Earthquake loading – Brief

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All designers are accustomed to evaluating moments due to eccentric and wind loads, but there are a few who may not be familiar with the method used for estimating moments due to earthquake. Therefore, the following brief outline is presented because this method is recommended as a design procedure for vessels where dynamic considerations are required. The weight of each vessel element (shell, head, tray, or internal part) is calculated. and then multiplied by the vertical distance from the circumferential seam (or horizontal plane) under
consideration to the center of gravity of the element. The summation of the moments so found ismultiplied by the seismic factor for the area where the vessel is to operate, thereby yielding a moment due to earthquake or seismic disturbance. For vessels, the seismic factor will usually have a value of 0.03 to 0.12, depending upon the geographical location. Expressed mathematically,

Thickness Calculation for Combined loading

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It is customary for most vessel designers to establish the minimum vessel shell and trend thickness according to the pressure temperature conditions and then calculate the thickness required at the bottom head seam due to bending moments imposed by wind or earthquake forces. Stresses in the longitudinal direction are involved nod the following notation may
be used to summarize the thickness required :

Here, The terms within the absolute value signs are positive for tensile stresses and negative for compressive stresses. The first term gives the thickness required for the longitudinal stress resulting from internal pressure and is positive for pressures above atmospheric and negative for pressures below atmospheric. The second term is the thickness required to resist the longitudinal bending stress and both positive and negative values exist at the same time. The third term is the thickness required for the weight of the vessel above the seam being investigated and, since this is a compressive stress, it has a negative value. The combination giving the highest value establishes the thickness required to resist the longitudinal stresses.

This formula hold good when the units are in Psi, ft & lb, if the units are in MPA, mm, N then remove the ’48’ from the formula.

Design : 2 : Shell (External Pressure)

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External pressure can be due to internal negative Pressure, or external loading like wind, earthquake etc. or live load, snow load etc.

One can design Pressure vessel for either sever combination of various load or for most possible occurrence of load combination.

Load combinations are given in respective ‘Building Code’ like API, UBC, IS etc.

Yes, If you noticed I’ve said ‘Building Code’, why? as most of these loading decides how the pressure vessel, and if it fails, it can harm the occupants. hence design of such loading will be governed by building codes.

These Building code will give su way to calculate loading on tank/vessel, and then our design code like ASME, BS,EN will tell us how to derive the thickness from them.

one should note that, nearly all the design code talks about +ve Internal pressure & -Ve internal pressure & its design rules, but none of the code talk about how to do thickness calculation for the loads specified above.

In latest edition of ASME it does talk about these loading, and ask user to use Engineering practices to calculate loadings (UG-22), loading listed in this sections are
(a) internal or external design pressure (as defined in UG-21);
(b) weight of the vessel and normal contents under operating or test conditions;
(c) superimposed static reactions from weight of attached equipment, such as motors, machinery, other vessels, piping, linings, and insulation;
(d) the attachment of:
(1) internals (see Appendix D);
(2) vessel supports, such as lugs, rings, skirts, saddles, and legs (see Appendix G);
(e) cyclic and dynamic reactions due to pressure or thermal variations, or from equipment mounted on a vessel, and mechanical loadings;
(f) wind, snow, and seismic reactions, where required;
(g) impact reactions such as those due to fluid shock;
(h) temperature gradients and differential thermal expansion;
(i) abnormal pressures, such as those caused by deflagration;
(j) test pressure and coincident static head acting during the test (see UG-99).

In next section, we will discuss how to calculate final thickness of a vessel considering all loadings

Design : 1 : Shell

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We design pressure vessel for longitudinan and circumferencial stresses. Now whats that?
Ok, longitudinal stresses comes on circumferencial joints, where as circumferencial stresses comes on longitudinal joints.
And to add cherry on top circumferencial stresses are twise that of longitudinal stresses.
If you are not yet twisted your tongue, and to avoid that we generally call them c’seam and l’seam and c’stress and l’stress.

In below pictures, one can identify types of Seams and Stress.

here p is design pressure, R is internal Radius, L is Length of shell, t is thickness of shell.

Hence with the basic mechanical formula,
t for Hoop stresses = pL (2R)/2 xStressx L = pR/Allw. Stress
t Longitudinal stresses = px3.14xR^2 / (2×3.14xRxAllw. Stress) = PR/2 Allw. Stress

& what ASME Says..
t for Hoop Stress = PR/(SE – 0.6P)
Where S > All. Stress, E > Joint Eff. or factor of safety
why 0.6 P, because, its factor of safety set by ASME People

&
t for Longitudinal Stress = PR/(2SE+0.4P)

Note the ‘2’ in denominator..

if E remain the same for both cases, then thickness given by Hoop stress will be twice that of by Longitudinal stresses


For Calculation Sheet Visit http://sumitmechsoftware.blogspot.com/

Pressure Vessel catagory (Design Persay)

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OK..

The Billion dollar Question…

Just to make it easy, we will break the pressure vessel in two category..
1. Pressure Vessel without external loading &
2. Pressure Vessel With external Loading

For Category 1, we can use ASME Section VIII Div. 1 directly, where as for Category 2, we need to consider additional loadings. And for additional loading we need to follow the basic engineering practices.

Hence, in future section, 1st we will discuss the Sizing of Pressure vessel as per ASME Section VIII Div. 1, and then we discuss the additional loading part.

“Terms” in Pressure Vessels

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Failure: Failure of a structure is an event, the transition from a normal working state, where the structure meets its intended requirements, to a failed state, where it does not meet its requirements
Limit states: A limit state is a structural condition beyond which the design performance requirements of a component are not satisfied. Limit states are classified into ultimate limit states and serviceability limit states
Elastic limit states: An elastic limit state is a structural condition associated with the onset of plastic deformation. This term is usually used in connection with monotonic actions, and it relates to virtual structures, usually with zero initial stress distribution
OPERATING PRESSURE : The pressure which is required for the process, served by the vessel, at which the vessel is normally operated.
DESIGN PRESSURE : The pressure used in the design of a vessel. It is recommended to design a vessel and its parts for a higher pressure than the operating pressure. A design pressure higher than the operating pressure with 30 psi or 10 percent, whichever is the greater, will satisfy this requirement, The pressure of the fluid and other contents of the vessel should also be taken into consideration.
MAXIMUM ALLOWABLE WORKING PRESSURE : The internal pressure at which the weakest element of the vessel is loaded to the ultimate permissible point
HYDROSTATIC TEST PRESSURE : One and one-half times the maximum allowable working pressure or the design pressure to be marked on the vessel when calculations are not made to determine the maximum allowable working pressure.

There are many more terms than this… but to start with.. these are sufficient.

What is Pressure Vessel?

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Pressure vessels are probably the most widespread “machines” within the different industrial sectors. In fact, there is no factory without pressure vessels, steam boilers, tanks, autoclaves, collectors, heat exchangers, pipes, etc.
More specifically, pressure vessels represent components in sectors of enormous industrial importance, such as the nuclear, oil, petrochemical, and chemical sectors.
There are numerous Codes & Laws present in each country to control the usage of this ‘machines’! its useful but its dangerous!
What one need to design a Pressure Vessel?
● good workmanship with regard to the tools used,
● knowledge of the basic engineering principles and the phenomena involved,
● fantasy and creativity with regard to the selection of the models used,
● fair knowledge of the legal requirements pertaining to design,
● fair knowledge of manufacturing and testing procedures, and especially
● extreme carefulness in each step, from the design specification to the design
report.

Pressure Vessel mostly are made-up of ‘Steel’ Steel behaves in an elastic fashion even beyond the proportional limit, as long as another characteristic point corresponding to stress called elastic limit is not exceeded.In practice, we typically equate the proportional limit to the elastic limit.

we always discuss the steel’s behavior at room temperature. It is, however, of the greatest importance to be aware of the influence of temperature on the mechanical characteristics of the material. As we shall see, not only temperature but also time may have a strong influence.

Next we will discuss some “TERMS”

What Next?

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Now, let us start new topic, on design of pressure vessel!

I think this one of the interesting subject, and we find lots of data over here and their defining how to design pressure vessel.

Some people take advantage of this and confuse you some time.. end result you may get over size vessel (high cost) or under size vessel (Dangerous!)

Solution.. buy High end software!! Sure. thats the last thing I’ll recommend!

Hence onward, for few more post we will discuss this subject, and I’m open for suggestions and lengthy discussions over this 🙂

So next post.. Over view on Pressure Vessel (Simplified 🙂

Visit to Drinktec

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Hello All from Munich!
Right now I’m at drinktec,and sharing my experience with you all.
Drinktec (website : www.drinktec.com) Drinktec is an exhibition which happen at Munich germany after each 4 year! Its for people from Brewery & Beverages.

The exhibitors are manufacturers & supplier from/ for same Industry.
Its a good experience for new bee like me, who is very new to this industry. you will find here huge stalls from all equipments around the world. I’ve seen huge bottling plant made alive in Stall at exhibition.

I’ll share few pictures from exhibitions soon.

Chao!