Test safety critical valves during normal operation and under real-life conditions.
Prevent failure of emergency and plant protection valves
Emergency Shutdown Valves (ESD), High-Integrity Pressure Protection System Valves (HIPPS), and other fail-safe or critical valves typically incorporate a pneumatic, hydraulic, or electric actuator that must stroke the valve in the event of an irregular process condition. Typically, the components of such safety systems remain in one position and without mechanical movement over a long period of time. The danger of mechanical stiction and unpredicted component reaction can have a severe impact on the functionality of the safety system. If the critical valve does not perform its function at the time of the distressed event, the consequences to life, property and the environment can be catastrophic. Preventing failure of emergency valves is of utmost importance.
Full stroke valve testing
Critical valves are tested at regular intervals to ensure their functionality and maintain their Safety Integrity Level (SIL). One method of proving the degree of valve reliability is to fully stroke the valve to the closed (or open) position, providing immediate confirmation of the operability and performance. However, when performing a full-stroke test while the plant is operating, the process flow would come to a halt. To minimize impact on the process, operators usuallychoose to perform an offline full stroke test during a
Different results in ‘hot’ and ‘cold’ conditions
A downside of a full stroke testing during shutdown is that these tests are performed in a ‘cold’ and therefore abnormal process condition. There are examples where ESD valves and actuators were tested only during shutdown and then worked properly, but when tested in operation they failed. Clearly, the valves reacted differently in ‘hot’ and ‘cold’ conditions.
Partial stroke valve testing
Partial stroke testing prevents failure of emergency valves, and helps determine the likelihood that an emergency shutdown valve will operate on demand. During a partial stroke test, the system is activated but the valve is only partially closed or opened to a pre-set degree. Thereby the performance of the entire actuator and valve package is tested at real speed and force – and under actual operation conditions. The process flow is maintained and the impact on the production is minimal. Whilst a partial stroke test never replaces the need for full stroke testing, by doing partial stroke tests between full stroke ones, the facility may lower the Probability of Failure on Demand (PFD) of their safety system, for example by lowering the risk of an increasing break-loose torque, and/or extend the time interval between full stroke tests.
Electronic versus mechanical partial stroke testing
Standard electronic partial stroke test devices use an electronic module that connects between the supply from the ESD system and the solenoid valve. To perform a test, the timer de-energizes the solenoid valve to simulate a shutdown and re-energizes the solenoid when the required degree of partial stroke is reached. These systems are fundamentally a miniature PLC dedicated to the testing of the valve.
A mechanical partial stroke test device allows a true system test. All the actual components, controls and elements used in an ESD valve will be activated. The user has real information about the exact controls that will be relied upon, to protect plant and personnel.
The downside of electronical partial stroke testing
Most non-mechanical partial stroke test devices assume a relatively smooth movement of the valve actuator. They also reckon that the automated valve will act in a consistent manner, independent of environmental conditions, such as temperature and humidity or the length of time the valve rests between test cycles. This is rarely the case, however. One of the advantages of mechanical partial stroke test devices is that they stop valve movement mechanically at a specified percentage or degree of valve closure. This metal-to-metal blocking prevents overshooting of the valve past the set point due to spurious trips or other extraneous conditions.
When using controls-driven partial stroke test systems it is assumed that the test device or system will indeed prevent the actuator from driving the valve past the set point or to the fully closed position. However, actuators don’t always act smoothly and ‘stiction’ can prevent a spring return actuator from stroking immediately upon release of air pressure from the actuator cylinder. The result is the possibility of stored energy driving the valve past the set point and allowing the valve to nearly or fully close, potentially forcing a process shut down.
Advantages of mechanical partial stroke testing
- Mechanical partial stroke testing has some distinctive advantages over electronical testing.
- It is a ‘true system test’, all the actual components, controls and elements used in an ESD valve package will be activated under reality conditions.
- The actuator’s full force is tested, and the entire system will stroke in its ‘real world’ time sequence and speed of operation.
- The metal-to-metal blocking system prevents stroking past the specified point, eliminating the danger of overshooting.
- No additional power or wiring required, resulting in cost savings and reduction of system complexity.
- No need for calibration of the mechanical test device after installation.
- The ‘human-machine’ Interface assures that valve movement can be ‘eyeball’ verified.
The benefit of human involvement in mechanical partial stroke testing
In a typical mechanical partial stroke test scenario, the key to initiate the test mode is kept in a supervised location, such as a key cabinet in the control room. When it is time to test the valve, the operator inserts the key into the device, engages the device and then informs the control room that the valve is now ready to test. When the partial stroke test is initiated, the person witnesses the valve movement and reports the event, usually by radio, to the control room (this can also be verified by limit switches). After completing the test, the key is removed, disengaging the device, and the key is returned to supervised control.
By design, the key cannot be removed from the mechanical testing device while it is in the test (or ‘engaged’) position. So, if plant operations personnel know the key is in their control, then they also know the device cannot be engaged.
Sofis FAITH mechanical stroke limiter: partial stroke testing with visual verification
With the FAITH mechanical stoke limiter you can see if the valve will close upon request. You don’t need complicated software or additional wiring and you don’t have to interpret diagnostics. With the FAITH, you can test the entire system under real life conditions, without bypassing components or stopping your operation. Unlock the device with the dedicated key, activate it via the operating lever and start the shutdown simulation. You will see if the valve moved or not through the perforated windows on the rotary FAITH device. After completion of the test return the device back to inactive mode, remove the key from the device and store it in a safe location. The mechanical blocking system is securely fixed in open position, and the actuator valve package is free to move and not restricted in any way.
In the very unlikely event of an emergency happening exactly at the time of a partial stroke test, you can reset the FAITH unit immediately back to inactive mode via the operating lever. This allows the actuator to perform as intended and fully close (or open) the valve.
The dedicated key ensures that partial stroke tests are only initiated when planned and authorized. Furthermore, the key system allows the setup of an interlocked sequence with other manual valves. Using interlocks, you only allow the correct valves to be opened or closed in the correct sequence, providing safety and guidance for the operators every step of the way.
The FAITH device is ATEX-certified, suitable for SIL 3 environments and custom fit to the specific valve and actuator package.