005 BASIC PIPING: Pipe Stress Analysis

Main source: Teknologi dan Perencanaan Sistem Perpipaan by Ir. Raswari (Penerbit Universitas Indonesia, 2007)

Oh, it has been so long! How is it so far? Hope you guys are still with me. Okay, so now the favorite topic for most Piping personnel (is it? well, at least for me); the Pipe Stress Analysis.

In all of piping system, cryogenic system would be the least having stress. You know why? Because they are always chill! You know what I mean? Chill...no stress? No, you don't understand? It's a joke. Okay, fine. ?? Lets get down to business. ??

Pipe Stress Analysis is like a whole different division in piping engineering. That is why there are commonly 2 types (sometimes divided into 3) of engineer in Piping; that is Piping Engineer who deals with materials, layout and drawings and Pipe Stress Engineer who deals on analyzing the flexibility of the piping system. However, being in this field, it will be great if we could at least understand the basic nature of each division. It will be very helpful in our job deliverables as we could understand each other's point of view. Here, I will only cover basic concept and principle of Pipe Stress Analysis as I am not Pipe Stress Engineer myself.

Please feel freely to correct or add any information required.????

WHAT IS PIPE STRESS ANALYSIS?

In general, Pipe Stress Analysis is when we want to analyze the behavioral of the piping system based on its layout, material build up, pressure-temperature given, and fluid services they carries within the restraints/ supports that we designed.

In my personal opinion, we have to treat the piping system like a living thing. They have to be allowed to 'grow' but not too much until it could harm others that are interconnected with them or adjacent to them (nozzles, supports, structures, etc.). We also need to provide sufficient supports for them to avoid deflections due to its weight and load caused during operations, but not too much restraints until it can stress them out with too many limitations that not allowed them to expand or contract.

In short, we want the piping system to have a SAFE FLEXIBILITY in order to have an optimal performance throughout their life. Sounds like a human being, right? ????

Why Pipe Stress Analysis?

What is the purpose of having this pipe stress analysis? First and foremost, to ensure SAFETY. The piping system shall be in OPTIMAL design, with desired and required PERFORMANCE, and of course must be ECONOMICAL wise as well.

Why piping systems required flexibility? During operation, there will be thermal expansion or contraction that caused piping system to move; resulting displacement. Too much supports to restraint the movement from occur will caused damage as much as letting it freely happened.

1. A piping system must have:

* Safety assurance of the piping system in any condition designed for (sustain, operating) or foreseen (occasional - wind, earthquake).

* Passed on design code standard stress allowance, as an example ASME B31.3.

* Displacement checked in the allowed range especially if there is adjacent objects (another piping, structure, equipment, walkway, etc.)

* Flange leakage check.

2. A piping system must prevent:

* Excess stress that may lead to system failure like flange leakage.

* Excess deflection of piping by its own weight and fluid filled (sagging).

* Excess load action on pipe support.

* Excess movement (displacement) that may harm to equipment's connection or clashes with adjacent items such as structures or other piping.

Which Lines Should be Computerized Analyzed?

Not all lines are required to be analyzed using engineering software. Only lines that are listed in Critical Line List shall be analyzed by computer. Commonly, it will called as CATEGORY 3 lines. So which line is it? How do we categorized it? What are the criteria to be considered?

The exact criteria usually will be specified by Client (Owner). However, in general (not limited to), these lines are commonly categorized as critical (Category 3);

1. Lines that are connected to equipment.
2. Lines that subjected to potential vibration by internal or external loads.
3. Lines that have high design/operating temperature.
4. Lines that have extremely low design/operating temperature.
5. Bigger sizes of piping lines; 8" and above. 4" - 6" with certain design temperature that usually will be given by Client's requirement.
6. Thick walled thickness piping.
7. A long straight line piping system more than certain values specified (let say 20m).
8. Severe cyclic piping system.
9. Hazardous or flammable medium.

What Inputs are Required?

1. Major inputs are from Isometric drawings - to have all the data of its size, wall thickness, dimensional layout, location of basic pipe span supports, insulation thickness if it is insulated, material used, pressure test, and design pressure and temperature.
2. Usually if the material used are listed, the Specified Minimum Yield Strength (SMYS) will be automatically filled. If the materials are so rare and not listed, pipe stress engineer may required to manually filled the SMYS.
3. Weights of piping items.
4. From Vendors data - to have data that may not standard like weights of valves and sometimes its end to end dimensions due to special custom made.
5. Environmental Loads like Wind, Seismic or even Snow depending on the geographical location.
6. Nozzles Load at equipment.

How Stress Analysis Relates to Other Piping Division?

Can a Pipe Stress Engineer works solely in his/her world? No. Will the stress outcome (even if it passed) affect other piping division like Layout and Materials? YES, definitely.

How so?

1. As per stated above, critical lines usually involved either large bores, high/low design temperature, or critical services that may resulted in expensive material selection. Thus, in order to finalize and procure piping materials, one must have the stress analysis passed prior to buy it. Because in order to make it pass, one may have to adjust or change the layout. Hence, change the final MTO to procure. Unless the project management is aware that materials are procured in staggered phase and does not reflect the actual required MTO. Contingency can be used as backup plan, but there will still be possibility of insufficient materials supply.
2. Changes MTO does not limited to piping bulk only, but also the pipe supports. It may not be as critical if the lines are typically using carbon steel pipe supports. But, if it is required a special support like cryogenic shoes, spring hanger, etc., stress analysis shall be assured has passed the code compliance and finalized all supports that should be assigned on the piping system, prior to pipe supports procurement. As those special supports are not cheap and long lead items.
3. In order to get the piping system passed, one may have to change the routing or adding pipe supports or even change the locations of pipe supports. All of these outcomes shall be conveyed to design engineers/designers in order to reflect on their final Isometric drawings. As fabrication site will fabricate actual piping based on Isometric drawings and not pipe stress analysis report/stress isometric.
4. Even if the piping system passed but involved very large amount of displacement, one must notify to design engineer/designers to ensure clearance provided are sufficient to avoid any clashes at actual condition.
5. The same applies to fabrication or construction team. If any modifications are needed at Category 3 lines, it shall wait for the stress analysis to revise and updated prior to modify. Once the stress confirmed passed as per proposed modification (be it routing or pipe support location), then the work can be resumed as per intended. It will be an issue if the sequence are not following through. You might be lucky once or twice. Yeah, even thrice. But, one day you will be run out of luck, and a good engineering practice will beat LUCK.

I hope this article is helpful for none pipe stress engineer. It is very basic understanding, so that we have the idea in our working routine. Appreciate if you could Like and Share if you found this article is beneficial.

Till then, all the best and God bless! ????

#pipingnewb

Credit to other sources: csemag.com, whatispiping.com, piping-engineering.com