ISSN 2395-1621 Design Optimization for an Expansion Loop in Petrochemical Refinery #1 Sanjay S. Deshpande, #2 Hemant S. Sadafale, #3 Milind R. Joshi, #4 Yashodhan R. Soman, #5 Anagha A Pawar 1 deshpande.sanjay@gmail.com 2 hemant.sadafale@rediff.com 3 mrjoshi.piping@yahoo.com 4 somansir@yahoo.com 5 anagha.pawar6559@gmail.com #1234 Mechanical Engineering Department, Keystone School of Engineering, Savitribai Phule Pune University, Pune, India #235 Asian Academy Of Professional Training, K.K Market A wing,no.74-79, Dhankawdi,Pune-411043,India ABSTRACT The purpose of this study is to optimize an expansion loop in a petrochemical refinery in critical pipeline. Piping is the main transportation method for fluids from one location to another within an industrial plant. Design and routing of piping is heavily influenced by the stresses generated due to thermal effects and high pressure of the operating fluid. Pressurized fluids create critical loads on the supports and elbows of the pipe which increases the overall stresses in the piping. Moreover, long pipes operating under high temperature gradients tend to expand significantly. Therefore expansion loop is provided in order to relieve the pipe from the critical stresses. However, expansion loops require extra space, supports, elbows, bends, additional steel structure that could adversely affect the operating cost. There have been analytical methods by M W Kellogg and McKetta method (adopted by Piping design handbook by Mcketta). The design approach is conducted as per the guidelines of ASME B31.3 (Process Piping).Adeqquate piping system flexibility requirements through forces and moments at the equipment nozzles within allowable limits must be ensured for safety. This paper critically reviews the method given in Piping design Handbook by McKetta ( Originally published by M.S.Haqueand J.Starczeweski in the Journal of Hydrocarbon processing and Petroleum Refineries, in the year 1962) and simplifies the approach for a practical user. We find that the solution given by the method in terms of movement of guide supports is identical to increasing the height of expansion loop as given by M W Kellogg method. We examine alternative approaches and justify the use of this metod and are of the opinion that considering the cost of CAE Software such as CAESAR II and standard FEA pacages like ANSYS and NASTRAN and knowledge of physics of the problem and interpretation of results, the time to solve the problem and quick managerial decisions, the present approach is simple and novel to apply for practical problems of thermal expansions. ARTICLE INFO Article History Received: 28 th February 2016 Received in revised form : 1 st March 2016 Accepted: 3 rd March 2016 Published online : 5 th March 2016 Keywords Piping, ASME B31.3 MW Kellog s method, Optimization, Expansion loop. I. INTRODUCTION Expansion loops are commonly found in Petrochemical refineries and Power plant steam piping to absorb the thermal expansion of the system and ensure adequate piping system flexibility [4],[5],[6][7]. Expansion loop in pipe is used to reduce the effect of contraction and expansion in line due to thermal expansion. Reducing the number of loops in one single system or reducing the length of the loop itself is always favored as long as stresses are within safe limits. Results indicate that optimization reduces the dimensions and the number of expansion loops as well 2015, IERJ All Rights Reserved Page1
as the total number of supports. McKetta[2],[3] method is used to find out optimal solution as it is simple in calculation widely used in industrial purpose. This results in significant savings in the piping cost without any compromise on the safety. II. PROBLEM STATEMENT The following problem has been taken for design optimization.it refers to a line carrying a hydrocarbon fluid at a high temperature in a refinery Let us consider a 4 NPS SCH 40 pipeline with operating temperature 600. Thermal expansion to be absorbed 5 inch per 100 feet, allowable stress range 20000 psi. The initial geometry of the expansion loop can be taken as per fig.1. The stress intensification factor SIF (Stress Intensification Factor) value is computed as per Appendix D of code 31.3 and the value can be taken as 1.3.The objective is to : I. Use McKetta Method and determine Forces, Moments and Stresses developed in the expansion loop. The stress range factor (f ) is computed from the equation. This is computed from the number of cycles expected within the life of the process plant, the number of cycles over a useful life of 20 yrs has been considered around 55000 thousand cycles. For4NPS SCH 40:- Moment of Inertia (I) 7.23 in 4 Section Modulus (Z) 3.21in 3 DETERMINING THE FORCES, MOMENTS AND STRESSES FOR THE EXPANSION LOOP FOR ORIGNAL GEOMETRY Step 1- α α α β β β 1 α 1 and β 1 II. III. Discuss whether the present expansion loop is safe or not and suggest suitable remedies for making it safe. Use McKetta method to optimize the geometry of expansion loop and suggest suitable changes and justify them as a Piping Design Engineer. 100ft Fig 1 Original Geometry of the expansion loop III.GIVEN DATA AND ASSUMPTIONS Distance between two equipments 100 ft Height of expansion loop H 6 ft Width of expansion loop W 6 ft Distance between guide supports 18 ft i.e the guide supports are located at a distance of 6 ft from the corner of expansion loop. This is denoted by G Material of construction : A53 Gr B. Pipe Size 4 NPS SCH 40 Installation reference temperature( Cold condtion ) Operating temperature ( Hot condition ) 600 Thermal Expansion 5inch per 100feet Stress Intensification factor (SIF) 1.3 Allowable Stress range as given by the allowable stresses for the cold condition and hot condition can be taken from ASME 31.3 process piping code. 0.68 (1.25 x17300 +0.25 x 20000) 20000psi Fig 2:- Force graph for U type expansion loops (Force F in lbs) Value of Force corresponding to above parameters in the Force graph for U type expansion loops can be found out from the graph. This is a staircase type solution as indicated in the figure where we proceed in the sequence as β to α to b to H to F/I value. Force calculation:- α 1, β 1, H 6feet Thermal Expansion ( ) 400 lb f F 2892 lb f. (Graph) 2015, IERJ All Rights Reserved Page2
Moment calculation:- α 1, β 1, H 6 ft & F 2892 lb f Value of Moment corresponding to above parameters in the moment graph for U type expansion loops can be found out from the graph. This is also a staircase solution in which one proceeds from parameters βtoαto H force to get the Moment M as shown in Fig. [3] this means the design is not safe, this also means that the Expansion loop is not able to take the Thermal Expansion of 5 inch for which the developed stress is much larger. Now we propose different solutions for solving the problem practically for making the design safe. IV. DIFFERENT SOLUTIONS TO SOLVE THE PROBLEM SOLUTION 1 - INCREASING THE HEIGHT OF EXPANSION LOOP We can find out height by M.W. Kellogg method [1] L 18 ft k 1 L 6 ft k 1 0.34 k 1 0.34 Let us determine the value of the parameter and k 1 to get the value of k 2 from the graph. 0.028 Fig 3:- Moment graph for U type expansion loops (Moment in lb f in) M 80000 lb f in. (Graph) Bending Stresses calculation:- The maximum bending stress developed in the elbow is given by the expression Where (i) is the SIF (Stress Intensification Factor) 32398 psi Allowable Stress (S A ) 20000 psi Developed Stress (S B ) 32399 psi S B 32398 psi> S A 20000 psi Thus we observe that value of the Bending Stress is much larger as compare to an Allowable Stress range of 20000 psi Fig 4:- M W Kellogg Chart for determining the required height for Symmetrical Expansion Loop From the graph value of k 2 corresponding to k 1 and is k 2 0.6 H k 2 L 0.6 18 H 10.8 ft 2015, IERJ All Rights Reserved Page3
We can modify the design for the Expansion Loop to undertake a thermal expansion of 5inch by increasing the height from 6 ft to 10.8 ft. In this Design we have to weld three unequal pipe pieces as shown in fig. 5 Fig:-5 M W Kellogg solution and modifying the expansion loop by increasing the height. SOLUTION 2 : USE OF TWO EXPANSION LOOPS A second solution that is costlier but can be employed is to use two expansion loops with each expansion loop absorbing thermal expansion of 2.5 inch each. Let us now find out the forces, Moments & Stresses Determine whether the design is safe or not. calculations:- By following the same staircase solution but with only change of 2.5 in thermal expansion,we find that 210 lb f F 1518 lb f M 40000 lb f in The bending stress can also be computed as follows, SOLUTION3 : MOVING THE GUIDE SUPPORTS It is easy to observe that optimum fabricated expansion loop will have 3 equal pipe pieces which implies that the value of β is always fixed to unity i.e b 1.0) but the guide support can be movable and adjusted to absorb the required thermal expansion. Now we use chart for determining the location of the guide support. We suggest a very simple and novel understanding here so that even a junior design engineer can use the method. As per Original Geometry: - α 1, β 1 & γ 0.47 Where γ is the moment or stress ratio factor. We will call this as γ and this value ( 0.47) is based on the original Geometry of the Expansion loop Let us denote this by Let us compute the value of γ for the original geometry from the stress considerations That is we have to reduce the factor as the bending stress or moment is higher than that of the allowable moment or stress range. With, 0.29 & a fixed value of σ b σ b 16199 psi Allowable Stress (S A ) 20000 psi Developed Stress (S B ) 16199 psi S B 16199 psi< S A 20000 psi Thus we observe that value of the Bending Stress is lesser as compared to an Allowable Stress range of 20000 psi and this means that the design is safe. But the cost of fabrication is increased and this not acceptable to Process Engineer because the pressure drop will be on the higher side due to increase in the number of elbows from 3 to 6. Fig 6:- Graph for design optimization of expansion loop. From Fig 6 we get a value of which we will call as Fig. 6: Use of two expansion loops 1.8 2015, IERJ All Rights Reserved Page4
Now H 6 ft 6 ft Thus we observe that this method works to be very fast and provides a very effective solution which is as same as the height required for the expansion loop as worked by the M W Kellogg method. The final optimize Geometry of the expansion loop is given below:- 100ft Fig 7:- Optimal Design of Expansion loop V. COMPARISON OF THE SOLUTIONS Sr No. Solution Remark 1 Increasing the height of 3 unequal pipe pieces expansion loops 2 Two expansion loop Higher Pressure drop and high fabrication cost 3 Optimal Solution Easy to fabricate with 3 equal pipe pieces building up of the domain knowledge of the field of Piping. He is also thankful to every member of the academy for arrangements and also to his family members who can understand his passion for engineering and made the life enjoyable and meaningful. REFERENCES [1].M.W. Kellogg Company, Design of the piping system, Martino Fine Books,2007 [2]John J. Mcketta (ed), Piping Design Handbook, CRC Press,1992 [3]M.S.Haque and J. Starczeweski, Graphical shortcuts to Pipe Stress Analysis, Hydrocarbon Processing and Petroleum Refineries, 1962, p. 135 [4]Mohindar L.Nayyar,Piping Handbook, McGrawHill, 7 th edition,1999 [5]G.K.Sahu, Handbook of Piping Design, New Age International Publishes, 2014. [6]Americal Society of Mechanical Engineers, Process piping code 31.3, Version 2010 [7]Sanjay S Deshpande,Notes on Piping stress analysis for industrial training programme for various companies like ESSAR steels, Thermax, Larsen and toubro etc, Asian Academy of Professional Training, Pune, 2012. VI. CONCLUSION We have presented here a critical assessment of all possible solutions and also introduced a simple and novel nomenclature for the McKetta method. It is expected that amidst the use of somewhat blind use of FEA and CAE technology, the method of design optimization presented here is fast, simple and most important is reliable. The practical piping design engineers working in power plants, and process industry and petroleum refineries will benefit from this approach. ACKNOWLEDGEMENT Here the first author would like to thank all senior colleagues at the academy especially Prof. M R Joshi for his continuous moral support and made his life enjoyable at the academy. There were lot of technical thought sharing process on what is to be included as a syllabus content for the industry training ptogrammes.he would also like to thank the pioneers Mr. P. S. Joshi and Prof. Dr. S.M.Gadgil from whom he learnt a lot of API related inspection and process fluid mechanics and also gave the opportunity to the author for several corporate trainings such as stress analysis, understanding the B31.3 code, Piping materials, Fitness for Service,steam engineering and block and isolation valves which always contributed to the 2015, IERJ All Rights Reserved Page5