Manifold valves are fundamental components in process instrumentation, acting as the interface between the process line and a measuring instrument (like a pressure or differential pressure transmitter). Proper installation and operation of these valves are critical for the safety, accuracy, and longevity of the entire measurement system.

This guide focuses on the common 3-valve manifold, which is essential for differential pressure (DP) transmitters.

Part 1: Installation and Pre-Check

Correct installation ensures a secure, leak-free connection and allows for safe maintenance.

1. Pre-Installation Inspection

Before mounting, perform a thorough check:

Component Integrity: Visually inspect the manifold, tubing, and instrument flanges (if using a direct mount model) for any damage, scratches, or foreign debris. Ensure all valves operate smoothly. Reputable manifold valve suppliers like Trueway India provide products that are factory-tested up to 10,000 psi for leak-tightness.

Material Compatibility: Verify that the manifold's Material of Construction (MOC) is compatible with your process fluid and the pressure/temperature ratings are sufficient.

Cleaning: Use approved solvents to clean the pipe threads, fittings, and seating surfaces to ensure a perfect seal.

2. Mounting the Manifold

Manifold valves come in two main types: Remote Mount (R Type) and Direct Mount (T Type/H Type).

Remote Mount (R Type): The manifold is mounted remotely on a stand or panel, connected to the instrument via separate tubing (impulse lines). Ensure the manifold is easily accessible for operation and calibration.

Direct Mount (T/H Type): This type bolts directly onto the instrument's flange (e.g., a DP transmitter).

Alignment: Ensure the manifold and the transmitter flanges are perfectly aligned before inserting bolts. Misalignment is a primary cause of sealing failure.

Sealing: Use new gaskets or O-rings of the correct material between the two faces.

Tightening: Insert the bolts and tighten them in a diagonal sequence to ensure uniform pressure distribution. Use the manufacturer's specified torque or tighten moderately to prevent flange damage.

3. Final Inspection and Leak Check

After all connections are secured, perform a leak test by applying a safe, non-process pressure (e.g., Nitrogen) to the system while all manifold valves are closed. Use a leak detection fluid (like soapy water) to confirm there are no bubbles at any connection point.

Part 2: Operating and Calibrating (Valving Sequence)

The manifold valve sequence is a critical safety procedure that prevents the highly sensitive differential pressure cell from being over-ranged by applying pressure to only one side.

The 3-Valve Manifold Configuration

A typical 3-valve manifold consists of:

HP Block Valve: Isolates the high-pressure side.

LP Block Valve: Isolates the low-pressure side.

Equalizing Valve: Connects the HP and LP sides, balancing the pressure.

Procedure 1: Putting the Transmitter Into Service (Opening)

Isolate: Ensure the main process root valves (at the tapping points) are closed, and the HP, LP, and Equalizing valves on the manifold are closed.

Equalize: Slowly Open the Equalizing Valve (center valve, usually rotated counterclockwise). This creates a path between the HP and LP sides of the transmitter.

Apply High Pressure: Slowly Open the HP Block Valve. Pressure will now be applied equally to both sides of the transmitter (HP and LP) via the open Equalizing Valve, establishing a zero differential pressure.

Isolate & Measure: Close the Equalizing Valve. This separates the HP and LP sides again.

Apply Low Pressure: Slowly Open the LP Block Valve. The transmitter is now in service and will indicate the correct differential pressure ($ ext{HP} - ext{LP}$).

Procedure 2: Taking the Transmitter Out of Service (Calibration/Maintenance) Isolate Low Pressure: Close the LP Block Valve first.

Equalize Pressure: Open the Equalizing Valve next. This balances the pressure on both sides of the transmitter again before the HP side is closed, preventing a damaging pressure surge across the sensor.

Isolate High Pressure: Close the HP Block Valve.

Vent/Bleed: Close the main process root valves (if not already done). Use the dedicated vent/bleed valve (if present, or a separate connection) to safely release any trapped process pressure from the transmitter and manifold before disconnecting.

Following this precise valve sequence—Close LP, Open Equalizer, Close HP—is the foundation for safe maintenance and successful calibration.