Final control elements used in Safety Instrumented System (SIS) application shall be carefully engineered to meet the stringent requirements. As these valves are operated based on the safety requirements per LOPA, they’re termed as Automated Block Valve (ABV).
Usually, these valves play a critical role in isolating the process from any hazardous events, hence the valves need to meet the SIS design requirements and satisfy the testing needs.
The basic characteristics of Automated Block Valves (ABV) are as follows
A ball valve is a type of quarter-turn valve which uses a hollow and pivoting ball to control flow through it.
The valve turns full open when the ball’s hollow portion is parallel to the pipeline with the flow and the valve turns closed when the ball’s hollow portion is perpendicular to the pipeline (which is a 90-degrees turn).
The valve can be operated automatically with a pneumatic/electric actuator or manually by the valve handle.
A typical picture of Quarter-turn Ball valve assemblies with pneumatic actuation is shown above.
The butterfly valve has a disc and shaft arrangement, with the rotation of the valve disc controlling the fluid flow. In the closed position, the disc seats along the pipe bore, thus blocking the flow through the valve. Whereas in the open position, the disc is oriented perpendicular to the flow direction to allow flow.
There are 3 types viz.
Typical schematic of Butterfly valve
SIS Valves‘ performance is based on the below factors:
The below parameters decide the type of valve to be chosen in SIS applications.
SIS Valve (ABV) selection is based on the Application characteristic and Specific requirements for the respective applications.
Actuator sizing is very vital for the safe and fast response of SIS valves.
Actuator output torque > Required valve torque
Actuator output torque < Max allowable stem torque
Below parameters are required to be provided by the Valve manufacturer to the Actuator manufacturer for designing a suitable actuator for a particular valve.
The below parameters are required to be provided by the Actuator manufacturer to the Valve manufacturer for designing a suitable actuator for a particular valve.
Type of solenoid valve in SIS valve application also plays a vital role Default:
Namur type solenoid – Breather function: keeps spring chamber actuator clean
Pipe mounted solenoid – re-breather block
3/2-way for single acting
5/2-way for double acting
Direct Operated valves require Higher power consumption (>7 Watt), Bigger Cv-values (> ~5.0), and hence more reliable.
Pilot Operated valves have Lower power consumption (< 2 Watt), Smaller Cv values (< ~5.0), and current consumption is also very less, More vulnerable to non-clean air supply filters in air supply.
Solenoid valve coil assembly (ref figure)
The type of switch/ switch box shall be according to the actuator style to have better assembling and ease of maintenance.
Proximity type switch (non-contact switch)
Reed Switch: switch activated by an external magnetic field
Inductive Switch: detect magnetic loss due to eddy currents that are generated on a conductive surface by an external magnetic field.
Of all the above types Non-contact type proximity type switch is most preferred for SIS applications.
Mechanical types of switches shall never be used in SIS.
We must understand the type of valve, actuator, solenoid valve, switch, and accessories such as Air Filter/Regulator, Booster relays to be selected with a thorough understanding of the process fluid/gas application and valve response, leakage requirement.
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