Explosion Analysis 3

[Jsquare]

Has anyone performed blast analysis on piping systems?
I have a question about converting blast wind pressure to velocity and to equivalent force.

Thanks.

 

[Cockroach]

Yup, sure have!

My approach was strictly thermodynamics, not a subject for those not liking mathematics. You need to have an accurate understanding of the pipeline parameters and fluid conditions to use the method, which requires several iterations at points during the computation.

The details of the paper are too complex to get into with this limited space. Shoot me an e-mail!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Jsquare]

If you've used CFD programs, then it's over for me b/c I do not have access to such expensive program (and even if I did, it would take me months to learn it). My nick name is jone1939, and I have an account with yahoo. So if you could describe the general approach, that would be helpful for me to determine simplified yet conservative approach.

Thanks.

 

[Cockroach]

In the thermodynamic approach, the key is to recognize that an explosion IN THE ABSENCE of a chemical reaction (i.e. oxidation, secondary mixing, etc) is essentially an isentropic throttling process between two given states of a known working fluid. In my situation, the medium was compressed air so there was no fire, no source of internal energy converted to work on the control volume or system envelope. Air is about 78% nitrogen, I applied the nitrogen tables common to appendix of textbooks for courses in Thermodyamics. The assumption of an isentropic process can be justified noting that for given states of the working fluid, (P2/P1) = (V1/V2)^n, n=polytropic gas constant; I simply verified the polytropic constant was near 1.400 for the gas, justifying my assumption.

After the explosion, gas expansion is to state 2, atmospheric pressure and temperature. You can very accurately quantify this state. Therefore the idea is to define state 1, the initial conditions of the gas just prior to explosion known the process and final state of gas. Therefore the work done by the fluid in compressible gas expansion between two states, the second fully understood, equals the work done on the system, i.e. energy is conserved.

As mentioned, the process is isentropic so s1=s2. Compute s2 at given conditions; then this equals the value of entropy at state 1. There may be more than one condition for the fluid state, superheated or compressed, etc. Your process will dictate the given state when numbers start to make no sense, i.e. quality greater than unity, s1>s2, etc.

Recompute the value of enthalpy at state 2 with the quality understood. Extrapolate to state 1 given a inentropic process. In my case, I needed fluid velocity or the rate of explosion assumed to equal that of gas expansion, valid in the absence of oxidation. This was found quite simply from the First Law of Thermodynamics.

Therefore with state 1 fixed, it was possible to interpolate for pressure and temperature directly off the nitrogen tables at a known state, between two listed values. In my case, I had several interpolation processes since pressure and temperature where mid range. I found two listings close enough to assume linearity. One source of error would be if in reality, it was otherwise so I needed to minimize the possibility of error.

In my particular case, I had a sixty pound piece of steel go through the wall of a building. Measuring the field of debris, I deduced the range and altitude of the impression of the component going through the wall. Since flight is unpowered and can be closely approximated as a parabolic curve close to the surface of the earth, the equation for the flight path was computed and the traditional equations for rectilinear motion applied as energy statements. I found that the velocity of the steel component was 8.1% in agreement with the velocity of the gas computed at the instant of pressure vessel rupture. Getting a bit fancier and accounting for metallurgical effects of vessel stretch (remember I have homed in on state 1, pressure known) interpolating by error analysis methods brought the thermodynamic computation into correlation with the rectilinear equations of motion to within 1.35% after about fourteen (14) or so iterations.

Of consequence to the study, an operator at the facility found a chart recording on the line pressure connected to the compressor. The chart peaked at 2858 psi before a sudden loss of line pressure. My computations came in at 2847 psi, relatively in nice agreement. It is extremely rare that theoretical to practical results agree as such, I was extremely lucky in having a clean field of debris and luxury of thermodynamic tables of high accuracy. In life this is not the case, particularly pipeline applications were the chemistry of fluid has not much precession at the time of catastropic failure.

Certainly I hope this will help you out somehow. As mentioned, this website space does not lend itself well to academic discussion with mathematical treatment.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Jsquare]

Thanks for the detailed info. cockroach; however, I was looking more in terms of how supersonic overpressure by blast wave and dynamic pressure (blast wind) affect our piping systems. The explosion analysis such as gas expansion and atmopheric compressions have been performed by third party vendor who specializes in that field.

 

[bvi]

Presumably you are given overpressure and duration, impulse. Look towards building blast resistant design technology. The approach involves determining, in a simplified manner, the dynamic response of the piping system. Simplified, one D elastic-plastic dynamic analysis is used to evaluate the behavior of the system to the blast loading. The response will depend upon the frequency of the system and the characteristics of the blast wave. Higher, dynamic material strength properties are used. This is rather specialized, and you should consider engaging an expert in building blast evaluation, or be prepared to go up a steep learning curve.

 

[LSThill]

Hi Jsquare (Mechanical)

Do a search between the years of 1972 - 1974 for Piping System ASME PAPERS. The study was by BECHTEL Louisville. BECHTEL closed the Louisville Kentucky office in 1983. University of Louisvile may still have the report.

Leonard@thill.biz

http://www.thill.biz

 

[NeilatNEL]

Advantica (used to be British Gas R&T) did a lot of blast loading work on pipelines - try their website.

 

[bvi]

Here is a reference list

ASCE, Manuals of Engineering Practice, No. 42, Design of Structures to Resist Nuclear Weapons Effects, 1986.
J. M. Biggs, Introduction to Structural Dynamics, McGraw-Hill, Inc., 1964.
Center for Chemical Process Safety, AIChE, Guidelines for Design and Siting Criteria for Process Plant Buildings, New York.
Center for Chemical Process Safety, AIChE, Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and Bleves, New York, 1994.
Chemical Industries Association, Guideline, An Approach to the Categorization of Process Plant Hazards and Control Building Designs, London, 1979.
Chemical Manufacturing Association, Safety Guide, SG-22, Siting and Construction of New Control Houses for Chemical Manufacturing Plants, 1978.
Forbes, D. J., Design of Blast Resistant Buildings in Petroleum and Chemical Plants, Second Edition, John Wiley & Sons, 1982.
TM5-1300, Structures to Resist the Effects of Accidental Explosions, Department of Army, Navy and Air Force, Washington, D.C., 1990.
U.S. Army Armament Research and Development Center, Structures to Resist the Effects of Accidental Explosions, (6 volumes), 1990.
J. L. Woodward and P. Crossthwaite, “How to set explosion protection specifications for all process plant buildings,” Hydrocarbon Processing, November, 1995.
Benteftifa and Becht, Vapor-Cloud Explosions, Surviving, Encyclopedia of Chemical Processing and Design, Vol 61 Marcell Dekker, Inc., 1997 and also Hydrocarbon Processing, October 95 (Vol 74, No. 10)

 

[Jsquare]

Wow, thanks for all these great replies. I guess I better hit the books now.

 

[NGiLuzzu]

Jsquare,
please take also a look to Thread408-73599 and Thread311-73600!
I understood that blast load design requirements for piping systems, in our case, regarded possible explosions outside the pipe itself (submarine and/or in atmosphere). Is that correct?
And, most of all, are such requirements applicable to a valve and actuator assembly? How is this possible?

Many thanks, 'NGL