Virtual Redundant Transmitter (VRT) in Honeywell Turbomachinery Control Systems

Safe and effective operation in the field of turbomachinery control depends on constant availability of precise and dependable measurement signals. Honeywell introduced the Virtual Redundant Transmitter (VRT) capability to its Prodigy control system to improve signal dependability without requiring costly redundant hardware. In performance control systems and antisurge systems, where transmitter failure could cause controller fallback and insufficient compressor performance, this function is especially useful.

This article delves into how VRT works, its applications in real-world compressor systems, measurable benefits, and the engineering basis for the function. Included are diagrams showing important use cases in series and parallel compressor setups.
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Who is VRT for?

The VRT capability is meant for Prodigy users running simplex (non-redundant) transmitters in their turbomachinery control systems. While still wanting the safety and performance dependability linked with redundant transmitter systems, these users usually want to save capital and maintenance expenses.

Adding actual hardware redundancy could not be feasible or affordable in many brownfield or greenfield projects. VRT closes this gap by building a virtual redundancy layer using current process variables and recognized physical correlations, hence enabling compressors to remain online without needless trips to fallback mode.

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Problem Statement: Limitations of Simplex Transmitters

In conventional systems employing simplex transmitters:

• A single transmitter sends antisurge or performance control blocks vital process data e.g., pressure, temperature.
• Should this transmitter fail due to sensor malfunction, signal noise, or calibration drift the controller enters a Fallback mode.
• Fallback is a safety-oriented state meant to stop machine harm by making conservative assumptions.
• Although fallback safeguards assets, it often leads to lower performance, ineffective control, and possible process disturbances.

Must refer this: Pressure Transmitter Commissioning Checklist 

What is VRT and How does it Work?

In the Honeywell Prodigy system, the Virtual Redundant Transmitter (VRT) is a logic-based function block that: 

1. Tracks a principal measurement (such as the suction pressure at Compressor 1).
2. Constantly compares it with a secondary measurement anticipated to correspond (e.g., discharge pressure of the preceding stage or suction of a parallel compressor).
3. The VRT block automatically checks the validity of the secondary input should the main transmitter fail or deviate unexpectedly.
4. VRT avoids a fallback by switching the input to the secondary channel and maintaining the controller in normal mode if correlation is within specified parameters.

This approach ensures:

• No unsafe required switchovers.
• With least degradation, the control loop remains active.
• Employees may fix and replace broken transmitters without tripping the process.
• Functional Positioning 

Functional Positioning of VRT Function

The VRT function stands between the controller application block (Antisurge or Performance Controller) and the Analog Input (AI) block. As indicated in the conceptual Figure 1 from Honeywell documentation, it functions as a signal watchdog and switch-over manager.

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VRT in Compressor Systems – Diagram Explanations

Figure 1: Series Compressor Arrangement (Two-Stage Compression)

In this arrangement:

• PT 01 is measuring discharge pressure of the 1st stage.
• PT 02 measures the 2nd stage’s suction pressure..

Essentially, these two readings are monitoring pressure in the same pipe separated by a static component such an aftercooler or a check valve. This section’s physical behaviour is well known, hence one pressure may predict the other within reasonable bounds.

If PT O1 (primary) fails:

• VRT verifies the validity and correlation of PT O2.
• It replaces PT O2 into the controller input for the 1st stage if suitable.
• This guarantees operational continuity and stops antisurge fallback.

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Figure 2: Parallel Compressor Arrangement (Common Suction Header)

This picture depicts two parallel compressors, each with its own suction pressure transmitter:

• Compressor 1 uses PT 01 and PT 03.
• Compressor 2 runs on PT 05.
• Various discharge locations are PT 02, 04, 06.

Essentially, all suction transmitters (01, 03, 05) detect pressure in a shared suction header. VRT logic lets these naturally redundant signals act as backups.

If PT 01 fails:

• VRT verifies PT 03 or PT 05, whichever is defined as the secondary.
• If validated, the antisurge controller continues without fallback.

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Benefits of Virtual Redundant Transmitter (VRT)

1. Fault Tolerance Without Redundancy in Hardware

VRT’s main advantage is its virtual fault tolerance in systems employing only simplex transmitters. This allows for more control dependability without physical duplication.

2. Cost Savings

• Greenfield project: Hardware alone might run about $3,000 to install a genuinely redundant transmitter.
• Brownfield retrofit: Adding control room wiring, documentation, and cabling will increase expenses to $50,000.

VRT avoids all of this by employing current equipment and natural process

3. Operational Continuity

In vital antisurge or performance control loops, even a brief time in Fallback mode can cause:

• Load reduction.
• Process inefficiencies.
• Increased operator workload.

VRT ensures more seamless operations, hence preventing conservative fallback assumptions should another dependable signal exist.

4. Maintenance Flexibility

With VRT active:

• Operators have more time to identify and fix defective transmitters.
• Instrumentation teams can plan offline maintenance free of alerts or controller fallback mode activation.

Typical Applications of VRT

1. Multi-Stage Compressors

As shown in the first diagram(Figure1), VRT is very effective when:

• As indicated in the first diagram, when: Stage 1 discharge equals Stage 2 suction.
• One signal can reflect the other using physical knowledge of system behavior (pressure dips, valve positions).

2. Parallel Compressors with Common Headers

VRT is best shown in the second diagram(Figure2) using common headers for discharge or suction lines:

• Every transmitter sensing pressure in the same network segment can back up one another.
• Ideal for gas compression, refining, or chemical plants.

3. Multi-Point Instrumentation in Pipelines

VRT can validate or replace one signal for another in lengthy pipelines or manifolds if several transmitters are employed for cross-verification or averaging.

Safety and Validation Logic in VRT

VRT does not mindlessly flip to the secondary signal. The reasoning comprises:

• Thresholds for signal comparison: The secondary must to be within reasonable variance of anticipated values.
• Filters based on rate of change: Unless safe, avoid substitution during quick transients.
• Correlation history guarantees that replacement only occurs if the signal trends over time match.

If these validations fail, the controller returns to Fallback mode, keeping safety as the top priority.
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Engineering Considerations When Implementing VRT

Engineers employing VRT have to take into account:

• Physical process layout: Do the chosen transmitters really correlate?
• Signal quality: Is the secondary measurement consistent and preserved?
• Clearly specify switchover limits and override rights in DCS setup.
• Alarm management: Notify operators of substitution but no annoying alarms.

The Virtual Redundant Transmitter (VRT) function in Honeywell’s Prodigy platform is a smart and affordable way to increase the fault tolerance of turbomachinery control systems. Even when a vital measurement fails, it keeps compressors up and efficient by using natural process redundancies.

VRT helps customers to obtain more dependability, operational flexibility, and long-term cost reductions by means of increased VRT by avoiding needless fallback modes and lowering reliance on costly redundant equipment. Its use in both series and parallel compressor systems provides it a flexible complement to current industrial automation concepts.

VRT is a revolutionary invention for turbomachinery users looking for more durability with little expenditure.

Source: Honeywell