Flow Due to Sudden Failure of Compressed Air Pipe

Homework?

"A 10" pipe supply dry air to a plant 2 km away ..."

"3. Volume of piping is around 100 cu.m..."

These two things do not appear to go together.

Is this idle curiosity?

"These two things do not appear to go together."

I checked it before asking. It definitely goes together. ID of 10" pipe is 254.46 mm, its cross sectional area is 0.05085 sq.m, volume of 2000 m long pipe is 102 cu.m.

Now can you please answer my question, particularly the first one.

Insufficient information for adequate analysis.

Please suggest, what more information is need.

Why are you asking us? A system as expensive as this must have been designed & engineered by a Consultant. Ask him instead. He will tell you the velocity & pressure drop that he designed for. He approved the system of the fabricator. Did he not then check the hydraulic at 45 bar?

compressor discharge pressure dropped from 31 to 20 barg in 75 sec, from 20 to 15 barg in next 1 min, from 15 to 10 barg in next 2 min, from 10 to 5 barg in next 3 min...... then stopped) The pipe ripped open to discharge the air to the atmosphere or your safety valve (may be re-conditioned) kept opening and opening....to do this.

3. Volume of piping is around 100 cu.m, including a buffer vessel, total volume is 200 cu.m. What is the total quantity of air vented to atmosphere? Again why ask us? Check your Consultants drawing and ask him. Is he there? or run off?

Compressor was stopped after 7.5 minutes. I wanted to know: Why you have a man at the other end to drive down and then shut this manually? the time 7.5 min coordinates with the distance 2 km covered. Why was this not automatically shutoff with a simple pressure limit switch?

Dear ducon,

1. The system is quite old (over 25 yrs). Hydraulic check was at the time of erection. the consultant is no more available.

2. Compressor is operated by a different set of people. The failure was at the last consumer point. On the way, this pipe serves to some other consumers too. The pipe ripped open and moved backwards to discharge the air to the atmosphere (not the safety valve).

3. As I replied, he is not there. I want to calculate the total quantity of air vented to atmosphere in this way:

Total piping volume (200 cu.m) converted to NTP volume at 31 barg pressure and 35 degC temp + NTP volume fed to the system in 7.5 min.

Can you say whether it is OK? Air temp at feeding point is about 42 degC. Ambient is 29 degC.

4. Unfortunately, there is no auto shut off of compressor for low discharge pressure. It was stopped once the information reached to them about failure, until then they were puzzled what went wrong.

Sir, with due respect an engineering life-time is considered 7 years. You exceeded 3.5 life-times and yet took the risk of operating at such high operating pressure. There is something known as metal-fatigue. The only advice I can give you is get this re-designed or if you were satisfied with the operation last 25 years(which you were) then have all the pipelines & equipments erected new in parallel now with all safety features, limit switches, flow metering & monitoring/alarm system. What is this air used for? Where are you located?

"an engineering life-time is considered 7 years"

ducon, may I know the standard as per which above life time is proposed.

I tried to answer the 3rd question myself, I requested for check, whether it is in order? For me the 1st question is more important, if you can help.

1. What will be the air velocity at the point of failure? Will it decrease linearly with time?

The line burst and God knows at what pressure and "WHOOSH" the air discharges out and high speed and velocity. Regret I doubt if anybody can answer this.

As regards, the 7 year life time. Honestly, when I was a young instrumentation engineer I was informed that a life time is 14 years, counted day & night. So a life-time is considered 7 years. This is maximum you expect anything to last 60,480 hours. I never questioned this. However, some years back I read Time Magazines and they showed how air-craft existing life-time ripped off mid air and they said this is metal-fatigue which eventually takes over. For sake of argument >15 years is one hell of a time.

Dear - a - very well answered. Thank you for enriching my knowledge. GA

If

Output of compressor(s) = maximum design capacity of conveying media

and

Size of rupture = total flow capacity of design

then

Total quantity of air discharged to atmsophere = total output of compressor(s) minus conveying system dynamics losses. (turns, reductions, pipe length, etc.)

Else

Total quantity of air discharged to atmosphere = ratio of compressor output to flow capacity of system to size of rupture opening minus conveying system dynamics losses.

End

Lots of variables to identify and solve in order to answer your questions.

Identify and plug the numbers in to solve the algorythmic equation or submit WAG

"Total quantity of air discharged to atmsophere = total output of compressor(s) minus conveying system dynamics losses. (turns, reductions, pipe length, etc.)"

Why to minus conveying system dynamics losses? Whatever air fed to system must come out to atmosphere from the rupture point.

"Total quantity of air discharged to atmosphere = ratio of compressor output to flow capacity of system to size of rupture opening minus conveying system dynamics losses."

Could not get!!! Why ratio? Why to minus conveying system dynamics losses?

with reference to this and #17 and # 18, what will you gain from this stupid exercise. Your damage has been done due buried high pressure pipe over 25 years. Your process for whatever purpose it was designed for has come to stand still. You should be concentrating more on immediate designing/procurement /layout & commissioning. This will take you a good 4~6 months. Even more, depending what not have come on sites above ground en-route your underground lines during this 25 years span which you need to offset/take permission to dig/drill etc.

Spend your time here which is worthwhile rather than waste valuable re-erection days.

System dynamics equate to induced restrictions/resistance that slows the speed of the air being conveyed.

Slower speed => less flow

Ratio? How accurate do you really need to be?

If accuracy is not an issue, KISS it and base your numbers on the design capacity of the system and size of compressor (output) that was on line operating at the time of the event.

If this is critical such as an environmental excursion; then you will have to perform all of the required calculations with accurate system design DATA based upon actual media flow being produced at the time of the event. That number you must get from interrogating the flow at the point of generation either by active on-line flow measurement device(s) or knowing how worn the compressor(s) are and what the actual/real output of the unit(s) are. Then you will have to etierh knoe your system losses due to the dynamics or you will have to look up the formulae and plug the numbers in to obtain any sort of accuracy.

With all of the information on the web these day's. Why would you need to ask someone here to do your home work. How in the world do you think some of us older types got through school with nothing more than a slide rule and a grumpy professor?

Dear fixitorelse,

I'm also older type like you. I did search before posting my questions, could not get the answers (may be not enough search I did). I don't consider it as home work, in fact I don't like that some one should do home work here. It is for the experienced folk, who can visualise similar situations and readily have some thing to show the way towards the correct answer. Please see at #7, I tried to answer my own question and requested the folk to check whether it is correct way to do. I appreciate if you can try to answer my 1st question.

Another example of CR4 insulting and trolling a questioner instead of asking legitimate questions and attempting to help.

If you can't help the guy/gal, if you can't imagine a set of questions that will help him get closer to an answer, why post?

I understand this is a complex question that will likely require some basic modeling / major assumptions. Offer your help or be quiet.

For too many of us, this site is a place to rant and call people stupid. I know, allot of the questions ARE frustrating and kind of scary sometimes. Still, threads like this are the reason folks leave.

Is it the intention of the moderators of this site to allow it to become a private club that attacks people who are collectively determined to be "unworthy?"

I guess I'll know if I am banned for this post. If so, so be it.

-A-

GA & thanks -A- for comming to my rescue.

-A- they should have more then 2 scores to make a " Great Answer" also. So I could not vote

Ok make it simple!

How much air did the compressor compress before shut down, and how much air was in the pressurized system minus what remained in the system is you answer. Now why would you need to know the speed coming out from this system?

You probably need some temperature data and pressure readings because a little change there might be a lot of change on the other end.

Check your compressor details and you are almost there.

GA Ideasmith. I tried similar way at #7. Only thing I missed was "minus what remained in the system" which you pointed out correctly.

"why would you need to know the speed coming out from this system?"

I need to know it to calculate the propulsion force exerted by venting air at very high velocity. This force resulted reclining of pipe which hit a thick vessel and caused a big dent.

Pressure and temperature readings are available only at compressor discharge after cooler and drier which is 2 km away from rupture point (already given at OP & #7).

You should be able to roughly model this system in a spreadsheet.

First, you assume the leak was larger than the compressor can supply at design conditions. Otherwise, you have a steady state condition.

Then, starting at the compressor discharge, calculate the pressure drop in the piping and other equipment to the failure point 'orifice'. Assuming an orifice size, calculate the flow rate through the leak. Put all these calculations in one row. Select a time interval, maybe 30 seconds. Since the computer does all the calculations once you have the equations set up, you can use smaller intervals and all it costs you is pasting the equations in a few more times. Anyway, in your equations, you calculate what changed during the interval and use those to re-set the initial conditions for the next interval. Put each interval on a new row. It gives you a kind of piece-wise integration or the problem.

Years ago, I used a similar procedure to calculate the mass flow, remaining mass, pressure, and running cost vs time to use a railcar unloading compressor to pump-down the remaining product vapor after the liquid was empty.

GA bigg for nice suggestions.

But can we apply formula for "flow through orifice" here? Here whole 10" pipe got seperated after rupture and started venting to atmosphere.

I would have to research it. Maybe someone else can help.