Slug Periodicity:
Slug Periodicity can be defined as the time interval for two consecutive slugs hitting the same elbow. So mathematically it can be denoted as, Slug Periodicity = (Length of Liquid Slug + Length of Gas Slug)/Velocity of Flow.

Let’s assume that the calculated slug duration is 8 milliseconds and periodicity is 400 milliseconds as shown in Fig. 3. We will use these data for generation of spectrum profile.

Now Refer Fig. 4 and input the data as mentioned below:

Fig.4: Generation of Spectrum Profile for Slug flow Analysis

When you click on Enter Pulse data it will open the window where we have to enter the data for spectrum profile generation. From the above curve at time 0 the force is 2120 N the same force will be active for next 8 milliseconds till the slug crosses the elbow. Then at time 8.1 forces will be reduced to zero. And the same zero force will be there till 400 milliseconds. Then the next cycle will start. i.e, at time 400.1 seconds the force will be again 2120 N. That way enters data for at least two cycles as shown in Fig. 5:

Fig.5: Typical Spectrum Profile for Slug flow Analysis

Points to consider while checking a stress system using Caesar II:

In most of the organization there is a 3 tier process for every stress system for maintaining quality of analyzed stress systems. Normally stress system is performed by one (junior or senior), checked by some other (must be experienced enough) and finally approved by the lead stress engineer. Even though the main points which need to be considered is well-known to every piping stress engineers, but still some important points could be missed at the specific moments during stress analysis or checking. So a checklist can be prepared and referred during the process for proper quality control. The following article will provide an insight of the main points which a stress engineer must check during analyzing a system. Request you to inform me additional points which i may have missed while writing this article by replying in comments section.

Important points to consider while checking any stress system:

1. Whether the input for pipe material, pipe diameter, pipe wall thickness, pipe temperatures (operating, design and upset), pressures (design and hydrotest), insulation thickness, corrosion allowance, fluid density, insulation density is correct?

2. Whether the input for above design parameters for equipment and nozzles are correct?

3. Whether SIF’s for Tee, bend/elbow, cross and trunnions are taken correctly?

4. Whether flanged elbow is considered where required?

5. Whether actual weight of control valves/non standard rigid items/valve actuators are considered appropriate?
6. Whether equipment has been modelled with correct dimensions from general arrangement drawing?
7. Whether trunnion modelling is done following inhouse work instructions?
8. Whether settlements/displacements have been considered where required? Normally settlement is used for storage tanks and thermal displacements are used for compressors, turbines and packaged items?

9. Whether proper parameters have been used for seismic and wind analysis?
10. Whether friction has been included when significant?

11. Whether the expansion stress range has been checked in between maximum and minimum temperatures for which the piping system will be subjected?
12. Whether the effect of friction on sliding support loads been considered?
13. Whether the use of low friction pads been properly marked if used?
14. Whether the analysis is performed for the system with and without friction to check the effect of friction (to determine the worst case) as friction is not something that can be relied on? The harmful effects of friction need to be considered but not the benefits.
15. Whether the Caesar plot and isometric plot are matching with 3D plot?
16. Whether the loads on connected equipment are within allowable limit?
17. Whether the thermal effects of pipe supports, equipment supports been considered?
18. Whether the flange weight includes weight of bolting? In large size piping bolt weights become significant?
19. Whether all possible load cases (start up, shutdown, regeneration, any special process consideration) are considered in analysis?
20. Whether proper ambient temperature is used for the location?
21. Whether spring are modeled properly and selected considering all operating temperature cases?
22. Whether adequate documentation in case of gapped restraints (or any special consideration) are mentioned in isometric clearly to assure that supports will be installed in that manner in construction site?
23. Whether there is a possibility of elastic follow up or strain contentration condition?
24. Whether radial thermal expansion has been considered for line sizes greater than 24 inch NB?
25. Whether hot sustained check has been performed?
26. Whether pressure thrust has been considered while using expansion joints?
27. Whether a flanged elbow has been considered?
28. Whether sustained deflection and thermal displacements are within limit specified by project document?
29. Whether the SIF limitation been considered for large D/t piping?
30. Whether pressure stiffening of bends has been considered in analysis?
31. Whether flange leakage has been performed as per specification?
32. Whether change in pipe length due to internal pressure has been considered?
33. Whether all stresses are are within code limits?
34. Whether variability of springs is within 10% near rotary/critical equipments and 25% for others?
35. Whether thermal displacements more than 50 mm are marked on isometric?
36. Whether support loads are checked and discussed with layout/design?
37. Whether feasibility of all supports has been checked?
38. Whether routing change and special support requirements has been clearly marked in stress isometric and informed to layout/design group?
39. Whether spiders are modelled properly at appropriate intervals for jacketed pipes?
40. Whether weight of hot tapping machine and related equipments are considered in specific situations?
41. Whether alignment checking (WNC file) has been performed for all rotary equipments as per API RP 686?
42. Whether PSV forces are considered for open discharge PSV systems?
43. Whether Hot-Cold and Operating-Standby philosophy has been used when required?

Rigid Strut :

Rigid Strut

This article will explain the step by step methods for modeling the Rigid Strut using software Caesar II.

1 - Find out the direction in which restriction of movement is required (Assume X direction) and location of the strut installation. For reducing thermal loads to be carried by rigid struts it is preferable to choose thermal null points if feasible.

2- Double click on restraints check box in Caesar spreadsheet and model restraint X with 0 mm gap and with no friction. Keep stiffness K1 box blank.