Liquid control valve sizing

For a given liquid flow rate (Q), in gpm, and control valve pressure drop (dp), in psi, control valve sizing eoefScient (Cv) can be calculated by following equation  

Control valve sizing coefficient (Cv) is a measure of control valve capacity. Larger control valve has larger Cv value. For liquid control valve, Cv means the liquid flow in gpm thru a control valve at 1.0 psi control valve pressure drop. 

The mininium choke flow control valve pressure drop (dp choke) for a liquid control valve can be estimated by following equation. 

The maximum flow (Q max, in gpm) throu a liquid control valve is choke flow. It is calculated by following equation  

If there is reducer at control valve inlet and outlet, replace FL in Eq, (5) by FLp/Fp. FLp is calculated by following equation  

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Two liquid control valve sizing rules of thumb are... 

The above liquid control valve sizing equations are based on flow through valve is turbulent flow. Following procedures should be used to check whether the flow through control valve is turbulent or not. If the flow through control valve is not turbulent, difierent equations should be used to calculate the control valve Cv (2J, First, use following equation to calculate control valve s Reynolds number (Re) ... 

Cvs laminar flow liquid control valve sizing coefTicient see Eq. (10). 

When sizing liquid control valves first use... 

The careful selection and design of control valves is important good flow control must be achieved, whilst keeping the pressure drop as low as possible. The valve must also be sized to avoid the flashing of hot liquids and the super-critical flow of gases and vapours. Control valve sizing is discussed by Chaflin (1974). 

More detailed equations are available in publications of the control-valve manufacturers (for example, the Masonielan Handbook for Control Valve Sizing, Dresser Industries, 6th edition, 1977) that handle flows of gases, flashing liquids, and critical flows with either English or SI units. 

J4. Reactant liquid is pumped into a batch reactor at a variable rate. The reactor pressure also varies during the batch cycle. Specify the control valve size and the centrifugal pump head required. Assume a flat pump curve. 

For a complete treatment of the many aspects of control valve sizing, refer to the Instrument Engineers Handbook. For some basic equations to calculate the required valve coefficients (C ) for various liquid and gas flow applications using different valve types, refer to Tables 2.66 and 2.67. 

CONTROL VALVE SIZING FOR LIQUID, GAS, STEAM, AND TWO-PHASE FLOW... 

Control Valve Sizing for Liquid, Gas and Steam Conditions Program Inputs and Outputs... 

FORMAT (//,20X, CONTROL VALVE SIZING FOR LIQUID SERVICE, /, IH, 76(1H ))... 

The control valve is sized to be 10 percent open during summer operation, wiien the temperature of the subcooled liquid in the tank is 45°C. Use the control valve sizing formula... 

V. Control Valve Sizing Control valve sizing for liquid or vapor service will be discussed in this section. 

For sizing a flashing control valve add the Cv s of the liquid and the vapor. 

Whenever two-phase flow is encountered in facility piping it is usually in flowlines and interfield transfer lines. Some designers size liquid lines downstream of control valves as two-phase lines. The amount of gas involved in these lines is low and thus the lines are often sized as singlephase liquid lines. Oversizing two-phase lines can lead to increased slugging and thus as small a diameter as possible should be used consistent with pressure drop available and velocity constraints discussed in Volume 1. 

When equipment receives impulses at its own natural frequency of vibration, excessive vibration (resonance) occurs, and this can lead to rapid failure. A control valve was fitted with a new spindle with slightly different dimensions. This changed its natural frequency of vibration to that of the impulses of the liquid passing through it (the frequency of rotation of the pump times the number of passages in the impeller). The spindle failed after three months. Even a small change in the size of spindle is a modification [24]. 

B. SIZE. Sizing of control valves is one of the more controversial subjects in process control. The flow rate through a control valve depends on the size of the valve, the pressure drop over the valve, the stem position, and the fluid properties. The design equation for liquids (nonflashing) is... 

In the catalyst preparation area where the fire occurred, aluminum alkyl and isopentane are mixed in a batch blending operation in three 8000-gallon kettles. The flow rates of components are regulated by an operator at the control room. Temperature, pressure, and liquid level within the kettles are monitored by the control room operator. The formulated catalyst is stored in four 12,000-gallon vertical storage tanks within this process unit. Aluminum alkyl is a pyrophoric material and isopentane is extremely flammable. Each vessel was insulated and equipped with a relief valve sized for external fire. 

Pressure drop. Good control requires a substantial pressure drop through the valve. For pumped systems, the drop through the valve should be at least 1/3 of the pressure drop in the system, with a minimum of IS psi. When the expected variation in flow is small, this rule can be relaxed. In long liquid transportation lines, for instance, a fully open control valve may absorb less than 1% of the system pressure drop. In systems with centrifugal pumps, the variation of head with capacity must be taken into account when sizing the valve. Example 7.2, for instance, illustrates how the valve drop may vary with flow in such a system. 

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