【Discussion】Selection of actuator for quick shut-off valves in tank areas
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It has been learned that in many large tank farms in China today, such as those used for refined oil and crude oil by CNPC and Sinopec, electric actuators are generally used, and design institutes also prefer to use electric actuators when designing such tank farms. But given the fire that occurred at the Dalian port earlier, would it be safer to use pneumatic actuators in the tank area? Everyone, feel free to share your thoughts~
Pneumatic valves allow for quick opening and closing; therefore, double-cylinder double-acting valves can be used to achieve this rapid operation. For pneumatic valves, the solenoids that may generate sparks as well as the components for signal feedback should be purchased from foreign manufacturers that offer excellent explosion protection and robust safety features.
From the perspective of safety and routine maintenance, which is more advantageous, pneumatic actuators or electric actuators?
Reply to 3# zhaoliyin: I think electric valves have advantages, as they can operate as long as there is electricity available. In contrast, pneumatic valves require both electricity and air, which means an additional source of energy is needed; as a result, their cost and maintenance requirements are higher than those of electric valves
It is best to install electric shut-off valves outside the tank area cofferdam.
Many people believe that pneumatic systems are advantageous primarily because they are safe; larger valves require substantial air supply pipes for pneumatic shut-off valves. Electric motors have great power, but it is difficult to maintain position. I think pneumatic systems are better in chemical plants; if greater safety is desired, additional investment can be made in key areas to install electric systems as well.
Of course, pneumatic valves should be chosen – they have a fault indication location, whereas electric valves do not! I believe the greatest advantage of electric valves is the ability to bury cables, which ensures a stable power supply.
There’s no need to worry; pneumatic systems are used for tank areas, and cut-off valves are now very well-developed. There are also many ways of cutting off and linking. It is safer than electric versions, especially with first-level standardization inspections; it’s sufficient to use accessories that meet the explosion-proof standards
In CNPC’s tank farms, the cut-off valves are highly advanced in design; where there is a reliable source of instrument air, pneumatic actuators are used； Choose explosion-proof electric actuators for remote locations. Currently, there are also spring-return, single-acting models of explosion-proof electric actuators.
It must be difficult to achieve rapid opening and closing of electric valves, and the cost is very high; for safety purposes, it is recommended to use single-acting cylinders.
Note that since it is a tank farm isolation, according to safety instrument design specifications, pneumatic systems are preferred; electric systems would require additional energy storage, which may not be appropriate； Typical design institutes use pneumatic cutting; since rapid cutting requires a single-acting mechanism, a double-acting one cannot achieve fail-safe operation unless an air reservoir is added. That is, both power loss and air loss will cause it to stop in a safe position ； Solenoids and valve position feedback devices should be selected according to the explosion protection rating of the area in which they are used; it is advisable to choose solenoids with low power consumption, and their protection rating should generally be IP65 or higher. If the isolation valve is dedicated to the SIS system, a valve with the corresponding SIL rating must be used; and if it is equipped with a handwheel, it must be removed or sealed when the system is put into operation, as manual operation is not permitted during normal use.