Basic Control Valve Sizing and Selection

DeZurik Operation October 2005 Valve Sizing What is Valve Sizing? It is a procedure by which the dynamics of a process system are matched to the performance characteristics of a valve. This is to provide a control valve that will best meet the needs of managing flow within that process system.

Flow Coefficient (C V ) The valve flow coefficient, C V is the number of U.S. gallons per minute of water at 60 degrees F which will pass through a given flow restriction with a pressure drop of 1 psi. For example, a control valve which has a flow coefficient, or C V, of 12 has an effective port area that it passes 12 gallons per minute of water with 1 psi pressure drop.

DeZurik Operation October 2005 Valve Sizing Basic information requirements for effective valve sizing For the system: Pressure before and after the control valve, Δ P Flow rate, quantity and units, Q Process temperature with units, T Properties of the media, (viscous, fiber suspension, gaseous its vapor pressure, sometimes molecular weight) For the control valve: Flow capacity (Cv), inherent throttling curve, Kc (Cavitation Index), FL² (Critical Flow factor)

Delta P (  P)  P Sizing is the pressure drop across the valve used for control valve sizing at a specific flow rate . Most applications have multiple flows for sizing with different Δ P for each In a given system, higher flow rates generally result in lower pressure drop across the valve.

Flow Characteristics Control valve flow characteristics are determined principally by the design of the valve trim. The three basic flow characteristics available are: Quick Opening Linear Equal Percentage (=%) A modified characteristic (sometimes called modified percentage) generally falling between the linear and equal percentage characteristics may also be available.

Why Use Equal Percentage Provides equal percentage increases in rate of flow for equal increments of plug movement. Provides the best choice of flow characteristic for most systems.

DeZurik Operation October 2005 Valve Sizing - Cavitation What is cavitation and what does it do to valves ? Cavitation is a 2-stage activity where a portion of the liquid media drops below vapor pressure. This part will boil (vaporize). In stage two, slightly downstream, pressure recovery takes place and the vapor bubbles collapse. The condition known as “critical flow” is an extension of cavitation in that it simply gets worse as the pressure drop increases. This is to the point that changes (reductions) in downstream pressure no longer influence flow rate.

Fluid Recovery Factor (F L ) When a fluid passes through the valve orifice, the velocity increases. This velocity increase is accompanied by a proportional decrease in pressure. Velocity reaches a maximum and pressure a minimum at the smallest cross sectional flow area downstream of the orifice (the vena contracta ). Downstream of the vena contracta the fluid decelerates and consequently the pressure increases or recovers (giving us the term pressure recovery). Different valve types exhibit different recovery factors which becomes an important consideration in valve selection.

Fluid Recovery Factor (F L ) The fluid recovery factor (F L ) is effectively an index of pressure recovery in a control valve. High F L values indicate low pressure recovery where a low F L value is an indication of high pressure recovery . Higher F L values result in better resistance to cavitation. F L ~(P 1 -P 2 )/(P 1 - P VC )

Valve sizing - Cavitation Collapse of the bubbles created by cavitation produce noise, like gravel in the line. Bubbles that collapse in contact with the valve or pipe create damage. Cavitation can be controlled with special valve trims in some types of valves. Cavitation should be avoided in all circumstances.

DeZurik Operation October 2005 Valve Sizing What is “Flashing”? It is a condition where the downstream pressure is below the vapor pressure of the incoming fluid and allows some of the liquid to become vapor. Flashing does not create noise or damage in the valve as with cavitation. It can, however, create damage to the downstream piping due to high velocity. A control valve will function quit well under these circumstances. Yes!

DeZurik Operation October 2005 Valve Sizing Characteristics Systems driven by centrifugal pumps lose pressure two ways As the flow increases the pump pressure declines As the flow increases pipeline friction losses increase Using a valve with equal percentage or parabolic characteristics will produce an installed characteristic that is more linear.

Inherent vs. Installed Characteristics Inherent flow characteristic is defined as the relationship between flow and valve stroke at constant pressure drop. It is unique to valve construction and depends primarily on geometry of the throttling trim and body flow passages and is determined by flow testing. The inherent flow characteristic changes when installed in a piping system. The result is the installed flow characteristic. As system pressure drop absorbed by the control valve is reduced, the greater the deviation from the inherent state. Ideally a control valve should be sized to absorb 15%-25% of system pressure.

DeZurik Operation October 2005 Valve Sizing What is desired ? - After installation, generally it should be near linear. Why? - To provide a more predictable flow change in response to each incremental valve position change

Control Valve Seat Leakage It is becoming more common to expect a control valve to provide isolation duty, thus allowable leakage becomes more important. Control valve seat leakage is designated by the classification of ISA/ANSI-70-2 Leakage classes are class I-VI. Classes I-V are measured using water, class VI is measured using air. The most common class for metal seated control valves is class IV. (0.01% of rated Cv) Class VI generally applies to resilient seated valves only.

Fluid Velocity Excessive fluid velocity can create erosion, and accelerate corrosion damage. It also contributes to noise level and vibration. Valve users will generally define acceptable velocity in valves and pipes for the above reasons. Liquid velocity of up to 15 ft/sec . is common; for gas or steam 400 ft/sec. or higher, depending upon size, is perfectly acceptable.

Noise and Vibration Noise results from the conversion of the mechanical energy of the flow into acoustic energy as the fluid passes through the valve restriction. Vibration is a direct result of noise and can create issues with valve life. OSHA has designated 85-90 Dba as the generally acceptable limit for valves installed in most areas. Noise can be reduced through two means, internal at the valve trim, and external such as pipe insulation or downstream silencers. Cage guided globe valves offer the widest range of noise and vibration attenuation solutions.

Rangability Rangability is defined as the ratio of minimum to maximum controllable flow rates. Equal percent characteristic offers the best rangability of common types.

Flow Direction Many valve types have a preferred flow direction to minimize the effects of velocity, to create the tightest shut-off, and to optimize the response to input signal.

DeZurik Operation October 2005 Globe Valves

DeZurik Operation October 2005 Globe Valves How do they work? - By linear (stroke) action - A contoured plug is lifted out of a seat ring to allow flow to pass - In most (not all) cases flow comes in “under the seat” - Conversely, pushing the plug down into the seat causes closure Flow in

DeZurik Operation October 2005 Globe Valves Unbalanced / Balanced Trim - Unbalanced has system pressure acting against the full area of the seat Single seated plug styles are unbalanced - Balanced has provisions for offsetting system pressure Available in double seated plug or balanced cage styles

DeZurik Operation October 2005 Globe Valves Bodies & Bonnets - Configured as required to meet valve style and ANSI Class considerations - Removable bolted bonnets retain and/or support the trim components

DeZurik Operation October 2005 Globe Valves 3 Way configurations for Mixing and diverting. -Temperature control - Blending

DeZurik Operation October 2005 Globe Valves Trim Components - Stem, plug, seat, cage (if that style) & guiding components - Made of alloy materials - Optional hardened seating surfaces when required Top guided style Cage guided style

Noise and cavitation control trims By directing flow through a series of staged drops, these trims eliminate cavitation in liquid flow and provides multiple pressure breakdown for noise attenuation

DeZurik Operation October 2005 Rotary Control Valves

DeZurik Operation October 2005 Rotary Control Valves Design & Construction ANSI & ISA Face to Face

DeZurik Operation October 2005 Rotary Control Valves V ported valves : Uses a ball segment that is Always in contact with the seat. Up to 100:1 rangability. Eccentric type: Uses a plug that cams away from the seat during opening Ideal solution for erosive services Such as slurries and steam. Available with a range of flow capacities.

DeZurik Operation October 2005 High performance Butterfly Valve Ideal economic choice for larger line sizes 50:1 Rangeability Modified equal percent flow Characteristic Fast accurate response to Control signal

Actuators The actuator provides the muscle for the control valve and is responsible for moving the valve control element to the required stroke position. Spring diaphragm actuators are the most widely recognized and used by control valve suppliers. Reverse acting; fail closed and direct acting; fail open. Simplistic design ,few moving parts, and easy to maintain. Double acting pistons Smaller, lighter, and less expensive than diaphragm . Fail in last position .

Accessories Positioners – pneumatic input 3-15 psi Electro-pneumatic 4-20 ma (HART, Fieldbus, Profibus) Limit switches Position feedback

Review material ISA Practical Guide Publications Control Valves Manufacturers data and guides. Al Holton Nor’East Controls [email_address] 508-864-5984

Control Valve Noise What is noise? Noise is unwanted sound. Irritant to most people resulting in circulatory problems . Overexposure can result in hearing loss . Limits set by OSHA , Occupational Safety and Health Act; 90 dBA . Noise generated by control valves can cause damage . Three major forms of valve generated noise: Mechanical Hydrodynamic Aerodynamic

Hydrodynamic Noise Liquid flow noise, cavitation noise, and flashing noise can be generated by the flow of a liquid through a valve and piping system. Of the three noise sources, cavitation is the most serious because noise produced in this manner can be a sign that damage is occurring at some point in the valve or downstream piping. Select cavitation elimination or containment trim. Prediction techniques for subcritical liquid flow and cavitation noise are available. Flashing noise is not easily predictable and is not often a problem. Hard faced trims are often recommended.

Ratings Pressure and temperature ratings for pressure containment parts have been established for the more common materials by the American National Standards Institute ( ANSI ). Class 150, 300, 600, 900, 1500, 2500, 4500, and Special Class options. Since most materials have a reduction in allowable working stress at elevated temperatures, the pressure temperature rating must be considered in the choice of materials. Chrome-moly, carbon steel, stainless steel

Trim Materials Valve trim material selection is generally influenced by the factors of corrosion, erosion, wear, galling, pressure drop, and temperature, instead of pressure containment considerations. Commonly used materials are 304, 316, 416, 440, and 17‑4 precipitation hardened (PH) stainless steels . Other materials such as nickel alloys (i.e. Hastalloy, Monel…), and cobalt chromium alloy (i.e. Stellite), are sometimes required. A common practice is to use a base material, such as 316 stainless steel, faced with cobalt chromium alloy at points of expected wear such as seating surfaces and/or guide posts. Material selection is often limited by valve design. Turned contoured plugs and seat rings can often be made from readily available bar stock . Cast components with complex geometric shapes may have a more limited range of offering.

Materials of Construction Materials of construction are very important in control valve design since control valves are required to handle all types of fluids. Fluids can vary from clean, dry air to corrosive chemicals at temperatures ranging from near absolute zero to well above 1000 degrees Fahrenheit and pressures from near vacuum to 50,000 psig or higher. Most control valve materials can be placed in two categories: Pressure containment materials valve body, bonnet, bottom flange, and bolting Valve trim materials; seats, plugs, guides, stems, shafts.

Pressure Containment Materials Carbon steel is the most common material for the majority of applications. Other materials commonly used include chrome-moly, stainless steel and cast iron. Some services require the use of exotic alloys, high nickel content metals such as, monel, hastelloy, even inconel, duplex stainless to withstand corrosive fluids . Some manufacturers offer lined or composite valves to handle low pressure corrosive services.

End Connections Control valve end connections may be classified as follows; threaded, butt-weld, socket weld, flanged, and flangeless. Threaded connections (female NPT thread) are common in valve sizes 1 inch and smaller and are sometimes used for control valves up to 2 inches . Welded ends are common where high pressure, high temperature, or highly toxic fluids are encountered. Care should be taken to see that the valve body material specified is compatible with the adjoining pipe material.

End Connections (cont.) Flanged end globe bodies generally conform to the standardized face-to-face dimensions listed in ANSI B16.10 , with the exception of Saunders types (weir or diaphragm type) and angle bodies. The flange rating is determined by the type of service, required material, maximum pressure, and the maximum fluid temperature. Flangeless end connections have no flange connection as part of the valve body and are simply bolted or clamped between the adjoining line flanges. Face-to-face dimensions for these valves are listed in ISA S.75.04 (1979). Single flange designs available in butterfly valves.

Actuator Bench Range As it relates to spring diaphragms, the actuator bench range is the operating air pressure range under no load for the rated stroke; i.e. 3-15 psi, 6-30 psi, 10-24 psi. Increasing air pressure moves the actuator through it’s rated stroke; i.e. the actuator starts moving at 3 psi and comes to its rated stroke at 15 psi. Air supply = in a pneumatic actuator it is the minimum pressure supplied to the control valve actuator to obtain the necessary performance; i.e. shut-off, speed of response, full range of stroke.

Instrument Control Signal Generally a pneumatic or electronic impulse sent from a controller to the control valve; generally through a valve positioner to position the valve for the desired control objective. The most commonly used instrument signals are analog 3-15 psi or 4-20 milliamp . (10-50 ma) Analog control signals and point to point communications are being replaced by digital based, smart devices, communicating on plant wide networks: Analog to HART to FF networks.

Seat Leakage The control valve plug is the moving component of the valve which throttles flow by positioning itself within the seat orifice and shuts off flow by contacting the seat . Leakage as measured during factory testing is considered to be a qualitative value of seat leakage , even though quantitative measurements are the criteria for testing. Installed leakage normally exceeds that measured during factory testing. Process is often more hostile than test media.

Generic Control Valve Types Features Comparison

RAVEN Trim Design Features Multiple Flow Inlets Resist “Pluggage” Rapid Reduction of Velocity Flow Path Thin Wall Design Maximizes torturous path Assures uniform velocity control Open Flow Path Permits Flow Balancing Resists “Pluggage” Relief Points Provides alternate paths if blockage does occur Multiple Flow Outlets Eliminates jet impedance Lower noise Reduces body impingement Reduces wear 200:1 educing and ttenuating elocity rosion and oise

Noise Reduction Comparison At the same flow rate, Copes Vulcan noise levels are approximately 6 dBa lower than the competitors labyrinth trim design. This is due in part to a greater number of inlet paths providing a more uniform flow path through the labyrinth groves. 03/18/10

Basic Control Valve Sizing and Selection

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