Instrument Manifolds: Function & Types

An instrument manifold also referred to as a valve manifold includes manual valves to isolate and equalize pressure from the process to the transmitter (in this case the DP transmitter) or to isolate a gauge or absolute pressure transmitter for maintenance and calibration purposes.

Typically we have the following types of valve instrument manifolds:

2-valve instrument manifolds
3-valve instrument manifolds
5-valve instrument manifolds

The 2-valve manifolds are manufactured to be used for static pressure and liquid level applications. The 3-valve instrument manifolds as well as the 5-valve instrument manifolds are designed for differential pressure applications.

An example of 3-valve instrument manifold with a fourth valve referred to as a “bleed” valve that is used to vent trapped fluid pressure to atmosphere is shown below:

The 3-valve manifolds are fabricated as monolithic devices i.e. the three valves are cast together into one block of metal, attaching to the pressure transmitter by way of a flanged face with O-ring seals. The bleed valves are most commonly found as devices threaded into one or more of the ports on the transmitter’s diaphragm chambers.

During a normal operation, the two block valves are left open to allow the process fluid pressure to reach the transmitter. The equalizing valve is left tightly shut so no fluid can pass between the “High” and “Low” pressure sides.

To isolate the transmitter from the process for maintenance purposes, the block valves must be closed and the equalizing valve opened. The recommended sequence to follow is, to first close the high-pressure block valve, then open the equalizing valve, and then close the low-pressure block valve. It is important to keep in mind that, the equalizing valve(s) should never be open on any transmitter manifold while both block valves are open. If this happens, it will allow the process fluid to flow through the equalizing valve(s) from high pressure-side of the process to the low-pressure side of the process which can have a detrimental effect to the transmitter/manifold and or create a personal hazard.

This sequence ensures the transmitter cannot be exposed to a high differential pressure during the isolation procedure, and that the trapped fluid pressure inside the transmitter will be as low as possible prior to “venting” to atmosphere. Lastly, the “bleed” valve is opened at the very final step to relieve held back fluid pressure within the manifold and transmitter chambers.

For a 5-Valve manifold system, a built-in bleed valve allows the technician to vent trapped pressure through a tube to some remote location, instead of directly venting at the transmitter as with the case of 3-Valve manifold above.

Valve positions for normal operation and maintenance of the 5-Valve manifold is illustrated below:

Figure 1.6: 5-Valve manifold (valve positions in a normal and maintenance operation)

As indicated earlier the 2-valve manifolds used for static pressure and liquid level applications. A case in point is the pressure transmitter valve manifolds that come in single block-and-bleed configurations, for gauge pressure applications. In this case, the “Low” pressure port of the transmitter is vented to atmosphere, with only the “High” pressure port connected to the impulse line.

Figure 1.7: 2-Valve manifold application

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John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.