Tips on Getting a Good Large Petroleum Piping System

David Willoughby, POE, discusses what experienced piping system engineers take into account when they design piping for a petroleum terminal.

An individual needs a thorough understanding of hydraulics, the science and technology of the mechanics of fluids, to design an effective large piping system for a petroleum terminal. The hydraulic design must include an evaluation of the:

  • physical characteristics of the fuel;
  • the flow rates, the pipe route and distances;
  • the pressure required; and
  • environmental conditions.

Hydraulic profile
There are a number of mechanical designs that the designer can develop for any hydraulic design. Many choices for pumps, pipe sizes, pipe materials and pipe wall thickness are available. Good designers will usually select the best piping system available based on a particular set of conditions, including an economic analysis of several possible alternative systems.

In general, the hydraulic design for pipe in a terminal will provide a hydraulic profile with a discharge velocity of seven to 12 feet per second at full flow conditions. Pump suction velocities at full flow conditions are usually three to five feet per second. Suction piping design should ensure that the net positive suction head required by the pumps is available under all conditions of operation.

Terminal piping system pressures vary greatly among various applications. Under normal conditions, a good hydraulic design will allow the system to operate at the lowest possible pressure. Lower operating pressures permit smaller pipe wall thickness and system components with lower class ratings. These practices all result in lower system cost.

A function of pipe diameter and wall thickness, pipe cost is normally a significant part of the overall cost of the project. Consider the following factors in selecting pipe sizes:

  • Operating requirements of the facility.
  • Capital cost of the pipe.
  • Capital cost of pumping stations and attendant facilities.
  • Operating cost of the system.
  • Harmful effects of excessive velocity flow. Excessive flow is any flow that creates forces or pressure that exceeds the pipes’ strength characteristics—see the section on surge analysis for more information.
  • Fatigue failure caused by cyclic loading.

The objective of fluid flow design is to determine the minimum acceptable inside diameter of each segment of the piping system that will support the design flow rate, while maintaining the pressure drop and velocity within acceptable limits.

Several formulas exist for calculating the hydraulic profile of piping systems. Some of the most frequently used are Darcy-Weisbach, Fanning and Hazen-Williams. To cover them is beyond the scope of this article. However, all good hydraulic designs should make the maximum use of the available system energy.

For example, pumps normally introduce the energy (or “head”) available in the piping system, with the capacity of the pump determining the amount of energy. Based on the type of product, product flow rate and the system pipe size, the piping system will require a specific amount of energy (or head) to overcome the system pressure loss and to operate as intended. A good hydraulic system design will ensure a proper and efficient balance between pump selection and pipe sizing.

Manual calculations can be dangerous—especially when system surge pressures are crucial and the piping system is complex.

Surge analysis
Hydraulic transients are the most damaging of all the conditions that a pipe system may encounter while in service. One of the most common causes of hydraulic transients in terminal piping is waterhammer. If the velocity of the fuel flowing in the pipe is suddenly reduced, a pressure wave is created that travels in the liquid—up and down the piping system—at the speed of sound. This may cause a peak pressure that exceeds the system design.

Waterhammer frequently occurs in systems with rapid changes in the fuel flow rate. To design an effective piping system for terminal applications, the designer should conduct a complete surge analysis of the system operation. Today, most designers use computer programs to do surge analyses—especially when doing surge analyses for systems with quick closing valves and for aircraft hydrant and direct fueling systems with more than two outlets.

There are three methods to minimize fluid transients through effective system design:

  1. General design practices that either reduce or eliminate a possible transient.
  2. Addition of special systems to control or prevent a transient.
  3. Performance of time-history analysis of loads and the design of a pipe support system that will accommodate the transient loads.

When using general design practices, follow these two important practices:

  • Where allowed by system design, provide slow opening-closing valves.
  • Provide high-point vents or air release valves to allow system venting.

Use great care when using manual calculations instead of computer modeling— especially when system surge pressures are crucial and the piping system is complex. However, for simple piping systems that operate under 80 psi, you can use the following calculations to determine if surge is a problem:

  1. Determine the critical time of the system. Critical time is the time it takes for the first increment of the pressure wave to travel upstream, reflect and return to the valve.

Use the following equation:

Tc = critical closure time of system(s)
L = length of pipe (in feet)
a = surge pressure wave velocity (feet per second)

The values for “a” for liquid petroleum in ANSI schedule 40 steel pipe are as shown in the table below. These values are based on hydrocarbons with a specific gravity of 0.8 at 68 degrees F.

Piping Values Table

2. If valve closure time (T) is less than Tc, it is equivalent to instantaneous closure and will result in maximum surge pressure.

The equation used to calculate surge pressure rise for this situation is:

P1 = maximum pressure (in psig)
P = pump shutoff pressure (in psig) (equal to system static pressure)
V1 = initial velocity (in feet per second, or fps)
V0 = final velocity (fps)
w = specific weight of the fluid (lbm/ft3) (pounds per cubic foot)
g = gravitational constant (32.2 ft/s2) (feet per second squared)
C = unit constant (144 square inches divided by square feet)
a = surge pressure wave velocity (fps)

For example, a fuel storage facility has a truck loading rack located 2,000 feet away. The load rack is fed by a 600 gpm pump located at the storage facility. The load rack is equipped with a deadman apparatus that is tied to a hydraulically-operated diaphragm control valve at the rack. The valve has a closure time of 1.0 seconds. The pipe is six-inch diameter carbon steel, schedule 40, with an ANSI Class 150 flange. The pump shutoff pressure is 60 psig. Find the critical time of the system if the loading rack control valve closes.

From the Values Table, the value for “a,” the surge pressure wave velocity, is 3,692 feet per second. The maximum pressure in any piping system occurs when the total discharge is stopped in a period equal to or less than the critical time. Since the valve will theoretically close before this, Equation Two should be used to determine the pressure increase.

Many factors must be taken into account to compute the pressure flow at which these tank truck top loading arms can operate. Photo courtesy of OPW Engineered Systems.

Aboveground piping
Piping systems for terminal applications are either all aboveground or have a combination of aboveground and underground piping. Aboveground piping is less expensive to install and has easier access for maintenance than underground piping. Aboveground piping is usually required at pumps, filter separators, fill stands and other fuel system equipment.

Support aboveground piping so that the bottom of the pipe is at least 18 inches above the ground surface. In areas subject to flooding, greater clearance is desirable. At intersections with roadways, ensure the pipe is far enough off the roadway to allow enough clearance for the passage of tank trucks, cranes and similar heavy vehicles.

During the design and installation of aboveground pipe systems, consider the means of support. Good pipe support design begins with good piping system design and layout. For example, other considerations being equal, route piping to use existing structures to provide logical and convenient points of support, anchorage, guidance and restraint.

Place parallel lines spaced apart to allow room for independent pipe attachments for each line. Design supports to meet all static and dynamic operational conditions that the piping and equipment may encounter.

Aboveground pipes should rest on supports on a steel shoe welded to the bottom of the pipe. The shoe should be the same material as the pipe and be free to move on the support. Design pipe supports to meet the applicable requirements of ANSI/ASME B31.3, Chemical Plant and Petroleum Refinery Piping, or ANSI/ASME B31.4., Liquid Petroleum Transportation Piping System. (ASME is the American Society of Mechanical Engineers.)

Lay out aboveground pipe runs to provide the flexibility needed for pipe expansion and contraction caused by changes in the ambient temperature. Where possible, accommodate expansion and contraction by changes in direction in piping runs, offsets, loops or bends.

When this is not practical, flexible ball joint offsets are often used to provide the needed flexibility. Provide for pipe alignment on each side of the expansion joint. Do not use expansion devices that have packing, slip joints, friction fits or other non-fire resistant arrangements. Design expansion bends, loops and offsets within stress limitations in accordance with ANSI/ASME B31.3 and ANSI/ASME B31.4.

It is important to anchor aboveground piping at key points so expansion will occur in the desired direction. Anchors and guides may also be required to control movement in long runs of straight pipe or near a connection to fixed equipment, such as a pump or filter. It is also important to place anchors to provide the maximum amount of straight runs of piping from expansion points to the anchors.

In general, place anchors at all points of the system where only minimum piping movement can be tolerated, such as at branch connections and equipment connections. Key locations are pump houses or other buildings; manifolds, at changes of direction if not used as an expansion joint; at points where the pipe size is drastically reduced related to adjacent piping; and at all terminal points.

The bottom of the aboveground pipe needs to be at least 18 inches above the ground surface. Photo courtesy of Chevron.

Underground piping
Underground piping is used due to restrictions that require the pipe to be below ground, usually for beautification reasons, or to pass under obstacles such as roads or aircraft runways. Department of Transportation regulations 49 CFR 195, Transportation of Hazardous Liquids by Pipeline, and API RP 1102, Steel Pipelines Crossing Railroads and Highways, provide information on underground piping that passes under public roadways or railroad tracks. Before installing underground pipelines, review all local, state and federal regulations for double wall pipe, leak detection and corrosion protection requirements.

If possible, bury the pipe below the frostline. In all conditions try to maintain three feet of cover above the pipe. Less cover is permissible for occasional stretches where overriding conditions exist, such as the need to pass over other piping or a large culvert or beneath drainage ditches. At such locations, build sufficient flexibility into the piping to allow for vertical and lateral movement due to frost heave.

Provide a minimum clearance of 12 inches between the outer wall of any buried pipe and the extremity of any underground structures, including other underground pipe. In areas where multiple utilities are routed in the same area, make sure electrical and communication ducts/conduits are kept a minimum of 36 inches from all other underground utilities especially fuel, steam and high-temperature water pipes. Refer to ANSI C2, ANSI/ASME B31.4, and 49 CFR 195 for additional requirements.

For pipes in concrete trenches, provide a minimum clearance of six inches between flanges and the trench wall and between adjacent flanges. If there are no flanges, provide a minimum of eight inches between the pipe and the trench wall, and between adjacent pipes within the concrete trench.

Use steel casing sleeves only where sleeves are required by authorities having jurisdiction, where it is necessary to bore under the roadway or railroad tracks to avoid interference with traffic, or where boring is the most economical construction method. When planning construction of open trench crossings, consider the economics of installing spare casing sleeves to eliminate excavating for future fuel lines.

Ensure that the design isolates fuel-carrying pipes from contact with the casing pipes. Require a seal of the annular space at each end of the casing. Include a vent on the higher end of each casing.

When possible, locate crossings a minimum of 36 inches beneath the bottom of drainage ditches. If this depth cannot be obtained, install a six-inch thick reinforced concrete slab. It should be of adequate length and width above the casing or pipe to protect it from damage by equipment such as ditch graders and mowers. Refer to API RP 1102 for additional information.

Underground terminal piping is used only when aboveground piping cannot or should not be used—either for aesthetic reasons or to pass under obstacles. Photo courtesy of OPW Engineered Systems.

Piping connections
Piping connections and joints can have a major impact on the initial installed cost, the long-term operating and maintenance cost, and the performance of a piping system. When selecting pipe connections, consider material cost, installation labor cost, leakage and integrity, as well as specific performance requirements. Also, since codes do impose some limitations on joint applications, make sure that joint selection meets the applicable code requirements.

For steel piping systems, the most common type of connection is the use of weld neck forged or rolled steel flanges. These flanges have raised faces with a modified spiral serrated gasket surface finish.

Do not use cast iron flanges. Also, do not use grooved pipe type couplings or similar fittings in permanent fixed piping systems. Do not bury direct flanges, valves or mechanical couplings. If they must be used in an underground system, enclose them in an accessible pit. Welded connections are the preferred method for joining steel pipe. Flange connections are the preferred method for joining pipe to equipment.

Make branch connections with butt welded tees, except where the branch is at least two pipe sizes smaller than the run. In this case, make the branch connection with a forged or seamless branch outlet. Do not use wrinkle bends or mitered bends for changes in direction. Except for unions and control tubing couplings, do not use threaded joints in stainless steel systems.

Welding criteria
Welding often makes up the bulk of the work involved in the fabrication of terminal piping systems. It is essential that all designers have a working knowledge of welding practices and qualifications. The ASME Codes and API 1104 all have welding procedure qualification and welder qualification requirements. The purpose of procedure qualification is to assure that the welding process can produce a proper joint connection.

The purpose of the welder qualification is to assure that the welder can perform the welding operation according to the procedure. When installing terminal piping, ensure that the welding and welding inspections are in accordance with appropriate guide specifications and/or standard design.

All in all
Designing and installing petroleum terminal piping systems usually involve complex projects, extended liabilities, tight cost controls and strict quality standards. It is essential that all aspects and phases of a project be effectively controlled and executed. The available technology is extensive and growing every day. The applicable codes and standards are numerous. To be successful requires the concerted efforts and teamwork of professionals in a number of engineering and construction disciplines. This is the reality of the challenge.

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