Explain the need for equalizing valve in the manifolds
It is used to equalize the high and low pressure
A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged. It also allows isolation of the transmitter from the process loop. It consists of two block valves – high pressure and low-pressure block valve – and an equalizing valve
Mostly it is used to provide equal pressure for both sides of the transmitter
And why would anyone want to equalize the high side and low side pressure?
3 or 5 valve manifolds are used exclusively with Differential Pressure (DP) transmitters.
For initial configuration, or for a periodic calibration check, an offset adjustment must sometimes be made to the “zero”.
In order to zero the pressure while the transmitter is connected to the process, the 3/5 valve manifold allows for both high and low sides to be isolated from the process with isolation/block valves, but even after isolating, pressure remains in the high side and low side impulse tubing.
When the equalizing valve is opened, a connection is made between the high side and the low side, so the pressure between the high side and the low side equalizes, but it does not equalize at zero pressure (gauge). The equalized pressure is the same on the high as it is on the low side, which equals zero pressure differential. At that point, whatever steps are necessary to adjust the transmitter to read zero DP can be done.
There is no feature on a 3 or 5 manifold that allows for over-pressure protection. There are stand-alone pressure limiting devices that trip and block a given gauge pressure, but those are accessories used as needed by an application.
By design, most DP transmitters have static pressure ratings many times the DP range of the transmitter. A typical 1.0 barD industrial grade DP transmitter has static pressure limit Maximum Allowed Working Pressure) of 300barg.