How Blending Dispensers Make Big Tanks and Pipes Even Bigger

A combination of regulatory and market driven factors has resulted in the advent of the three-grade "blending"dispenser, as well as much bigger underground storage tanks and larger diameter piping.

A combination of regulatory and market-driven factors has resulted in the advent of the three-grade “blending” dispenser, as well as much bigger underground storage tanks and larger diameter piping. These dispensers “blend” regular and premium grades to form a blended “middle” grade, thus eliminating the need for a tank for the blended grade.

However, now, instead one of the tanks being sized for the expected throughput of regular gasoline, the tank must be bigger to include the volume blended into the middle grade. In addition, it follows that the regular grade pipe sizing (e.g., diameter) will need to handle a higher flow rate to simultaneously meet demand from both regular and blended grade customers.

Higher volume, lower number
Over the past 10 years, UST regulations have required leak-free technology in the form of corrosion protection, secondary-containment equipment and installation certifications. While effective, these improvements have increased costs for the traditional three-grade underground tank and piping system. Mandatory financial responsibility and the high risk associated with multiple tank installations have also escalated.

These increased investment and operating costs are, in my opinion, responsible for the fewer numbers of refueling facilities operating today. Recent EPA data, published on February 9, 1999, shows that fully 58 percent of the total number of regulated underground petroleum tanks in existence 10 years ago have been closed. In turn, the significant reduction in the number of fueling facilities, as well as an increase in vehicle population, have helped to produce much higher sales volumes at existing and new facilities.

A worker installs the bulkhead in what will be a 10-foot diameter, two-compartment 20,000-gallon underground storage tank. Photo courtesy of Containment Solutions.

“Blending” dispensers and bigger tanks
Dresser Wayne has manufactured blend-dispensers for many years. At first, the blending mechanism was mechanical, and Sunoco used them extensively. In the late 1980s and early 1990s, at least three companies—Dresser Wayne, Tokheim Corporation and Gilbarco Inc.—introduced electronically controlled blending dispensers.

These dispensers have increased in popularity. One manufacturer reports that 60 percent of its multi-product dispensers (MPD) sales are blending dispensers.

Tanks—High-volume fueling facilities that store three grades of gasoline are now installing 15,000-gallon tanks instead of the 12,000-gallon tanks installed previously. However, with the advent of the blending dispensers, most new tank-fields consist of two gasoline tanks. In these tank-fields, the regular grade tanks are much larger than usual. For example, one popular size is a 30,000-gallon fiberglass reinforced plastic (FRP) compartment tank configured with 20,000- and 10,000-gallon capacities.

In the past, 12,000-gallon tanks were typically eight feet in diameter and 32 to 37 feet in length (length varies for steel or fiberglass and single or double wall construction). However, a 15,000-gallon capacity increases the length of an eight-foot diameter tank by an additional 10 feet, which begins to create a space problem at a typical retail facility.

The solution has been to use 10-foot diameter tanks and reduce the length by some 35 percent. Thus, the requirement for larger tanks has prompted the use of 10-foot diameter tanks. The ratio of 10-foot to eight-foot diameter tanks is approaching 50 percent.

Larger capacity 10-foot diameter tanks require additional handling and installation precautions. First, 10-foot diameter tanks require “wide-load” transportation. Second, large capacity tanks are much heavier and require larger cranes for placement in the excavation which, is two feet deeper.

This is an end view of a 15,000-gallon tank with a 10-foot diameter on a truck trailer. Photo courtesy of Xerxes Corporation.

Large diameter piping
It is estimated that in a typical blending dispenser system, the regular grade piping will be called upon to supply 80 percent of the total volume dispensed to both regular and blended grade customers. Thus, piping flow capacity becomes more important.

Flow capacity in a pipe depends on two major factors, namely internal friction (e.g., Hazen-Williams co-efficient) and the pipe diameter (I.D.) inside. Assuming essentially the same Hazen-Williams factor is available in quality non-metallic piping, then pipe I.D. (i.e., internal cross section area) will control flow rate capacity. Thus, the larger I.D. will provide the higher flow rate capacity. For example, a two-inch nominal FRP pipe (i.e., 2.21-inch actual) will have a 115 percent greater flow capacity than a 1.50-inch flexible pipe.

In addition, a three-inch nominal FRP pipe (i.e., 3.32 inches) will have a 126 percent greater flow capacity than a two-inch FRP pipe, and a 176 percent greater flow capacity than a two-inch flexible pipe. Thus, smaller refueling facilities should use a minimum of two-inch pipe, and larger facilities should use a three-inch manifold and two-inch laterals.

Volumes of changes
While it was expected that the UST regulations would impact petroleum storage and dispensing systems, their implementation has also affected a number of other variables as well. Also affected are refueling facility volumes, dispensing equipment design, the number of gasoline grades stored and the capacities of tanks and piping.

These changes, in turn, are requiring more technically advanced operating equipment, a higher level of training for service technicians as well as improved tank and piping design and installation practices. While these change have occurred rapidly over the past few years, we have become conditioned to expect this and more!

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