Basic Theory of Chokes

Chokes will be divided into 4 main criterias, 1 types 2 reasons, 3 basic operations, and 4 application. there are few most common chokes used in the world of petroleum engineering, which is positive choke where the choke is have fixed orifice, and cannot change the bean size. and the second most common chokes is adjustable choke. the advantages of adjustable choe is provides variable orifice size through external adjusment

schematic of adjustable choke is a restrictions in a flowline that causes a pressure drop or reduces the rate of flow. it commonly uses partially blocked orifice or flow path. variable chokes is good for bringing wells on gradually and optimizing natural gas lift flow in some cases. prone to washouts from high velocity, particles, and droplets. the solution to fix this problems is hardened the choke using carbide components, chokes in series, or dual choke in well head

most comon positive choke/ bean choke is have id size 64 inch. there is a main function of chokes:
1. control flow to achieve liquid lift
2. maximize use - best use of gas
3. to protect equipment from abrasion and erosion
4. to make a cleanup -  best use of backflow energy
5. control circulation -  holds a back pressure
6. control pressures at surface during flow
7. control injection on injection line



the problems that may be occur in the choke is Pressure drop, some action need to be taken like increased velocity from gas expansion, doing vaporization or flashing of light ends to gas, the we can also do vaporization of water, caitation, cooling of gas, and some proper heating liquids. but some consequencies muste be a consideration whether choosing the action as mentioned above, for flashing action, hydrocarbon are more likely hc lights lost (value lost). for cavitation, erosion of solids, droplets, and bubbles in high velocity flow. for freezing - expansion of gasses cools the area (refrigeraton principle)

to make a pressure control using choke, we must take a careful action because if there is a larger difference pressure between inlet and outlet pressure, will cause the higher potential for damage the internals hardware of the choke. when delta P ratio rises above, is more likely damage the choke. look at the choke type, materials of construction and deployment methods

Cavitation during liquid flow, cavitation is happen because ultra low pressure region in and immediately below choke causes bubble to form vaporizing liquid, recovery of pressur causes bubble to collapse. the rapid collapse of the bubbles causes high velocity movement of liquid and damage around the site. we called this phenomena as Vena contracta phenomenon. the consequences of the of the low pressure region in the choke can lead to severe problems with cavitation and related flashing

Flashing during liquid flow, this problems is vaporization of light ends, but no significant  damage in this region since pressure recovery not above vapour pressure, hence bubble dont collapse. pressure recovery occurs downstream, damage location from high velocity?

Freezing component, expansion of gas and solution containing gas, cools the surroundings. excessive temperature losses and presence of water vapor can form an ice plug and block flow. temp drop accrose a choke is about 1 degree Fahrenheit for each atmosphere of pressure drop

Throttling method in chokes:
1. needle and seat
    (simplest and least expensie adjustable, best for pressure control, and have a high capacity)
2. multiple orifice
    (quick method for open and close, good rate and pressure control, an in line instrument)
3. fixed bean
    (best when infrequent change needed, and used mostly on trees)
4. plug and cage
    (own high capacity and good control)
5. external sleeve
    (superior dor erosion resistance, minimizes body erosion)

Choke sizing must be determine in order for control the flow to maximize the production, the to minimized vibration damage, and erosion damage. this selection process is based on application and sizing
consider the fluid - liquid gas or GOR mix
Pressure -  calculate the pressure drop and total pressure
temperature - range of acceptable temperatures during service
solids in flow
droplets, bubbles
scale and organic or deposit potential