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Back Flow Preventer

Back flow preventer is one of pilot operated safety relief valve accessory which functions to eliminate the reverse flow through the main valve in case the the back pressure at pressure safety relief discharge line exceeds main valve inlet pressure. In such condition, the back pressure would would lift the piston hence cause flow reversal from discharge to inlet line. Back flow preventer will provide a net downward force on the diaphragm or piston to keep the main valve closed as long as  the set pressure not achieved.

Back Pressure

Back pressure term is usually defined as the pressure that exists at downstream of instrument (e.g. at discharge line of pressure safety valve). With regard to pressure safety valve application, back pressure may consist of built-up back pressure and superimposed back pressure.

BALANCE 

The change in the position of an analog pointer from zero when the axis of the moving element moves from the vertical position to the horizontal position. The balance is expressed as a percentage of the scale length. 

BANDWIDTH (FREQUENCY RESPONSE) 

The frequency span where a constant amplitude input will produce a meter reading within a specified limit (usually 3db). In controllers, the region around the setpoint where control occurs. 

Basic Process Control System (BPCS)

Basic Process Control System (BPCS) is a system which handles process control and monitoring for the facility. It will take inputs from sensor and process instruments and provide output based on control functions in accordance with approved design control strategy.

Typically, Basic Process Control System (BPCS) performs the following functions:

 Control the process within pre-set operating condition, optimize plant operation to produce a good quality product and attempt to keep all process variables within its safety limit.

 Provide operator interface for monitoring and control via operator console (Human MachineInterface)

 Provide alarm/event logging and trending facilities

 Generate production data reports

Basic Process Control System (BPCS) is also considered as one of safety layer preceding Safety Instrumented System (SIS) within a facilities.

BAUD 

Digital transmission speed in bits per second. 

Baud Rate

The rate of information transfer in serial communications, measured in bits per second.

Blackbody

An ideal surface that absorbs all incident radiation, regardless of wavelength, the direction of incidence and polarization. It radiates the maximum energy possible for given spectral and temperature conditions. A blackbody has an emissivity of 1.00. See Emissivity.

Bumpless transfer

A situation in which the control output is maintained at the same level while transferring from auto to manual control.

BIAS CURRENT 

Current that flows out of an amplifier's input terminals which will produce a voltage drop across the source impedance. In a perfect amplifier this 

error term would be zero. 

Bridle (Stand Pipe)

Bridle, also named stand pipe, is a vertical pipe connected to the process tank or vessel so that the level of process liquid within bridle is always equal to the level of process liquid within tank or vessel. Level bridle is intended for installation of multiple level transmitter such as level gauge or level transmitter. These level instrumentation are installed to the bridle instead of the vessel or tank.

The use of bridle for level transmitter reduces the number of nozzles on the vessel or tank. If each level instruments has two nozzles, then they require six nozzles on the vessel. However by using bridle, the number of nozzle that should be  provided on the vessel or tank is only two (or three) for the bridle.

Bridle is commonly constructed from 4” pipe and provided with vent and drain connection for maintenance. Below picture shows bridle and its level instrumentation attached to.

Level bridle with multiple level instruments must have isolating valves installed between bridle and instrument for maintenance purpose.

Level Instruments served for Process Control System and those for Safety Instrumented System shall not be installed in the same level bridle. This is to avoid common cause failure.

BURDEN 

The input impedance of a measuring circuit (expressed in ohms) or the load on the secondary of a transformer (expressed in volt-amps or watts). In potential or current transformers, burden is the maximum load the transformer can support while operating within its accuracy rating.