field, there will be some failure problems. Today we will explain how to solve the problems encountered.

2.Problem analysis

(1)Problems with pneumatic actuators(Figure 1).

Figure 1 Corrosion or Damage to Pneumatic Actuator Parts

(2)Due to the harsh climate at sea, the equipment is corroded by seawater and cannot be maintained in time, the metal parts of the actuator are seriously corroded, and it is difficult to disassemble the parts with severe corrosion. Among them, the connecting bolts of the spring cylinder and the fork box are difficult to disassemble (Figure 2).

(a)Connecting bolt

(b) Fork box

Figure 2 Actuator

(3)The device is in the normally open or normally closed position for a long time, and there is no switch action. The failure of the seal causes the accumulation of liquid in the cylinder and the surface of the cylinder wall is seriously corroded (Figure 3).

Figure 3 Cylinder wall corrosion

3.Maintenance measures

(1)Disassembly of the spring cylinder

The cylinder block bolts of the actuator are corroded seriously, especially the outer hexagon nut used for connection, which cannot be removed with a normal wrench.

For this, removal methods such as kerosene infiltration method, heating loosening method, welding nut method and physical cutting method are adopted.

①The kerosene penetration method (WD-40 bolt loosening method) is suitable for connecting bolts and nuts that are not seriously corroded.

②Heating loosening method is used for parts that can be contacted by open flames, such as the cover of the fork box, connecting bolts and nuts with serious corrosion.

③ Welding nut method Weld a complete nut on the corroded connection nut, and then remove it with a tool.

④Physical cutting method Use physical cutting method to remove bolts for connecting screws that cannot be heated. There are pistons, piston rod seals, springs and other parts in the spring cylinder.

Among them, the spring is in a pre-loaded state, and it is extremely dangerous when dismantling and repairing. You can use the step-type nut loosening method to loosen the 4 screws respectively, replace them with longer screws with higher strength, and then slowly loosen the remaining 8 screws. After the springs completely release their elasticity, the disassembly process is completed.

(2)Removal of the fork box cover

Affected by the corrosion of the long-term marine environment and the deformation of the fork shaft, more than 50% of the fork box covers are difficult to disassemble.

The rotary dynamic dismantling method is adopted, the air cylinder is compressed with no dismantling, and the fork shaft rotates slowly + rising gradually under dynamic conditions come out. Heat loosening can also be used.

(3)Mapping and selection of seals

①Material selection refers to its working conditions, operating temperature, corrosion resistance, etc., and selects materials with similar properties.

②According to the structure and sealing principle of the seal, adopt the same reliable method and structure.

③Survey and map groove size, seal size, compression amount, and wear amount to determine parameters comprehensively.

④The selected material and structure can only be used for maintenance after verification.

(4) Repair of hydraulic cylinder

①Corrosion on the outer surface of the cylinder body should be repaired by manual arc welding AW+TIG, polished with sandpaper, and finally coated with a protective layer.

②The inner surface of the cylinder is corroded. The slight defects of the cylinder can be repaired by laser welding, argon arc welding, etc. Severely corroded cylinders that cannot be repaired by welding need to be discarded and replaced.