A three-valve manifold is a common instrument manifold used in process control systems to connect a differential pressure (DP) transmitter or other instruments to a process line. It consists of three valves arranged in a specific configuration to facilitate measurement, isolation, and venting of process fluids. Here's how a typical three-valve manifold works:

Components:

1. Isolation Valves: There are two isolation valves, one on each side of the manifold, typically referred to as the high-pressure (HP) valve and the low-pressure (LP) valve. These valves allow the DP transmitter to be isolated from the process fluid for maintenance or calibration purposes.

2. Equalizing Valve: The third valve, known as the equalizing valve or vent valve, is located between the isolation valves. It connects the high-pressure side of the process to the low-pressure side, allowing for equalization of pressure across the DP transmitter.

Working Principle:

1. Normal Operating Position:

   • In the normal operating position, both isolation valves are closed, effectively isolating the DP transmitter from the process fluid. The equalizing valve is also closed.
   • With all valves closed, pressure cannot pass through the manifold, and the DP transmitter measures zero differential pressure.

2. Calibration or Maintenance:

   • When calibration or maintenance of the DP transmitter is required, the equalizing valve is opened to equalize pressure between the high-pressure and low-pressure sides.
   • The HP and LP isolation valves are then opened, allowing the DP transmitter to be removed or serviced without exposing it to the full process pressure.

3. Process Measurement:

   • During normal operation, the equalizing valve remains closed, while the HP and LP isolation valves are opened to allow process fluid to flow through the manifold and to the DP transmitter.
   • The DP transmitter measures the pressure difference between the high-pressure and low-pressure sides, providing an indication of the process fluid level, flow rate, or other parameter.

Key Functions:

• Isolation: The isolation valves allow the DP transmitter to be isolated from the process for maintenance or calibration without interrupting the process flow.
• Equalization: The equalizing valve allows pressure to be equalized across the DP transmitter, ensuring accurate measurement and preventing damage to the instrument.

Applications: Three-valve manifolds are commonly used in industries such as oil and gas, chemical processing, and power generation for a variety of applications, including level, flow, and pressure measurement.

In summary, a three-valve manifold provides a convenient and efficient means of connecting a DP transmitter or other instruments to a process line, allowing for isolation, equalization, and measurement of process fluids.