Functional Safety Terms and Acronyms

2oo3

Two out of three logic circuit (2/3 logic circuit) A logic circuit with three independent inputs. The output of the logic circuit is the same state as any two matching input states. For example a safety circuit where three sensors are present and a signal from any two of those sensors is required to call for a shut down. This 2oo3 system is said to be single fault tolerant (HFT = 1) in that one of the sensors can fail dangerously and the system can still safely shut down. Other voting systems include 1oo1, 1oo2, 2oo2, 1oo3 and 2oo4.

Actuator

A device responsible for putting a mechanical device into action such as a valve. Single acting actuators act in only one direction such as in a spring and diaphragm actuator where the spring acts in a direction opposite to the diaphragm thrust. Double acting actuators have a power supply that acts to move the actuator in two normally opposite directions. Pneumatic actuators converts the energy of a compressible fluid, usually air, into motion. Vane actuators are typically fluid-powered devices where the fluid acts upon a movable pivoted member (the vane) to provide rotary motion to the actuator stem.

ALARP

As low as reasonably practicable. The philosophy of dealing with risks that fall between an upper and lower extreme. The upper extreme is where the risk is so great that it is rejected completely while the lower extreme is where the risk is, or has been made to be, insignificant. This philosophy considers both the costs and benefits of risk reduction to make the risk “as low as reasonably practicable”.

Algorithm

A prescribed set of well defined rules or processes for the solution of a problem in a finite number of steps

Analogue I/O

Input or output signals to or from the filed that vary continuously over a range of values. Typically voltage, electric current, temperature, or pressure signals are analogue.

Annunciator

A device or group of devices that call attention to changes in process conditions that have occurred. Usually included are sequence logic circuits, labeled visual displays, audible devices, and manually operated acknowledge and reset push buttons.

Architectural constraints or AC

Limitations that are imposed on the hardware selected to implement a safety-instrumented function, regardless of the performance calculated for a subsystem. Architectural constraints are specified (in IEC 61508-2-Table 2 and IEC 61511-Table 5) according to the required SIL of the subsystem, type of components used, and SFF of the subsystem’s components. Type A components are simple devices not incorporating microprocessors, and Type B devices are complex devices such as those incorporating microprocessors. See Fault Tolerance.

Architecture

The voting structure of different elements in a safety instrumented function. See Architectural Constraints, Fault Tolerance and 2oo3.

As-built

A document revision that includes all modifications performed as a result of actual fabrication or installation. Note for safety systems, that where the actual installation does not conform to the design information, then the difference shall be evaluated and the likely impact on safety determined. If the difference has no impact on safety, then the design information shall be updated to “as built” status. If the difference has a negative impact on safety, then the installation shall be modified to meet the design requirements.

Asynchronous communication

Circuitry or operation without common clock or timing signals. Often called start/stop transmission; a way of transmitting data in which each character is preceded by a start bit and followed by a stop bit.

Auto-tuning

Controller feature that calculates proportional, integral and derivative (PID) output settings based on calculations using measured process dynamics and combining those with the parameters of a PID controller. Calculations may be based on transient responses, frequency responses or parametric models.

Availability

The probability that a device is operating successfully at a given moment in time. This is a measure of the “uptime” and is defined in units of percent. For most tested and repaired safety system components, the availability varies as a saw tooth with time as governed by the proof test and repair cycles. Thus the integrated average availability is used to calculate the average probability of failure on demand. See PFDavg.

Basic process control system

System which responds to input signals from the process, associated equipment, and/or an operator and generates output signals causing the process and its associated equipment to operate in the desired way. The BPCS can not perform any safety instrumented functions rated with a safety integrity level of 1 or better unless it meets proven in use requirements. See proven in use.

Batch process

A process that manufactures a fixed quantity of material by subjecting measured quantities of raw materials to a time sequential order of processing actions using one or more pieces of equipment. Typically used for small volume production of high value materials.

BLEVE

Boiling liquid expanding vapor explosion. A specific type of fireball that can occur as the result of the situation where a vessel containing a pressurized liquid comes in direct contact with external flame. As the liquid inside the vessel absorbs the heat of the external fire, the liquid begins to boil, increasing the pressure inside the vessel to the set pressure of the relief valve(s). The heat of the external fire will also be directed to portions of the vessel where the interior wall is not “wet” with the process liquid. Since the process liquid is not present to carry heat away from the vessel wall, the temperature in this region (usually near the interface of the boiling liquid), will rise dramatically causing the vessel wall to overheat and become weak. A short time after the vessel wall begins to overheat, the vessel can lose its structural integrity and a rupture will occur. After vessel rupture, a fireball will usually result with the external fire available as the ignition source.

BMS

Burner management system. The control system designed to improve combustion safety and assist the operator in starting and stopping the burners. It also should prevent mis-operation and damage to the fuel preparation and burning equipment. The BMS can include: interlock system, fuel trip system, master fuel trip system, master fuel trip relay, flame monitoring and tripping systems, ignition subsystem, main burner subsystem, warm-up burner subsystem, bed temperature subsystem, and duct burner system.

BPCS

See Basic Process Control System.