Application Scenarios:

In a chemical reactor,maintaining pressure within strict limits is critical to prevent over-pressurization and ensure product quality.A pressure transmitter sends a 4-20mA signal proportional to the vessel pressure.This signal is wired to a YOKOGAWA MAC2-B​module mounted in the control panel.Engineers set a”High-High”alarm limit on the module’s front dial.Under normal operation,the relay remains de-energized.If the pressure exceeds the set limit,the MAC2-B​instantly energizes its relay.This relay’s dry contact can be wired to sound a horn,illuminate a beacon,or,most critically,initiate an automatic sequence to open a vent valve or shut down a feed pump.This simple,reliable,and completely independent hardwired safety action provided by the MAC2-B​acts as a crucial last line of defense,independent of the main DCS,ensuring a safe and predictable response to a dangerous condition.

Parameter:

Main Parameters

Value/Description

Product Model​

MAC2-B​

Manufacturer​

Yokogawa

Product Category​

Dual-Loop Isolated Alarm Trip Module

Input Signal​

4 to 20 mA DC(two-wire transmitter compatible)

Number of Channels​

2 independent alarm channels per module

Alarm Function​

Independently settable as High or Low limit for each channel

Setpoint Accuracy​

High precision,typically±0.1%of span or better

Output Relays​

2 SPDT(Single-Pole Double-Throw)electromechanical relays

Relay Contact Rating

5A 250V AC/30V DC(resistive load)

Power Supply​

24V DC(standard)

Isolation​

Input-to-output,channel-to-channel,and power supply isolation

Setpoint Adjustment

Front-accessible dial with clear scale

Mounting​

Standard 35mm DIN rail

Technical Principles and Innovative Values:

The YOKOGAWA MAC2-B​embodies the principle of simple,robust,and independent safety interlocks.Its core innovation is providing highly reliable,hardware-based protection that operates separately from complex software-based control systems.

Innovation Point 1:True Independence from the Control System.Unlike software alarms in a PLC or DCS that share the same CPU and power supply,the MAC2-B​is a physically separate device.It has its own power input,signal conditioning circuitry,and relay outputs.This independence is a fundamental safety design principle.If the main control system fails,becomes corrupted,or is undergoing maintenance,the MAC2-B​continues to monitor the process and can still execute its critical shutdown or alarm function.This separation of safety functions is a cornerstone of many Safety Instrumented System(SIS)designs.

Innovation Point 2:High Density and Dual-Channel Efficiency.The module’s design packs two completely independent alarm channels into a single,slim DIN-rail module.Each channel has its own input terminal,its own front-accessible setpoint dial,and its own dedicated SPDT relay output.This dual-channel design allows one compact unit to monitor,for example,both a high-pressure and a low-pressure limit on the same vessel,or to provide separate alarms for two different pumps.This saves valuable panel space,reduces wiring complexity,and simplifies inventory management compared to using two single-channel units.

Innovation Point 3:Exceptional Reliability and Fail-Safe Design.The MAC2-B​uses a proven,straightforward comparator circuit and an electromechanical relay.The relay is typically configured in a de-energized-to-trip(normally de-energized)state.This is a fail-safe design:if the module loses power,the relay de-energizes,and its contacts change state to initiate the safe action(e.g.,an alarm sounds,a valve closes).This predictable failure mode,combined with high-grade signal isolation to prevent ground loops or noise from causing spurious trips,makes it an exceptionally reliable component for critical applications.

Application Cases and Industry Value:

Case Study:Offshore Water Injection Pump Protection.On an offshore oil platform,high-pressure water injection pumps are critical for reservoir management.A sudden loss of discharge pressure could indicate a catastrophic pump failure or line rupture,risking equipment damage and production shutdown.

Each pump was fitted with a discharge pressure transmitter.Its signal was routed to a YOKOGAWA MAC2-B​module configured for a”Low-Low”pressure alarm.The relay contacts were wired directly into the pump motor’s emergency stop circuit.During normal operation,the relay remained energized,allowing the pump to run.When a seal failure caused a rapid pressure drop,the MAC2-B’s comparator circuit detected the signal falling below the set limit within milliseconds.It de-energized its relay,whose contacts immediately broke the motor circuit,shutting down the pump before severe damage occurred.The platform’s control system logged the alarm,but the trip action was executed independently and with hardware speed.The maintenance supervisor credited the MAC2-B​with preventing a multi-million dollar pump seizure and weeks of downtime,highlighting its value as a cost-effective,ultra-reliable layer of mechanical protection.

Related Product Combination Solutions:

The MAC2-B​is a component in a monitoring and safety loop.Essential related products include:

Field Transmitters:​The signal source.Yokogawa’s EJA​series pressure transmitters or other 4-20mA output devices provide the input to the MAC2-B.

Power Supply:​A stable 24V DC supply,such as a Yokogawa SPS​series or other industrial PSU,to power the module and the connected two-wire transmitter.

Annunciators or Beacon/Horns:​Devices like the Yokogawa UT​series annunciator or standard beacons that connect to the relay outputs of the MAC2-B​for audible/visual alarm indication.

Final Elements:​Solenoid valves,motor starters,or other actuators wired to the relay outputs to execute the physical shutdown action(close valve,stop motor).

Signal Conditioners/Isolators:​In complex systems,a Yokogawa SFP​or similar signal converter/isolation amplifier might be used to split or condition a signal before sending it to both the DCS and the MAC2-B.

DIN Rail and Enclosure:​A standard DIN rail​for mounting and an appropriate NEMA/IP-rated enclosure​for field or panel installation.

Installation,Maintenance,and Full-Cycle Support:

Installation is straightforward.The module snaps onto a 35mm DIN rail.Wiring involves connecting the 24V DC power,the 4-20mA input signal(s)from the transmitter(s),and the output relay contacts to the alarm or shutdown circuit.The front dial is used to set the precise trip point.Commissioning involves applying a known mA signal(using a calibrator)to verify the trip point accuracy and checking the relay action.A bypass switch is often installed in series for safe testing.

Maintenance is minimal but critical.The primary activity is periodic functional testing to ensure the module trips at the correct setpoint.This is done by simulating the trip condition with a calibrator and verifying the relay activates the correct alarm or shutdown sequence.The electromechanical relays have a finite mechanical life,so contact inspection or replacement may be needed after millions of cycles.We provide full lifecycle support for the MAC2-B.Our units are sourced from trusted channels.Prior to dispatch,each module undergoes basic functional verification,including power-up,signal response check,and relay operation test.We offer technical guidance for integration and testing procedures,helping you maintain the integrity of your critical safety and alarm loops.

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