battery production - pressurized vessels

7/25/2019 Battery Production - Pressurized Vessels

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10B-1

The Lease Pumpers Handbook

Chapter 10

The Tank Battery

Section B

PRESSURIZED VESSELS

This section focuses on the pressurized

vessels and related equipment commonly

used for tank batteries:

The flow line. From the well to the tank

battery.

The header. A manifold of all flow lines

to the first pressurized vessel.

The separator. Typically, the first

pressurized vessel in the system.

The heater/treater. Typically, the

second pressurized vessel in the system.

The information presented in this section

is general in nature due to the many

variations in equipment configurations andspecific uses by different companies.

B-1. The Flow Line.

A flow line is laid from the well to the

tank battery. Where there is a long distance

to the tank battery and the production is low,

the lines may be joined at some convenient

spot so that one line is laid from that point to

the tank battery. This does not present a

problem except when one of the wells needsto be tested. To test wells individually, a

well tester, mounted on a small double-axle

trailer, can be brought to each location. The

second option is to shut one well in while

the other is being tested. Limiting one flow

line to several wells is never an ideal

situation but may be the cheapest alternative.

The material selected for the flow lines

depends on many factors. Options include:

Steel is usually preferred for high-

pressure and flowing wells. It can be

standard line pipe, heavy-duty line pipe,

or new or used upset tubing. It can also

be welded, use pipeline couplings, or

have groove clamps. Steel can be plastic-

coated to combat corrosion and scale

accumulation.

Polyethylene may be selected for

medium- to low-pressure lines and can be

practical also when paraffin or scale is

present. It is especially satisfactory when

steel lines deteriorate rapidly. Fiberglass can also be considered when

extremely corrosive conditions are

encountered.

Figure 1. Polyethylene line to be laid as a

flow line.


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Laying new flow lines. When laying new

flow lines, the weather must be considered.

If a line is laid straight from one point to

another in hot weather, when it gets cool it

will shrink several feet in length. When itgets extremely cold it will shrink many feet

in length. This will cause it to pull so hard

that it will part, or in extremely hot

situations it may buckle. Placing correct

slow bends in flow lines can accommodate

this problem, with plastic pipe add a few

slow curves to provide extra line near the

destination points and road crossings.

Road crossings. When crossing a lease

with a flow line, it is always best to lay a

joint of casing across the road and run the

line through it (Figure 2). If the ends of the

casing are to be sealed or buried, a vent is

welded to the casing to remove gasses in

event of a line leak. A riser is welded into

the conduit line at each end before it is

buried to allow this to occur. All steel lines

that go through a conduit should be coated

and wrapped.

Figure 2. A road crossing for flow line.

B-2. The Header.

As the flow lines approach the tank

battery, they are lined up about 18 inches

apart and enter the tank battery parallel to

each other. As they come up through a riser,

a check valve is installed. In the event a

hole should develop in a flow line and this

check valve does not seal, all of the

production entering the header can flowback through this line and result in a large

oil spill. If the check valve on the casing of

the well should become locked open, not

only will the well begin to circulate by

losing the produced oil back down the well,

but produced oil from the header can flow

back down the flow line and also be lost

down the well.

A tee will be installed in multiple well

batteries, so that the flow may be diverted

into the production or the test separator. The

line to the heater/treater will have a

connection in it to allow the injection of

treating chemical. When injecting the

chemical at the tank battery instead of at the

well, it is always injected into the header and

usually after the header but before the first

vessel. This is standard procedure.

Quarter-round opening valveseither plug

or ballare more appropriate for the tank

battery than multiple round opening valves

because the lease pumper can determine if

they are open or closed at a glance.

The header should also have a line

installed to permit the flow to the separator

to be diverted through a bypass around the

vessel. Most vessels must be bypassed while

repairs or changes are made.

B-3. Pressurized Vessels.

Tank battery typical operating pressures.

Separators. Separators will have a

maximum operating pressure of about

100 pounds with a test pressure of 150

pounds. Normal operating pressure is

from 15-50 pounds, 25-35 pounds is

about average. The pressure must be


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great enough to push the liquid from the

separator into the heater/treater with a

small safety margin.

Heater/treaters. These vessels are larger

around than separators, so it takes athicker shell to hold the same pressure.

Thicker shells raise the cost of the vessel

to a much higher level. Fifty pounds

operating pressure is about average with a

test pressure of less than 100 pounds for a

heater/treater. A higher pressure

separator is usually located ahead of it to

lower the operating pressures to save

money during construction. Vertical

heater/treaters are taller than the stock

tank and the oil outlet is about the same

height. One pound of pressure will lift

oil about three feet, so 10-15 pounds of

pressure will usually be satisfactory at the

heater/treater.

There are several openings and lines that

all pressurized vessels have in common

(Figure 3). There are also a few openings

and lines that are normally limited in use

and are installed on specific purpose vessels.A good understanding of the purpose and

locations of specific special purpose lines

for pressurized vessels is important to the

lease pumper.

The emulsion inlet. The emulsion inlet is

located on the side of the vessel near the top

and above the fluid level in the vessel. Some

pressurized vessels, such as the separator,

will have a diverter plate on the inside to

give the fluids a swirling motion upon entry.This allows the gas to break out of the liquid

phase and reduce liquid carryover into the

gas sales. Emulsion inlet lines are usually

above the operating liquid level to prevent

loss of liquids from the vessel in the event of

line leaks before the line gets to the vessel

and siphoning effects.

The gas outlet. The gas outlet is always

located at the center of the top. A mist

extractor will be installed inside the vessel

to further limit liquid carryover.

The drain outlet. The drain outlet is

located in the center of the bottom.

The high oil outlet (optional). If the oil

outlet is located high on the upper side of the

pressurized vessel as it is in the

heater/treater, this will be the oil outlet and

also the height of the fluid in the vessel.

Figure 3. Typical line Openings in a

pressurized vessel.

The lower liquid outlet or water leg. If a

liquid outlet is located near the bottom on

the side of the vessel and is utilized as a

water leg, this will indicate that the vessel is

a three-stage separator. The fluid will beseparated into the base contents, gas, oil, and

water. If a fire tube is also added for heat,

then it will become a heater/treater.

Heater/treaters are also three stage

separators.

During the summer months, most

companies try to use the heater/treater as a


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three stage separator, operating it without

heat with only chemical treating in order to

save and sell the gas. This procedure works

well for lighter (higher API gravity crude)

oils. If the crude oil is heavy and has a highparaffin and water content, the oil may not

treat without heat.

Floats.Floats in vessels may be discriminate

or indiscriminate, according to need and

design. The purpose of a float is to control

the level of liquid inside a vessel by means

of an outside arm or pneumatic mechanism.

Indiscriminate floats. Indiscriminate

floats are made to float on both oil or

water indiscriminately. The size of the

float depends upon the power that it

takes to operate the control arm, the

turbulence of the liquid, the volume

moving through the vessel, the length of

the arm, the weight of the ball, and

several other factors. The ball float is

the most common indiscriminate float.

Discriminate floats. Discriminate floats

in the oilfield are floats that have adensity or weight that will allow them to

sink in oil but float on water. The float

will operate on this interface. Since the

weight per gallon of oil will vary

according to the viscosity of the oil, and

the weight of water will vary according

to how much salt is contained in the

water, discriminate floats are available

with several densities. Oil will weigh

over 7 pounds per gallon and water will

weigh 8.3-9.6 pounds per gallon. Freshwater weighs 8.3 pounds per gallon and

at about 9.6 pounds it begins to reach

maximum natural salt saturation.

Ball floats are usually weighted with sand

to allow them to sink through the oil phase

but float in the water phase.

B-4. The Separator.

There are several styles of separators,

which are classified by shape and by

separation method. The basic shapes ofseparators are:

Vertical separators

Spherical separators

Horizontal separators

The basic separation methods are:

Two-stage

Three-stage Metering

Operation of the two-stage vertical

separator. Two-stage vertical separators

have historically been manufactured in three

styles: right-hand, left-hand, and the

combination. The combination vessel has

two identical emulsion inlet openings,

located opposite of each other about two-

thirds of the way up the sides of the vessel.

All of the other openings are standard andthis is the only difference between them.

One of the inlet openings will be selected as

the most appropriate to face the inlet

manifold, and the other will be plugged,

usually with a four inch bull plug. The right

and the left hand separators will only have

one inlet opening, and these are opposite

from the other. The design of a two-stage

vertical separator is shown as a cutaway

drawing in Figure 4 and as a photograph in

Figure 5 on the next page.The gas will rise to the top, go through a

mist extractor which is built into the top of

the separator, and enter the gas line. This

gas pressure is not really high pressure in a

literal sense, because it is usually no more

than 20-50 pounds.


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Figure 4. Cutaway drawing of a vertical

separator showing the operation of the

vessel.

Since the atmospheric vessels will have no

more than a few ounces of backpressure

usually 2-4 ouncesthe pressure is high in

relation to the atmospheric vessels, and this

pressure is never directed toward an

atmospheric pressured vessel.

The dumping of oil and water is regulated

by a float-controlled dump valve. In newer

designs, this valve draws the liquid from

much closer to the bottom than previous

styles, reducing corrosion problems. This

was achieved by adding a short line inside

the separator that ends near the bottom,

reducing the amount of corrosive water that

is present below the dump valve outlet.

A pressure gauge is added above the

dump valve and a sight gauge glass is

mounted to the left of the dump valve. The

Figure 5. A typical two-stage

vertical separator with a high

liquid level alarm.


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pressure gauge line turns upward inside a

second short section of pipe that has a cap

on top. If the liquid level rises all the way to

the gas outlet, none will enter the gauge. A

hail guard is added to protect the sight gaugeglass from being accidentally broken by

people, animals, or hail. An O-ring is placed

on a new sight glass at the fluid level. A

glance at this periodically gives an instant

reference to several operating problems. A

string may be tied on the glass to replace the

O-ring when it becomes weather cracked

and drops off.

The emulsion inlet has a device that

forces the oil to swirl as it enters. This

circular action creates turbulence in the

liquid phase and accelerates gas breakout

from the liquid. This lowers the amount of

mist lost into the gas line.

As the fluid is dumped out of the

separator, it will be directed toward the

heater/treater, gun barrel, or a stock tank.

The dump valve should be gently tested by

hand periodically to be sure that it is still

working freely and is not becoming frozen

in one position. Care must be exercised,

because a harsh push may break the float off,

necessitating repairs.

The two valves on the sight glass have a

built-in reamer on the valve seat to keep the

hole into the tank open and to remove

paraffin or scale that might accumulate.

These should be carefully closed and opened

periodically to ream them clean and to keep

the fluid level in the gauge accurate. The

lease pumper can close the bottom gauge

valve and leave the top one open, then openthe drain cock that is screwed into the

bottom opening of the bottom valve to flush

out the gauge glass. Experience at each

separator will help determine how often this

may be required. It may be necessary to

occasionally adjust a valve stem packing if

leakage or oil staining should occur.

Some separators have pneumatic instead

of mechanical controls (Figure 6). Note the

gas line coming off the top of the vessel to

supply gas pressure to operate the diaphragm

operated dump valve. Since the diaphragmis a medium- to low-pressure automation

device, a pressure regulator is installed in

the line to reduce the pressure and protect

the valve.

Figure 6. A separator with pneumatic

controls.

Pressure safety devices. Two types of

safety devices are placed on top ofpressurized vessels: the pop-off or relief

valve and the rupture disc or safety head (see

Figure 7 on the next page).

The pop-off valve is set near the limit or

maximum pressure that might cause the

vessel to become dangerous. The pop-off

valve has stainless steel springs, ball and


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seat, and other parts to prevent rust from

changing the setting or making them fail to

operate correctly. The pop-off valve can

release the excessive pressure and has the

ability to automatically shut off when thepressure returns to a safe level. It is an

automatic valve requiring little or no

maintenance.

The safety rupture disc is a thin, dome-

shaped disc made of stainless steel,

aluminum, or, in the case of some older

separators, brass. The rupture pressure of

the disc is several pounds higher than the

safety release valve. When the safety release

valve fails to open and the pressure

continues to rise above a safe limit, the

rupture disc will burst, releasing the pressure

on the vessel. When the rupture disc bursts,

the pressure will continue to be released into

the atmosphere until the lease pumper comes

by and shuts in the well or switches from the

vessel. For this reason a line from the

rupture disc may be run to the pit to prevent

ground contamination. But if a line is

plumbed from the rupture disc to a pit, it

must be run so that there are no low places

where water can collect and freeze during

cold weather.

Figure7. Examples of safety release

valves and pressure rupture discs.

B-5. The Heater/Treater.

The heater/treater (Figure 8) is a three-

stage, pressurized vessel with heating

capabilities, although they are alsomanufactured as atmospheric vessels. To be

atmospheric, the stock tanks must be low

enough to allow the oil and water to gravity

flow to the stock tank and the water disposal

system.

Figure 8. A typical vertical

heater/treater. The lines can be seen

entering the vessel. The site gauges and

firebox are on the opposite side.


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Three stage means the fluid is removed

through three lines. In the heater/treater

pictured nearby, the oil, water, gas emulsion

coming into the vessel enters through the

highest line on the right side of the vessel.This should not be confused with the gas

line that comes off the center of the top.

Thenatural gasflows out through the gas

line coming off the center of the top of the

vessel through the line to the right. A

backpressure valve is installed nearby to

control the pressure inside the vessel.

Theproduced oil flows out the lower line

on the right side of the vessel. about eight

feet from the top. This is the fluid level

inside the vessel. Note that a liquid valve

with a back pressure weight is installed

approximately four feet from the ground.

This valve controls the amount of oil

standing in the down comer line and does

not control the height of the liquid in the

tank.

The water comes out of a line on the

lower left side of the vessel at 90 degrees

from the other lines, and the water goes up

and spills over the water leg a small distancelower than the oil outlet. The height of the

water in the water leg is controlled by a

valve identical to the oil outlet control valve.

The heater/treater is usually the only

pressurized vessel in the tank battery system

where the heights of the fluids in the vessel

are controlled by line height. The fluid

levels in the gun barrel are also controlled by

line height. The water level in the bottom of

the heater/treater is maintained at

approximately one foot above the fire tube.Oil does, however, come into contact with

the fire tube.

Most operators try to heat the produced

crude oil only in the winter. The use of

appropriate chemicals and the heat from the

sun is usually satisfactory for treating oil in

the summer months. The firebox can

consume a lot of gas. Selling the gas

provides income toward the purchase of the

chemicals. It is a give-and-take situation in

determining when heat is necessary.

Controlling the height of the water.

Earlier, the use of the beam balance scale in

weighing water and oil was reviewed.

Calculations for computing the height of the

water leg is reviewed in Appendix F,

Mathematics.

The height of the water in the

heater/treater should be approximately one

foot above the top of the fire tube.

The amount of water in the heater/treater

is controlled by raising or lowering the

height of the side boot on the water leg. If

the weir nipple is raised in the water leg

boot, the amount of water retained inside the

heater/treater will be higher. If the weir

nipple is lowered, the water retained will be

lower. The total fluid column height will

remain constant, controlled by the height of

the oil outlet opening.

The fluid sight glasses. The lower fluid

level sight glass should display water in the

lower half and oil in the top half. If the

lower valve is occasionally closed, it is a

good practice to close and re-open the top

valve, and then bleed the lower petcock into

a container. This will effectively clean the

glass to allow the liquid level to correct

itself. Tie a small rag at this level to indicate

future level changes.

The top sight glass will have oil in the

bottom and gas in the top. It can be cleanedin the same manner.

Backpressure valves for pressurized

vessels. The Kimray diaphragm-operated

backpressure valves are widely accepted by

the petroleum industry as a dependable

automatic operating valve (Figure 9). It is a


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very popular selection for controlling the

back pressure for the separator, the

heater/treater, the emergency vent line, and

the gas purchasing company, and on several

other vessels where a back pressure valve isinstalled. The term backpressure refers to

the pressure inbackof the valve. The valve

that controls pressure after the backpressure

valve or downstream is usually referred to as

aregulator.

Figure 9. A diaphragm-operated

backpressure valve.

(courtesy Kimray, Inc.)

The treater oil and water valves (Figure

10) are used to control the dumping of oil

toward the stock tanks and the water toward

the water disposal and injection system.These are basically liquid dump valves. The

installation of the valve is shown in Figure

11. The drawing shows one method of

securing the necessary gas pressure to the oil

or water valve that prevents fluctuations in

gas pressure from affecting the operation of

the valve.

Figure 10. Treater oil and water valves.


Figure 11. How the treater liquid valve is

installed.


As the oil spills out of the heater/treater

into the oil line, or water spills over the

equalizer riser on the water outlet line, these


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10B-10

liquids create a downward pressure against

the valve. When the column of liquid builds

up to about four or five feet in the line, the

valve opens, allowing part of it to flow

toward the next vessel. The height of theline, not the valve, controls the liquid level

in the vessel. As the pressure gets lower, the

valve closes again.

A gas line comes off the top of the vessel

and connects to this valve. The purpose of

this line is to equalize the gas pressure inside

the vessel across the liquid dump valve. If a

flowing well opens up or someone opens a

valve and a surge of additional pressure

comes into the vessel through the inlet line,

this change in pressure will also occur on

both sides of the diaphragm. This change in

pressure will have no effect upon the

operation of the valve.

The height of the water leg is determined

by the weight per square inch of the column

of salt water and the weight of the oil on top

of the water. The height of this column of

liquid is from the oil overflow outlet to the

bottom of the vessel.

In Appendix F, Mathematics, the

procedure for calculating the height of water

legs is reviewed.

The float-controlled separator dump

valve pictured in Figure 12 is a popular

valve. The pressure below the seat of the

valve is transferred to the top mechanism of

the valve. This removes any stress or

unusual pressures from being exerted against

the diaphragms in the valve and results in

smooth and dependable valve action.

If the dump valve arm needs to operate inthe opposite direction, remove the bolts in

the top, rotate the top one-half turn, and

replace the bolts. Now the mechanism will

operate in the opposite direction. The

direction of the arm can also be turned in the

opposite direction according to need.

Figure 12. A float-controlled separator

dump valve.


B-6. Interior Design of the VerticalHeater/Treater.

In the past, two types of heater/treaters

have been constructed. During the past few

years, several innovative changes have made

the basic design more efficient, so only one

type is illustrated as Figure 13 on the next

page. This style is designed for high water

production.

Inlet line. The inlet line enters above thefluid level. The gas, being lighter, flash

separates and goes up through the mist

extractor and into the gas line. The liquid

falls to the bottom of the vessel through a

tube.


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Figure 13. Interior view of a

heater/treater.

(courtesy of Sivalls, Inc.)

At the bottom. At the bottom, the free

water that has flash separated continues on

down and out of the vessel through the water

leg, absorbing very little of the vessel heat.

The oil migrates up through the watersection, past the fire tube, and is heated as it

continues to migrate up to the oil section of

the heater/treater.

The oil trip upward. As the oil travels

upward, it moves from one side to the other

passing upward through openings, while

any suspended water contacts the plates,

separates, and moves back downward

toward the water leg and on to the disposal

system. When the oil reaches the top, it falls

into the oil outlet and on to the gun barrel or

the stock tank. Any new gas that breaks out

passes upward through a provided tube up to

the mist extractor and into the gas system.

This tube also equalizes pressure between

the upper and the lower section of the vessel.

The water leg. The operation of the water

leg is also visible, and the weight of the

water in the water leg is exactly the weightof the water and oil column inside this

vessel. This three-stage separation is fully

controlled by line heights instead of a

discriminate float as it is in the free water

knockout. This vessel is one of the most

useful vessels in oilfields.


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battery production - pressurized vessels

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