The purpose of a choke valve (or choke) is to control the release of fluids from a well.
· It controls the rate of flow of produced well fluids.
· It keeps a back pressure on the producing reservoir formation.
There are two basic types of choke.
· Fixed (positive) choke
· Adjustable choke
Fixed (positive) Choke
Most fixed chokes have a choke nipple or choke box into which devices called 'choke beans'
are screwed. The choke beans have an accurate hole (orifice) through the centre.
The choke bean must be changed if we wish to change the flow. The production flow must be stopped when the bean is being changed.
Adjustable chokes are the most widely used chokes.
They work by a pointed stem and seat arrangement.
Other types have a two plate set‑up with one plate rotating on a sleeve with bored holes that moves when opening or closing the choke.
A disadvantage of the adjustable choke is that the seat wears away (erodes). This is due to the higher velocity of flow that takes place through an adjustable choke.
|Adjustable Choke and Equipment|
Because chokes have to withstand high flow they are normally made of wear resistant
materials such as:
· tungsten carbide,
· stainless steel.
Even with wear resistant materials, chokes can be eroded. This is called 'wash‑out'.
Wash‑out can be identified as it happens. The pressure upstream of the choke decreases,
and the flow rate downstream of the choke increases.
It is possible for a choke to become plugged. Solids smaller than the hole through the choke
can plug it. This is called 'bridging'.
On an adjustable choke the plugging can normally be removed by opening the choke and
then resetting it. This breaks the bridge.
It is necessary to isolate a fixed choke and to bleed off pressure before the plugging can be
OPERATING CHOKE VALVES
Flow should be opened slowly and progressively through choke valves to prevent pressure
shocks. This is very important on high pressure, high GOR (Gas to Oil Ratio) oil wells or on
When pressure upstream of a choke is two or more times the down stream pressure, critical
flow exists through the choke.
Any increase in upstream pressure will have no effect on downstream pressure or flow. Any decrease in downstream pressure will have no effect on upstream pressure, or flow.