Centrifugal compressor

Why is there an alarm for low pressure in the balance pipe?

The balance drum is located after the last-stage impeller and serves to balance the axial forces acting on the rotor. The principle by which the rotor of a centrifugal compressor generates axial forces is the same as that in a centrifugal pump; the direction of these forces is from the back side of the impeller toward the inlet. The commonly used balancing measure is the balance drum structure. This structure is similar to the balance drum used in centrifugal pumps, and it is also referred to as a balance piston. The pressure on the left side of the balance drum is approximately equal to the discharge pressure at the last stage; the right side is connected to the compressor inlet via a balance pipe. The unequal pressures on both sides of the drum generate a force that acts from the inlet toward the outlet and is parallel to the rotor’s axis. This force acts in the direction opposite to the axial force, thereby balancing it. It should be noted that this balance disk cannot counteract all axial forces; a certain amount of axial force must still be allowed to act on the bearing to prevent rotor movement. To prevent excessive gas leakage between the outer circumference of the balance drum and the casing, a labyrinth seal is used here.