How the differential pressure controller works

Differential pressure controller automatically maintains the preset pressure difference between the impulse take-off points (impulse tube connections) by throttling the flow.

The operating principle of the differential pressure controller is illustrated using the example of a normally open direct-acting differential pressure controller with a diaphragm sensor, a spring actuator, a seat valve, and two external impulse lines.

The hydraulic actuator of the controller includes a chamber divided by a diaphragm. The diaphragm is rigidly connected to the valve plug in such a way that any displacement in either direction moves the plug and changes the water flow through the controller. The diaphragm is influenced by water pressure from the impulse take-off points through the impulse lines and an additional spring.

The pressure difference is balanced by the compression force of the spring acting on the diaphragm, causing it to assume a middle position. The stronger the spring is compressed, the higher the differential pressure the controller maintains.

Deviation of the pressure difference from the set value disrupts the balance in the diaphragm chamber, and the water pressure on the diaphragm exceeds the spring pressure – the diaphragm deflects and moves the valve plug, which restricts the water flow.

Depending on the design, differential pressure controllers can either open or close the valve plug when the controlled parameter increases.

In normally open differential pressure controllers, an increase in the higher water pressure tends to close the valve, while an increase in the lower water pressure tends to open the valve. That is, an increase in the controlled value closes the valve, while a decrease opens it. In contrast, in normally closed controllers, an increase in the controlled differential pressure tends to open the valve, while a decrease closes it.

Changes in the flow rate through the controller should affect the differential pressure in the regulated section. For more details on the correct installation of a differential pressure controller, refer to the section describing application schemes.

The differential pressure controller can operate in fully open, fully closed, and partially open positions.

Normally open controllers – in the absence of pressure and when the differential pressure in the regulated section is lower than the set pressure, the controller valve remains fully open (the flow passage is maximally open).

Normally closed controllers – in the absence of pressure and when the differential pressure in the regulated section is lower than the set pressure, the controller valve remains fully closed (the flow passage is completely shut).

Direct-acting proportional differential pressure controllers are so called because the speed and degree of valve plug opening are proportional to the speed and degree of differential pressure change relative to the set value.