Hello
I have modeled a raw water pumping line from a low to a high elevation using Hammer to compute up surge and down surge.
My problem is even when using a hydronumatic tank i am getting negative pressures in the pressure envelope at the tail end of the line even though the individual nodes (along the negative section) does not show a negative pressure when i check their time history. Is this possible?
The hydronumatic tank does not go empty during this simulation.
Suminda
Hi Suminda,
This is certainly possible if the hydropneumatic tank isn't set up correctly. Since you say that the tank doesn't empty during the simulation, check that there isn't too much headloss across the tank outlet. Also check that there is enough compressed gas in the tank at the start of the simulation (since the compressed gas provides the energy to force water out of the tank when pumps shut down).
Regards,
Mal
Here comes question; how to size the hydropneumatic tank?? I know it depends on case by case, but should be general notes or general guide steps to select the hydropneumatic tank!! and what if the the tank is undersize or oversize?
how we can judge about the tank; i.e. what are the signals of judgements in general??
what are best sites preferred to select hydropneumatic tank??
Regards...........Mohamad Azzam
There are many factors that come into play with a hydropneumatic tank, so I think that engineering judgment will play a big part. Each of these are outlined in the below technote, and can all be a factor in the effectiveness of the tank:
Modeling Reference - Hydropneumatic Tanks
Your space or budget requirements may dictate the total tank size or how many tanks you can use. The engineer can decide on the location, whether a bladder will be used, the initial gas pressure/volume and orifice headlosses.
The tank should probably be sized such that it appropriately protects the system against transients, but does not become empty. The engineer can quickly check the effectiveness of various combinations of the above factors by running multiple scenarios in HAMMER and comparing the results. If the tank does not provide adequate protection and/or becomes empty, then the engineer will need to adjust the size, initial pressure/volume, location and orifice headlosses. It's also possible that the tank will work best in conjunction with other protective equipment, but I think it's up to the engineer to decide what is appropriate for the given project.
Now, for the model that Suminda posted, there are a few things that I feel are worth mentioning. First, I am able to see negative pressures in a time history for the junctions that show negative pressure in the profile (for example J-7). Could you identify a particular element that you're looking at?
Next, as Mal mentioned, there are some excessive headlosses through the tank orifice. This can be seen by looking at the table at the bottom of the Transient Analysis Detailed Report (note: you've entered the report period for the tank, but you'll need to choose "true" for the "Generate Detailed Reports?" calculation option):
As you can see, once flow leaves the tank, there is a significant head loss through the tank orifice (as indicated by the difference between the gas head and pipe head).
If you adjust the orifice diameter and minor loss coefficient such that headlosses are reduced, the tank will drain more slowly, but it will still drain down enough so that the downstream end of the system still experiences negative pressures. The particular negative pressure that you see are due to the topology of the pipeline. Just before the downstream end of the system, there is an uphill slope to the pipe, which is at a highest elevation than the hydropneumatic tank:
So, as you can see, just because a tank doesn't become empty doesn't mean that it will keep a positive pressure everywhere in the system, especially if some elements are at a higher elevation. If you animate the profile path, you'll see that a relatively sharp downsurge wave passes over this uphill section of pipe after the pump shuts down, which causes the negative pressures. So, you may need to try a larger tank (to drain slower) or a new location (such as the hill in the middle, where AV-2 is currently located) if you must keep the pressure positive for the entire network.
I hope this helps.
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Jesse
Thanks for the reply. I was worried about the negative pressures developed between nodes J-8 and J-6.
I have varied the minorloss coefficient while keeping the same volume of the tank but the difference in results are marginal. I have also increased the volume of the tank up to 12m3 but still the results obtained are comparatively marginal.
I then introduced several AVs ( while keeping the same volume of the tank 10m3) in between these nodes and i was able to further minimize the negative pressure to a acceptable level.
It is best as you said to have the air tank at AV-2 location but the remote access to AV-2 location does not permit this and increasing the tank volume by a large margin is not economical when compared to the results i am getting from increasing it. Increasing the volume also creates other problems such as the availability of space and transportation problems.
So therefore i guess i have to decide on a appropriate buffer capacity for the tank and choose good quality AV for this system.
Try changing the preset pressure of the gas in the hydropneumatic tank to a larger number - for example 50 m H2O and you should see a big difference. I am not saying that a 50 m preset pressure is possible with the particular bladder tank you are using (you will have to check with the manufacturers), but I wanted to highlight the big affect it has.