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Control Valves In most processes, the the final final control element element is is an automatic valve. (See the figures in Riggs on pp. 45-48) Most industrial control valves are globe are globe valves valv es,, so called because of the shape of their body. Within the body, the valve stem is stem is moved up and down ( strokes strok es)) by the actuator . This opens and closes a gap between the valve plug and and the valve seat . The parts of the valve that come in contact with the process fluid including the valve seat, plug, etc. are collectively known as the valve tri t rim m . Trims are designed and machined to regulate and "shape" the flow according to a specific characteristic. characteristic. Valve actuators are usually usually pneumatic/spr pneumatic/sprin ing g devi de vices ces,, although although pi p iston sto n (air on both sides), sides), electri electricc motor, motor, and and hy hydraul draulic actuators actuators are made. The signal from the controller passes through an I/P transducer and is converted to a stream of pressuri pressurizzed air. air. Thi Thiss air air push pushes es agai again nst one one side side of a diaphragm and works against a spring to move the valve stem to the desired position. If the air signal is lost, the spring drives the valve to its failure position position.. Depending on whether the spring is above or below the diagphragm valves can be Fail be Fail Closed (a.k.a. (a.k.a. Air-to-Open Air-to-Open or AO) or Fail or Fail Open Open (Air-to-Close or AC). The failure position of a valve is a significant safety consideration and is determined early in the control system design. The valve, actuator, and I/P transducer all influence the system behavior (Riggs lumps them together as the "actuator system"). Other types of valves (notably rotary or butterfly valves) are used, but the globe type control valve described is most common. Other final control elements include furnace dampers, variable speed drives, etc. A valve positioner valve positioner is is a pneumatic device which precisely positions a valve. It is essentially a small controller which senses the stem position, compares it to the controller output, and adjusts the pressure to the actuator. Positioners can be used to overcome stem friction, high fluid pressure, viscous or dirty fluids, or to improve response when system dynamics are slow (cf. ( cf. valve valve position controllers).
Sizing Valves Undersized control valves cannot pass the required flow. Oversized valves cost more than is necessary and may not make a large enough impact on the system. Neither case permits precise regulation of the process, hence control valve sizing is an important engineering task. facstaff.cbu.edu/r pr ice/l ectur es/val ve.html
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The valve sizing equation should look familiar -- after all, a valve is just a flow restriction.
The equation shows that the volumetric flow through the valve is proportional to the square root of the pressure drop, adjusted for the density of the fluid. The proportionality factor is the valve size coefficient . This equation, and particularly the size coefficient, is written in various ways (Riggs includes a specific "units" term) but the form is good for any incompressible fluid. The size coefficient has two parts -- a constant value that relates to the maximum flow the valve can pass and a characteristic function that function that describes how the valve open area varies with stem travel. This function goes to 1.0 when the valve is wide open and to 0.0 if the valve is completely closed. Most valve manufacturers do not separate the valve characteristic into parts, but instead tabulate values of C v in in their their catalog c atalogss and a nd software (cf. (cf. Riggs Riggs Table 2.1). When selecting a control valve, you first estimate a body size (equal to or slightly smaller than the line size), a characteristic, and then use the table to determine which valve will provide the required size coefficient. The sizing equation shown is limited to incompressible fluids. Different equations are needed for compressible and two phase flow. As a comparison, one steam sizing equation is:
Valve Characteristics The trim of a valve is designed to produce a defined characteristic relationship between the stem travel (opening) and the flow through the valve. Three common characteristics are linear, equal percentage, and quick opening; equal percentage is the most common. A linear valve is the easiest to understand: the flow rate is directly proportiona proportionall to trav travel el.. The The charact characteri eristi sticc fu function ction is sim simple: ple:
where travel is given as a fraction (0 to 1.0). An equal percentage valve percentage valve is machined so that an equal increment of travel produces an equal change in flow (so it will be linear on a semilogrithmic plot). For example, it might work something like: Trave Tr avell ((fr fraction) action) Flow (% of Max) Ma x)
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0.0
0.0
0.4
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0.6
25.0
0.8
50.0
1.0
100.0
A typical characteristic function for an equal percentage valve might be:
Inherent
vs.
Installed Characteristics
The design of the trim is only part of what sets the flow through a valve. In particular, the flow depends on the pressure pressure drop across across the the val valve. Manu Manufacturers acturers test test val valv ves in in a rig rig where where the the pressu pressure drop is is kept cons constan tant, t, thu thus the performance they see is the inherent characteristic of characteristic of the valve. In a real plant, pressure drop varies as the flow changes, so the characteristic relationship seen between travel and flow will not be the same as that seen in the test rig. This installed installed characteristic is characteristic is what really matters to a process eng engineer. To understand the difference between inherent and installed characteristics, visualize a family of linear valves.
Notice Notice how how each has a dif different erent slope slope and and max maxiimum flow. In many real systems, pressure drop increases with flow. This can be visualized by picking the appropriate operating point from each valve of the family:
and the "installed" characteristic of a linear valve in this situation looks more like equal percentage. Oddly enough, when an inherent equal percentage valve is used in this situation, the installed characteristic is nearly linear. We usually want the installed characteristic to be as linear as possible to make for consistent control, so equal percentage valves are typically purchased for use when pressure drop varies with flow.
Fixing Valve Pressure Drop facstaff.cbu.edu/r pr ice/l ectur es/val ve.html
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The pressure drop in a process system comes from a variety of sources -- friction in lines, static head, pumps, and control valves. When the total flow is low, control valve pressure drop tends to be a large fraction of the total system pressure loss; but at high flows this may not be true. A good design will respond well over the full range of conditions, hence it is important to pick the right characteristic for your system and size the valve for the right amount of pressure drop. For good control, it is nice to take a fairly large pressure drop across a control valve. This way it will have a big influence on the total system, making the operators and control engineers happy. However, design engineers will worry that increasing valve pressure drop will tend to increase pumping and other operating costs. Compromise is necessary. As a rule of thumb (your mileage may vary), design the system and size the valve so that 25% of the total system pressure pressure drop (including the valve) is taken across the control valve, with a minimum of 10-15 psig. (Another version of this rule suggests taking 1/3 of the system drop excluding the valve).
Valve Valve Sizing/Des Sizing/Design ign Procedur Proc eduree Typically, a process engineer sizing a control valve will follow a set of steps like the following: 1. Size for maximum maximum flow flow Estimate the maximum required flow Calculate system pressure drop without the valve Choose a valve that will pass the maximum flow when about 90% open Check for cavitation, flashing, etc. 2. Size for minim minimum um flo flow Choose a valve that will pass the minimum when about 10% open 3. Size for for "nor "normal" mal" flow flow Choose a valve that will pass the normal flow when about 60- 70% open You want to maintain valve turndown and turndown and operability across the full range of anticipated conditions. A valve that is closed or wide open cannot modulate the flow, so you should avoid any plans to operate with the valve "pegged". You also should note that automatic control valves are not designed to produce "tight shutoff", and will likely be facstaff.cbu.edu/r pr ice/l ectur es/val ve.html
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prone prone to sm small all amou amount ntss of leakage eakage when when closed. closed. Consequ Consequen entl tly, y, most control control val valve in install stallations ations inclu clude block valves, valves, manual valves which can be turned when complete shutoff is needed.
Cavitation Liquid flowing through a valve speeds up because flow is restricted. This causes the pressure within the valve to drop.
If the minimum pressure of the flowing fluid falls below the vapor pressure, vapor bubbles form. These can pose problem problems. s. If the exit pressure remains below the vapor pressure, the bubbles can choke the choke the valve so that changes in valve pressure pressure drop no no lon longe gerr eff effect th the fl flow rate. rate. If the exit pressure comes back above the vapor pressure, the vapor bubbles collapse in tiny detonations, produci producin ng noise, oise, vi vibration bration,, and and phys physiicall cally damag damagiing the the surf surfaces. aces. Thi Thiss cavitation can cavitation can be very destructive. Control valves should always be designed to avoid cavitation and flashing. In extreme cases, it may be necessary to use specially designed valve trims to prevent problems.
References:
1. Luyben, Process Luyben, Process Modeling, Modeling, Simulation and Control Cont rol for Chemical Engineers Engineers (2nd (2nd Edition), McGrawHill, 1990, pp. 213-22. 2. Marlin, Marlin, T.E., Process T.E., Process Control: Cont rol: Designi Designing ng Processes Processes and Control Systems for fo r Dynamic Dynamic Performance, Performance , McGraw-Hill, 1995, pp. 541-47. 3. Riggs, Riggs, J.B., Chemical Process Control (2nd (2nd Edition), Ferret Publishing, 2001, pp. 43-64.
R.M. Price Original: 10/25/93 Mo dified: dified: 4/29/2003 4/29/2003
Copyright 2003 by R.M. Price -- All Rights Reserved
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