piping interview tips

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Inspection Engineer Interview points PipingCommonly used Construction codes: ASME B 31.1 (Power piping) ASME B31.3 Process Piping, ASME B31.4 Liquid transportation piping ASME 31.8 Liquid petroleum transmission piping API 1104 Welding of pipeline What are the main deference between ASME B 31.3 & ASME B31.4? The allowable stress is not the same. Minimum thickness formula is not same. One is for the piping inside the plant and the other one is for transportation. Inspection: API 570, API RP 574 Material: ASTM A53, ASTM A106, SA 135, SA 333, SA 671, SA 672, API 5L, SA 268, SA 213, SA 312, SA 790, etc., a) Carbon Steel Pipe API 5L, Grade A or B, seamless API 5L, Grade A or B, SAW, str. seam, Ej 0.95 API 5L, Grade X42, seamless API 5L, Grade X46, seamless API 5L, Grade X52, seamless API 5L, Grade X56, seamless API 5L, Grade X60, seamless ASTM A 53, seamless ASTM A 106 ASTM A 333, seamless ASTM A 369 ASTM A 381, Ej 0.90 ASTM A 524 ASTM A 671, Ej 0.90 ASTM A 672, Ej 0.90 ASTM A 691, Ej 0.90 (b) Low and Intermediate Alloy Steel Pipe ASTM A 333, seamless ASTM A 335 ASTM A 369 ASTM A 426, Ec 0.90 ASTM A 671, Ej 0.90


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ASTM A 672, Ej 0.90 ASTM A 691, Ej 0.90 (c) Stainless Steel Alloy Pipe ASTM A 268, seamless ASTM A 312, seamless ASTM A 358, Ej 0.90 ASTM A 376 ASTM A 451, Ec 0.90 (d) Copper and Copper Alloy Pipe ASTM B 42 ASTM B 466 (e) Nickel ASTM ASTM ASTM ASTM and Nickel Alloy Pipe B 161 B 165 B 167 B 407

(f) Aluminum Alloy Pipe ASTM B 210, Tempers O and H112 ASTM B 241, Tempers O and H112 Low temperature service material & fittings Product Form Pipe Tube Fittings Forgings Castings Bolting Plate ASTM Spec. No. A 333 A 334 A 420 A 350 A 352 A 320 A 20


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The primary elements in determining the minimum acceptable diameter of any pipe network are system design flow rates and pressure drops. The design flow rates are based on system demands that are normally established in the process design phase of a project. The hydraulic design of a piping system is premised on selecting the optimum pipe size (diameter) and thickness (schedule), for the design flow rate and the allowable pressure drop in the system.89358063.doc Page 3 of 39

For a specific flow rate, as the pipe diameter increases, the pressure drop increases at a fast rate (pressure drop varies approximately with the square of the velocity). This means that the pumping horsepower required would increase resulting in a higher pump cost and on-going operating energy costs. On the other hand, as the pipe diameter decreases, the installed cost of the piping system (pipe, fittings, valves) also decreases. The optimal solution is to find the pipe size that would result in the lowest life cycle costs (initial installed cost plus operating costs over the life of the piping system. Piping and Instrumentation Diagram (P&ID) The primary element of a piping design is the piping and instrumentation diagram (P&ID). Both the process engineer and the instrument and controls engineer provide design information. The first version should represent all major equipment, process piping with sizes, utility piping with sizes, and line-mounted instrument hardware. The piping design team is responsible for drafting the P&ID. The P&ID is commonly referred to as the control document for design and construction and subsequently for operation and maintenance. For the pipe designer, the diagram is a complete mechanical description. It is an engineering expression of the processscope of work. It is imperative to maintain the P&ID up to date and to control and communicate the revisions to the design group to ensure that the design is based on the latest revision. Piping components shall be designed for an internal pressure representing the most severe condition of coincident pressure and temperature expected in normal operation. This condition is by definition the one that results in the greatest required pipe thickness and the highest flange rating


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MATERIAL SELECTION Materials selection is an optimization process, and the material selected for an application must be allowed by the applicable code; and chosen for the sum of its properties (strength, toughness, corrosion resistance, etc), availability, and cost. Thus, the selected material may not rank first in each evaluation category; but it should be the best overall choice In practice it is usual to select materials which corrode slowly at a known rate and to make an allowance for this in specifying the material thickness. Material Selection Process Piping material is selected by optimizing the basis of design. 1. Eliminate from consideration those piping materials that: a. Are not allowed by code or standard; b. Are not chemically compatible with the fluid; c. Have system rated pressure or temperatures that do not meet the full range of process operating conditions; and d. Are not compatible with environmental conditions such as: external corrosion potential, heat tracing requirements, ultraviolet degradation, impact potential, and specific joint requirements. 2. The remaining materials are evaluated for advantages and disadvantages such as capital, fabrication and installation costs; support system complexity; compatibility to handle thermal cycling; and cathodic protection requirements. The highest ranked material of construction is then selected. 3. The design proceeds with pipe sizing, pressure integrity calculations and stress analyses. If the selected piping material does not meet those requirements, then the second ranked material is used and the pipe sizing, pressure-integrity calculations and stress analyses are repeated.


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PRESSURE-INTEGRITY DESIGN The pressure-integrity design of a piping system normally requires the consideration of at least two issues. The first is the determination of the minimum or nominal pipe wall thickness, and the second is the determination of the pressure rating of the in-line components such as valves and fittings. The design process for consideration of pressure integrity uses allowable stresses, thickness allowances based on system requirements, and manufacturing wall thickness tolerances to determine minimum wall thickness Current Basis for Determining the Allowable Stress (S) As a result of the introduction of new materials and increases in service temperatures, use of Safety factors was abandoned and the factor became part of the allowable stress for a material at any temperature. The allowable stress is based on the least of the following: Room-temperature tensile strength / 3.5 Room-temperature yield strength / 1.5 The stress required to cause a creep rate of 0.0001%/1000 hours The average stress to cause rupture at 100,000 hours / 1.5 The minimum stress to cause rupture at 100,000 hours / 1.25

Today, fracture mechanics allows an engineer to establish the minimum toughness required in a material based on the stress applied and the maximum credible size flaw. These changes eliminated concern over brittle fracture. In addition, Section VIII requires that hydrostatic testing be performed at the minimum design metal temperature plus atleast 30F, ensuring that brittle failure will not occur during hydrostatic testing

Design Margin (Safety Factor) in the ASME Boiler and Pressure Vessel Code In the 1999 addenda of the ASME Boiler Code, the design margin (formerly known as the Safety Factor) was changed from 4.0 to 3.5. ASME B31.3 has had a design margin of 3.0 for more than 20 years ASME B31.1 is in the process of changing to 3.5, but may change to 3.0.


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PIPING SYSTEM DESIGN PROCEDURE The following flowchart shows the typical procedure used in designing piping systems

PIPING LAYOUT AND ISOMETRIC DRAWINGS Flow diagrams, line lists and design specifications are all used by the piping designer to lay out the piping and generate design drawings. Piping of the size and schedule must be routed between the appropriate pieces of equipment as shown on the flow diagrams. Routing will be affected by system operating temperature, pipe weight, installation and material costs, applicable code requirements, pressure drop requirements and equipment and building structure locations. Piping isometrics are three-dimensional representations of the piping system using a 30 orientation of the two horizontal axes. Piping not running parallel to one of the main axes is shown by its components in each direction. Isometrics need not be drawn to scale; the piping segments may be drawn as long as is necessary for clarity


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Bolting material: ASTM A 193 grade B7 bolting for ordinary service. ASTM A 193 grade B7M bolting for areas exposed to hydrogen sulfide serviceHydro test pressure: 1.5 X design pressure What is the difference between pipe & tube? The word pipe is used, as distinguished from tube, to apply to tubular products of dimensions commonly used for pipeline and piping systems. Pipes NPS 12 (DN 300) and smaller have outside diameters numerically larger than their corresponding sizes. In contrast, the outside diameters of tubes are numerically identical to the size number for all sizes. Minimum required thickness calculation: Pipe the minimum required thickness is t, the nominal thickness is t + corrosion allowance The t as per API RP 574 is t=PD / 2SE where P-Design Pressure, D-OD of pipe, S-allowable stress as per the code, E-quality factor (for Seamless pipe it is One). This formula is used for in-service piping. The t as per ASME B31.3 is t=PD / 2(SE+PY) where P-Design Pressure, D-OD