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Waterlegs – What you need to know!
Striving for Better Facilities Understanding in 2015
INTRODUCTION
Somewhere in the oilfield an oilfield tank is overflowing … right now! The
associated costs can be staggering. The loss is almost criminal! Adding insult to
injury, the environmental impact can be terrible too. Sometimes, tank nozzles
and waterlegs are at fault. Understanding both may help us understand why
tanks, particularly Gunbarrels, overflow … and how to prevent these costly
events!
As an industry we have been buying and using tanks and Gunbarrels since the
1800s! The API was founded on March 20th, 1919 and soon took control of
standardizing oilfield tank design. Not all tank manufacturers are sanctioned by
the API, but most follow API’s published Recommended Practices (RPs). Today,
API RP 12F governs the design and manufacture of steel tanks, while API RP
12P governs the same for fiberglass tanks. These RPs are for typical oilfield
storage tanks. All “standard” nozzles and locations are specified in these RPs
with the caveat that the end user may change sizes and locations if they so desire.
Few ever do!
Furthermore, there are no API RPs for Gunbarrels, Skim Tanks, or Water Legs.
Therefore, fab shops are left to their own devices in these areas, and normally
they don’t fabricate Gunbarrel water legs at all. They leave it up to the end users
to do this themselves. So the questions become:
1. How do you do size a tank nozzle, and
2. How do you size a water leg?”
We need the answer to these questions if we’re going to prevent tank overflows,
but we rarely do! Instead, we assume the manufacturer, or fab shop, or
installation crew somehow “knows”. These details easily slip through the
cracks, and the result is tank overflows. If we take charge we can do it right. If
we don’t, it’s a crap shoot!
So how do we take charge? These and more answers are found in this paper.
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WHAT DO NOZZLE SIZES AND WATERLEGS HAVE TO DO WITH TANK
OVERFLOWS?
Nozzle sizes and waterlegs control the flow of fluid out of all tanks! Water legs are real-world
monometers (liquid balancing tubes/pipes) designed to control the oil-water interface inside
Gunbarrels (oil-water separation tanks) … or they are supposed to. Maintaining a constant
water-oil contact point, or interface, is critical to efficient separation. If an oil-water or
emulsion-water interface is allowed to rise and fall, the separating oil droplets accumulating at
the interface can be re-entrained back into the water phase rather than rising into the oil phase
where they may be piped off to oil storage and sales tanks. As interfaces move up and down
with varying flow rates, the smaller oil droplets often re-entrain in the water and carry over into
the next tank or tanks. They tend to then migrate with the water, and may be lost to water
disposal or a water injection system. This oil translates to lost revenue and often to plugged
disposal or water injection wells.
In 2105 most oil producers are agreeing that they lose about 1% of their produced oil to
carryover; oil entrained in their waste water stream. At today’s crude oil production rate and
oil prices that 1% loss equates to 79,000 barrels of oil per day; oil worth $3.95 million dollars
each and every day! At least 25% of this, or about one million dollars per day, can be attributed
to improperly sized tank nozzles, and improperly constructed or improperly adjusted waterlegs!
Let’s focus on nozzle size first.
TANK NOZZLES – PROPER AND IMPROPER SIZING
API 12F and 12P specify the typical nozzles in all sizes of commonly used oilfield tanks. While
these are only recommendations, and completely changeable, few ever actually get altered or
changed regardless of actual field conditions.
In larger API 12F and 12P tanks most fittings (nozzles) are 4”. In smaller tanks, most are 3”.
This simply means that left unchanged, these nozzles must accommodate the actual field flows
from the well streams feeding them. It also means that when the instantaneous flows exceed the
capacity of the nozzle the nozzle reaches a “flood state” and the tank level rises and the tank can
overflow.
The consequences are well known to all of us!
So, how do we size tank nozzles to prevent this? For decades most engineers used the “Crane
Handbook”. This is a Crane Company publication titled “Technical Paper 410”, and it gives us
the long-hand formulas needed to size nozzles. This “bible of the industry” has been replicated
by others (see Engineer’s Toolbox and others on line) and has been augmented by PC based
software that takes most of the time and effort out of this exercise.
However, since we may not have access to these tools all the time, here are a few real-world
rules of maximum instantaneous flow in barrels per day for common nozzle sizes:
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TABLE 1
SIZE
FLOW (B/D) AT
CENTERLINE
FLOODED
FLOW (B/D)
FLOW (B/D) 6”
ABOVE FLOOD
FLOW (B/D) 12’
ABOVE FLOOD
FLOW (B/D) 24’
ABOVE FLOOD
3”
49
65
127
189
314
4”
85
118
215
280
507
6”
248
366
485
841
1316
8”
701
1088
1668
2247
3704
The key to using the Table 1 is to remember that the flows published are in barrels per day
instantaneous! If, for instance, a facility processes 1,000 barrels per day, but the fluid is moved
from tank to tank using a pump, it is the pumping rate, NOT the 1,000 b/d that must be
considered. If the pump moves, for instance, 250 GPM, it is moving the equivalent of 8,571 b/d.
While we can see from the table on Page 2 that a 6” nozzle would handle 1,000 b/d if flooded
just less than 24” above the top of the nozzle, at a pumping rate of 250 GPM (8,571 b/d) the
pumping rate would force the tank level to rise and the tank would predictably overflow!
In the real world most oil tanks have 4” overflow connection, and if built according to API 12F
or 12P, those connections are only 6” from the top of the tank. Given this, we can expect most
full oil tanks to overflow onto the ground when we flow into them at a rate exceeding 215 b/d or
6.27 GPM. Since we may occasionally exceed this flow rate it is not surprising that we
occasionally overflow tanks with 4” overflow lines.
Now, let’s shift the focus to water leg design and sizing.
WATER LEGS – DESIGN AND SIZING
Most oilfield tanks store either water or oil, but one tank, the Gunbarrel, and another tank, the
Skim Tank, store both. In order to maintain the contact point between the oil and water, known
as the “interface”, we need a level controlling device. This is most often a “water leg”.
A water leg is a manometer, or liquid balancing tube, like
the one at the left. In our case, the tank is the right side
and the water leg is the left. When the tank has only
water, the tank level and the water leg level are equal like
the example far left. As the tank establishes an oil layer
the overall liquid height in the tank rises. The higher tank
level of a mixture now of lighter oil and the water pushes
the water up inside the water leg, the side on the right of
the graphic.
In the design of water legs we decide the depth of oil. Its maximum vertical oil height in any
tank is depth is set by the elevation of oil outlet/spillover nozzle … again usually a 4”. Once this
decision is made, the remainder of the tank below the oil will obviously be water. In order to
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maintain this oil layer we must establish the height of the water on the right. In other words, we
need to calculate the spillover height of the water leg. This is done with the following formula:
WATER LEG SPILLOVER ELEVATION = (HEIGHT OF OIL) (SP.GR. OIL) + (HEIGHT OF WATER) (SP.GR. WATER)
SPECIFIC GRAVITY OF WATER
Since we deal in API gravity in the oil patch, and not specific gravity, you’ll need to Google the
conversion table needed to convert API gravity to specific gravity. Once you have solved the
equation, you have the ideal spillover elevation of the water leg.
There are several designs for water legs, and each is unique. Most are simple pipe risers with a
tee located at the ideal spillover elevation. The tee, however, is a horizontal nozzle, so the flows
through it are limited to the flows in table above.
As you might imagine, most water legs built with spillover tees are too small. The result is that
the tee floods, and when it does the level in the right side of the manometer rises, taking the level
in the left side with it! Since the left side is the tank, the tank overflows!!
To keep this from happening, the use of a larger spillover area is suggested. Increasing pipe
size is one solution, but the least effective since all flow through horizontally oriented pipe is
quite limited, as we see in the table above! The more effective approach is to enlarge the
overflow area. The restriction water flows through is called a “weir”. This weir area is the
limiting factor, so increasing it increases flow. In a typical pipe water leg, for instance,
increasing from 6” to 8” increases the weir area from 6” to 8”, or about 33%. This results in
33% more flow.
This is okay, but there is a better way!
The better way is the use of an “engineered” water leg. This water leg is a pair of concentric
pipes, one inside the other. In this case the water flows up the inside pipe and spills over into the
annular area between the two pipes. The size of the weir in this configuration is NOT the size of
the pipe, but the circumference of the pipe. An 8” horizontal pipe has a maximum cross
sectional weir of only 8”, but when this pipe is configured vertically, the water now spills over
the circumference of the pipe. The circumference of an 8” pipe is just over 50”… 6.3 times as
large as the pipe’s cross section of 8”! Therefore, the flow is increased by the same ratio, and
the levels in tank are far less mobile, and far less likely to overflow the tank!
Finally , since the specific /API gravity of oilfield fluids varies, making water legs adjustable,
like the one pictured at the left, adds real value and flexibility. For
instance, a 6’ oil layer in a Gunbarrel or Skim Tank might be necessary
in winter to assure complete dehydration when the viscosity of the oil is
higher and water separation rates are lower, but 4’ layer might be
adequate in summer. Since two feet of oil in a standard 15’6” diameter
Gunbarrel or Skim Tank is equal to 62.5 barrels, moving two feet of oil
to sales adds $3,125 to cash flow (with oil @ $50.bbl). Adjustable
water legs make this possible, and are becoming the standard of the
industry for this reason.
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In conclusion, properly sizes tank nozzles and properly designed (engineered) water legs can
avoid most oil spills.
ABOUT THE AUTHOR AND HTC
Bill Ball is the founder and owner of HTC, Inc., founded in 1993. Bill has a long
history of oilfield separation system design experience, which when coupled with
his hands-on oilfield experience and career portfolio, make him one of the
industry’s leading separation authorities today. After his university studies his
career started in a 1,000,000 b/d waterflood operation where he was responsible
to evaluate and improve the performance of all surface facilities. Through this
hands-on effort, he learned the modifications that help improve process
efficiency, and those that do not. In the 50 years since Bill has accumulated a
lifetime of knowledge and experience in oilfield separation. He holds several
patents in the field.
The culmination of this work is the HWSB© Skim Tank, aka the “HWSB© Gunbarrel”. This
unique design achieves the highest level of hydraulic efficiency known to exist in any design.
The results are unparalleled oil-water separation efficiency in high water cut applications. Bill
says, “The HWSB© really gets the last squeak out of the pig!” He means the oil out of the
water, of course.
Today, HTC, Inc. is one of the industry’s leading low-cost surface facilities design firms. Bill’s
team of seasoned veterans specialize in salt water disposal (aka SWD) plant, flowback water
treatment plants, and crude oil processing and dehydration/desalting plant designs worldwide.
Most of Bill’s patents focus on SWD Plants. HTC affiliates blanket every field of engineering
discipline making HTC a full service firm capable of complete facility designs.
Bill invites you to visit HTC’s website at www.hightechconsultants.net or www.hitec1.com where
you can find over a dozen illuminating technical white papers like this one.