Chapter 4(c) - Separation Problems

8/8/2019 Chapter 4(c) - Separation Problems

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Production Engineering II

Separation Process


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Potential Operational Problems

The following are the potential operating problems which

can apply to two-phase and three-phase separators

(i) Foamy Crude

(ii) Paraffin(iii) Sand

(iv) Liuid Carryover

(v) !as "olwby

(vi) Liuid Slugs


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i. Foamy Crude

Foam is caused by the impurities in the crude oil which is impractical to

remove before the stream reaches the separator#

Foaming in a separator results in $

%ggravated &echanical control of liuid level because the control

device must deal with essentially three phases instead of two#

'educed space for liuid collection or gravity settling as foam has a

large volume-to-weight ratio (it occupies a large amount of the vessel

space)

ifficulties in removing separated gas or degassed oil from the vessel

without entraining some of the foamy material in either the liuid or gas

outlets#


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Foaming tendencies of an incoming stream can be determined via laboratory

tests#

Foaming cannot be predicted ahead of time without laboratory tests#

"y comparing the foaming tendencies of a nown oil to a new one* the

operational problems which may be e+pected with the new oil can be analy,ed#

Foaming can be e+pected where C. is present* even in small uantities# (one

percent to two percent)#

The amount of foam is dependent on $

(i) Pressure drop to which the inlet liuid is sub/ected#

(ii) Characteristics of the liuid at the separator conditions#


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Changing the temperature at which a foamy oil is separated has two effects on

the foam#

a) Change is viscosity

b) Change in oil-gas euilibrium

0t is difficult to predict the effects of temperature on foaming tendencies* but

some general trends can be identified#

)For heavy oils with a low !'* an increase in temperature will typically

decrease foaming tendencies#

)Similarly* for light oils with a high !'* temperature increases typically

decrease foaming tendencies#

)1owever* for light oils with a low !'* a temperature increase may

increase foaming tendencies# (because its rich in intermediated which

have tendency to evolve to the gas phase as temperature is increased)


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Foam-depressant chemicals can be utili,ed to increase the capacity of a given

separator#

0n si,ing a separator to handle a specific crude* the use of an effective

depressant may not be of the same type as characteristics of the crude and of

the foam may change during the life of the field#

The cost of foam depressants for high-rate production may not be costeconomical#

uring the design phase* sufficient capacity should be provided in the separator

to handle the anticipated production without use of a foam depressant or

inhibitor#

nce the foam depressants are used in the operation* it may allow more

throughput than the design capacity#


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ii. Paraffin

The accumulation of paraffin in the separator can adversely affects its

operation#

Coalescing plates in the liuid section and mesh-pad mist e+tractors in the

gas section are particularly prone to plugging by accumulations of paraffin#

2ane-type or centrifugal mist e+tractors should be used in events where it is

determined that paraffin is an actual or potential problem#

&anways* handholes and no,,les should be provided to allow steam*

solvent or other types of cleaning of the separator internals#


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iii. Sand

Sand causes cutout of valve trim* plugging of separator internals and

accumulation in the bottom of the separator#

%ccumulations of sand can be minimi,ed by periodically in/ecting

water3steam in the bottom of the vessel to suspend the sand during

draining#

Plugging of the separator internals is a problem that must be considered

during the design stages of the separator#

% design that will promote good separation and have minimum traps for

sand accumulation may be difficult to attain#

This is because the design that provides the best mechanism for separating

the gas* oil* and water phases probably will also provide areas for sand

accumulation#


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iv. Liquid Carryover

ccurs when free liuid escapes the gas phase which results in $

0ndication of high liuid level amage to vessel internals Foam

Plugged liuid outlets Flowrates which e+ceeds the vessel4s design rate

Can usually be prevented by installing a level safety high (LS1) sensor that

shuts in the inlet flow to the separator when liuid level e+ceeds the normal

ma+imum liuid level by 56-578 (usually)#


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Potential Operational Problemsv. Gas Blowby

)!as "lowby occurs when free gas escapes with the liuid phase which can be anindication of $ Low liuid level 2orte+ing Level control failure

)0f there is a level control failure and the level dump valve is open* the gas will e+it

the liuid line and will have to be handled by the ne+t euipment in the process#

)9nless the ne+t euipment is designed for gas blowby conditions* it can be over

pressured#

)Can be prevented by installing a level safety sensor (LSL) tat shuts the inflow when

the liuid level drops 56-578 below the lowest operating level#

)ownstream euipment should be euipped with PS1 sensor3 PS2s si,ed for gas

blowby


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vi. Liquid Slugs

)Two phase flow lines tend to accumulate liuids in low spots in the lines#

):hen the level of liuid in these low spots rises high enough to bloc the gas

flow then the gas will push the liuid along the line as a slug#

)epending on the flow rates* flow properties* length and diameter of the flow

line* and the elevation change involved* these liuid slugs may contain large

liuid volumes#

)Situations in which liuid slugs may occur should be identified prior to the

design of a separator#

)The normal operating level and the high-level shutdown on the vessel must be

spaced far enough apart to accommodate the anticipated slug volume#


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0f sufficient vessel volume is not provided* then the liuid slugs will trip the high-

level shutdown#

The location of the high-level set point may be established to provide the slug

volume between the normal level and the high level#

The separator si,e must then be checed to ensure that sufficient gas capacity

is provided even when the liuid is at the high-level set point#

This chec of gas capacity is particularly important for hori,ontal separators

because* as the liuid level rises* the gas capacity is decreased#

For vertical separators* si,ing is easier as sufficient height for the slug volume

may be added to the vessel seam-to-seam length#


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Q & A

Sessio

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Chapter 4(c) - Separation Problems

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