Treat Tanks with Care

A variety of easily avoided problems can cause vessel failure.

By Roy E. Sanders, chemical process safety consultant

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Storage tanks play a vital role at chemical plants, oil refineries and other facilities, and represent major investments. Yet, despite all-too-frequent incidents of wrinkled roofs, collapsed walls and other failures, many people who design, construct, operate and maintain low-pressure storage tanks don't appreciate how frail they are.

In his popular book "What Went Wrong? Case Histories of Process Plant Disasters" [1], Trevor Kletz starts the chapter on storage tanks by saying: "No item of equipment is involved in more accidents than the storage tank, probably because storage tanks are fragile and easily damaged by slight overpressure and vacuum. Fortunately, the majority of accidents involving tanks do not cause injury, but they do cause damage, loss of material and interruption of production."

Dramatic tank failures feature in about a quarter of the case histories presented in the Beacon [2], a free Internet publication of the Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers. The incidents covered stem from causes such as overfilling, vacuum damage and explosions resulting from flammable mixtures in the vapor space.

So, here we'll look at factors that can compromise such vessels — and how to address them.

TANK CLASSIFICATIONS
Tanks come in a variety of designs. Some low-pressure storage tanks, sometimes called atmospheric storage tanks, are built to American Petroleum Institute (API) 650 specifications. Many large API-650 tanks have a flat to slightly coned roof and appear similar to a can of tuna fish or soup. However, a can of tuna fish or soup will withstand many times more pressure than a typical low-pressure tank [3]. The weight of the roof often limits the pressure rating of low-pressure tanks. (Some low-pressure tanks have a floating roof but we won't discuss these here.) Another popular choice is the API-620 tank, which is limited by code to 15 psig. Its top resembles a puffed cupcake. There also are small shop-built tanks, transportation vessels, plastic tanks and an array of non-code vessels.

In addition, a wide range of pressure vessels are designed and fabricated to Section VIII of the American Society of Mechanical Engineers (ASME) Pressure Vessel Code. Such pressure vessels come in a variety of shapes — the more popular ones are like a fat straight sausage or a sphere. The ASME Code covers vessels with internal design pressures from 15 psig to 3,000 psig.

The largest low-pressure storage tanks often are the most fragile and most vulnerable to failure from both over-pressure and vacuum [3]. Basically that's because a slight pressure over a very large area creates a large force.

SOME COMMON PROBLEMS
Simple operational situations often cause tank failures. Accidental overfilling, impeding exiting vent flow, and not allowing in-breathing as a tank is being pumped out are cardinal sins.

Overfilling. A recent CP article, "Don't Underestimate Overfilling's Risks,"  focused specifically on the hazards posed. It cited three major industrial accidents resulting from overfilling — including a massive fire in December 2005 at the Buncefield Oil Storage Depot in Hertfordshire, England. The tank that caused the incident had an independent high-level alarm and interlock but the system didn't work. The September 2006 Beacon provided details on that disaster and related that overfilling has contributed to a number of serious incidents in the chemical and oil industries in recent years.

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