Sizing of Control Valves

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. 

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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... 

Sizing of control valves is important because, if the valve is oversized or undersized, accurate control of the flow rate can be poor and a contrain for the flow rate can be contrained. A simplified valve flow equation for an incompressible fluid is given by... 

Mote This table may serve as a rough guide only since actual flow capacities differ between manufacturer s products and individual valve sizes. (Source ISA Handbook of Control Valves" Page 17). 

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. 

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). 

The design of a chemical engineering system always involves a number of trade-ofls. We have already discussed the conflicts between the process engineer and the control engineer in the question of control valve sizing. There are many other such conflicts between what would be the optimum from only a steadystate standpoint and what is needed to handle the dynamics of the process. 

Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually arc shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. 

No. Req d. Size— Diaphragm Control Valve Valve to With Failure of Air... 

Correctly sized control valves achieve a high quality of control. A valve that is too small will not allow the required flow, and a valve that is too large will cost more than a eorrectly sized smaller valve and the flow will be uncontrollable because its full control range will not be used. The procedure for sizing a control valve is [8] ... 

The pressure drop (AP j j g) used for sizing the control valve is the actual pressure drop across the valve at the sizing flow rate. This is obtained by starting with the system pressure drop (AP y ). The pressure drop at the sizing flow rate of all pipe, fittings, and equipment in the flow system is subtracted from The remainder is the actual... 

Sizing a control valve for liquid-gas mixtures introduces incorrect valve coefficient, C, if for each of the two fractions is calculated separately, and then added. This method assumes that the liquid and gas are passing through the valve orifice independently and at greater different velocities. 

Hooper, W. B., The 2-K Method Predicts Head Losses in Pipe Fittings, Chem. Eng., p. 97, Aug. 24 (1981). Hooper, W. B., Calculate Head Loss Caused by Change in Pipe Size, Chem. Eng., p. 89, Nov. 7 (1988). Hutchison, J. W., ISA Handbook of Control Valves, Instrument Society of America, Research Triangle Park,... 

Control system. For subsequent selection and sizing of pumps and compressors, we need to map out the number and location of the control valves. Since the number of control valves is related to the number of control degrees of freedom, identify the control degrees of freedom. For example, a typical hydrodealkyllation process with a reactor, furnace, vapor-liquid separator, recycle compressor, two heat exchangers, and three distillation columns has 23 control degrees of freedom (Luyben et al., 1997). This requires 23 control valves whose location affects the rest of the design and the safety and hazards (see Section 16.7). 

Based on the high elasticity of PDMS, the elementary microfluidic unit operation is a valve which is typically made of a planar glass substrate and two layers of PDMS on top of each other. One of the two elastomer layer contains the fluidic ducts while the other elastomer layer features pneumatic control channels. To realize a microfluidic valve, a pneumatic control channel crosses a fluidic duct as depicted in Fig. 9a. A pressure p applied to the control chaimel squeezes tlie elastomer into the lower layer, where it blocks the liquid flow. Because of the small size of this valve in the order of 100 x 100 pm, a single integrated fluidic circuit can accommodate thousands of valves. Comparable to developments in microelectronics, this approach is called microfluidic large scale integration (LSI) [122]. 

The control valve adjusts the flow rate by changing the size and shape of the flow area offered to the fluid passing through it. There are two main categories of control valve used on process plant globe valves and rotary valves. The flow area in a globe valve is dictated by the position of the valve plug relative to its seat, while the flow area in a rotary valve depends on the... 

The historical dominance of US valve manufacturers has led to valve characteristics often being given in standard US units (US gallons, standard cubic feet per hour, etc.). A particularly important influence because of its extensive research into and testing of control valves was the Fisher Controls Company of Marshalltown, Iowa (now part of Fisher-Rosemount Ltd.) This appendix converts the basic liquid-flow equation from US to SI units. The appendix then goes on to derive Fisher s Universal Gas-Sizing Equation ( FUGSE ) from the basic liquid-flow equation and shows how this may be converted into SI units. 

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