Why pull the plug on drains?

— Published Nov/Dec 2015 issue of Tank Storage Magazine —
DISCUS IFR in Tank 337

DISCUS leg-supported deck under a dome roof

 

The design philosophy behind a significant shift in floating cover technology: decks without drains.

Background

Cover drains are defined by the US EPA in AP-42 as either “open” or “closed”. Closed drains are not typically used on aluminum floating covers. Open cover drains can be either “flush” or “overflow”. Overflow drains are not typically used on aluminum floating covers either. We are left with open drains that are flush with the cover and extend to the liquid under the cover to allow liquid to drain off the floating cover. These are also called “stub drains” or “automatic drains”. Since they are open, this type of drain is a source of emissions.

F Topside at Seal and Funnel

Topside view showing the liquid-mounted seal a support leg, anti-rotation cable, and gauge funnel

Objective of drains

Remove water from on top of the cover

Drains are often used as the preferred mechanism to keep water from collecting on, and possibly overloading the cover by draining the water into the stored product. Do we really want water to drain into product? Water tends to sink to the tank bottom, collect around the corner weld or sump and corrode the tank walls and bottom. Microbial induced corrosion tends to occur at the product / water interface. Also, custody transfer is complicated when there is a water bottom.

Remove product from on top of the cover

If there is product on the cover, the situation needs to be addressed. But drains can only function effectively if they are sized and spaced correctly, and are of the correct number. Drain quantity, size, and spacing is not addressed in API 650.

10-D-4-Way-Joint-Topside-200x200

Topside view of a 4-way joint with non-welded seams

Drains and live load

According to API 650 Annex H, if your design includes “automatic drains” the live load is reduced from 12.5 psf to 5 psf. The premise is that the drains will remove liquid from anywhere on the cover. AP-42 estimates that there will be one drain per 98 square feet (115 on a 120’ tank). A cover with ineffective drains and/or not enough drains, or plugged drains, but still built to the lighter live load requirement, is MORE in danger of being overloaded. These arrangements may meet the letter of the standard, but may violate the intention of the standard. If the drains are ineffective and the cover is designed to the lighter live load, the situation can quickly become unsafe as cover supports and connections may fail from liquid loads.

To summarize the downsides of drains:

  • Drains are a substantial source of emissions and odors. Per AP-42 and Tanks 4, each has a loss factor of 1.2 pound-moles per year.
  • Drains include a liquid/air interface and become cover corrosion hot-spots.
  • Drains allow the product to be contaminated from above.
  • Water settles to the tank bottom, encouraging corrosion.
  • Drains are only effective if they are at the low point when liquid is on the surface of the cover. Being in the right place is a matter of chance.
  • Drains can be a safety hazard as they pump vapors into the work space when the floating cover is walked on in service.
  • Drains can be weak points of the structure.
  • Drains can become clogged.
  • Ball-checks are unproven to restrict vapor flow upwards tend to limit liquid flow downwards.
  • The many downsides of drains led to a shift in floating cover design. For DISCUS, this meant creating additional strength and buoyancy.

Potential scenarios that lead to liquid on top of a cover, and what to do about them

Since the DISCUS cover does not have drains, and is built as a series of bulkheaded open compartments, we’ve done a lot of work to identify various scenarios that can lead to liquid on the top of a cover, and our recommendations on how to deal with it.

1. Water on top of a floating cover, due to the fixed steel roof or aluminum dome leaking. Consider the scenario below if a dome panel is torn off during a storm:

  • Dome panels are about 50 ft²
  • Assuming two panels are torn off, there would be 100 ft² of opening
  • If the storm resulted in 2 feet of rain/ft², 200 cubic feet of water would get on the cover
  • 200 cubic feet of water weighs about 12,500 lbs
  • Each (DISCUS) panel holds about 1,500 lbs of water so this would result in filling about 8-9 panels
  • The DISCUS cover has 460% excess buoyancy, meaning relatively few panels are needed to keep the cover afloat. Even on smaller tanks, 8-9 panels full does not endanger the cover. Therefore the concern in the scenario above is limited to removing unwanted water by pumping or evaporation. Since DISCUS panels are bulkheaded compartments, the water is limited to a few panels, making removal easier.

Who wants water in their tank anyway? It contaminates the product, corrodes the tank bottom, etc.

2. Product on top of a cover is usually the result of a mistake, design error, operational issue or other preventable incident.

3. If from product fill, it is usually an upset condition, likely preventable with the installation of an adequate liquid only or liquid + gas inlet diffuser.

4. If from the use of a mixer (jet or propeller), engineering and/or operational controls can mitigate or eliminate such occurrences.

In each of these cases, the incident is likely preventable but if it does occur, it would be only by chance that drains would be in exactly the low point and completely effective. So in most cases, removal of product from the cover would be required with or without drains, rendering the drains unnecessary. The bulkheaded DISCUS cover minimizes evaporation by reducing the surface area of the liquid, therefore more can be cleaned up, as compared to a “flat” style cover, where much of the liquid can evaporate.

5. What if product burps up onto the roof and gets stuck on top of the cover?

If a significant amount of product were to burp on top, removal and cleaning on a bulkheaded cover would be easier than a “flat” style cover. Liquid runs to low points, however drains will often be high points because they are in raised, thicker, flatter skins, and rise as they work. The separate compartments characteristic of the DISCUS cover provide containment and limit the surface area that can evaporate. Because the DISCUS cover is very stiff and its seals offer true +/- 4″ of expansion all around, there would have to be a major upset before the cover would tilt far enough to make any gaps in the seals. This limits the problem to simply a clean-up issue. On a DISCUS cover, liquid would be more likely to burp up through column wells rather than PV vents because its PV vents are heavily weighted vs the “flapper door” style, and as such it is likely that the liquid would be towards the center of the cover. Identifying and cleaning of top side liquid can be done more safely, easily and quickly if the liquid is localized, as it would be in the compartments of the DISCUS bulkheaded cover.

By prioritizing strength and buoyancy and building to a heavier live load, the DISCUS floating cover functions more safely and effectively without relying on drains.

Have more questions that weren’t answered in this article? Contact us.

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