From Gas Lift Manual

2.5.5 Flow through restrictions

Similar to single-phase cases (see Section 2.4.5), flow restrictions are used to control well production rates at the wellhead where multiphase flow usually takes place. Additional functions of wellhead or production chokes include:

  • maintaining a stable wellhead pressure by preventing downstream pressure fluctuations to act on the well

  • protecting surface equipment from severe slugging

  • limiting pressure drawdown on the well's sandface

Wellhead chokes are divided in two broad categories: fixed or positive and variable or adjustable chokes. Positive chokes have a replaceable, bean-type orifice as shown in Figure 2-57. The housing commonly used in the field has an elbow upstream of the choke bean that facilitates removing and replacing the bean. Flow beans are usually 6-in. long and have an inner bore in fractional increments of 1/64 thof an inch with a maximum size of in. Where sand or corrosive fluids are produced, stainless steel, tungsten carbide or ceramic beans are used to extend operational life.

Figure 2-57: Typical arrangement of a wellhead choke.

Just like in single-phase flow through a restriction, multiphase flow also can occur in a critical or a sub-critical state. In critical flow, velocity in the bean's throat attains the sonic velocity valid for the multiphase mixture at the prevailing conditions. Flow rate through the choke reaches its maximum value and downstream pressure fluctuations cannot propagate upstream. Under these conditions, changes in downstream pressure do not involve changes in the choke's throughput rate.

In sub-critical flow conditions, i.e. when...

Products & Services
Wellhead Equipment
Wellhead equipment is used to connect the tubing and casing to an oil or gas pipeline.
Gas Compressors and Gas Compressor Systems
Gas compressors and gas compressor systems circulate gas through a process, enhance conditions for chemical reactions, provide inert gas for safety or control systems, recover and recompress process gas, and maintain correct pressure levels by either adding and removing gas or vapors from a process system.
Artificial Lift Equipment
Artificial lifting equipment refers to the systems which increase the flow rate of a production well. This is beneficial when reservoir pressure are low or nonexistant to increase the flow rate of water or crude oil.
Inductors, Coils, and Chokes
Inductors coils and chokes are passive components that are designed to resist changes in current and store energy in the form of a magnetic field. 
Butterfly Valves
Butterfly valves control flow through a circular disc or vane by turning the valve's pivot axis at right angles to the direction of flow in the pipe. They are normally used as throttling valves to control flow.

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