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...