Piping Flow Rate Design For Retail Refueling Facilities

Piping design calculations has always include flow rates. Sully Curran explains the differences in flow rates for piping systems in use at petroleum marketing facilities.

For practical reasons, piping systems for retail motor fuel dispensing facilities should be designed to fuel vehicles at the maximum EPA allowable limit of 10 gallons per minute during peak traffic periods, when the maximum number of nozzles are in use.

Going with the flow
Designing a piping system for a retail application requires calculating hydraulic flow rates for a specific site, based on a specific set of characteristics. Determining the flow rate of a piping system must take into account the major sources of friction that influence flow rates, such as piping material, the inside diameter, pump type and horsepower, specific gravity of the fuel being pumped, the number and types of fittings, the piping layout, as well as the size and type of the dispenser, hose and nozzles.

This column describes the flow rates that can be expected for steel, FRP and the five most widely used UL Listed flexible pipes; it also describes common methods of piping layouts for multiple dispensing islands. This information is designed to provide some rules of thumb for small, medium and large retail vehicle refueling facilities. The calculations are based on commonly used piping systems.

First, a quick overview. Underground piping materials fall into four general categories: cast-iron, copper, steel and non-metallic. Three of them are not widely used for petroleum: cast-iron, copper and steel. Cast-iron is not practical or recommended for small diameter pressure or flammable liquid applications. Copper may be used but is not price-competitive. Only steel piping with malleable iron fittings was used until the development of fiberglass piping in the late 1960s.

Fiberglass reinforced thermosetting plastic (FRP) was the first nonmetallic piping developed to overcome some of the inherent problems with steel piping, mainly, corrosion and problems with swing-joints. UL Listed the first fiberglass piping for use with flammable and combustible liquids in the late 1960s. (See my article “Fiberglass pipe—past, present and future” on in the July/August issue of PE&T for more on FRP piping, and my column “The fundamentals of fiberglass,” in the May/June issue for more on fiberglass.)

Flexible thermoplastic piping was developed in the late 1980s, and is UL Listed for petroleum products, alcohol and alcohol-gasoline mixtures. (See “UL 971: an evaluation of flexible piping” 22 in the July/August issue of PE&T and Joe Hartmann’s article in this issue for more on flexible piping.)

Friction loss
In theory, calculating the flow rate should be a fairly straightforward process in so far as it is based on friction. The lower the friction, the quicker the flow. Friction loss in piping is typically expressed in terms of “feet per 100 feet” of pipe, and differs for fluids of different specific gravity, different pipe inside diameters (IDs) and different piping materials. Calculations in the table below are for gasoline. They are based on pumping a petroleum solvent with a flash point of 100 degrees F with a specific gravity of 0.78 to 0.85, relative to water.

Friction loss tables have been developed for liquids with different specific gravities, pipe materials, inside diameters and pumping rates. The table below follows with examples of friction losses for a 3/4 hp submersible turbine pump, with a designed flow rate of 40 gpm. The table can be used to compare friction losses of certain sizes of fiberglass, flexible piping and steel piping:

Pipes: fitting loss
Pipe fittings are used to change direction and make connections. Unfortunately, they cause friction and turbulence, which cause slower flow rates. Fittings that contribute the most significant losses, or slowdowns in fiberglass piping systems are tees’ and elbows, and, in some flexible piping products, pipe end attachments. Pipe end attachments in some flexible piping systems create losses because of the attachment methods utilized. Metal fittings are used with steel and flexible piping systems, whereas only fiberglass wound or cast fittings are used with fiberglass piping.

Typical configurations
The three common methods of piping layouts for multiple dispensing islands are described below:

1. Pipe in series from the pump to the first island, then on to the second island second, etc.

2. Pipe in a series to the first three islands: (a) run a separate line from the pump to the fourth or more island; or (b) branch out with a separate pipe at the second island to serve additional islands.

3. Pipe in a main line to the furthest island and “tee” off this “manifold” to the other islands.

Flexible piping systems typically use one of the first two methods; and rigid systems use the third, “manifold” approach.

Tee losses
In a typical four island facility, the flexible piping configuration will use the layout described in #2a or 2b, with a total of five tees per product line to achieve the maximum downstream flow rate. Rigid piping will use the layout described in #3 and a total of four tees per product line.

Elbow losses
While rigid steel and FRP piping systems use elbows to change direction in a piping run, flexible piping is curved if there is room to accommodate the bending radius. As a result, flexible piping will typically use two fewer elbow fittings than a steel rigid or FRP system.

Rules of thumb
The following guidelines on the piping sizes necessary to achieve 10 gpm dispensing rates during peak traffic periods at a minimum cost are general at best. This is because other unknown factors affect flow rates, such as internal dispenser plumbing (e.g., 1-1/2 inch shear valve, meter resistance),and hose and nozzle (e.,g., vapor recovery) equipment restrictions. However, the following design criteria are generally applicable:

1. For a three dispenser, six nozzle facility, with a 3/4 hp submersible turbine pump and the tanks approximately 100 feet from the dispensers:

• design for 40 gpm flow rate (i.e., four of six nozzles in service)
• use 2-inch FRP or 1-1/2 inch flexible piping If the dispensers are further than 100 feet from the tank:
• Use 2-inch FRP or 2-inch flexible piping

2. For a four dispensers, eight nozzle fueling facility, with a 1-1/2 hp submersible turbine pump, design for 60 gpm flow rate (i.e., six of eight nozzles in service): If the dispensers are further than 100 feet from the tank with approximately 400 feet of piping run:

• Use 2-inch FRP or 2-inch flexible piping.

3. For six or more dispensers, 12 or more nozzles, one or two 1-1/2 hp submersible turbine pumps, and approximately 600 feet of piping run:

• Design for 100+ gpm flow rate, and use 3-inch FRP manifold and 2-inch FRP lateral piping.

References:

1. API/RP #1615, PEI/RP100

2. May 1995, Consulting Specifying Engineer, article “Intuitive Pipe Specification Requires All the Facts,” Randall L. Pool, P.E.

3. UL 971, “Standards for Nonmetallic Underground Piping for Flammable Liquids”

4. March, 1995, Apogee Research, Inc. “Survey of Flexible Piping Systems”

5. Steel Construction Manual of the American Institute of Steel Construction and API Spec 5L, 41st Edition, April 1, 1995.

6. Ameron Dualoy 300/L and Smith Fiberglass Red Thread II product data.

7. Ameron Fiberglass Pipe; Smith Fiberglass Products and EBW–ref. Dwg. 275-103.

8. March 2, 1994, Engineering and Design Guide, Smith Fiberglass.

9. Cameron Pump Operator’s Guide, Ingersoll-Rand Company.

10. Calculation and pump test data.

11. Ken Wilcox Associates, Inc., Blue Springs, MO.

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