A Brief Discussion on the Application of Partial Stroke Testing in Emergency Shut-off Valves
25-05-09
In order to enhance the production efficiency of factories, increase economic benefits and maximize profits, in recent years, the continuous operation cycle of large-scale oil refining plants, ethylene plants and other technological processes has been prolonged. The regular maintenance time has generally been extended from once every 2 to 3 years to once every 5 to 6 years. This leads to the pneumatic actuators in SIS (Safety Instrumatic System), such as the emergency Shutdown Valve (ESD), remaining in a "dormant" state for a long time. Due to the fact that the process often has the characteristics of high temperature, corrosion, erosion, vibration and smoke dust, and the valve is constantly affected by such an environment, the ESD valve may develop a "fixed" fault mode and fail to respond to the operation (safety protection) instructions from the SIS (Safety Instruminstrumentation System). When the ESD valve needs to perform safety protection actions, it may experience a "refusal to act" phenomenon. The occurrence of this situation will lead to the failure of the SIS system, posing a threat to safe production, and the consequences may even be disastrous.
To solve the problem of the ESD valve's failure to operate, it is envisaged to insert the action of partial stroke testing during the regular maintenance cycle and perform periodic actions at certain time intervals, so as to better check the movement of its valve stem and determine whether the working state of its function is normal.
I. What is PST-Paitial Stroke Testing?
The so-called PST-Paitial Stroke Testing of the ESD valve is a safety measure for an ESD-emergency Shutdown valve. The method of detecting faults in the ESD valve system is to make the ESD valve perform periodic pST-paitial stroke actions. For instance, the ESD valve partially closes by 10-30% of its travel from the fully open position. Periodic actions are set at certain time intervals to better inspect the movement of its valve stem and determine whether its functional working condition is normal. PST is a reliable detection method that does not affect the normal operation of ESD valves.
Ii. Application of PST Partial Stroke Test on Emergency Shut-off Valves
During the normal operation of the process production equipment, the ESD valve is usually in the fully open state. In the PST test, if the ESD valve is mistakenly closed, it will cause the production equipment to malfunction and stop, resulting in an accident. Therefore, there should be a limit on the opening degree of the ESD valve to prevent it from accidentally closing. Under normal circumstances, based on the flow characteristics of the ESD valve, when conducting PST tests, the opening degree of the ESD valve is mostly set at 70-90% (i.e., the closing stroke is 10%-30%). There are many methods to implement PST. Here, only the ESD valve with a pneumatic actuator is used for PST testing, and the specific method of conducting PST testing is introduced.
The pneumatic actuator type PST partial stroke test introduced in this article, due to its simple and unique mechanical system, does not contain electronic devices (microprocessors) and does not require control software. This pneumatic actuator is equipped with the PST function, which becomes a feature of the entire system.
Introduction to the principle of partial stroke testing for linear pneumatic actuator type PST
The solenoid valve has been out of power for a long time and is in a normally open state, while the cylinder is always open (with ventilation in the rod cavity).
It is applicable to linear emergency shut-off valves. When used for partial stroke detection, turn the key valve. At this time, the key valve is in the exhaust state, the pneumatic control valve changes direction, and the actuator moves in the closing direction. When the actuator moves to the set position, the preset collision block will hit the CAM valve. At this time, the CAM valve changes direction, and the instrument air passes through the CAM valve to give the key valve a pressure signal. The key valve will return to its initial position to maintain the air intake state. The pneumatic control valve will reset, and the actuator will quickly move in the open direction to return to the state before the test, completing part of the detection process. In addition to the partial stroke detection function, this system also has an additional gas cut-off protection circuit to ensure the safety of the entire system in case of gas supply failure. Its working principle is as follows: During normal operation, one gas channel passes through the three-way valve and the filter pressure reducing valve to supply gas to the main circuit. The other one is the auxiliary circuit, which supplies air to the air storage tank through a filter and a check valve. When a gas supply failure occurs, the pneumatic three-way valve (under the action of the spring force) automatically changes direction and supplies air to the air storage tank instead, completing the process operation before the failure and stopping to meet the production requirements.
2. Introduction to the principle of partial stroke testing for angular stroke pneumatic actuator type PST
It is suitable for rotary emergency shut-off valves and can perform partial stroke detection. The solenoid valve in the airway diagram is in a non-excited state, and the valve state is closed. A 2-position 3-way solenoid valve with manual reset is adopted. When opening the valve, it is necessary to confirm whether the manual button of the solenoid valve is in the reset state. The pneumatic control valve is used for rapid air intake and exhaust to achieve emergency valve cut-off. In case of power or gas failure, the valve will close.
When partial stroke detection is required, simply turn the key valve. At this time, the key valve is in the exhaust state, the pneumatic control valve and the actuator exhaust, and the ESD valve gradually closes. When the actuator moves to the set position, the rotation of the actuator shaft will drive the rotation of the CAM. When the CAM curve turns to the set point, it will touch the valve stem of the CAM valve at the preset position, and at this time, the CAM valve changes direction. The instrument air sends a pressure signal to the key valve through the CAM valve, and the key valve will return to its initial position to maintain the air intake state. The cylinder supplies air, returns to the valve open state, and completes the detection process of part of the stroke. To prevent rainwater and impurities from entering the key valve, the key valve can be placed in the pneumatic control box to provide better protection.
Iii. Advantages and Disadvantages of the PST Part Stroke Test of ESD Valves
The advantages of partial stroke testing of ESD valves
① Partial stroke testing (PST) reduces risks. Compared with full-stroke testing, it lowers the PFD (Probability of Failure) value, leads to an increase in SIL(High Safety Integrity Level), and enhances the reliability of the ESD valve.
② Partial travel testing can extend the time interval of full travel testing, but it cannot replace full travel testing (FST); it is merely a supplement to full travel testing.
③ Provide a basis for maintenance;
④ Due to the PST test conducted, there is a possibility that the bypass valve of the ESD valve may be cancelled.
⑤ Reduce testing costs;
⑥ Automated partial stroke testing can enhance the safety of the test and reduce the probability of human errors caused by manual testing.
2. Disadvantages of partial stroke testing of ESD valves
① PST testing cannot detect all ESD faults. Some faults need to be solved by full-travel testing (FST).
② The device operates abnormally, causing the ESD valve to oscillate. This situation is not suitable for PST testing.
③ PST tests usually only conduct 20% of the travel tests. If the situation requires an increase, the test travel can only be increased by 10%.
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福斯拓科,无锡福斯拓科科技有限公司
Wuxi Force & Torque Technology Co,Ltd
Add: No.8 Dongyue Road, Xishan District, Wuxi, China
Fax: +86-510-88550590/91/92-8020
Wuxi Force & Torque Technology Co,Ltd
Add: No.8 Dongyue Road, Xishan District, Wuxi, China
Tel: +86-510-88550590/91/92
Fax: +86-510-88550590/91/92-8020
Email: sales@force-torque.cn
Website: www.force-torque.cn