Centrifugal fans are driven fluid machinery that increase air pressure and transport gases by inputting mechanical energy. This article mainly shares preventive measures for the installation and pipeline layout of centrifugal fans.

When purchasing a fan, try to avoid a performance curve with a “saddle” shape, i.e., a curve that lacks the lower left corner, since surge primarily occurs in this part of the performance curve. Even if the fan only has a performance curve with a lower right corner, try to move the high point (K) of the curve to the left, meaning the flow at the pressure peak (Q(K)) should be as small as possible, or the lower left part of the performance curve should be as flat as possible, which helps to prevent surge.

To ensure the air pressure inside the fan is greater than the surge flow rate, an exhaust valve can be set in the fan’s exhaust pipe to release a portion of the gas (QB), while the remaining portion (QC) is transported to the pipeline network (this flow can be below the surge flow rate). This surge flow prevention method is simple and widely used in fans, but its disadvantage is that it discharges a portion of the energy-rich gas from the fan’s impeller, reducing the efficiency of the equipment.

During the installation and use of the fan, adjusting the fan speed or changing the installation angle of the inlet guide vane can move the fan’s performance curve to the low-flow area, and the surge vibration boundary will also move to the low-flow area, thus increasing the stability range of the fan’s operating conditions.

The two-valve operation method involves setting two valves in the fan exhaust pipe and fixing the volume between the two valves to a constant value, which acts like a gas reservoir. The first valve is installed close to the centrifugal fan outlet and serves as a throttling valve. A slight throttling can prevent surge vibration. The second valve, located farther from the fan outlet, serves as a resistance. This method has been proven to stabilize the system and reduce noise.

Additionally, to stabilize the entire system, try to choose short pipes, and all necessary pipeline valves, flow measurement nozzles, dust filters, etc., should be placed as close to the fan as possible. With lower pipeline resistance, surge is less likely to occur in a short period.

Preventive and control measures during operation and daily maintenance include controlling the inlet blade (or inlet damper) to small openings and small operational currents. The system must control the inlet blade (or inlet damper) to small openings and low current to avoid situations where the fan cannot generate enough pressure to maintain airflow when the flow decreases to a certain point, causing reverse airflow and resulting in surge.

When the centrifugal fan operates at a large flow and full load, the fan cannot overcome the system resistance until the fan functions to overcome the resistance. This situation leads to surge because the fan’s characteristic curve is severely deviated from the design state, and the designed discharge pressure is already in the surge region. The effective method for preventing this surge phenomenon, especially in fans with only small-flow automatic surge protection, is to reduce the fan’s large operating current value.

When the system’s gas consumption changes rapidly over a short time, the controller sometimes cannot respond in time and fails to open the intake valve promptly to prevent surge. The solution is to set the intake valve’s set point to a more conservative value. The only side effect is reduced energy savings, as the intake valve opens earlier.

Additionally, regularly check on-site instruments and replace instruments that are inaccurately calibrated or counting incorrectly. Even if the control system detects no surge, the centrifugal fan may still experience surge, possibly due to insufficient driving gas, inaccurate valve sensor calibration, or poor reset performance of the intake and exhaust valves. If the valve does not respond to a signal to execute a certain action, the control system may not accurately control exhaust pressure, leading to surge.