What is the maximum allowable deflection between the span for the tubes according to a boiler code?

[Bhupinder1]

Another related question, is it necessary to use hot modulus to calculate the deflection? The hot gases are flowing around the tubes, and there is hot water at high pressure inside the tubes.

 

[davefitz]

I am not sure I have seen this explicitly written in the boiler code, but a vertical deflection of a horizontal tube due to gravity loads implies it will develop a bending moment and such a moment can be described as axial forces on the tube top and bottom . One possibility is that the maximum additive axial stress due to this bending moment not exceed 50% allowable stress , since another 50% of allowable stress is imposed by fluid pressure in the axial direction.

Separate from gravity loads is the vibratory deflection caused by fluid elastic forces , and that deflection would be limited based on high cycle fatigue requirements.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[XL83NL]

B31.1 is not my area of comfort/experience, however ... B31.1 para 121.5 specifies

121.5 Hanger Spacing
Supports for piping with the longitudinal axis in
approximately a horizontal position shall be spaced to
prevent excessive sag, bending and shear stresses in the
piping, with special consideration given where components,
such as flanges and valves, impose concentrated
loads. Where calculations are not made, suggested maximum
spacing of supports for standard and heavier steel
pipe are given in Table 121.5. Vertical supports shall be
spaced to prevent the pipe from being overstressed from
the combination of all loading effects.

If you do not calculate sag and/ or bending e.g. by means a flexibility or pipe stress analysis, table 121.5 can be used. Nevertheless, the amount of deflection should not impose any stress greater than the Code allowable.

As per your 2nd question

119.6.4 Stresses. Calculations for the stresses shall
be based on the least cross section area of the component,
using nominal dimensions at the location under consideration.
Calculation for the reference displacement stress
range, SE, shall be based on the modulus of elasticity,
Ec, at room temperature, unless otherwise justified.

As your calculating displacement stresses for deflection, youre not allowed to use the hot modulus, unless otherwise justified. If the hot modules provides a greater sag with a greater detrimental effect, I would imagine you could opt for using the hot modulus, if so allowed by the governmental jurisdiction and the owner.

 

[Bhupinder1]

Thanks for the reply. In para 121.5, it mentions the excessive sag but doesn't define what the excessive sag is? In addition, general notes of table 121.5, it mentions the sag is limited to 2.5 mm but again that only applies when the calculations are not made. What is the detrimental effect of a sag other than aesthetic if the tube is not over stressed?

 

[XL83NL]

One effect you may not want is drainability.

 

[chicopee]

So the main question is, what condition caused the sag? My instinct tells me that you had low water condition and upon discovery by the operator, relatively cold water was introduce which permanently set the tubes to sag. I am interested to hear your version.

 

[Bhupinder1]

The sag is caused by the long length of the span between the supports, tube weight, and inversely proportion to the tube thickness. My question is what is the limition of the sag that you can have, and what are the detrimental effect of having a sag?

 

[chicopee]

Can you tell us about the type and manufacturer of the boiler. I don't ever remember the ASME Power Boiler I ever putting a restriction on tube length or ever giving a formula for tube length when I took my program to become a NB boiler and pressure vessel inspector back in the '70's, however, this could have changed over the years. Now I have seen boiler failures due to low water conditions resulting in permanent tube "sags" when the operators decided to flood the boilers.

 

[chicopee]

I do not believe that you can apply ANSIB31.1 to boiler tubes. I have checked several of my references on boiler tubes and there is no apparent precise information perhaps due to the several reasons: 1) tube thickness is based on the boiler MAWP and stress values at 700 dF making sag negligible (I suspect that being so but I don't know for a fact). 2) tubes under boiler pressure and temperature will stretch about the same value as the boiler shells, therefore, controlling sag. 3) boiler design has reached optimum size based on the economy of raw material, fabrication time, steam generation and boiler room size. Firetube boilers which will have horizontal tubes can not be excessively long, therefore, sag does not appear to be an issue.

 

[Bhupinder1]

Thanks chicopee for your response.

The boiler is an economiser (water tube boiler). ASME-I doesn't explicitly mention sagging or bending stress, however, looking at PG-22 Loadings it requires to consider any additional loading, hence bending stress needs to be considered.

Again, I can't find anywhere on how to consider it in ASME-I but I have found out that it's being considered in B31.1.

 

[davefitz]

To avoid mid span sag, there has been developed double barreled ( as with over/under shotguns) tubes for use in economizers, which allow unsupported horizontal spans up to 6 m. These are usually finned tubes and are sometimes called H-pattern tubes.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[chicopee]

Thanks for the wild goose chase. In your OP you are talking about boiler but then you are referring to unfired PV suggesting isolable economizers.