Sizing, selection, and adjusting control valves often causes confusion for process and control system designers. Improper valve application can cause operating problems for plant staff and waste blower power. Basing the airflow control system design on fundamental principles will improve valve and control system performance.
The Law of Conservation of Energy and the Law of Conservation of Mass govern the behavior of control valves. When a concept or a conclusion seems questionable, or unfamiliar technology is being examined, these two fundamental principles must form the basis of the evaluation.
In this first of a two-part article, we will examine valve operating and analysis basics.
Creating Pressure Differential
The function of any valve is to create a pressure differential between the upstream and downstream piping. If the valve is employed as a shut-off device the differential is equal to the full upstream pressure. In aeration applications, valves are also used to create the pressure differential required to control airflow rates – a process known as throttling.
The pressure differential across a valve is dependent on many factors. Fluid properties are significant but are generally outside the control of the system designer. The mechanical design of the valve and the nominal diameter are also important, and they are amenable to designer selection. In most aeration applications butterfly valves (BFVs) are used for control, but alternate designs are available.
Regardless of the type of valve, or its size, the restriction to flow can be quantified by the flow coefficient, Cv. This is defined as the gallons per minute of water flowing through a valve with a pressure differential of 1.0 psi. The greater the flow coefficient the lower the restriction to flow the valve creates. The coefficient increases as valve diameter increases, or as a given valve moves open. Most valve suppliers publish the Cv data for various diameters and positions as shown in Figure 1 and Figure 2.
Shown in Figure 1 is an example of tabulated flow coefficient data for a butterfly valve. Figure 2 is an example of graphical flow coefficient data for a butterfly valve.
If the conditions of flow are known the airflow rate for a given Cv can be calculated:
Qs = airflow rate, SCFM
Cv = valve flow coefficient from manufacturer’s data, dimensionless
pu = upstream absolute air pressure, psia
Δpv = pressure drop (differential) across the valve, psi
SG = specific gravity, dimensionless, = 1.0 for air
Tu = upstream absolute air temperature, °R
In ISO units, the flow coefficient is expressed as Kv. This is defined as the flow in cubic meters per hour of water at a pressure differential of 1 bar.
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