I to P Converter is a Field Mounted Instrument that is used to Convert an Electrical Signal into a Pneumatic Signal. Pneumatic signal means the Air Pressure Signal.
In a Process Plant, I to P Converter usually Converts an Electrical Signal of 4 to 20ma from a Controller into an Air Pressure Signal of 3 to 15PSI, which is used to Operate a Pneumatic Control Valve or valve Positioner.
When a controller decides to open or close a control valve, accordingly an electrical signal of 4 to 20 ma is sent to the control valve.
As pneumatic control valve operate only on pneumatic air pressure signal, so an I to p converter is used to convert this electrical signal from the controller to proportional pneumatic signal.
This pneumatic signal can be used to open or close the pneumatic control valve.
Working principle of I to P Converter
Current to Pressure converters mainly works on the principle of Electromagnetic Force Balance, that is the process of converting electric current in the force and then to pressure.
There are many designs for the I to P converters but the basic working always involves the use of a Flapper Nozzle System Arrangement.
Working of a Flapper Nozzle System.
The Main elements of this arrangement is Flapper, Nozzle and a Permanent Magnet with a Coil wound around it.
The Flapper is Pivoted in one End and free at the other end so that it has some degree of flexibility to move up and down.
A Ferromagnetic Material is attached to the free end of the flapper and a Nozzle is present near the Flapper. Their is some gap between the Nozzle and the Flapper.
A standard supply of about 20PSI is maintained as supply pressure. As there is a gap between the Nozzle and the Flapper, air flows out through the nozzle.
if the Nozzle is Covered (Fig below) and no air is allowed to pass through the Nozzle, then all the 20PSI supply pressure will be received as the Output pressure to control valve.
if the Nozzle is not completely covered, the air escapes (Fig below) through the Nozzle and the Pressure output to control valve will be less than 20 PSI.
An Air Regulator is used to regulate the supply pressure in this arrangement (Fig Below).This air regulator regulates the supply pressure to constant value of 15PSI
When the current from the controller passes through the coil wound around the permanent magnet, a magnetic field will be produced. The Electromagnet then tends to attract the ferromagnetic material attached to the free end of the flapper. (Fig below)
It causes the Flapper to move downwards towards the coil. Stronger the current flowing through the coil stronger will be the attraction between the coil and the ferromagnetic material.
Thus the flapper will be pulled towards the nozzle and when it moves downwards the gap between the nozzle and the flapper get reduced.
Less gap means less air will pass through the nozzle end and thus most part of the supply pressure will move out from the end opposite to that of the nozzle. which means most pressure will be received as the Output pressure to control valve.
now, if a weak current passes through the coil, the gap between the nozzle and the flapper increases and relatively more air can pass through the nozzle.(Fig below). which means that very less pressure will be received as the pressure to control valve.
The system is set up in such a way that if 4ma current passes through the coil, then the pressure at the end opposite to that of the nozzle is 3PSI and if 20ma current passes through the coil then the pressure at the end opposite to that of the nozzle 15 PSI.
Now lets see an Example of I to P converter manufactured by FOXBRO
In this model two pressure gauges indicate supply pressure and output pressure respectively. A view with the cover removed shows the balancing mechanism used to generate a pneumatic pressure signal from the electric current input.
The nozzle is located at the end of a bend tube facing the flapper which is located on the surface of the circular coil unit.
As the electric current passes through the coil, it produces a magnetic field which generate a torque. this torque causes the coil unit to rotate counterclockwise.
As flapper is located on the surface of the coil unit the flapper also moves in response to current. The more current through the coil the more the coil unit moves.
The nozzle faces this flapper so when the flapper begins to move towards the nozzle, back pressure within the nozzle rises.
This Rising pressure is amplified by a relay and its output pressure is applied to a bellow.
The motion of one is matched by the motion of the other making this a Motion Balance Instrument.