Selection of Electric Actuators

In the petroleum and petrochemical industries, problems with electric valves are common, and the causes are multifaceted, including the selection of valves and electric actuators, torque matching, and other production and manufacturing factors. Since the operating environment of electric valves is relatively complex, the harsh conditions can significantly impact their performance. If quality control is not strengthened during production, manufacturing, and inspection, the electric valves may experience significant operational failures due to both external and internal pressures. To avoid similar accidents and minimize harm and losses, it is necessary to strictly control the production of electric valves, summarize the problems that occur during production and use, investigate their causes, and find reasonable solutions. Initially, the focus was on the selection of electric actuators.
 

Electric actuators are classified into two types of operation: manual and electric. The actuator can be switched between manual and electric modes using a manual/automatic switch. The motor's rotation drives the worm through the coupling. The worm, in turn, drives the worm wheel, which then rotates the output shaft via the clutch. When the switch handle is set to the manual position, the clutch disengages the worm gear from the handwheel, allowing the handwheel to rotate and drive the output shaft. Electric operation is generally preferred. In the event of a fault, the system switches to manual operation. As the output shaft rotates, the bevel gear also rotates, and the valve position, along with changes in the valve core within the valve body, is detected by the valve position sensor.
 

When selecting the built-in components of the electric actuator, factors such as the location of use, medium type, pressure, temperature, and material should be fully considered. Otherwise, failures may occur and unnecessary losses may result. The analysis is as follows:

(1) Due to the low ambient temperature on site (with the local monthly average minimum temperature being -26℃), the actuator's storage battery discharges too quickly. Additionally, if the power supply is cut off, the battery cannot be charged in time. The LCD displays data from the last valve position.

 

(2) The built-in battery of the electric actuator powers the LCD display and stores the valve position parameters. However, if the electric operation fails due to a working voltage lower than the battery setting, the valve can only be operated manually. The valve position during manual operation is not recorded, resulting in inconsistencies between the valve position data and the LCD display data from the last electric operation, which causes the valve position to drift.

 

(3) After power is restored, the electric valve is restarted. During the gate lifting process, even though the valve opening has reached 100%, the motor continues to operate due to valve position drift, causing it to stall. Additionally, the over-torque protection and limit switch fail (see Table 4). The torque protection range of the electric actuator exceeds the maximum torque of the valve stem, which should prevent damage to the valve stem. However, due to valve position drift, this results in stalling. The stalling value is twice the rated torque. As the current and power increase, the continuous lifting force causes the valve stem to twist until it breaks.

 

(4) The electronic limit switch of the electric actuator uses a Hall effect sensor. The built-in battery serves as a backup power supply for the actuator’s LCD screen display, parameter settings, and status queries when external power is lost. It also powers the handwheel-operated valve position detection component when external power is lost. However, if the built-in battery loses power, this type of sensor may malfunction.

 

(1) Add a fault alarm for the electric actuator (including battery failure). Upon receiving the alarm signal, the operator should address the fault promptly.

 

(2) Replace the limit switch of the electric actuator with an absolute value encoding sensor. The principle of this sensor involves multiple optical channel lines on the code disk, with each line arranged in configurations such as 2, 4, 8, 16 lines, etc. This arrangement allows for generating a set of unique binary codes at each encoder position by reading the states of each line, ranging from 2 to 2n−1. This type of encoder is determined by the mechanical position of the photoelectric code disk and is not affected by power outages. Valve position detection does not require backup battery power. Even if the external power supply is lost or the battery is exhausted, the valve position will be preserved, preventing issues such as valve position drift. Once the valve reaches the fully open position, the limit stop action is executed immediately to prevent the actuator from continuously applying torque to the valve.

 

(3) Test the electric actuator in a low-temperature environment (-40°C). Allow it to acclimate for 24 hours, then perform functional tests such as torque and switch setting values again (see Table 5).

 
 
Table 5 Test content and results of electric actuators

 

In summary, fault analysis of electric valves has become an essential area of research in the industrial field. To prevent failures of electric valves during production and operation, the following three points should be addressed:

(1) The design, selection, and installation of valves and electric actuators are critical factors affecting their performance. Therefore, quality control must be stringent during the initial design and selection stages to enhance equipment reliability and reduce the failure rate during later use.

(2) To address situations where valves suddenly fail and require component replacement, repairs should be performed promptly, and the impact on normal production should be minimized. Special attention should be given to storing relevant spare parts.

(3) Investigate the reasons from a management perspective, regularly organize training for staff, and strengthen the maintenance of electric valves.