Analysis of Piping System Used In Chemical Plant

Transcription

1 1 Analysis of Piping System Used In Chemical Plant Brijesh M. Bhavsar, Design Engineer, Reliance Industries Ltd, Navi Mumbai Abstract Piping System is a network of Pipes by using Pipe Fittings and other special components to perform the required mode of transferring fluids (Liquids/ Gas/ Slurry) from one location to another location. Designing of piping systems are governed by Industrial/International Codes and Standards. Piping codes defines the requirements of design, fabrication, use of materials, along with tests and inspection of piping systems and the standards are more on defining application design and construction rules and requirements for piping components. The basic design code used in this paper is ASME B31.3. This code includes Process piping code for petroleum refineries, chemical plants, textile plants and paper plants. Stress in pipe is also main concern while designing any piping system. Stresses in pipe or piping systems are generated due to loads like expansion & contraction due to thermal load, seismic load, wind load, sustained load, and reaction load etc. In this paper, methodologies for the designing the piping systems within chemical plant is explained. The analytical study of piping systems is done for loop length derived by using Guided cantilever method. The configuration for loop is made using M. W. Kellogg method. The piping system is modelled and analyzed in CAESAR II software. Optimum loop configuration is selected on the based on result derived from above methods. Index Terms Stress analysis, ASME B 31.3, Expansion loop, CAESAR II P I. INTRODUCTION ipelines are the most common means of transporting any kind of fluid. To connect the various process and utility equipment contained within a process plant, it is necessary to use an assortment of piping components that, when used collectively, are called a piping system. The individual components necessary to complete a piping system are Pipes, piping fittings, Valves, Bolts and nuts (fasteners and sealing), special items, like steam traps, pipe supports, and valve interlocking. These pressure-containing and non-pressurecontaining components combine to form the ingredients of a piping system. [7] Pipe is the main artery that connects the various pieces of process and utility equipment within a process plant. Although Pipe can be consider least complex component within a piping system, but it is not without knowing its importance peculiarities. Pipe used within a process plant are designed to one of the ASME B31 codes. Generally it is of a metallic construction, such as carbon steel, stainless steel etc and Nonmetallic pipe such as one of the plastics like PVC, glass reinforced epoxy, or glass-reinforced plastic, each has its own set of characteristics. Circular in shape, pipe is well-known in the various industry codes, standards, and specifications as a nominal pipe size (NPS), in U.S. customary units, or in diameter nominal (DN) metric units, or in nominal bore (NB) with a wall thickness quoted in one of the following ways: Standard weight (STD), extra strong (XS), double extra schedule (XXS). Carbon steel pipe in schedules, SCH 20, SCH 30, SCH 40, SCH 60, SCH 80, SCH 120, SCH 160. Stainless steel pipe in schedules, SCH 5S, SCH 10S, SCH 40S, SCH 80S, SCH 160S. Pipe and pipe fitting components complement straight pipe, and within a piping system, they must be chemically and mechanically compatible. Pipe fitting components are used for one or more functions. For change of direction we use 90 degree or 45 degree elbows, for reduction in pipe size we use eccentric and concentric reducers, swages, for reduction in pipe size and change of direction we use reducing tee or equal tee respectively, for pipe joints we use flange, coupling, union, Reinforced branch fitting like Weldolet, Sockolet, and Threadolet etc. Pipe fittings used for projects designed by ASME code and are made to standard dimensions, based on their size and wall thickness. These fixed dimensions are essential to allow a piping designer to layout or route the piping system efficiently. All these piping components can be connected together by several welding and mechanical methods like butt-weld, socket weld or threaded ends, flanges with bolts, nuts and gaskets. [7] II. LITERATURE REVIEW Suyog U. Bhave, have focused in his paper on the, calculation methodologies developed in order to do quick analysis of the most common loop configurations, according to the codes like ASME B31.3, allowing improvements on the flexibility of the projected piping systems. His method was used to calculate the forces and the moments due to thermal expansion using Spielvogel Method, it is more versatile than other methods which are dependent of tables and charts. Pipe loops are a very efficient way to increase system s flexibility. He have concluded that from the different configurations, for pipe loops, the best is the one that follows the relation 2=2 3.Where L2 is the Height and L3 is width of the loop. [1] N.A.Alang, N.A.Razak, have studied the effect of metal loses due to corrosion on burst pressure of steel pipes using nonlinear finite element (FE) method. The nonlinear finite element method coupled with stress modified critical strain model was used to predict the failure of the pipes. In their paper, the corrosion defects were simplified to rectangular shape. The paper has presented the effect of longitudinal corrosion defects on burst pressure of API X42 steel pipes. The results obtained are as follow: The burst pressure of corroded pipelines is affected by the length and depth of the defects. The depth of corrosion defect is more influential parameter that would affect the burst pressure of pipes. The

2 2 width of the longitudinal corrosion defects affect insignificantly on burst pressure. [2] Navath Ravikiran, V. Srinivas Reddy, G. Kiran Kumar, have discussed about the flare line connected from equipments to the knock out drum. In their paper stress values, forces and deflections are analyzed at each node to make the design safe. The stress analysis is done by help of software like Caesar II. And a Flare pipe line is designed and 3D model was prepared in PDMS software. They concluded that Flare pipeline between equipment s to knock out Drum is safe in design. As per ASME 31.3 Knock out drum nozzles are within the Allowable stress range, Nozzle loads, Restraint loads, all are within the limits after providing an expansion loop as per standards. [3] Vansylic Israel Pintu, Dr. Manivannan and Jeremiah JothiRaj, have focused in their paper about cryogenic piping circuit to handle the Liquid hydrogen. Cryogenic fluid servicing pipelines are tend to develop thermal stress due to contraction or expansion of piping material during chilling and warming from ambient to cryogenic temperature or vice versa. It consists of piping elements like expansion joints and loops with optimal placement of supports. This paper mainly discusses about the thermal stresses produce in the piping circuits when liquid hydrogen flows through it and how these stresses can be reduced by incorporating various expansion loops. They have designed three piping system and selected optimum from them, based on lowest stress values induced in the piping system. Thus the structural analysis done by them provides flexibility. From the graphical results, they concluded with the best suited pipe structure among the three analyses. From the three pipelines with goose neck design gives the lowest stress value when compared with other piping system. [4] Dr. D.P. Vakharia, Mohd. Farooq, have discussed about straight cross-country pipelines which are supported throughout the length of pipeline on different forms of supports at regular spans. Maximizing the distance between supports will minimize the number of supports required, which in turn reduce the total cost of erecting these pipe supports. They tried to maximize the distance between supports keeping the values of stresses and deflection within safe limits. They have concluded by reducing the number of supports which indirectly reduce the total cost of erection. [5] Payal Sharma, Mohit Tiwari and Kamal Sharma, have focused in the paper about the basic design code used for Process piping like petroleum refineries, chemical plants, textile plants, paper plants and semiconductor plant. The objective of their paper was to explain the basic concept of flexibility such as flexibility characteristics and flexibility factor, and also stress intensification factor (SIF) referring to the code. They have made use of CAD Packages like CAEPIPE for the comprehensive analysis of complex systems. This software makes use of Finite Element Methods to carry out stress analysis. They have concluded by saying that CAEPIPE gives more accurate and precise results as compared to other software. They have described in brief about the advantages of CAEPIPE software. [6] III. OBJECTIVE The carbon steel ERW pipe of ASTM A56 pipe containing ammonia fluid at 450 F has been routed on pipe rack between the distillation column and process equipment. The objective of our paper is to reduce the stress induced within the pipe by providing expansion loops to absorb the stress. Different configuration was derived and optimum loop has been selected from them. Finite element analysis is done using Caesar II software by dividing pipe into 300 node points. Various configurations were also analysed and bending stress is compared derived from the software results. In analysis we refer pipeline as line for ease of understanding. Optimum expansion loop configuration is selected after results derived from analytical and software results. A. Methodology The flow chart shown below describes the process carried out in day today piping industry. This paper has focused the similar methodology to route the ammonia line and providing optimum expansion loop to reduce the stresses induced within the pipe. Analytical calculations are performed using Guided cantilever method to decide the expansion loop length. Different Loop configurations are derived using Kellogg method, and optimum loop configuration is selected from them. Similar configuration of pipe is modelled and analysed using Caesar II software. Figure 1: Work flow Chart B. Importance of Stress Analysis Pipelines were initially used in agriculture, due to the growing need for cultivation. Piping systems also had a vital role in the development of big cities especially during the industrial revolution with the discovery of steam power systems. Piping engineering turned out to be important in the exploration of oil from earth surface. In the present stage, piping systems are constantly present, either in residential and commercial buildings. In oil refineries and others industrial plants, pipelines represent between 25% and 50% of the total cost of the facilities. Since piping systems are allied with facilities of high degree of responsibility, stress analysis represent a

3 3 fundamental stage of the piping design, in order to prevent failures and cause of accidents. We define pipe stress as force per unit area applied to cross section of piping component. There are many causes of pipe stress in the piping system. The two most common causes are weight and thermal load. These two causes are also common reason for loads on equipment nozzles. Weight: It causes the pipe to sag, which put stress on the piping material and forces onto the equipment nozzle. Proper spacing and design of supports and careful attention of concentrated load can take care most of the problem. Thermal: When piece of pipe gets hot, it grows. The pipe itself become physically longer as the temperature of pipe material gets hotter. As the pipe grows it pushes against the nozzle and supports which restrain it from moving. With an improperly stress analyzed system this pushing will cases pump bearing to wear out quicker, vessels nozzle to leak and perhaps even pipe or vessel themselves to rupture. Also restrain of the growth causes pipe to deflect in the direction different from unrestrained pipe, This "Unnatural" deformation causes addition stress in the pipe. Even in present days, pipe stress analysis covers much more than flexibility analysis, it still is one of the main tasks of the engineers that work in this area. Many times due to the absence of a quick method that allows a verification of the flexibility of projected systems, system turns out to be too stiff or too flexible. Stiff pipe will increases the stress in the pipe and too flexible pipe will cause sagging in the pipe which is also not desirable. [9] Neglecting the stress occurring in the pipe, the impact of high piping stress on operating piping system can be dramatically costly. A thoroughly analyzed plant will last longer and be more cost effective. Neglecting the impact of weight of pipe and thermal expansion of hot pipe can cause significant maintenance problem. Some of the typical examples of typical maintenance problem due to high piping stress are Pump bearing wearing out. Hairline crack developing in vessel and nozzle junction. Flange leaking flammable liquids. Permanent deformation in Pipe. The various codes and standards covering the design of piping system puts a limit to maximum stresses which the system can be subjected when pipe is loaded. This limit is said to be the allowable stress range for expansion. Stress analysis shall be performed according to ASME B 31.3 Para A general guidance, for a line which shall be subject to Stress analysis if it falls into any of the following categories: 4" NPS and larger at design temperature above 130 C. All lines at design temperature above 180 C. 16" NPS and larger at design temperature above 105 C. All lines 3" NPS and larger connected to sensitive equipment such as rotating equipment. However, lubrication oil lines, cooling medium lines etc. for such equipment shall not be selected for the analysis. All piping subject to vibration due to internal forces such as flow pulsation or slugging or external mechanical forces. All relief lines connected to pressure relief valves and rupture discs. Lines subject to external movements, such as abnormal platform deflections, bridge movements, platform settlements etc. [9] IV. PIPING FLEXIBILITY Flexibility denotes the measurements of presence of necessary piping length in proper direction. The purpose of piping flexibility analysis is to construct a piping layout that causes neither excessive stresses nor excessive end reactions. To accomplish this, piping layout should not be stiff. It is also not advisable to make the system unnecessary flexible because this requires excess materials, which increases initial cost. More length with many bends increases pressure drops, which increases the operating cost. We know that the pipe expands or contracts due to temperature changes in the pipe. When a pipe expands it causes the entire piping system to make room for its movement. This creates forces and stresses in the pipe and on its connecting equipment. If the piping system does not have adequate flexibility to absorb this expansion, the force and stress generated can be large enough to damage the piping and the connecting equipment. When the temperature of straight pipe with two end having anchor changes, it causes the pipe to expand. However, the anchors at the ends prevent it from expanding. The resistance of the anchor develop force on the anchors from which the same force is reflected back to the pipe. There is no force or stress generated in the free expansion state. However, if there is neither of ends loose in two anchor case, the force generated on the anchor is equal to the force required to push the free expanded end back to its original position. [8] Therefore piping system should have sufficient flexibility, so that expansion or contraction or movements of supports and terminal points will not cause: (i) Failure of piping or support from overstress or fatigue. (ii) (iii) Leakage at joints. Detrimental stresses or distortion in piping or in connected equipment (pumps, vessels, or valves) resulting from excessive thrusts or movements in piping. A. Methods of Providing Flexibility There are two main categories of methods for providing piping flexibility: the flexible joint method and the pipe loop method. Flexible joints, including expansion joints, ball joints etc. Pipe loop method is to provide expansion loop within the piping system. We know that the huge thermal expansion force and stress on an anchored straight section of pipe are the result of squeezing the free expansion of the pipe. This is very difficult,

4 4 as we can experience by squeezing the ends of a wooden stick. Instead of this direct squeezing, we can absorb the same amount of movement in much simple way by bending the stick sideways. This is the principle of providing piping flexibility. [8] The flexibility is provided by adding a portion of the piping that runs in the direction perpendicular to the straight pipe connecting two terminal points. Figure shows an expansion loop used in a long straight pipe run. Adding the loop, the pipe expands into the loop by bending the loop length instead of squeezing the pipe axially. The longer the loop length, the lesser the force generated in absorbing a given expansion. From the basic beam formula, we know that the required force is inversely proportional to the cube of the leg length, and the generated stress is inversely proportional to the square of the leg length. A small increase in loop leg length has a considerable reduction effect on force and stress. [8] Total length of Pipe l = 60 m Deflection in Pipe at 450 F is, ε = 2.63 x 10-3 m [8] Total deflection within the pipe, δ = l x ε δ = x 2.63 x 10-3 = 160 mm σ allowable = psi = N/mm 2 Now by Guided cantilever method, expansion loop length to be provided within the pipe to avoid deflection due to stress is, Let L be expansion loop length [8] L = L = L = mm = 12 m Our answer confirms with the Nomograph chart to determine the loop size. Now by M.W. Kellogg Method, Figure 2: Expansion loop Loops provide necessary leg to piping in perpendicular direction to absorb the expansion. They are safer as compared with expansion joints but take more space. Expansions loops may be symmetrical or non symmetrical. Symmetrical loops are advantageous to use because leg H is used to absorb an equal amount of expansion from both side. Non symmetrical loops are used to utilize the existing support steel or to locate the loop over the road crossings. Vertical direction supports are provided to support the gravity weight at calculated span. When several piping loops are laid side by side on pipe rack, the size of loop may be modified to lay the loops one inside other. But the final size of the loop must be larger than the calculated bend length. Hotter and larger lines are placed outside as outer loops because longer height is needed. Smaller lines with lower temperature are placed as inside the loops. Because this loop arrangement may change entire pipe rack layout, it is advisable to estimate the loop s size at early stage of project. Guides on both sides of loop are important for proper functioning of the loop. Guide support directs the expansion into the bend along the axis of pipe. This avoids the shifting of lines in sideways. A practical problem often encountered in the interference when sufficient gap was not provided for the guide in the design. V. DESIGN AND ANALYSIS TOOL A. Analytical calculations for Loop length The Carbon steel pipe of 6in outer diameter with pipe thickness 40 SCH at temperature 232 deg C is analyzed for 60 meter pipe length. Pipe diameter, d = mm Figure 3: Loop configuration using M.W. Kellogg Method Considering case (i), W = H Let W = 6000mm and H = 6000mm K 1 = W / L = 6000 / K 1 = 0.5 K 2 = W / L = 6000 / K 2 = 0.5 L C = 0.5 x L x (1 K 1 ) L C = 0.5 x x (1 0.5) L C = 3000 mm Stress within the loop of by case (i) configuration is σ 1 = (E d δ) / L 2 σ 1 = x 10 6 N/m 2 Considering case (ii), H = 2W Let W = 4000 and H = 8000 K 1 = W / L = 4000 / K 1 = 0.33 K 2 = 2W / L = 8000 / K 2 = 0.66 L C = 0.5 x L x (1 K 1 ) L C = 0.5 x x (1 0.33) L C = 4020 mm = 4200 mm Therefore the resultant values by M. W. Kellogg method will be L = = mm W = 4200mm and H = 8400mm Stress within the loop of by case (ii) configuration is

5 5 σ 2 = (E d δ) / L 2 σ 2 = x 10 6 N/m 2 Therefore analytically we can see that configuration using case (ii), i.e. W = 2H reduces stress within the pipe as compared to the configuration using case (i), W = H. Therefore we will provide loop within the pipe using above configuration W = 2H and analyze the system using Caesar II software. C. Analysis Using CAESAR II CAESAR II is pipe stress analysis software. Process piping or power piping are typically checked by pipe stress engineers to verify that the routing, nozzle loads and supports are properly placed and selected such that allowable pipe stress should not exceed in different loads such as sustained loads, operating loads, pressure testing loads etc, as stipulated by the ASME B 31.3 or any other applicable codes and standards. It is essential to evaluate the mechanical behavior of the piping system under regular loads (internal pressure and thermal stresses) as well under occasional and intermittent loading cases such as earthquake, high wind or special vibration, and water hammer. In cryogenic pipe supports, most steel become more brittle as the temperature decreases from ambient or specific operating conditions, so it is important to know the temperature distribution for cryogenic conditions. Steel structures will have areas of high stress that may be caused by sharp corners in the design, or inclusions in the material. This evaluation is usually performed with the assistance of a specialized pipe stress analysis computer program. CAESAR II is a pipe stress analysis software program developed, upgraded and sold by INTERGRAPH Engineering. This software package is an essential engineering tool used in the mechanical design and analysis of piping systems. The CAESAR II user creates a model of the piping system and defines the loading conditions imposed on the system. With this input, CAESAR II produces results in the form of displacements, loads, and stresses throughout the system. Additionally, it compares these results to limits specified by recognized codes and standards. We will check the line providing different load case condition. Various types of load cases which can be can be calculated by CAESAR software are as below Sustained load: These loads are expected to be present throughout normal plant operation. Typical sustained loads are pressure and weight loads during normal operating conditions. Occasional load: These loads are present at infrequent intervals during plant operation. Examples of occasional loads are earthquake, wind, and fluid transients such as water hammer and relief valve discharge. Thermal expansion load Expansion loads are those loads which are due to displacements caused on piping system. Examples are thermal expansion, thermal contraction of pipe. Now we will model the line of 150 NB using CAESAR II, dividing lines into various numbers of nodes. The line will be analyzed for all the three load cases i.e. Sustained load, Occasional load and Thermal expansion load condition. Figure 4: Line divided into number of nodes Figure 5: Stress report of the Line If the line fails in any of the above load condition than we implement following solutions. Provide proper supporting with correct span length between the two supports. Addition of expansion loops in the line to absorb thermal expansion movements within the pipes. Addition of expansions joints, like single expansion joint (single bellow), double expansion joint etc. Re-routing of line such that it minimize the stress in the line. Increases the nozzle load of the connecting equipment. Now the line was routed providing expansion loop to absorb the stresses within the pipe. The line was supported by providing distance less or equal too standard support span of 5200mm. Line was re-analyzed for above two cases which were derived analytically. Case (i) Routing the line with loop configuration as, Width and Height are of equal length i.e. W = H. Following line was modeled and analyzed using CEASAR II software. The result is as shown in Figure 8

6 6 Figure 6: Line with Loop configuration (i) Figure 9: Line with Loop configuration (ii) Figure 7: Close view of Line with Loop configuration (i) Figure 10: Close view of Line with Loop configuration (ii) Figure 11: Stress report of Loop configuration (ii) Figure 8: Stress report of Loop configuration (i) Case (ii) Routing the line with loop configuration as, Height is twice of width length i.e. H = 2W. Following line was modeled and analyzed using CEASAR II software. The result is as shown in figure 11.

7 7 VI. CONCLUSION The objective of this paper was to develop methodologies for the design of piping systems within chemical plant. There are several codes and standards that can be used so as to assure the integrity of the system. We have analyzed the pipe using ASME B 31.3 used specifically for Petroleum refinery or chemical process piping system. According to the ASME B31.3, the stresses to which a piping system is subjected may be separate in three main classes, the stresses caused by sustained loads, the stresses caused by occasional loads and the stresses caused by thermal expansion. The paper concentrates on the stress occurring within the pipe due to high temperature fluid flowing within the piping system. The analytical study of piping systems is done for loop length derived by using Guided cantilever method. Our answer is in well agreement with the standard chart Nomograph which is used to determine loop size. The optimum loop configuration is selected on basic of M. W. Kellogg method. The system is modelled and analysed using software CAESAR II to check out its behaviour under sustained load, Occasional load and thermal expansion load condition. From the stress report generated by software and by analytical calculations, we have verified that, loop configuration having height as twice of width, within the expansion loop reduces bending stress as compared to the loop configuration having height and width of same length. Therefore using similar methodology other piping system line can be analyzes within the chemical plants. REFERENCES [1] Suyog U. Bhave, Calculation Methodologies for The Design of Piping Systems, Int. Journal of Engineering Research and General Science, Vol. 2, Issue 6, October-November, [2] N.A.Alang1, N.A.Razak, Finite Element Analysis on Burst Pressure of Steel Pipes with Corrosion Defects, International Conference on Fracture, June 2013, pp [3] Navath Ravikiran, V. Srinivas Reddy, 3D Modeling and Stress Analysis of Flare Piping, Int. Journal of Engineering Trends and Technology, Vol. 16 Number 3, October [4] Vansylic Israel Pintu J & Dr. Manivannan, Flexibility Analysis of A Bare Pipe Line Used For Cryo Application, Int. Journal Of Engineering And Computer Science ISSN: , Volume 4 Issue 5 May 2015, pp [5] Dr. D.P. Vakharia, Mohd. Farooq A, Determination of maximum span between pipe supports using maximum bending stress theory, Int. Journal of Recent Trends in Engineering, Vol. 1, No. 6, May [6] Payal Sharma, Mohit Tiwari, Design and Analysis of a Process Plant Piping System, Int. Journal of Current Engineering and Technology, Special Issue-3, March [7] Peter Smith, The Fundamentals of Piping Design, Vol.1, Gulf Publishing Company, USA, 2007, pp [8] Sam Kannappan, Introduction to Pipe Stress Analysis, A Wiley- Interscience publication, John Whiley and Sons, USA,1985, pp [9] Author Profile Brijesh Bhavsar received the M.E. Degree in Design Engineering from Dhole Patil College of Engineering, University of Pune. He has completed his B.E. degree in Mechanical Engineering from MET college of Engineering, University of Pune, in year He is working as a Piping Design Engineer from last more than four years till today s date.