Tank types: fixed roof, floating roof and bolted tanks

On her first morning at Bayou Junction, Owen Kask walks Kiera along the tank farm road. There are twenty-two tanks in the terminal, built over fifty years, and almost every common type is here. "Every one of these was chosen for a reason," Owen says. "The product, the vapor pressure, the size, the climate, the money. If you know why a tank is the type it is, you know what matters when you build it or repair it." This lesson follows that walk and then looks at how the shell of a welded tank is designed, because the shell course thicknesses decide most of what the erection crew does.

Fixed roof tanks

The simplest tank has a fixed roof. A cone roof is a shallow cone of thin plate, usually about 3/16 in thick, sloping up from the top of the shell to the center. Small cone roofs are self-supporting: the plate and a few rafters span from the shell to the center. Larger cone roofs rest on rafters that run from the shell to a ring of columns inside the tank, and the columns stand on the bottom. A dome roof is curved like a shallow sphere and supports itself from the shell, which keeps the inside of the tank free of columns. Aluminum geodesic domes, made of triangular panels bolted to a frame, are often fitted over old tanks to keep rain and snow off a floating roof.

A fixed roof tank has a vapor space between the liquid and the roof. When the tank is filled, the liquid pushes vapor out through the vents; when it is emptied or the sun heats the roof, air is drawn in. For products that give off little vapor, such as diesel, heavy fuel oil, asphalt or water, this does not matter much, and a fixed roof is the cheapest choice. For crude oil and gasoline, the vapor lost through the vents would be a large loss of product and a large source of air emissions, so those products go into floating roof tanks or into fixed roof tanks with a floating roof inside.

Fixed roof tanks also have a design feature that matters to the erection crew: the frangible roof joint. In many cone roof tanks, the weld between the roof plate and the top angle of the shell is made deliberately small, so that if an explosion inside the tank raises the pressure, the roof tears away from the shell before the shell tears from the bottom. That keeps the product inside the shell. The erector must make that weld exactly the size the drawing shows, no larger, and must not add strength around it, because a stronger roof joint can turn a roof failure into a bottom failure.

Floating roof tanks

A floating roof sits on the product and rises and falls with it, so there is almost no vapor space. An external floating roof tank has no fixed roof above the floating one; the floating roof is open to the sky. The roof is a steel deck stiffened by a ring of watertight compartments, the pontoons, around the outside, or by a double deck for very large tanks. Rainwater that falls on the roof drains through a hinged or flexible pipe inside the tank and out through a valve at the bottom of the shell. A gap of about 8 in between the roof rim and the shell is closed by a seal, usually a primary seal of steel shoes or foam with a secondary seal above it. Legs hold the roof up when the tank is emptied, so that the crew can work under it.

An internal floating roof tank is a fixed roof tank, often a dome or cone, with a light floating roof inside, usually aluminum pontoons with a thin deck. The fixed roof keeps rain and snow off the floating roof. Many older external floating roof tanks have been converted by adding a dome. Tank 15 will be an external floating roof tank with a pontoon roof, like Tank 14 beside it.

Bolted, shop-built and water tanks

Not every tank is welded in the field. In oil and gas production, small tanks that hold crude and produced water at well sites are often bolted tanks built to API 12B, made of curved steel panels bolted together with gaskets in the seams. They can be taken apart and moved when the wells are depleted. Small tanks up to about 12 ft in diameter are often welded in a shop and trucked to site, to API 12F, API 650 Annex J or a fabricator's listed standard. Water tanks for towns and fire protection are built to AWWA D100 if they are welded and AWWA D103 if they are bolted, with coating systems approved for drinking water.

Owen points out the terminal's two oldest tanks, which are riveted. Riveted tanks were common until the 1950s. A boilermaker today will not build one, but may well repair one, and the repair standards treat riveted seams with special care because the rivets cannot be welded.

Choosing the type: vapor pressure and emissions

The product's vapor pressure decides most roof choices. Vapor pressure is the pressure at which a liquid boils at a given temperature; a liquid with a high vapor pressure gives off more vapor. In the United States, federal rules for volatile organic liquid storage vessels, such as the New Source Performance Standards in 40 CFR 60 Subpart Kb, require a floating roof or a closed vent system for tanks above set sizes that store liquids above set vapor pressures, and they set the seal gap limits that lesson 6 uses. Products with a very high vapor pressure, such as butane or propane, are stored in pressure vessels or spheres, not in atmospheric tanks.

Spill containment is the other rule every tank farm lives by. Tanks stand inside dikes or berms that hold the contents of the largest tank plus a margin, and many have leak detection under the bottom or a double bottom. In the United States, the EPA's Spill Prevention, Control, and Countermeasure rule in 40 CFR 112 requires plans, containment and regular tank inspection, and fire codes based on NFPA 30 set spacing and venting. The erector does not design the containment, but must not damage the dike, the liner or the leak detection, and must leave the tank ready for the inspection program that will follow it.

How the shell is designed: the one-foot method

Whatever the roof, the shell of a welded tank is designed for the hydrostatic pressure of its contents. The pressure at any depth is the product's density times gravity times the depth. A thin cylinder carries that pressure in hoop tension, and the required thickness is the hoop stress formula: pressure times radius divided by the allowable stress. API 650 sets the allowable stresses for each plate material and gives two methods for working out each course's thickness. The simpler one, the one-foot method, calculates the thickness at a point 1 ft above the bottom of each course, because the stiff course below helps hold the bottom of each course and the critical stress occurs a little above the seam. API 650 allows the one-foot method for tanks up to about 200 ft in diameter; larger tanks use the more exact variable design point method.

Here D is the diameter in feet, H is the design liquid level above the bottom of the course in feet, G is the design specific gravity of the product, Sd is the allowable design stress in pounds per square inch and CA is the corrosion allowance in inches; the answer is in inches. The standard also requires a hydrostatic test thickness, calculated the same way with water (G of 1), no corrosion allowance and the test stress St. The plate must be at least the larger of the two, and never less than a minimum thickness that depends on the diameter.

Owen hands Bernie the design for Tank 15, and Bernie asks Kiera to check the course thicknesses against it before the plate order is released. The design uses a plate grade with an allowable design stress of 28,000 psi and a test stress of 30,000 psi, values that are illustrative here, and the owner has asked for a corrosion allowance of 1/16 in and a design specific gravity of 0.9.

# One-foot method shell thicknesses for Tank 15 (US units, illustrative)
import math

d = 160.0              # diameter, ft
H = 48.0              # design liquid level, ft
course = 8.0        # course height, ft
G = 0.9              # design specific gravity of the crude
ca = 0.0625            # corrosion allowance, in
Sd, St = 28_000, 30_000  # allowable design and hydrotest stresses, psi
tmin = 0.3125        # minimum nominal thickness for this diameter, in

def plate(t):
    """Round up to the next available plate thickness."""
    return math.ceil(t / 0.0625 - 1e-9) * 0.0625

print("course  H at bottom   td      tt      plate")
for n in range(1, int(round(H / course)) + 1):
    hb = H - (n - 1) * course                 # liquid height above the bottom of this course
    td = 2.6 * d * (hb - 1) * G / Sd + ca
    tt = 2.6 * d * (hb - 1) / St
    t = max(plate(max(td, tt)), tmin)
    print(f"{n:>4}   {hb:8.0f}   {td:6.3f}  {tt:6.3f}  {t:6.3f} in")

course  H at bottom   td      tt      plate
   1         48    0.691   0.652   0.750 in
   2         40    0.584   0.541   0.625 in
   3         32    0.477   0.430   0.500 in
   4         24    0.370   0.319   0.375 in
   5         16    0.263   0.208   0.312 in
   6          8    0.156   0.097   0.312 in

The calculation gives a bottom course of 3/4 in, stepping down to 5/16 in for the top two courses, where the diameter's minimum thickness governs rather than the liquid. Owen's design matches. Kiera notices that for every course the design thickness, with its corrosion allowance and specific gravity, is greater than the hydrotest thickness, so the design condition governs. For a tank storing a light product with no corrosion allowance, the hydrotest thickness can govern instead.

The erection crew cares about these numbers for practical reasons. Each course is a different thickness, so each has its own welding procedure details, its own bevels and its own number of passes. The bottom course is the heaviest to handle and the thickest to weld. The thickness also decides how much radiography each course needs, as lesson 11 shows, and whether the vertical seams need preheat in cold weather.

Reading a tank data sheet

Every new tank starts from a data sheet, a standard form that the owner's engineer fills in and the tank designer and erector work from. API 650 includes a data sheet in its annexes, and most owners use it or their own version of it. For Tank 15, Owen's data sheet gives the diameter, height and capacity, the product and its design specific gravity, the design and test temperatures, the design wind speed and snow load, the corrosion allowances for the shell, bottom and roof, the plate materials, the roof type and seal types, and every appurtenance with its size, orientation and elevation. It also states the edition of API 650 and any annexes that apply, such as the annex for floating roofs. Kiera reads it before she reads anything else on the job, because every other drawing and calculation must agree with it, and when two documents disagree, it is usually the data sheet that settles the question or shows that someone has to ask.