Choke Valve Recommendation

Index

 

1.0 Valve Design
Design methodology, qualification and performance verification for production chokes is covered by the API specification 6A (ISO 10423) for surface equipment and API 17D for subsea equipment. These valves may be supplied with API, ASME or other (manufacturer specific) end connections. Valve ratings must specify pressure, temperature, material classification and capacity. These valves are typically pressure or flow restriction valves, therefore manufactured with reduced bores resulting in higher fluid velocity within the body bore. The higher velocity can have a dramatic effect on erosion and corrosion rates. Typically, the higher velocity within the valve requires special consideration be given to material selection since corrosion inhibitors may not be effective. Choke valves typically experience high velocity at the throttling mechanism; accordingly, the materials selected for the trim components must consider the service conditions. Applicable specifications will include API 6A, API 17D, ANSI B16.34, ANSI B16.5, ISA S75.01, ISA S75.02, ANSI/FCI 70-2, ASME IX and NACE MR 01 75 (ISO 15156)
1.1  Valve design methodology shall conform to API 6A latest edition.
1.2 Valve performance verification shall conform to API 6A PR2 and/or API 17D as applicable.
1.3 Valve internals including trim components shall be held in position by the bonnet bolting. Valve designs that utilize threads to secure the seat to the body are not recommended.
1.4 Valve design shall incorporate means for galling prevention on all dynamic sealing components. Metal to metal contact between sliding or rotating sealing components shall be avoided. In the case of metal seals the stem surface shall be protected with a suitable Tungsten Carbide coating to prevent galling.
1.5 Valve selection and design shall consider both purchase (CAPEX) and operation (OPEX) costs. Designs which incorporate the stem with trim components as a single unit should be avoided.
1.6 Capacity “Cv” rating shall be tested per ISA S75.02. Test results shall be accurate to within 5% of published data.
1.7 Valve designs shall incorporate a sliding stem design. Designs that rotate stems within seals shall be avoided.
1.8 Long valve travel is preferred for applications which require good controllability. For actuated applications long valve travel is required. Valve designs shall consider serviceability. Valves should be field serviceable without special tools.
1.9 Valve designs shall consider serviceability. Valves should be field serviceable without special tools.
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2.0 Body Design
Choke bodies can be either a right angle design or an inline orientation; inline typically referred to as control valves. The angle type orientation is preferred for severe service applications. This orientation will allow improved fluid management since the fluid exiting the trim will travel directly into the valve outlet. The valve model is typically associated with the valve trim size as assigned by the valve manufacturer and therefore should not be the governing factor in valve selection. Proper valve selection should be based on valve capacity “Cv” as tested per ISA S75.02 verses flow condition requirements as calculated by ISA S75.01 methodology. Valve models may be smaller than the end connections supplied.
Example: a 4″ valve may be fitted with 6″ end connections.
2.1 Body outlet profile shall be free of steps or abrupt change in diameters. Outlet sleeves, if necessary, must be accommodated with smooth transitions.
2.2 Screw-in seat assemblies shall not be used. Wetted threads should be avoided where possible.
2.3 Welded end connections are acceptable provided material selection and weld design conform to sound Engineering practice. All weld procedures must be ASME IX qualified. All alloy steel welds shall be stress relieved after welding.
2.4 Design may utilize a Hammer Union Nut Bonnet or Bolted Bonnet connection provided the Bonnet connection does not require special tools for valve disassembly.
2.5 Valves supplied with rotary actuators must utilize bolted bonnet designs.
2.6 In erosive applications a right angle body design is the preferred configuration.
2.7 End connections bores shall match mating flange bores/schedule to prevent unnecessary protrusions within the transition.
2.8 In corrosive applications the selected body material should be suitable for operation without the assistance of corrosion inhibitors. Higher velocities may diminish the benefit of inhibitors in valve bodies.
2.9 Valve model/size selected shall consider the effect of velocity on erosion and corrosion control.
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3.0 Trim design
Trim designs may be classified as needle and seat, disk, or cage designs. Important factors to consider in trim selection are controllability defined as the “turn down ratio”; pressure recovery factor defined as the “FL factor”; valve capacity defined as the “Cv.”
3.1 Design shall incorporate cage type trim. Trim design can be cage with external sleeve or internal plug design. Erosive or high pressure applications shall utilize an external throttling sleeve. Internal plug designs are not recommended for erosive or high pressure drop applications.
3.2 Valve trim should be designed with equal percentage opening characteristics for improved controllability. Quick opening characteristics shall not be supplied for modulating service.
3.3 For erosive service the ports should be positioned on the valve axis to reduce turbulence around the cage. For non-erosive service multi-ported cages are acceptable.
3.4 The shut-off mechanism is to be segregated and upstream of throttling ports. The seating face shall be metal to metal.
3.5 Materials used for erosion protection shall be solid tungsten carbide for erosive conditions and hardened stainless steel for non-erosion applications. Stainless steel carriers are acceptable for the flow sleeve; however the entire turbulent area and high velocity zone shall be protected with hardened trim material. The Cage shall be of solid hardened trim material. Hard facing or overlay shall not be used.
3.6 Tungsten Carbide trim materials shall utilize composite binders considering both erosion and corrosion properties. Minimum hardness for the tungsten carbide shall be Ra 93. Binder constitutes shall be Nickel, Chromium and Cobalt at a minimum.
3.7 Multi-Stage trim designs shall utilize a throttling mechanism on each stage. Designs which throttle on the initial or final stage only are not acceptable.
3.8 Trim design shall consider the potential for plugging and the design shall incorporate suitable means for passing normal sand and proppant that may be present within the fluid media
3.9 Designs should consider life cycle costs. Designs which incorporate the trim components into the valve stem should be avoided.
3.10 Trim design shall be capable of Class V shutoff as defined by ANSI/FCI 70-2
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4.0 Valve Actuation
Valve actuation may incorporate a manual operator (hand wheel) or pneumatic, hydraulic or electric actuators. For remote modulating operation the actuator is typically fitted with a positioner that will accept an input signal (either 3-15 psi, 6-30 psi or 4-20mA) to position the valve respective to the input signal. Linear actuators can incorporate fail positions defined as “fail open”, “fail closed” or “fail last”. This action is typically achieved through a mechanical spring, however in the case of linear hydraulic actuators; this can be accomplished through incorporation of an accumulator.
4.1 Typical positional accuracy for linear actuated valves shall be 2% of valve travel for pneumatic applications and 1% for hydraulic applications.
4.2 Typical positional accuracy for rotary actuated valves shall be 1% of valve travel.
4.3 The number of moving parts external to the valve and actuator shall be minimized. Exposed linkages shall be avoided where possible.
4.4 Actuation mounting shall consider service. Adequate provision shall be made for operator safety and environmental protection.
4.5 Where linear actuation is utilized the actuator design should be double acting.
4.6 Where modulating service is specified, actuators are to be fitted with positioners.
4.7 Actuators shall be sized to ensure proper valve operation at shut-off and flowing conditions. In the case of “fail open/close” operation the actuator shall operate the valve to the desired position against worst case conditions.
4.8 Electric and rotary actuators shall provide suitable means to “torque seat” to maintain Class V shut-off capabilities.

 

 

Source :  masterflo