10 replies to this topic

[ingeprox]

There is a pipe system presurized with N2 at 120 barg to be vented across a 10" globe valve. How you can calculate the flow across valve with a variable upstream pressure while the pipe system pressure is decreasing with time venting across the valve?.

The idea is to determine how much flow will pass across the 10" valve while the system is depresurizing with the valve 100% opened venting to atmosphere knowing there is a total of 12,000 Nm3 of Nitrogen at 120 barg initially calculate how much time the system will take to achieved atmospheric pressure.

[Andree]

I think the option is to calculate instantaneous velocity from Bernouli eqnation (taking into accout entrance and local head loss for valve used in this equation) and then solve the problem in time discretized domain, i.e. assume short time step during which constant pressure inside the vessel and venting velocity is assumed, calculate amount of N2 released (velocity * cross section area * time) and then update the initial pressure inside the tank (calculated from EOS for updated amount of N2) for the next step... from this you will obtain change of pressure and velocit in time... :-)

[ingeprox]

thanks for answer my enquiry, but recently doing more investigation i find out in that conditions the Nitrogen vent will be at chocked (sonic) regime, so that's means there will be a máximum flow corresponding to a sonic flow across the 10" valve.

How to calculate flow rate over time will be a good help to have an idea.

[Art Montemayor]

Ingeprox:

What you have is an application very much in line with what Milton Beychok - a venerable and respected Chemical Engineer that has many years of experiene - has attacked in order to resolve it. Go to Milton's website: http://www.air-dispersion.com/

Then proceed on to his page on "Calculating Accidental Release Flow Rates From Pressurized Gas Systems". Milton is a past master at explaining compressible flow - especially sonic (or Choked) flow. In fact, if I were you, I would download all the information and literature I could find in Milton's website.

I don't know why it took you so long to realize that what you have is sonic flow. Any time that your have a gas (or vapor) expanding from a high (P1) to a lower (P2) absolute pressure and the ratio of P1/P2 is 2 or greater, then you have sonic flow. Read how Milton arrives at that magic ratio. The ratio varies for different gases and mixture; but if you employ 2 as your guideline, you are generally correct in identifying a constant mass flow rate of gas occurring.

[ingeprox]

Thanks you for your answer and reference of the literature related to this matter.

The reason for take me so long is because I was not involved with this type of application before at such a higher pressure for pneumatically tested a plant piping system, but I have 20 years experience in Methanol Plants EPC contract and operations, and now I'm assiting to an LNG company in doing a pneumatic pressure test in SS piping at 121 barg and if you know what we are talking about you will like to be pretty sure of your calculations to define the design basis of the test and the equipment you should be used for this operation.

This is for real life application, not only to share in a forum, so I'll like to find somebody who knows what we are talking about to share my case.

Anyway I doing the flow calculations on sonic regime after verify that will be the case in this situation and for a 187 mm efective vent nozzle diameter it can be a flow rate of 765 kg/s, which is a lot of flow, more than what I need to have a despressurization of the all system in a reazonable period of time.

I have now all the data to design the procedure to doing the pneumatic pressure test and fix the design basis for the vent silencer we need to consider for venting the N2.

You asked for my background which is Chemical Engineer graduated in the University of Concepción in Chile and since 1988 to 2007 I'm been working as Process Engineer, Chief Process Engineer, Senior Process Engineer and Project Manager of EPC Contract for design, build and Start-Up 2 of the 4 Methanol Plants that Methanex Corporate has in his complex at Punta Arenas, Chile working as a Methanex representative with engineering companies like Akersolutions, Lurgi, Jhonson Matthey among others.

My expertize is in the Catalyst field for Methanol Plants with strong experience in pre-commissioning, commissioning and Start-Up of the Process Plants and I'm been involved on this type of operations since 1988.

Hope this will give you an idea of my background and why because besides if this case appear as a trivial compressible fluid problem, at the time where you need to perform this operation in a real LNG Plant which will be tested pneumatically at 121 barg with N2, better you will make sure your are rigth in your calcualtions rather to trust just in your backgound or experience because all times whatever the experience or background you could have all times you can learn somenthing new.

Regards,

Alex

[sheiko]

Ingeprox:
I don't know why it took you so long to realize that what you have is sonic flow. Any time that your have a gas (or vapor) expanding from a high (P1) to a lower (P2) absolute pressure and the ratio of P1/P2 is 2 or greater, then you have sonic flow. Read how Milton arrives at that magic ratio. The ratio varies for different gases and mixture; but if you employ 2 as your guideline, you are generally correct in identifying a constant mass flow rate of gas occurring.

Hello,

Just to add my 2-cents:

I think this rule of thumb breaks down when pipe friction becomes a factor. It cannot be used to predict the supply and discharge pressures necessary for sonic choking unless the piping has negligible friction loss.

In fact, what Mr Montemayor said is true if P1 is the pressure at the upstream side of the shock wave, but it is usually not known. If there is any pressure drop in the pipe from the supply pressure to the shock wave, then the supply pressure cannot be used as P1 in the above mentionned magic ratio.

Hope this help

[fallah]

I think the type of expansion here to be discussed, is sudden expansion through, e.g., nozzles, valves, RO, ... and the expansion due to pipe friction is not considered.

Sonic flow due to gas moving along the pipe is another story and occurs because of pressure loss leading to expansion of gas in fixed cross sectional area (pipe).

[Neelakantan]

hi all and imgeprox,

we all would like to see the results of your calculations. A simple xl sheet would do

i have done the reverse of the same. pressure a well with N2 from a membrance nitrogen generator (filling the flow line upto SSSV and later the well) and depressuring the same through test separator.

i even generated a nice xl graph of showing the pressure vs time in the well bore.
i plan to give this exercise of yours to my juniors for solving using xl and also by dynamic study using hysys and revert with their experience

regards
neelakantan

[SafetyUser]

Hi

let me add other 2-cents...

If I'm well understanding the problem, there's a pipe pressurized volume with a valve/orifice for venting.

My thought is that if the pipe volume is isolated at boundaries, well this is the usual blowdown problem through a 10" vent valve (?).
For this, the equation suggested by the master Montemayor appear to be very close to the usual Saville & Richardson equation for blowdown of gas vessels (you can find in F.P. Lees "Loss Prevention in the Process Industries").

If the pipe has boundaries open to flow, then the pipe frictions will define the upstream pressure and the flow through the vent will experiment a transient time before settling on a new steady-state flowrate and upstream pressure.
Should this be the case, the only way I found to solve a similar problem (a hole in a pressurized pipeline) was with the support of HYSYS.

hope this help

[astro]

Hi

let me add other 2-cents...

If I'm well understanding the problem, there's a pipe pressurized volume with a valve/orifice for venting.

My thought is that if the pipe volume is isolated at boundaries, well this is the usual blowdown problem through a 10" vent valve (?).
For this, the equation suggested by the master Montemayor appear to be very close to the usual Saville & Richardson equation for blowdown of gas vessels (you can find in F.P. Lees "Loss Prevention in the Process Industries").

If the pipe has boundaries open to flow, then the pipe frictions will define the upstream pressure and the flow through the vent will experiment a transient time before settling on a new steady-state flowrate and upstream pressure.
Should this be the case, the only way I found to solve a similar problem (a hole in a pressurized pipeline) was with the support of HYSYS.

hope this help

Is the attached what you're referring to?

Attached Files

•  Blowdown.jpg   111.01KB   66 downloads

[SafetyUser]

Is the attached what you're referring to?

yes it is. Do you think could be useful to Ingeprox question?