9 replies to this topic

[ecovarrubias]

Hi all,

I'm starting using Piping Systems Fluidflow Software to do an hydraulic simulation of the cooling system for the local refinery, but I can't specify a determined water flow at a determined pressure (420 m3/h @ 50psig), I can only specify either the flow or the pressure (with the boundary icons). I don't think so this could not be possible only I don't know how to do it.

If someone can help we I'll be appreciated.

Thanks.

[Pingue2008]

Dear,
I am not quite sure I understand your question. Do you want the program to back calculate the inputs given the outputs?
What software are you using? I usually use a program called fathom and it is quite good with systems such as yours.
Please clarify your thread.

Thank you,
Alan

[ecovarrubias]

Hi Alan,

Thanks for your reply.

What I'm trying to calculate is the output pressure given the input conditions. I have a water distribution system with a pump discharging 420 m3/h at 50 psig and I want to evaluate the pressure at the end of the system and if the pressure drop is to high I would require to modify the piping increasing the pipe diameter. The distribution system consist of condensers and coolers located at 100 m. of the water pump.

The software I'm using is Piping Systems Fluid Flow V.3.21.

Please let me know if everything is clear now.

Thanks again.

[Pingue2008]

Dear,

It is very clear and I think it is rather a simple simulation. unfortunately I am not quite familiar with your software.
what we can do is this: attach a detailed sketch (fitting etc..) so i can run with the software I told you about.

Thank you,
Alan

[Pingue2008]

Dear,

I just reminber a discussion we have on this forum. see the attach if it will be of any help.

Attached Files

•  Cooling Water Model.xls   105KB   158 downloads

[katmar]

I don't know this particular program, but no program can calculate the pressure at a point unless you have told it what piping and equipment is between that point and a downstream point of known pressure (for example a discharge to atmospheric pressure).

I would imagine that you will have to do it by trial and error by specifying a pressure at the end of the system and also a pressure at your pumping point, and then let the program calculate the flow rate. Depending on the flow rate that is calculated adjust your guess of the end pressure and run the program again. Repeat until the flow matches your known flow.

Alternatively you could probably specify the flowrate and not the pump pressure (but of course still supply a guess for the end pressure), and then use the program to calculate the inlet pressure. Again guess new values for the end pressure until your inlet pressure is matched by the calculation.

[Art Montemayor]

Eduardo:

I can’t add anything to the logic and correctness of Katmar’s valued comments. But I can attest to the fact that his recommendations are valid.

I can also add that it is not possible to determine a water flow rate through a pipeline system without you knowing what the pressure drop (or driving force) is. Simply knowing what ONE of the two pressures is (you don’t tell us if it is the initial or the final pressure) is not enough information to make the calculation. The way the hydraulic logic works in your case is that (if the pressure you cite is the initial pressure) you must determine the anticipated pressure drop through the given system at the flow rate you cite. To obtain this pressure drop you must find the pressure losses through the piping system. These pressure losses are created by resistance to flow: valves, fittings, turns, piping length, piping roughness, equipment, etc. This is indicated by Katmar and the Stephen Hall spreadsheet is an excellent example of this logic.

Like the other members, I also don’t know anything about your fluid flow program. This doesn’t mean it isn’t any good; it just means we are not familiar with it. Nevertheless, if it is a worthy program, it must follow the same fluid flow logic I have mentioned above. That is something that can’t be avoided. In order for the program to truly be a “simulation” program, it MUST include the ability to vary the various types of anticipated flow resistance in your system – as the ones I mention above. If it doesn’t do this, then it can’t simulate the fluid flow in your system.

[ecovarrubias]

Thank you all for your reply. About the system I do have the number of fittings, valves and the known pressure loss through out the heat exchangers so what's next is to evaluate the system considering Katmar's suggestions.

The software has a friendly interface and a detailed sketch of the system can be done in isometric view so It seems to be very interesting analize hydraulic systems using this tool. I almost finished sketching the systems so I will do some tests and let you know about the results.

Thanks again for your help!!!

[Steve Hall]

Be sure that you convert your "known pressure loss through out the heat exchangers" to an equivalent length of pipe, unless your software does this for you automatically based on your inputs of pressure drop at specified flow rate. As you change flow rates in your model, the pressure drop through the exchangers will change.

[S.AHMAD]

1. Other members have made workable suggestions. Let me suggest another approach.
2. I am sure you are aware that we have two characteristic curves that is pump characteristic which is independent of system configuration and the system characteristic curve that is depending on the piping configuration. However, once you have decided the system configuration, the system characteristic curve will also fixed but for this study, make the pipe size be the variable.
3, Do several cases of various std pipe size for the header and the branch to each exchanger.
4. Let the software determine the pump pressure required for each case.
5. Select the case such that the required pumping pressure is 50 psig or slightly lower at 420 m3/h total flow.
6. The software will also give you the flowrate at each branch, check that the flowrate matches with the required flowrate to each exchanger, otherwise you need to select other pipe size for that branch and re-run the case until everything satisfies the system needs.
7. This analysis can also be easily done using spreadsheet. I have done similar analysis of a firewater system network that is connected to three pumps at several locations, using excel spreadsheet. IT WAS FUN!

Edited by S.AHMAD, 13 December 2012 - 05:12 AM.