PIPING DESIGN / STRESS ENGINEERING NOTES:

1. PIPING STRESS ANALYSIS.

PIPING STRESS ANALYSIS CALCULATES THE STESS IN PIPING SYSTEM SUBJECTED TO THE FOLLOWING LOADS.

A. NORMAL OPERATING LOADS SUCH AS PRESSURE, WEIGHT AND THERMAL EXPANSION.

B. OCASSIONAL LOADS SUCH AS WIND, EARTHQUAKE AND WATER HAMMERING.

PIPING SYSTEMS ARE CONNECTED TO ROTARY AND STATIC EQUIPMENTS THEREFORE THE PIPING CALCULATIONS OR STRESS ANALYSIS INVOLVES THE EVALULATION OF THE EFFECT OF PIPING FORCES AND MOMENTS TO THE CONNECTING EQUIPMENT.

2. HOW TO CONTROL PIPING STRESS.

PIPING STRESS GENERALLY CONTROLLED BY ADDING SUPPORTS, RESTRAINTS, LOOPS AND BELLOWS TO THE PIPING SYSTEM.

3. VERIOUS ENGINEERING ISSUES IN PROCESS PIPING THAT AN ENGINEER MUST KNOW.

THERE ARE VARIOUS ENGINEERING PROBLEMS OCCUR WHILE DESIGNING A PIPING SYSTEM THAT IF AN ENGINEER HAS AN IDEA ABOUT IT HE OR SHE COULD SWIFTLY COME TO A REASONABLE SOLUTION AND AVOID REWORKING FOR INSTANCE THEY ARE.

A. THERMAL STRESS

B. PRESSURE THRUST FORCE

C. THERMAL BOWING

4. WHAT IF APPROACH OF ENGINEERING CALCULATION DEVELOPMENT.

NOWADAYS, MAKING A CALCULATION WITH A COMPUTER IS SO FAST THAT WE OFTEN HEAR ABOUT THE WHAT-IF APPROACH IN ENGINEERING. WHAT ALL THIS WHAT-IF APPROACH ACCOMPLISHES IS MAKING NUMEROUS RANDOM TRIALS AND THE WISH THAT ONE OF THESE TRIALS WILL HIT THE MARK SOONER OR LATER. THE PROBLEM IS THAT AFTER A FEW TRIALS, MOST PEOPLE LOSE THE ABILITY TO MAKE SENSE OF THE TRIALS. THE MORE THEY TRY, THE MORE THEY GET CONFUSED. IN CONTRAST, THIS BOOK PUTS EMPHASIS ON THE WHAT, WHY, AND HOW TO GUIDE THE READERS INTO THIS 3-W APPROACH THAT IS, TO BE AWARE OF THE PROBLEM, UNDERSTAND THE CAUSE OF THE PROBLEM, AND TO SOLVE THE PROBLEM OR PREVENT IT FROM HAPPENING.

5. BRIEF INTODUCTION OF PIPING AND PIPING SYSTEMS.

A PIPING SYSTEM IS THE MOST EFFICIENT AND COMMON MEANS OF TRANSPORTING FLUIDS FROM ONE POINT TO ANOTHER. WITHIN A PETROCHEMICAL COMPLEX, ACRES AND ACRES OF PIPING CAN BE SEEN RUNNING IN EVERY DIRECTION AND AT MANY DIFFERENT LEVELS. PIPING CONSTITUTES 25% TO 35% OF THE MATERIAL OF A PROCESS PLANT, REQUIRES 30% TO 40% OF THE ERECTION LABOR, AND CONSUMES 40% TO 48% OF THE ENGINEERING MAN-HOURS [1]. THE ACTUAL IMPORTANCE OF PIPING, HOWEVER, CAN FAR EXCEED THESE PERCENTAGES. AN ENTIRE PIPING SYSTEM IS COMPOSED OF A LARGE NUMBER OF COMPONENTS. THE FAILURE OF JUST ONE SINGLE COMPONENT HAS THE POTENTIAL TO SHUT DOWN THE ENTIRE PLANT OR, WORSE YET, CAUSE SERIOUS PUBLIC SAFETY PROBLEMS. IN SPITE OF THIS, PIPING IS GENERALLY CONSIDERED A LOW-TECHNOLOGY SUBJECT IN THE ACADEMIA. VERY FEW COLLEGES TEACH THE SUBJECT, LEAVING ENGINEERS TO GAIN THIS KNOWLEDGE ONLY THROUGH ACTUAL PRACTICE IN THE FIELD.

6. PIPING DESIGN ENGINEERING AND THE ROLE OF DIFFERENCE DICSIPLINES.

TO FIND OUT EXACTLY WHERE PIPE STRESS FITS IN THE PIPING DESIGN PROCESS, LET US FIRST FIND OUT WHAT PROCEDURES ARE INVOLVED IN DESIGNING A PIPING SYSTEM. A PIPING SYSTEM IS DESIGNED IN THE FOLLOWING STEPS BY DIFFERENT ENGINEERING DISCIPLINES:

A. PROCESS ENGINEERS.

PROCESS ENGINEERS, BASED ON PROCESS REQUIREMENTS AND PLANT CAPACITY, DETERMINE, AMONG OTHER THINGS, THE FLOW PATH, THE FLOW MEDIUM AND QUANTITY, AND OPERATING CONDITIONS. THEY THEN PUT ALL THIS INFORMATION INTO PROCESS FLOW DIAGRAMS.

B. MATERIAL SPECIFICATION ENGINEERS.

MATERIAL SPECIFICATION ENGINEERS ASSIGN SUITABLE CATEGORIES OF SPECIFICATIONS FOR THE PIPING SYSTEM BASED ON THE PROCESS FLOW AND REACTIVITY OF THE CONTAINED FLUID. EACH SPECIFICATION APPLICABLE TO CERTAIN COMBINATIONS OF FLUID TYPES, TEMPERATURE RANGES, AND PRESSURE RANGES. MATERIAL SPECIFICATIONS NORMALLY INCLUDE PIPE MATERIAL, PIPE WALL THICKNESS FOR EACH PIPE SIZE, THE CORROSION AND EROSION ALLOWANCES, FLANGE CLASS, VALVE TYPES, FITTING AND BRANCH CONNECTION TYPE, BOLT MATERIAL, GASKET TYPE, ETC.

C. SYSTEM ENGINEERS.

SYSTEM ENGINEERS COMBINE PROCESS FLOW DIAGRAMS, MATERIAL SPECIFICATIONS, AND EQUIPMENT DATA SHEETS TO CREATE OPERATIONAL PIPING DIAGRAMS. THEY SELECT THE APPLICABLE MATERIAL SPECIFICATION AND DETERMINE THE SIZE FOR EACH LINE BASED ON FLOW QUANTITY, ALLOWABLE PRESSURE DROP, AND FLOW STABILITY. PIPING DIAGRAMS ARE GENERALLY COMBINED WITH THE NECESSARY INSTRUMENT AND CONTROL CIRCUITS TO BECOME PIPING AND INSTRUMENT DIAGRAMS (P&IDS). SPECIAL ITEMS SUCH AS POTENTIAL TWO-PHASE FLOW AND SLUG-FLOW ZONES ARE ALSO IDENTIFIED ON THESE DIAGRAMS FOR SPECIAL CONSIDERATION IN DESIGN AND ANALYSIS. IN ADDITION TO THE P&IDS, A LINE LIST COVERING ALL PIPE SPOOLS IS ALSO CONSTRUCTED. THIS LINE LIST CONTAINS MOST OF THE DESIGN, UPSET, AND OPERATING PARAMETERS TO BE USED IN THE LAYOUT, ANALYSIS, AND FABRICATION OF THE PIPING SYSTEM.

D. PIPING DESIGN ENGINEERS.

PIPING DESIGN ENGINEERS IN COORDINATION WITH THE DISCIPLINES LIKE PROCESS AND SYSTEM ENGINEERING, THEY CONCIEVE AN OVERALL PLANT LAYOUT, PERFORM PIPING ROUTING STUDY, DETERMINE THE PIPE RACK LOCATIONS, AND PLACE THE ACTUAL PIPING AND CONNECTS TO THE DESIGNATED POINTS.

THEY LAY OUT AND SUPPORT THE PIPING BY FOLLOWING THE RULES AND PROCEDURES SET UP BY EACH INDIVIDUAL COMPANY. IN GENERAL, THREE SETS OF DRAWINGS ARE PREPARED.

THE FIRST SET IS THE SCHEMATIC PLANNING DRAWINGS, USED AS A COMMUNICATION BOARD BETWEEN DIFFERENT DEPARTMENTS. ACTIONS AND COMMENTS FROM RELATED DISCIPLINES ARE ALL RESOLVED AND RECORDED IN THESE DRAWINGS. PIPE SUPPORTS ARE ALSO RECORDED IN THIS SET OF DRAWINGS.

THE SECOND SET IS THE COMPOSITE DRAWINGS, CONSISTING OF TO-SCALE DRAWINGS OF ALL PIPES AND EQUIPMENT IN THE AREA. THESE DRAWINGS, TO BE USED IN THE CONSTRUCTION, ARE EVOLVED FROM THE PLANNING DRAWINGS.

THE THIRD SET OF DRAWINGS IS THE ISOMETRICS OF THE PIPING, USED FOR STRESS CHECKS AND SHOP FABRICATIONS.

E. PIPING STRESS ENGINEERS.

PIPING MECHANICAL ENGINEERS CHECK THE STRESSES AND SUPPORTS OF THE SYSTEMS. USING THE P&IDS, THEY DEVELOP OPERATING MODES SO THAT ALL THE EXPECTED OPERATING CONDITIONS ARE PROPERLY ANALYZED. PROPER SUPPORTS AND RESTRAINTS ARE SELECTED AND PLACED TO OPTIMIZE THE OVERALL COST AND PERFORMANCE OF THE SYSTEMS. THEY ALSO DESIGN OR SPECIFY PIPING SPECIALTY ITEMS, SUCH AS EXPANSION JOINTS, FLUE HEADS, SPECIAL CONNECTIONS, SPRING HANGERS, VIBRATION SUPPORTS, AND SO FORTH.

7. SCOPE OF PIPING STRESS ENGINEER.

PREVIOUSLY IN EARLY 50s AND 60s PIPING STRESS ENGINEERING WAS ONLY CONSISITING HOW TO CATER THE THERMAL EXPANSION, IN OTHER WORDS THEY WERE ONLY CHECKED WHETHER PIPING SYSTEM WAS ENOUGH FLEXIBLE TO ABSORB THE THERMAL EXPANSION DUE TO TEMPERATURE CHANGES AND THIS ANALYSIS WAS CALLED AS PIPE FLEXIBILTY ANALYSIS.

LATER ON, AS TECHNOLOGY ADVANCED, PIPING STRESS ANALYSIS WAS MORE THAN JUST CHECKING FLEXIBILTY, EVEN NOW A DAYS MANY ENGINEERS STILL REFER TO PIPE STRESS ANALYSIS AS FLEXIBILTY ANALYSIS. HOWEVER, THE CONCEPT THAT FLEXIBILITY IS THE ONLY CONSIDERATION IN PIPING STRESS ANALYSES CAN LEAD TO AN EXPENSIVE, AND UNSAFE, SUB-STANDARD DESIGN. FOR INSTANCE, MANY ENGINEERS TEND TO CONSIDER THAT PROVIDING ADDITIONAL FLEXIBILITY IN THE PIPING IS A CONSERVATIVE APPROACH. IN REALITY, ADDITIONAL FLEXIBILITY NOT ONLY INCREASES THE MATERIAL COST AND PRESSURE DROP, IT ALSO MAKES THE PIPING PRONE TO VIBRATION, THE BIGGEST PROBLEM AREA OF THE PIPING IN OPERATION. SINCE THE PUBLICATION OF THE 1955 PIPING CODE [10] AND KELLOGGS [3] BOOK, FAILURES DUE TO INSUFFICIENT FLEXIBILITY HAVE BECOME VERY RARE. NOWADAYS, MOST FAILURES ARE CAUSED BY VIBRATION, THERMAL BOWING, CREEP, THERMAL FATIGUE NOT RELATED TO FLEXIBILITY, STEAM/WATER HAMMER, EXPANSION JOINTS, AND SO FORTH. THESE FACTS SHOULD SERVE AS CLUES TOWARD DESIGNING BETTER PIPING SYSTEMS.

AN EXAMPLE TO ILLUSTRATE THE PIPING STRESS ANALYSIS.

EXAMPLE FIG.

FOR INSTANCE FOLLOWING LOADS AND STRESS CASES ARE TO BE CONSIDERED WHILDE DESIGNING A PIPING SYSTEM

A. SHELL DISPLACEMENT AND ROTATION DUE TO HYDROSTATIC BULGING.

THIS TEMPERATURE-INDEPENDENT DISPLACEMENT AND ROTATION WILL EXERT A GREAT INFLUENCE ON THE CONNECTING PIPING. FURTHERMORE, THE TANK NOZZLE CONNECTION IS FAR FROM RIGID. ITS FLEXIBILITY HAS TO BE ESTIMATED AND INCLUDED IN THE ANALYSIS. THEN, AFTER THE PIPE FORCES AND MOMENTS AT THE CONNECTION ARE CALCULATED, THEY HAVE TO BE EVALUATED FOR THEIR ACCEPTANCE.

B. MOMENTS AND SHEAR FORCES ACROSS FLANGES.

THE NEXT ITEM TO BE VARIFIED IS WHETHER FLANGES AND VALVES CAN MAINTAIN TIGHTNESS UNDER FORCES. CAN VALVES OPERATE PROPERLY UNDER THE INFLUENCE OF FORCES AND MOMENTS.

C. SUPPORT FRICTION.

D. THERMAL BOWING.

DUE TO TEMPERATURE GRADIIENT BOWING EFFECT DOES OCCUR AT BENDS.

E. INTERACTION WITH ROTARY EQUIPMENT.

PIPING CONNECTED TO ROTARY EQUIPMENT SUCH AS PUMPS AND COMPRESSORS THE PIPE LOAD SHELL BE IN RANGE THAT IS PROVIDED BY MANUFACTURER OF THE EQUIPMENT TO AVOID EXCESSIVE VIBRATIONS, WEAR AND OVER HEATING.

THE PIPING CONNECTED TO THE ROTARY EQUIPMENT SHALL WITHSTAND THE EFFECTS PRODUCE BY THESE EQUIPMENT SUCH AS WATER HAMMER, PULSATION AND OTHER DYNAMIC EFFECTS.

F. VIBRATIONS

USE THE PROPER SPRING HANGERS AND SUPPORTS TO CATER PIPING VIBRATIONS.

G. OCCASIONAL LOADS

PIPING SUPPPORTED STRUCTURE SHALL BE ENOUGH STRONG THAT SUSTAINS THE WIND, SEISMIC, HURRICANE AND SNOW LOADINGS.

H. AT NOZZLE

AS PIPING IS TERMINATED ON THE NOZZLE ON THE OTHER VESSEL OR ANY OTHER EQUIPMENT THEREFORE NOZZLE SHALL BE ADEQUATELY REINFORCED TO SUSTAIN THE ALOWABLE PIPING LOADS.

IN GENERAL THE PIPING STRESS ANALYSIS Is:

A. ENSURE STRUCTURAL INTEGRITY.

B. MAINTAIN OPERATION STABILITY.

8. PIPING COMPONENTS.

FOLLOWING DIAGRAM WILL DEFINE THE PIPING COMPONENTS.

A. PIPES

B. WELDS

C. WELD REDUCTION FACTORS

IN ADDITION TO THE JOINT EFFICIENCY THAT AFFECTS THE GENERAL STRENGTH OF THE PIPING, THE WELD ALSO HASTENS CREEP FAILURE AT CREEP TEMPERATURE. THE ADDITIONAL REDUCTION OF CREEP STRENGTH OVER THE NON-WELD-AFFECTED BODY IS CALLED THE WELD STRENGTH REDUCTION FACTOR. THIS IS THE FACTOR APPLIED, OVER THE JOINT EFFICIENCY, AT HIGH TEMPERATURE RANGES. THE SAME FACTOR IS APPLIED AT BOTH LONGITUDINAL WELDS AND CIRCUMFERENTIAL WELDS. HOWEVER, LONGITUDINAL WELD AFFECTS ONLY THE CALCULATION OF WALL THICKNESS, WHICH IS GOVERNED BY THE CIRCUMFERENTIAL HOOP STRESS. ON THE OTHER HAND, CIRCUMFERENTIAL WELD AFFECTS ONLY THE SUSTAINED LONGITUDINAL STRESS DUE TO PRESSURE, WEIGHT, AND OTHER MECHANICAL LOADS. THE WELD STRENGTH REDUCTION FACTOR IS NOT APPLICABLE TO OCCASIONAL STRESS DUE TO THE GENERALLY SHORT DURATION OF THE STRESS. IT ALSO DOES NOT AFFECT THERMAL EXPANSION AND DISPLACEMENT STRESS RANGE DUE TO THE SELF-LIMITING NATURE OF THE STRESS. GENERALLY, THE TEMPERATURE THAT REQUIRES THE APPLICATION OF THE WELD STRENGTH REDUCTION FACTOR STARTS FROM 950F (510C). HOWEVER, B31.1 AND B31.3 TREAT IT SLIGHTLY DIFFERENTLY.

D. FLANGES

E. BRANCHES

FORGED TEE, UNREINFORCED FABRICATED TEE, REINFORCED RABRICATED TEE, EXTRUDED TEE THERE IS A TABLE THAT DEFINES THE STRESS INTENSIFICATION FACTOR, FLEXIBILITY FACTORS.

F. VALVES (GATE VALVE, GLOBE VALVE, BALL VALVE, BUTTERFLY VALVE, NEEDLE VALVE, PLUG VALVE CHECK VALVE)

G. FLEXIBLE JOINTS (TIED BELLOW EXPANSION JOINT, HINGED BELLOW JOINTS, GIMBALED BELLOW JOINTS, BALL JOINTS, FLEXIBLE COUPLINGS, AND FLEXIBLE HOSES)

H. TERMINAL CONNECTION

I. SHELL CONNECTION

J. TUBE BUNDLE HEADER CONNECTION

K. ROTARY EQUIPMENT CONNECTION.

9. MODES OF FAILURE.