Oil and Gas Choke Valve
IN THIS ARTICLE OIL AND GAS CHOKE VALVE
TABLE OF CONTENTS
- OBJECTIVES
- INTRODUCTION
- GENERAL APPLICATION
- • Fixed (positive) Chokes
- • Adjustable Chokes
- CHOKE MATERIALS
- CHOKE EROSION
- CHOKE PLUGGING
- OPERATING CHOKE VALVE
- CRITICAL FLOW
OBJECTIVES
After reading this article you will be able to:
Identify and explain the functions and purpose of choke valves in production applications.
INTRODUCTION
All fluids that come out of a well go through a choke valve. Choke valves control the fluid flow.
GENERAL APPLICATION
The purpose of an Oil and Gas 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
Figure Flange Bodied Choke
Fixed (positive) Chokes
Figure Fixed 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
Adjustable Choke
Adjustable chokes are the most widely used chokes. See figures 3‑3, 3‑4, and 3‑5.
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.
Choke Operation
Adjustable Choke and Equipment
CHOKE MATERIALS
Because chokes have to withstand high flow they are normally made of wear-resistant materials such as:
- ceramic,
- tungsten carbide,
- satellite,
- stainless steel.
CHOKE EROSION
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.
CHOKE PLUGGING
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 removed.
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 gas wells.
CRITICAL FLOW
When pressure upstream of a choke is two or more times the downstream 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.
