CONTROL VALVE
INTRODUCTION:
A control valve is a device capable of modulating flow at varying degrees between minimal flow and full capacity in response to a signal from an external control device. The control valve often referred to as “the final control element,” is a critical part of any control loop, as it performs the physical work and is the element that directly affects the process.
A control valve is comprised of an actuator mounted to a valve. The valve modulates flow through movement of a valve plug in relation to the port(s) located within the valve body. The valve plug is attached to a valve stem, which, in turn, is connected to the actuator. The actuator can be pneumatically, electrically or hydraulically operated.
Control valve parts:
- Body - That part of an automatic control valve in which the flowing medium is contained. The body provides the pipe connecting ends, the fluid flow passageway and may support the seating surfaces.
- Actuator - That part of an automatic control valve, which causes the valve stem to move
- Bonnet: those portions of the valve pressure-retaining boundary, which may guide the stem and contains the packing box and stem seal. It may also provide the principal opening to the body cavity for assembly of internal part of the valve body. It may also provide for the attachment of the actuator to the valve body. The bonnet can be bolted, threaded or welded to or integral with the body. Types of bonnets are Bolted, flange bonnet, Extension bonnet, Finned bonnet and bellows
- Bonnet Gasket: A deformable sealing element between the mating surfaces of the body and bonnet.
- Yoke: A structure by which the diaphragm case assembly is supported rigidly on the bonnet assembly
- Packing: A sealing system consisting of deformable material of one or more mating and deformable elements in a packing box which may have an adjustable compression means to obtain or maintain an effective pressure seal. Common materials used are Teflon; Teflon impregnated asbestos and Graphite asbestos
- Packing Box: The chamber in the bonnet, surrounding the stem and containing packing and other stem sealing parts.
- Packing follower: A part, which transfers mechanical load to the packing from the packing flange or nut
- Lantern ring: A rigid spacer assembled in the packing box with packing normally above and below it and designed to allow lubrication of the packing or access to a leak-off connection.
- Plug: The moveable part of the valve that makes contact with the valve seat when the valve is closed, and which varies the area controlling the flow. The three most common types of control valve plugs are contoured, V-port, and quick opening. A contoured plug controls flow by a shaped end and is usually end-guided at the top or bottom (or both) of the valve body. A V-port plug has a cylinder (called a skirt), which rides up and down in the scat ring. The skirt guides the plug, and by virtue of shaped openings in the skirt, varies the flow area. Quick-opening plugs are machined to provide maximum flow quickly when the plug lifts from its seat. These plugs can be either end-guided or guided by wings riding in the seat ring.
- Seat Ring: A part that is assembled in the valve body and may provide part of the flow control. The seat ring may have special material properties and may provide the contact surface for the closure member.
- Cage: A part in a globe valve surrounding the closure member to provide alignment and facilitate assembly of other parts of the valve trims. The cage may also provide flow characterization and or a seating surface for globe valves and flow characterization for some plug valves.
- Guide: That part of the valve plug that keeps the plug aligned with the valve seat. Top or bottom guides on a valve plug are usually located so they do not influence flow but merely accomplish the centering function. Valve guides often have the added function of determining the valve flow characteristic. These are known as skirt guides and usually have notches or Vs cut into them to characterize flow.
- Port: This term refers to the flow-controlling opening between the seat and the disc when the valve is wide open. It does not refer to body size or end connection size. Standard valve ports are the sizes normally used in the valves. Valves with reduced ports have flow equal to a smaller valve with a standard port.
- Trim: Trim consists of all parts of a valve that are in contact with the flowing medium but are not part of the valve shell or casting. Thus, plugs, seats, discs, stems, packing rings, etc. are all trim components. The term "trim" is usually used in connection with trim materials.
Capacity: The rate of flow through a valve under stated test conditions
Flow coefficient: It is a constant related to the geometry of a valve. The quantity of water in gal/min at 60 F (16 C) that flows through a given wide-open valve with a pressure drop of 1 lb/in2. The symbol for capacity index or flow coefficient is Cv. Once the Cv of the valve has been determined, the flow of any fluid through the same valve can be calculated provided the characteristics of the fluid and the pressure drop through the valve are known.
Flow characteristic: The relationship between the flow rate through a valve and travel of the closure member as the closure member is moved from the closed position to rated travel with a constant pressure drop across the valve, is called the inherent flow characteristic. The same relationship, when the pressure drop across the valve varies as influenced by the system is called installed characteristic. The important flow characteristics are:
- Equal percentage: An equal increment of stem travel produces equal percentage in existing flow. The change is always proportional to the quantity flowing before the change. They are mostly used a pressure application. A combination of linear and equal percentage is known as modified parabolic characteristic and a combination of linear and quick opening termed as modified linear are other two types of characteristic available.
- Quick-Opening Characteristic: This characteristic provides larger flow variation at low stem travel while maintaining a linear relationship through most of the stem travel. In figure 90 percent flow rate obtained at 30% valve opening and a straight-line relationship to that point. They are used primarily in on-off service.
- Linear Characteristic: A valve, which produces flow directly proportional to the valve lift. These are specified for level control and application, which requires constant gain.
- Inherent Flow Characteristics For Common Valve Trim Designs
Travel Coefficient: The ratio between the flow at a given valve stem position and the flow through the valve at its wide-open position, usually expressed as a decimal fraction.
Rangeability: The ratio of the maximum controllable flow to the minimum controllable flow. For instance, a valve with a rangeability of 50 to 1 and having a total flow capacity of 100 gal/min, fully open, will control flow accurately down as low as 2 gal/min. The valve may or may not have tight shutoff. However, for this particular valve you could not depend upon getting steady flow of, for instance, 1.5 gal/min if this were necessary to maintain stable load conditions. Generally, rangeabilities in the range of 50 to 1 or 40 to 1 are considered excellent for extreme precision of control. Valves with high rangeability are very expensive to manufacture since very close tolerances are involved between the disc and the seat. Competitively priced control valves ordinarily have rangeability of less than 30 to 1. In many cases wide rangeability compensates automatically for deviations from anticipated conditions without loss of control.
Turndown: The ratio between maximum usable flow and the minimum controllable flow; usually less than the rangeability. For instance, as stated above, after the 100-gal/min valve has been applied at a job, it might turn out that the most flow you would ever need through the valve is 68 gal/min. Since the minimum controllable flow is 2 gal/min, the turndown for this valve is 34 to 1. In comparing rangeability and turndown, we may say that rangeability is a measure of the predicted stability of the control valve, and turndown is a measure of the actual stability of the valve
Shut –Off: The shut -off rating of a valve is the maximum allowable pressure drop to which the valve may be subjected while fully closed. This rating is usually a function of the power available from the valve actuator for holding the valve closed against pressure drop, but structural parts such as the stem sometimes are the limiting factor. The shut -off rating is independent of the actual valve body rating.
Direct / Reverse acting valve: A control valve that travels to the closed position when the signal to the actuator increases is called direct acting and the valve that travels to the open position when the signal increases is called reverse acting.
Seat Leakage
Control valves are designed to throttle. However, this is not a perfect world, and control valves are also usually expected to provide some type of shut-off capability. A control valve's ability to shut off has to do with many factors. The type of valves for instance. A double-seated control valve will usually have very poor shut-off capability. The guiding, seat material, actuator thrust, pressure drop, and the type of fluid can all play a part in how well a particular control valve shuts off
There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2-1976. But for the most part you will be concerned with just two of them: CLASS IV and CLASS VI. CLASS IV is also known as METAL TO METAL. It is the kind of leakage rate you can expect from a valve with a metal plug and metal seat. CLASS VI is known as a SOFT SEAT classification. SOFT SEAT VALVES are those where either the plug or seat or both are made from some kind of composition material such as Teflon
Valve Leakage Classifications:
Class I. Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made.
Class II. Intended for double-port or balanced singe-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.
Class III. Intended for the same types of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity.
Class IV. Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01% of rated valve capacity.
Class V. Intended for the same types of valves as Class IV. The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 X 10 ml per minute per inch of orifice diameter per psi differential.
Class VI. Intended for resilient-seating valves. The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.
Nominal Port Diameter Allowable Leakage
(Inches) (ml Per Minute) (*Bubbles Per Minute)
1 0.15 1
1.5 0.30 2
2 0.45 3
2.5 0.60 4
3 0.90 6
4 1.70 11
6 4.00 27
8 6.75 45
10 9.00 63
12 11.5 81
Bubbles per minute as tabulated are a suggested alternative based on a suitable calibrated measuring device, in this case a 0.25-inch O.D. X 0.032-inch wall tube submerged in water to a depth of from 1/8 to 1/4 inch. The tube end shall be cut square and smooth with no chamfers or burrs. The tube axis shall be perpendicular to the surface of the water. Other measuring devices may be constructed and the number of bubbles per minute may differ from those shown as long as they correctly indicate the flow in milliliters per minute.
Type of valves
There are basic two types of control valves as per the motion of the closure member. They are rotary and linear. Linear-motion control valves commonly have globe, gate, diaphragm or pinch type closures. Rotary-motion valves have ball, butterfly or plug closures. Each type of valve has its special generic features, which may, in a given application, be either an advantage or a disadvantage.
a) Linear Valve Features
• TORTUOUS FLOW PATH
• LOW RECOVERY
• CAN THROTTLE SMALL FLOW RATES
• OFFERS VARIETY OF SPECIAL TRIM DESIGNS
• SUITED TO HIGH-PRESSURE APPLICATIONS
• USUALLY FLANGED OR THREADED
• SEPARABLE BONNET
b) Rotary Valve Features
• STREAMLINED FLOW PATH
• HIGH RECOVERY
• MORE CAPACITY
• LESS PACKING WEAR
• CAN HANDLE SLURRY AND ABRASIVES
• FLANGELESS
• INTEGRAL BONNET
• HIGH RANGEABILITY
Control Valve Classification
a) Single ported Globe Valve
• Simple in construction
• Used in sizes 2” and below
• Can get tight shut off
The balancing in this valve is achieved by porting through the body or plug so that the same pressure acts on both sides of the valve plug.
b) Double ported valves
- High flow capacities
- Smaller stem force required when compared to single seated valve
- Should not be used where tight shut off is needed.
C) Three-way valves
It is used to mix or divert flowing steam. In blending (mixing) service there are two inlets and one outlet in diverting service two outlets and one inlet in mixing, it is modified double port body.
d) Angle Valves
Used where space is at a premium.
In services where the pressure drop is high.
Where the effect or turbulence, contention percent problems.
e) Butterfly valve
• It has a shaft supported vane or disc rotating with in a cylindrical body.
• Used in high-pressure drop, high static pressure and tight off services.
• Economical in larger size ranges due to their simple design.
f) Diaphragm valves (Saunder’s valve)-
• Well suited for slurries and viscous Fluid.
• Very high capacity.
• Low cost.
• Very poor control characteristic and Low turn down ratio.
Actuators
Actuators must overcome the unbalance force caused by the pressure drop across the valve, friction in between and weight of the moving parts and stem unbalance. There are pneumatic, hydraulic or manual actuators available. Most of the control valves are pneumatically actuated. There are two types of pneumatic actuators: (a) Spring /diaphragm types and (b) piston type.
In spring type depending on whether air to open or air to close vent is required, direct acting or reverse acting actuator can be used.
The main advantage of a piston actuator is its capacity for longer travels and large possible effective areas. It also generally tolerates higher actuating pressure. As with the diaphragm style, the piston can be spring opposed, air to extend or air to retract. Piston actuators can also be used in double acting mode with pressure on either side of the piston by using a double acting positioner.
Accessories
Manual override or Hand wheel, Limit switches, Position transmitter, solenoid valves, air regulator, volume booster, positioner.
The I to P converter receives current signal (4-20 mA signal) from the controller (control room) and convert it to a proportional pneumatic signal (3-15 psi) and feeds the positioner. A control valve positioner is basically a relay that senses both instrument signal and control valve position. The primary function is to have the valve stem position proportional to the input signal.
Advantages:
• To minimize friction in the movement of the stem.
• To overcome hystresis
• As pressure boosters in a split range valves
• A control valve signal can be reversed using a positioner.
Site specific information:
In the refinery a large number of control valves are used as the final control element for flow, level, pressure and temperature. They belong to different type, size and manufacturer.
Instrumentation Ltd, MIL / Dresser Masonielan & Fisher are the common manufacturers. For certain applications few valves of the following make is also available like Motoyama, Kitamura, Neles Jamesburry, Copus Vulcan, Blakeborough, Fouress, Gulde, Continental, Mazda, Dempla, RK engineering, Dezurick, Valtek, Virgo, Brooks, Microfinish, Sereg Schlumberger etc.
Generally air to close valves are painted green and air to open valve are painted red.
Critical valves are generally serviced during shutdown using in house expertise or outside agencies. Routine problems encountered are listed below:
• Problems associated with the fault of electrical signal from control room, air pressure, faulty I to P, faulty air regulators and positioner.
• Gland leak and bonnet leak can generally be attended on line but in certain cases may require opening of the valve.
• Body leak can be attended by welding or body replacement.
• Valve put on high temperature services after servicing may develop bonnet leak and may require hot bolting.
• Calibration of valve may also be required to be done in certain cases.
• Actuator casing damage due to corrosion, spring and diaphragm damage,
• Bending of stem
• Damage of internals like plug & seat due to erosion. Corrosion and cavitation.
• Damage of diaphragm in case of diaphragm valves (mainly in DM plant)
Whenever valves are attended for maintenance jobs on line, it has to be ensured that valve isolation and trapped quantity draining is done.
For shutdown valves positioner are not used since they do not have any intermediate position other than full open or full close. Solenoid valves are used to on / off air supply to the valves.
In the truck-loading gantry, Brooks digital valves are used where the fluid pressure itself is used to operate a spring opposed piston. The pressure on the piston is varied by operating a NO and NC solenoid each for varying times, which either admits fluid from valve upstream into the valve actuator or drains fluid from the actuator to valve downstream and thus controls the valve opening.
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