The generation of static electricity
Static electricity is a common physical phenomenon, which is closely related to our life. Our bodies and clothes will also generate a lot of static electricity, especially in winter. Any two objects of different materials come into contact with each other and then separate; the contact is accompanied by friction, and static electricity will be generated. There are two main key reasons for static electricity: one is the different materials and the other is friction or contact. When two different materials are rubbed, they will be triboelectrically charged. Electrons will be transferred. The object that gains electrons is negatively charged, and the object that loses electrons is positively charged. Two objects of the same material rubbing against each other will not generate static electricity.
Why should the valve be anti-static?
When the valve is soft sealed, the opening and closing parts (balls and valve seats) will generate static electricity at the opening and closing states. Sparks are easily generated when there is static electricity. If the medium is inflammable and explosive, the static spark may cause the medium to burn or explode. The anti-static device can discharge static electricity from the opening and closing parts of the valve to nature. There will be no sparks without static electricity. The fire source required for combustion and explosion will be eliminated without sparks.
API 6D requirements for anti static electricity for valves
On page 23 of API 6D-2014, 5.23 Anti-static device states that an anti-static device should be provided for soft-sealed ball valves, plug valves and gate valves.
API 6D-2014 only stipulates that an anti-static device should be provided for valves with soft seals, because only the non-metallic valve seat (PTFE, PPL, NYLON, DEVLON, PEEK, etc.) of the soft seal is easy to generate static electricity when rubbing against the metal valve core (spheres). The hard-sealed valve seat and valve core (spheres) are all made from metal, and the frictional contact will not generate static electricity. Therefore, there is no requirement.
Antistatic tests
On page 23 of API 6D-2014, 5.23 Antistatic Device states that if specified by the purchaser, this device shall be tested in accordance with Section H.5."
On page 82, H.5 Antistatic Test states that the resistance between the closing member and the valve body, between the stem or shaft and the valve body shall be tested with a DC power supply not exceeding 12V. The resistance measurement should be carried out on a dry valve with a resistance value not exceeding 10Ω before the pressure test.
Anti-static devices
The anti-static device consists of a spring and a steel ball, forming a static spring. Ball valves are generally composed of two sets of static springs. One is distributed on the contact surface between the valve stem and sphere, as shown in Figure B above, and the other is distributed on the contact surface between the valve stem and the valve body, as shown in Figure A above. Because there is a gap between the valve stem and the sphere, the built-in compression spring of the static spring group can ensure that the valve stem and the sphere are always in contact. There will also be a gap between the valve stem and the valve body due to the effect of packing, O-ring and other seals on the center. The built-in compression spring of the electrostatic spring group can ensure that the valve stem and the valve body are always in contact.
In the opening and closing process of the valve, the static electricity generated by the sphere and the valve seat is conducted from the two sets of electrostatic spring groups and the ball to the valve stem and then to the valve body; the valve body is grounded, and finally the static electricity is completely discharged.
Static electricity is a common physical phenomenon, which is closely related to our life. Our bodies and clothes will also generate a lot of static electricity, especially in winter. Any two objects of different materials come into contact with each other and then separate; the contact is accompanied by friction, and static electricity will be generated. There are two main key reasons for static electricity: one is the different materials and the other is friction or contact. When two different materials are rubbed, they will be triboelectrically charged. Electrons will be transferred. The object that gains electrons is negatively charged, and the object that loses electrons is positively charged. Two objects of the same material rubbing against each other will not generate static electricity.
Why should the valve be anti-static?
When the valve is soft sealed, the opening and closing parts (balls and valve seats) will generate static electricity at the opening and closing states. Sparks are easily generated when there is static electricity. If the medium is inflammable and explosive, the static spark may cause the medium to burn or explode. The anti-static device can discharge static electricity from the opening and closing parts of the valve to nature. There will be no sparks without static electricity. The fire source required for combustion and explosion will be eliminated without sparks.
API 6D requirements for anti static electricity for valves
On page 23 of API 6D-2014, 5.23 Anti-static device states that an anti-static device should be provided for soft-sealed ball valves, plug valves and gate valves.
API 6D-2014 only stipulates that an anti-static device should be provided for valves with soft seals, because only the non-metallic valve seat (PTFE, PPL, NYLON, DEVLON, PEEK, etc.) of the soft seal is easy to generate static electricity when rubbing against the metal valve core (spheres). The hard-sealed valve seat and valve core (spheres) are all made from metal, and the frictional contact will not generate static electricity. Therefore, there is no requirement.
Antistatic tests
On page 23 of API 6D-2014, 5.23 Antistatic Device states that if specified by the purchaser, this device shall be tested in accordance with Section H.5."
On page 82, H.5 Antistatic Test states that the resistance between the closing member and the valve body, between the stem or shaft and the valve body shall be tested with a DC power supply not exceeding 12V. The resistance measurement should be carried out on a dry valve with a resistance value not exceeding 10Ω before the pressure test.
Anti-static devices
The anti-static device consists of a spring and a steel ball, forming a static spring. Ball valves are generally composed of two sets of static springs. One is distributed on the contact surface between the valve stem and sphere, as shown in Figure B above, and the other is distributed on the contact surface between the valve stem and the valve body, as shown in Figure A above. Because there is a gap between the valve stem and the sphere, the built-in compression spring of the static spring group can ensure that the valve stem and the sphere are always in contact. There will also be a gap between the valve stem and the valve body due to the effect of packing, O-ring and other seals on the center. The built-in compression spring of the electrostatic spring group can ensure that the valve stem and the valve body are always in contact.
In the opening and closing process of the valve, the static electricity generated by the sphere and the valve seat is conducted from the two sets of electrostatic spring groups and the ball to the valve stem and then to the valve body; the valve body is grounded, and finally the static electricity is completely discharged.
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