Selecting Pipe Wall Thickness
The fluid flow equations and formulas presented thus far enable the engineer to initiate the design of a piping or pipeline system, where the pressure drop available governs the selection of pipe size. (In addition, there may be velocity constraints that might dictate a larger pipe diameter. This is discussed below in the section on velocity considerations for pipelines).
Once the inner diameter (ID) of the piping segment has been determined, the pipe wall thickness must be calculated. There are many factors that affect the pipe-wall-thickness requirement, which include:
- The maximum and working pressures
- Maximum and working temperatures
- Chemical properties of the fluid
- The fluid velocity
- The pipe material and grade
- The safety factor or code design application
If there are no codes or standards that specifically apply to the oil and gas production facilities, the design engineer may select one of the industry codes or standards as the basis of design. The design and operation of gathering, transmission, and distribution pipeline systems are usually governed by codes, standards, and regulations. The design engineer must verify whether the particular country in which the project is located has regulations, codes, and standards that apply to facilities and/or pipelines.
The basic formula for determining pipe wall thickness is the general hoop stress formula for thin-wall cylinders, which is stated as
where:
| Hs | = | hoop stress in pipe wall, psi |
| t | = | pipe wall thickness, in |
| L | = | length of pipe, ft |
| P | = | internal pressure of the pipe, psi |
| do | = | outside diameter of pipe, in |
Piping Codes
The following standards from the American Natl. Standards Inst. (ANSI) and the American Soc. of Mechanical Engineers (ASME) specify wall-thickness requirements.
ANSI/ASME Standard B31.1, Power Piping. This standard applies to steam piping systems.
ANSI/ASME Standard B31.3, Chemical Plant and Petroleum Refinery Piping. This standard applies to major facilities onshore and offshore worldwide.
ANSI/ASME Standard B31.4, Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia, and Alcohols.This standard applies to onshore oil pipeline facilities.
ANSI/ASME Standard B31.8, Gas Transmission and Distribution Piping Systems. This standard applies to gas transmission, gathering, and distribution pipelines onshore.
In the U.S, piping on offshore facilities is mandated by regulation to be done in accordance with ANSI/ASME Standard B31.3. Most onshore facilities are designed in accordance with ANSI/ASME Standard B31.4 or B31.8, depending on whether it is an oil or gas facility. respectively. Some companies use the more stringent ANSI/ASME Standard B31.3 for onshore facilities.
In other countries, similar standards apply with minor variations. For simplicity, we will discuss only the U.S. standards in this chapter. The engineer should check to see if there are different standards that must be applied in the specific location of the design.
Pipe Materials – Basics
There are some applications where plastic, concrete, or other piping materials are both desirable and acceptable. Utility systems such as those for water, sanitary or storm water, air, draining or low-pressure oil or gas service applications often use the nonsteel piping material systems. However, for the vast majority of the “pressure” piping systems encountered, steel pipe is required.
For petroleum applications, pipe materials that meet American Petroleum Inst. (API), American Soc. for Testing and Materials (ASTM), ASME, and ANSI standards are used most often. All of these standards have very rigid design, specification, chemistry, and testing standardization and manufacturing requirements. Modern steel pipe manufactured to these exacting standards assures both high quality and safety in design.
Steel pipe is available in a variety of commercial sizes ranging from nominal 1/8 up to 60 in. or greater. Table 1 illustrates a number ANSI pipe schedules, for reference. The “nominal” commercial pipe sizes from 1/8 through 12 in. refer to the approximate ID measurement of Schedule 40 or “standard” wall, whereas nominal 14 in. and larger sizes refer to the outside diameter. A variety of steel pipe sizes, wall thicknesses, and material grades are available for petroleum piping and pipeline applications.
Wall Thickness Calculations – Using B31.3 Code
ANSI/ASME Standard B31.3 is a very stringent code with a high safety margin. The B31.3 wall-thickness calculation formula is stated as
(Eq. 2)
where:
| t | = | hoop stress in pipe wall, psi |
| te | = | pipe wall thickness, in |
| tth | = | length of pipe, ft |
| P | = | internal pressure of the pipe, psi |
| do | = | outside diameter of pipe, in |
| S | = | allowable stress for pipe, psi (Table 3 and 4) |
| E | = | longitudinal weld-joint factor [1.0 seamless, 0.95 electric fusion weld, double butt, straight or spiral seam APL 5L, 0.85 electric resistance weld (ERW), 0.60 furnace butt weld] |
| Y | = | derating factor (0.4 for ferrous materials operating below 900oF) |
| Tol | = | manufacturers allowable tolerance, % (12.5 pipe up to 20 in. –OD, 10 pipe > 20 in. OD, API 5L) |
Under ANSI/ASME Standard B31.3, the allowable pressure can be increased for certain instances. The conditions for the permissible increases in allowable pressure, according to Standard B31.3, are given next.
- When the variation lasts no more than 10 hours at any one time and not more than 100 hours per year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by no more than 33%.
- When the variation lasts no more than 50 hours at any one time and not more than 500 hours per year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 20%.
Wall Thickness Calculations – Comparisons
Additional comparison of Standard B31.3 to both B31.4 and B31.8 indicates the following:
- ANSI/ASME Standard B31.3 is more conservative than either Standard B31.4 or B31.8, especially relative to API 5L, X-grade pipe and electric-resistance-welded (ERW) seam pipe.
- ANSI/ASME Standard B31.8 does not allow increases for transient conditions.
- The ANSI/ASME Standard B31.3 specification break occurs at the fence, whereas B31.8’s occurs at the “first flange” upstream/downstream of the pipeline.
Using ANSI/ASME Standard B31.3 criteria for oil- and gas-facility piping will assure a very conservative design. However, the cost associated with the Standard B31.3 piping design may be substantial compared to the other codes and may not be necessary, especially for onshore facilities.
Minimum Wall Thickness – Pipe And Fittings
The pressure and temperature requirements, and the chosen wall-thickness calculation formula, dictate the resulting pipe wall thickness required for the piping or pipeline design. The specification and grade of pipe and fitting materials selected for the design must be compatible with each other chemically (e.g., carbon content) so that the fittings can be welded to the pipe. In Sec. IX of the ASME Codes for Welding, base metals (pipe and fittings) have been assigned P-numbers and group numbers. Within the P-number groupings, ferrous base metals, which have specified impact test requirements, are classified in groups. The assigned P-numbers and group numbers are based essentially on comparable base-metal characteristics, such as:
- Composition
- Weldability
- Brazeability
- Mechanical properties
The ASME/ANSI and ASTM material specifications for pipe and fittings will list the P-numbers and group numbers within the data. The group number and P-numbers for the materials to be welded should be compatible (see ASME Sec. IX, QW-420 to verify material compatibility). Typically, for most oil and gas production-facility and pipeline applications (Group 1), P-1 materials will be required. The various codes and standards may prescribe allowable tolerances for pipe-to-fitting thickness variances. The allowable operating pressure and temperature and wall thickness of the fitting must be compatible with the pipe design. The maximum allowable operating pressure and temperature of the weakest piping-system component will determine the maximum allowable operating pressure for the system.
In small diameter threaded piping systems, for mechanical strength, impact resistance, and corrosion resistance, the pipe should have at least a 0.25-in. wall thickness. For smaller pipe, it is recommended that 3/4-in. and smaller pipe be Schedule 160, 2 to 3 in. Schedule 80, and 4 to 6 in. Schedule 40. ANSI/ASME B31.3 recommends that 1 1/2-in. and smaller threaded pipe use a minimum Schedule 80 and 2 in. and larger use Schedule 40.
sumber: http://petrowiki.org/Pipeline_design_consideration_and_standards
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