Do your positive displacement pumps and fans have outlet valves to control flow rate?

1. The flow rate of a metering pump should be adjusted using the flow control knob; it cannot be adjusted with the outlet valve, as this will cause overpressure. 2. A previous vortex pump had a return line to the inlet (tank return inlet), and under such conditions it could be regulated using the outlet valve. 3. An old Roots blower had flow control at its outlet, but there was also a pressure control valve for venting before the flow control device. 4. In the past, a Roots blower was used, with the inlet valve used to control the outlet pressure and the outlet valve used to control the flow rate. It was found that the current flowing through the motor was very high, which caused the system to trip. Later, the inlet valve was opened, and the outlet pressure was controlled by venting through the outlet; as a result, the flow rate remained unchanged, and the current dropped from 30A to 21A. 4. Centrifugal fans can use inlet valves to control the outlet pressure, which helps to reduce their surge flow rate. It is normal for centrifugal fans to use outlet valves to regulate flow; however, when the flow rate is too low, it is still necessary to open the outlet vent to avoid reaching the fan’s surge point. 5. All media that are not allowed to be vented can be returned via a backflow valve before the outlet control valve.

Positive displacement pumps cannot have their flow rate adjusted using an outlet valve, as this will cause pressure buildup.

Centrifugal fans can have their flow rate adjusted using an outlet valve.

So, under normal circumstances, outlet butterfly valves are not used directly to control the flow rate of positive displacement pumps, right?

If the outlet valve is a butterfly valve, it generally cannot be used for flow regulation, as the valve disc of a butterfly valve cannot handle it. Positive displacement pumps (such as diaphragm pumps) have a stroke that can be used to control flow, and for fans, the outlet valve can still be used to regulate flow.

Can positive-displacement fans such as rotary vane fans regulate flow rate through the outlet valve?

For positive-displacement blowers such as Roots blowers, the adjustment of the outlet valve has little impact on their flow rate; the flow rate can be controlled by means of variable frequency or bypass systems.

I’ve never seen a Roots blower with flow control; I have seen centrifugal blowers with that feature, though

For positive displacement pumps, it is not allowed to adjust the outlet valve; regulation is usually achieved by varying the motor frequency or the stroke length. Fans allow for outlet adjustment, but this is not economical; nowadays, they generally also come with inlet adjustment options (constant pressure, constant flow) or motor frequency control~~