Hello,
Can someone give me a criteria to evaluate sustained deflections?
Actually I have a maximum vertical deflection of 9 mm in sustained case in a 6" sch 80 steam line (empty pipe+insulation) located on an elbow.
Normally I check my support arrangemet to avoid deflections above 0.1 inch (2.5 mm) but I am not sure if that is too much conservative.
Comments of the stress comunity shall be apreciated.

Hello JMD

1.Steam pipes can flood.
2.Steam pipes which allow pocketing of liquid is bad due to water hammer.
3.large deflections can allow the pipework to 'wobble'
4.'Wobbly' pipework is bad in external/dynamic situations
5.(Probably most important)large deflection look bad.
6.Spans are indicates in MSS SP69 so why change a good thing?
7.If support is not possible and stress is OK, Then it is generaly accepted that YOU as the stress engineer can overide the standard pipespans
as long as you have good reason and there are no adverse affects.
8.Some engineers use the building codes as a rule of thumb for deflection (ie 1/360 span)

Dear JMD,

For steam piping, you can go for high piping slope like 1:100(BS 806:1993, Section 4.12) to 1:250(Rudomino & Remzhen, Page-23) & provide high sustained deflection, ensuring that no low point exists at the middle of a support span at any operating condition, including start-up & shutdown.

regards,
sam

Hello JMD,

Good advice above.

Keeping the calculated stresses of the piping under sustained loading below the allowable stress is of course important. Eliminating "low spots" in which liquids can accumulate (and then sometimes become "slugs") is important. In large steam lines we try to slope (or "pitch") the pipe at about 1/8 inch per foot (in the operating condition). Next we would want to be careful about cantilevered horizontal elbows as they are prone to vibrate when the right (or wrong) conditions are present. Sometimes we will add "extra" supports to "tune" the natural frequency of the system to a frequency higher than the vibration drivers (pumps, fluid flow perturbations, etc.). This of course makes for a stiffer, higher frequency system when it is done correctly.

It is likely that the steam pipe will be hydrostatically tested so it is prudent to analyze the system (at least one "case") as filled with water. If temporary supports are required for these tests they should be noted and care should be taken to assure that they are removed/inactivated after the test.

Grinnell's "Pipe Hanger Design and Engineering" booklet provides a pipe span chart as do many other sources. They note in their chart that the spans are chosen based upon a combined bending and shear stress of 1500 psi when the pipe is filled with water and 1/10 inch deflection is permitted between supports. And of course the suggested spans are simply supported with no concentrated loads (e.g., valves) in the span.

If you are in a position to recommend to your client the preferred periodic maintenance of the support system, some very good recommendations can be found in B31.1, Non-mandatory Appendix (Roman numeral) V. EPRI has presented a three session "webcast" program on that topic so if the utility is an EPRI member they could download a useful set of notes.


Regards, John.