Most Common Choke Valves:

•Positive:
–Fixed orifice.
–Disassemble to change bean.
•Adjustable:
–Provides variable orifice size through external adjustment.

A choke is a restriction in a flow line that causes a pressure drop or reduces the rate of flow. It commonly uses a partially blocked orifice or flow path.

Variable Chokes – good for bringing wells on gradually and optimizing natural gas lift flow in some cases.
Prone to washouts from high velocity, particles, droplets.
Solutions – hardened chokes (carbide components), chokes in series, dual chokes on the well head.

Beans are fixed (non adjustable) orifices – ID size is in 64ths of an inch.

Choke Uses
•Control Flow – achieve liquid lift.
•Maximize use – best use of gas (lift).
•Protect equipment – abrasion and erosion.
•Cleanup – best use of backflow energy.
•Control circulation – holds a back pressure.
•Control pressures at surface (during flow).
•Control injection – on injection line.

Pressure Drop

•Action
–Increased velocity (from gas expansion).
–Vaporization (flashing) of light ends to gas.
–Vaporization of water.
–Cavitation.
–Cooling of gas.
–Some heating of liquids.
•Detriments
–Flashing – hydrocarbon light ends lost (value lost).
–Cavitation – erosion of surfaces in and around choke.
–Erosion– solids, droplets and bubbles in high velocity flow.
–Freezing – expansion of gasses cools the area – refrigeration principle.

Pressure around the choke valve

Problems:
•The larger the difference between the inlet and outlet pressures, the higher the potential for damage to the internals of the choke.
•When DP ratio (= DP/P1) rises above 0.6, damage is likely. Look at choke type, materials of construction, and deployment methods (multiple chokes needed in series )

Cavitation During Liquid Flow:

VENA Contracta Phenomenon:

The consequences of the low pressure region in the choke can lead to severe problems with cavitation and related flashing (vaporization).

Flashing During Liquid Flow:

Vaporization of light ends, but no significant damage in this region since pressure recovery not above vapor pressure, hence bubbles don’t collapse.
Pressure recovery occurs downstream, damage location from high velocity.

Freezing:

Expansion of gas (and solutions containing gas) cools the surroundings. Excessive temp losses and presence of water vapor can form an ice plug and block flow.Expansion of gas (and solutions containing gas) cools the surroundings. Excessive temp losses and presence of water vapor can form an ice plug and block flow.

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Throttling Methods
•Needle and seat.
•Multiple orifice.
•Fixed Bean.
•Plug and Cage.
•External Sleeve.

Needle and Seat
•Simplest and least expensive adjustable.
•Best for pressure control.
•High Capacity.

Multiple Orifice
•Quick open and close.
•Good rate and pressure control.
•An in-line instrument.

Fixed Bean
•Best when infrequent change needed.
•Used mostly on trees.

Plug and Cage
•High capacity.
•Good control.

External Sleeve
•Superior Erosion Resistance.
•Minimizes Body Erosion.

Choke Sizing
•Control the flow – maximize production.
•Minimized vibration damage.
•Minimize erosion damage.
•Choke Selection – based on application and sizing.
•Fluid – liquid, gas, or GOR of mix.
•Pressure – both pressure drop and total pressure.
•Temperature – range of acceptable temperatures during service.
•Solids in flow.
•Droplets, bubbles.
•Scale and organic deposit potential.

Choke Sizing
•Cv = coefficient value
–Number of gallons of water per minute that will pass through a restriction with a pressure drop of 1 psi at 60oF.
–Used as the “flow capacity index”.
–Does not correspond to a specific throttling method.

Erosion:
Erosion – damage caused by impingement of particles, droplets, bubbles and even liquid on any solid surface at high velocity.
To reduce erosion, slow down the velocity.
A choke is required for throttling, never use a gate valve. If wells must be brought on line without a choke, use the outer wing valve if rated for the job. Erosion in a positive of bean choke from micron sized fines and high velocity gas flow.