Stress Analysis Training - Tank Bulging.pptx

DETAIL DISCUSSION ON TANK BULGING EFFECT

1. Prepared By: STRESS ENGINEER Dated: Piping Stress Analysis

2. Contents Basis for deciding Stress critical lines What is the Tank Bulging? How Tank Bulging is calculated?

3. Stress analysis is a complex task and in any process unit there are a huge number of lines exist which runs from one location to other. Analysing all lines will take a lot of time which in turn will increase the engineering time and corresponding cost. So every engineering organisation in this field has set up some guidelines for deciding which lines are to be stress analysed using software (Caesar II, Autopipe , Caepipe or Rohr II). The main factors which decide stress critical lines are as follows: Line design/operating/upset temperature Equipment connection Pipe and Equipment material Pipe condition Pipe thickness Design/Upset pressure Basis for deciding Stress critical lines:

4. Every organisation has their own guidelines and the guidelines vary from project to project. The following write up will provide few criteria for deciding stress critical lines. This is only a idea of how the differentiation occurs. Mostly the critical lines for which stress analysis is to be performed by formal computer analysis consists of the following lines: All Pump (Centrifugal-API/ANSI, gear pump, Screw pump) suction and discharge piping (4 inch and larger). Centrifugal Compressor inlet and outlet piping. Lines to and from steam generators. Reciprocating pump and compressor suction and discharge piping. Piping requiring expansion joints or other proprietary expansion devices. Steam and Gas Turbine inlet and outlet piping. Air Cooler inlet and outlet piping (3 inch and larger). Process Heater inlet and outlet piping Lines classified as category M as per ASME B31.3. Piping subjected to high cyclic temperature conditions. All jacketed lines. Basis for deciding Stress critical lines:

5. Lines that require nozzle load compliance as stipulated per applicable codes or equipment Vendor allowable (Heat exchanger, Pressure Vessel Connected systems). Lines subject to dynamic loading (relief lines, line with large pressure drop at control valves, surge pressure, slug flow, churn, two phase flow, water hammer, flashing, etc.) All Fiberglass, aluminium alloy, refractory or elastomer lined piping. All piping systems connected to FRP, plastic, glass lined steel or brittle equipment Lines subjected to non-thermal movements (Expected differential settlement between structures, structure-equipment, etc., process equipment growth, header growth, tower growth or other significant displacements, etc.) All lines 8” and larger operating above 150 deg. C (300 deg. F) and greater. All lines 20” and larger operating above 80 deg. C (200 deg. F) and greater. All lines 36” and larger. All lines operating below -45 deg. C (-50 deg. F) which require special “cold” supports. All plastic lined piping systems. Special attention shall be given to add enough additional supports to limit the external forces and moments in the flange connections to avoid an extra risk of flange leaks Lines with special design requirements All Safety pressure relieving systems 4 inch and larger Basis for deciding Stress critical lines:

6. Lines judged by the lead piping engineer/stress engineer as not having sufficient inherent flexibility In addition, the piping effects of other conditions such as temperature gradients that could cause thermal bowing or where piping is connected to equipment with significant thermal growth may warrant detailed computer analysis. For thin wall piping, if the D/T ratio exceeds 100, following requirements are applicable: Design and support of piping systems using this specification should be reviewed by a stress engineer. Support and spans of thin wall piping systems are not covered by current Project practices and therefore must be designed for each application. Stub-in connections per 304.3.2 thru 304.3.4 of ASME B31.3, are not allowed for run pipe with D/T greater than or equal to 100 and the branch diameter is greater than one half of the header diameter. Lines connected to non-ferrous equipment. Underground process lines with more than 30 degree difference in between design and ambient temperature. All vertical lines connected to vertical vessels that require pipe supports or guides from that vessel. Basis for deciding Stress critical lines:

7. All lines 4 inch and larger subject to external pressure or vacuum conditions. All lines subject to vibration, as specified by Process, due to high velocity flow, high pressure drop, water hammer or mixed phase flow. All lines that are connected to equipment constructed of thermoset or thermoplastic materials or that is glass, refractory, or elastomer lined. All pressure containing non-metallic lines. All flare line headers Lines for which an Alternative Leak Test has been specified. Basis for deciding Stress critical lines:

8. Stress analysis of lines connected to API tanks is very critical. Most of us have done stress analysis of lines connected to equipment nozzles. However when it comes to tank nozzle, there are some differences, due to which the approach followed for equipment nozzle cannot be followed. In the Stress analysis of lines connected to normal Equipment nozzle (Vessel, Column, Heat Exchanger etc.), generally there are only 2 things which we have to account during Caesar modelling. Nozzle’s thermal movements, and Nozzle flexibility But in additional to those two things, there are two additional points which we have to account in the Caesar modelling during analysis of tank connected piping system. These are, Nozzle rotations due to tank bulging, and Tank settlement About the first two, i.e.  Nozzle’s thermal movements and Nozzle flexibility, we all are well aware. Tank Bulging Effect

9. In case of tank, tank is filled with liquid. This liquid has varying height. Due to this, there is varying liquid pressure on tank wall. It has more pressure at bottom. Due this, tank wall try to expand more at bottom But the bottom plate prevents this expansion and holds the bottom end of shell in position. Due to this, actual shape of tank is formed similar to as shown in Fig. 1. This is called bulging of tank shell. We will see other two effects, about which we may not be aware, or if aware, not very clear how to model these in Caesar and take care of these along with Nozzle’s thermal movements, and Nozzle flexibility. So first in the current presentation we will see Nozzle Rotation due to tank bulging. What is the Tank Bulging?

10. What is this Tank Bulging? Fig . 1

11. Due to tank shell bulging, the nozzle on the shell moves radially outward, and rotates in vertical plane, depending upon their position. The nozzle on lower portion of the tank rotates downwards whereas nozzle on upper portion rotates upwards. This effect is not seen in other equipments , mainly because Equipment diameter is relatively much small (up to 3 m). Therefore the amount of radial growth is much less. Whereas tank diameters are generally large, of the order of 10 m to 60 m. Due to this the amount of radial growth is significant. Also, equipment has internal pressure, not only pressure due to fluid weight. Thus pressure variation from top to bottom is not so much where as in tank, pressure on top is zero. At the same time, the bottom of equipment is not flat like tank, which does not deflect but acts like stiffener, to holds the shell ends. However the main difference is due to tank diameter only. What is this Tank Bulging?

12. Confidential Property of Rishabh Software Pvt Ltd What is this Tank Bulging?

13. In the design code API 650, which governs the design of tank, this bulging effects is covered in Appendix – P. This Appendix – P is mandatory for tanks greater than 36 m diameter and for tank with diameter 36 m & below, it is optional or mandatory only if specified by purchaser. The intent of 36m diameter condition is to inform the user that the bulging effect is significant in large diameter tanks, which code has considered as above 36m diameter, hence put as mandatory. For smaller diameter it is considered as insignificant, hence kept as non-mandatory. Calculation of Radial movement and rotation due to tank bulging is provided in API 650. How Tank Bulging is calculated?  

14. How Tank Bulging is calculated?   If you calculate the outward radial movement and rotation using the above formulas it can be found that the effect of tank bulging on nozzle at higher elevation is insignificant. Due to bulging, nozzle at lower levels rotates downward. This causes pipe to move vertically downwards.  To minimize the amount of this movement: Piping shall be rotated through 90° as close to the tank wall as practical. 2D (D=outer diameter of pipe) spool may be provided to avoid elbow stiffening due to flanged elbow.

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