Model 90 Low Pressure Selector Relay

The Model 90 Low Pressure Selector Relay is designed to select the lower of two signal pressures to provide a continuous output pressure to a control device. The Model 90 is recommended for dead-end or low-flow service in critical applications such as control loops requiring precise, automatic monitoring of signal pressures.

Application Paper: The Fairchild Model 90 Low Pressure Selector Relay in Maximum Allowable Setpoint Limiting for Pneumatic Control Loops

Abstract

The Fairchild Model 90 Low Pressure Selector Relay is a pivotal pneumatic logic device widely recognized for its role in selecting the lower of two input pressures to form a single stable output signal. Among its many uses, one of the most prevalent and critical applications is in maximum allowable setpoint limiting within pneumatic control loops. In such scenarios, the Model 90 ensures the output signal governing a control valve or actuator never exceeds a predefined “safe” pressure limit, even if a primary control signal attempts to push beyond it. The result is a fail-safe, protective mechanism that prevents overpressure conditions, maintains system integrity, and safeguards process equipment and personnel.

This paper provides a comprehensive overview of how the Fairchild Model 90 is employed in this popular application. It details the principles of operation, design considerations, installation best practices, and integration strategies into broader control architectures. A case study example is included to illustrate tangible benefits in an industrial setting.

1. Introduction

In many industrial processes—chemical, petrochemical, energy generation, water treatment, and food & beverage—pneumatic control loops regulate critical variables such as flow, pressure, level, and temperature. These loops rely on control valves, pneumatic actuators, and regulators to maintain desired operating conditions. Often, operators require an upper bound or “cap” on the allowable control signal to prevent the valve from opening too far or a regulator from delivering excessive pressure, which can lead to equipment damage, unsafe operating states, or compromised product quality.

The Fairchild Model 90 Low Pressure Selector Relay is well-suited to this task. By taking in two pressure signals—one being the primary control setpoint and the other a maximum allowable limit—the Model 90 ensures the output line always tracks the lower signal. This approach effectively enforces a hard limit on the control pressure and thereby the actuator or valve position.

2. Principles of Operation

The Model 90’s fundamental principle is often described as a “lowest pressure wins” logic function. Consider the following scenario:

• Primary Input (P1): The controller’s output signal, which can vary between a low and high pneumatic range (e.g., 3-15 psig) to adjust the position of a valve based on process conditions.

• Secondary Input (P2): A fixed or adjustable limit pressure source set at the maximum allowed value (for example, a stable 10 psig line).

The Model 90 continuously compares P1 and P2. Internally, it uses diaphragms and flow paths that direct the output to track whichever input is lower. If the controller tries to drive the valve to 12 psig (exceeding the 10 psig limit), the relay will disregard the higher signal and pass only the 10 psig from P2 to the output. If the controller requests 8 psig, which is below the limit line, the relay outputs 8 psig, granting the valve full control down to safe operational pressures.

3. Device Characteristics in the Maximum Setpoint Limiting Application

3.1 Accurate and Stable Limit Enforcement

The Model 90’s sensitive diaphragms and precision-valve mechanism ensure that even slight deviations above the allowable limit are blocked. The output tracks changes instantaneously, maintaining tight control and stable operation.

3.2 No Electrical Power Required

As a purely pneumatic device, the Model 90 enforces maximum setpoint limits without electricity. This makes it highly reliable in hazardous areas and appealing in intrinsically safe designs.

3.3 Minimal Pressure Drop

Due to its optimized internal flow paths and direct diaphragm-to-valve design, the relay imposes negligible pressure drop. Thus, the presence of the Model 90 does not compromise the responsiveness of the control loop.

3.4 Robust Construction and Material Selection

Built from corrosion-resistant metals and elastomeric materials suitable for a wide range of process conditions, the Model 90 can withstand harsh environments, including outdoor installations and exposure to chemicals or humidity.

4. Design and Integration Considerations

4.1 Selecting the Limit Pressure Source

The limit pressure (P2) typically originates from a stable reference regulator. Ensuring this source is clean, stable, and within a known, safe range is crucial. Many facilities use a fixed pressure regulator set slightly below the maximum allowable setpoint (e.g., 10 psig in a 3-15 psig system).

4.2 Determining the Limit Value

The chosen limit should align with mechanical and operational constraints. For example, consider a valve rated for a certain maximum flow or pressure differential. Engineers set the limit line to ensure the valve is never forced beyond its specifications.

4.3 Placement in the Pneumatic Loop

The Model 90 is typically installed downstream of the controller’s output but upstream of the valve or final control element. The line carrying the limit pressure signal is routed into the second input port. This direct, in-line configuration enables the Model 90 to act as a gatekeeper, ensuring the outgoing line never surpasses the predefined limit.

4.4 Avoiding Turbulence and Fouling

Like all pneumatic instruments, the Model 90 should be installed in a location free of excessive vibration or contaminants. Proper filtration of the supply air and ensuring dry, clean instrument air extends the device’s service life and accuracy.

5. Commissioning and Calibration

5.1 Establishing Baseline Conditions

Before introducing the Model 90 into a live control loop, baseline tests are conducted. With the limit pressure line set at the desired value, technicians verify that the relay correctly selects the lower input. For instance, applying a 12 psig signal at P1 and a 10 psig limit at P2 should result in a 10 psig output.

5.2 Functional Testing

Simulate normal and limit conditions. Lower the controller setpoint incrementally and observe the output track correspondingly. Raise the setpoint above the limit and confirm that the output caps at the limit pressure.

5.3 Fine-Tuning System Parameters

If output stability, response times, or limit thresholds need adjusting, ensure the regulator feeding P2 is accurately set. Although the Model 90 itself has no moving mechanical adjustments besides installation orientation, the limit pressure source can be tuned to a more precise value if needed.

6. Integrating with Broader Control Architectures

6.1 Redundancy and Safety Instrumented Functions

In critical applications, the Model 90 can be part of a larger safety strategy. For example, a Safety Instrumented System (SIS) may provide an emergency shutdown signal. The Model 90 ensures that during normal operation the valve never exceeds a safe limit. In emergency conditions, a separate low-limit signal could override the controller’s command, forcing the valve into a safe state.

6.2 Combined Use with Other Pneumatic Logic Devices

In complex pneumatic schemes, the Model 90 may function alongside volume boosters, reversers, and other pneumatic relays. As part of a pneumatic logic network, it ensures safe, stable operation without reliance on electronic controls.

6.3 Diagnostic Feedback

While the Model 90 is a passive device and does not provide electronic diagnostics, integrating it into a control system that includes pressure transmitters can provide insight. Monitoring output line pressure remotely helps operators verify correct functioning of the limit control strategy.

7. Case Study

Background:

A petrochemical refinery faced chronic issues with a large control valve that regulated steam injection into a distillation column. Under certain upset conditions, the process controller attempted to open the valve beyond its safe operational limit. This caused mechanical stress, premature wear, and increased maintenance costs.

Solution:

Engineers installed a Model 90 Low Pressure Selector Relay. The primary input was the control signal from the plant’s pneumatic controller (3-15 psig). The second input was a stable, regulated 10 psig line serving as the maximum allowable limit.

Results:

• Prevented Overstress: The valve never saw a pneumatic signal above 10 psig, preserving its mechanical integrity.

• Improved Reliability: The valve’s lifetime increased, and scheduled maintenance intervals lengthened.

• Enhanced Process Stability: The control loop stabilized because the valve no longer surged beyond intended operating conditions, improving product quality and operational efficiency.

• Reduced Costs: Avoiding overpressure conditions reduced both downtime and repair costs.

8. Conclusion

The Fairchild Model 90 Low Pressure Selector Relay, by enforcing a maximum allowable setpoint limit, represents an elegant solution to maintaining safe, reliable, and cost-effective control operations. Its well-established popularity in this application stems from its simplicity, durability, and precision. With no moving mechanical linkages apart from resilient diaphragms and no need for external power, it provides a robust line of defense against overpressure scenarios.

By integrating the Model 90 into their pneumatic control loops, plants ensure their valves and actuators function within safe parameters, reduce equipment wear, and improve operational stability. This leading application exemplifies the Model 90’s value as a cornerstone device in the pneumatic logic toolkit—delivering reliable performance and peace of mind in demanding industrial environments.

References

1. Fairchild Industrial Products Company, “Model 90 Low Pressure Selector Relay Data Sheet.”

2. ISA (International Society of Automation), “Pneumatic Control Systems and Components,” ISA Publications, 2020.

3. API (American Petroleum Institute) “Recommended Practices for Instrumentation and Control,” API RP 551, 2019.

4. Smith, A., “Improving Safety and Reliability in Pneumatic Control Loops,” Chemical Engineering Progress, 2021.

Acknowledgments

The widespread success of the Fairchild Model 90 in maximum allowable setpoint limiting applications owes to decades of engineering development, user feedback, and continuous improvement. The contributions of instrumentation engineers, control system integrators, and technical support teams have refined and validated the Model 90’s role as a fundamental device for safe and stable pneumatic control.