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Technical Recommendation
for Production Chokes |
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| 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) |
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| 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. |
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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. |
| 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 coefficient"; and trim characteristics defined as "linear, equal percentage or quick opening". |
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| 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 the 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 |
| 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. |
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| 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. |
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| 5.0 Choke Valve Specification | |