In the first of this two-part series on the basics of aeration control valves we examined valve fundamentals and basic equations for analysis. Here, we look at interactions between valves and discuss new flow control technologies.
Basic Control Valve Principles
Most aeration systems have multiple diffuser grids drawing air from a common blower discharge header. Control valves are used for isolation and modulating airflow to match process demand.
Let’s use Figure 1 to illustrate the basic principles. It shows an aeration system with two parallel tanks, identical diffuser grids, and 8-inch drop legs. The blower output will be regulated to equal the total demand of the two tanks. The air is assumed to be at 8.5 psig pd and 180 °F Td, and V1 and V2 are butterfly valves (BFVs).
If pressure drops in piping and diffusers are ignored, the downstream pressure is identical for both tanks because submergence is equal. Differences in diffuser pressure loss are negligible. The common air header creates equal upstream pressure at both tanks. Therefore the pressure drop across both valves is identical.
In systems with several tanks the valve restriction and airflow will vary from tank to tank, but the Δp will be approximately the same. This is true whether the distribution system contains two valves or twenty. It is true regardless of the type of control device being used for throttling flow.
The upstream pressure of the system is determined by the valve at the position creating the lowest pressure drop necessary to meet the required airflow. This is the “most open valve.” In automatic control systems sophisticated programming is required to establish the most open valve. For this simplified example V1 is established as the most open valve. At 1,500 scfm to Tank 1 and V1 set at 70% open the pressure drop will be 0.06 psi.
To create an airflow rate of 750 scfm at a Δp of 0.06 psig V2 must create a Cv of ≈700. With a typical BFV this is achieved at 56% open. Any control valve, regardless of type, would have the same Cv and Δp at these conditions.
It is instructive to analyze the position response of V2 when V1 is in different positions as depicted in Figure 2, which depicts valve positions in the two-tank system. As V1 is closed the pressure differential at 1,500 scfm increases. This in turn requires V2 to move further closed to create the Cv needed to maintain the desired airflow. The data shows a wide range of flow rates can be accommodated so long as the position of V1 is within a reasonable range.
The example assumes the blower is controlled to deliver the airflow required to meet the total process demand. If that’s not the case the system can go awry quickly. As the valves at the basins throttle back the system pressure rises. If pressure control or direct flow control isn’t used to reduce blower airflow the system will eventually shut down - from high pressure with Positive Displacement (PD) blowers or surge for centrifugal blowers. Control coordination between aeration systems and blowers is mandatory.
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