Bladder vessel - Same result for max and min pressure

1) When the valve closes, there is nothing downstream to maintain the pressure in the system, so the tank will eventually drain out. Downstream of the valve you have a Discharge-to-Atmosphere element, whose pressure drops to zero when the valve closes: it simulates a pressure-dependent demand (outflow), so zero outflow = zero pressure. Zero pressure means a hydraulic grade of about 7.3 m.

So, regardless of the hydropneumatic tank settings, not long after the valve closes, the tank will drain out and the HGL will be equal to the Discharge-to-atmosphere element's elevation. This is why the minimum pressure result does not change as you change the preset pressure.

2) When you say that the "volume of the vessel increased", do you mean that the calculated maximum volume of gas in the tank increased? I set up two scenarios in your model, one for the preset pressure set to 0.35 bar and the other with the preset pressure set to 0.5 bar. The difference in the maximum volume of gas in the tank did not change by much and in both cases they were under the 1.0 full tank volume specified in the tank properties. See the below screenshot showing the gas volume time series between the two scenarios. Note that you appear to be using an old version of HAMMER: 08.11.05.61, but I tried this in both the latest version (10.00.00.55) and your version and saw the same results. If there is a step I am missing or if I have misunderstood, please clarify.

In general, you can read about how the preset pressure works in this wiki article:

Modeling Reference - Hydropneumatic Tanks

A preset pressure of 0.35 bar is 3.5 m, compared to the initial steady state pressure of 96.5 meters, so the preset pressure is quite small. Accordingly, the gas inside the bladder compressed a lot, to an initial volume of about 0.15 m^3 compared to the full tank volume of 1.0 m^3. This establishes the relationship between pressure and volume, which impacts how fast the tank drains out with a respective change in volume and vice-versa.

With a preset pressure of 0.5, the bladder compresses to a slightly greater initial volume and also changes that relationship between pressure and volume, hence the volume that it expands to at the given change in pressure (see #1 above) is different (~0.92 m^3 vs. ~0.85 m^3)

The missing screenshot has been added to my previous reply.

Dear Sir 

first of all thank you for your reply , secondly i have the following points 

Hello Mohammed,

1) In a system like this where the tank will always drain down to match the HGL of the final discharge point, the protection that the tank provides is for the transient that occurs when the valve is first closed. Meaning, the initial downsurge or upsurge pressure spikes. So, even though a small tank will end up draining down just like a large tank, and may accordingly have the same minimum transient pressure, the small tank may not provide protection against any initial surge from the valve closure. For example you may find that the tank drains out so fast that the downsurge wave downstream of the closed valve was even more severe and caused negative or sub-vapor pressure conditions downstream as a result of that first transient wave. If you do not find this to be the case, you may want to analyze the system without any protection (if you have not done so already) and see if the tank is even needed at all. If even the small tank does provide good protection against negative pressure or high pressures resulting from this transient event, then the small tank may indeed be a good option.

2) The preset pressure is what the bladder is compressed to before it is installed in the pipeline. I am not certain if all manufacturers only have specific standard preset pressures, or if it something that you can tell them (and they will then compress it accordingly), or if some tanks have the ability for you to precharge.

I had a quick look at the website of one manufacturer (Charlatte Vessels) and according to this PDF, it sounds like the precharging can be done by the customer. (in which case you can prescharge to whatever you decide on in your design).

In general I have heard some reference values of precharge pressures somewhere between 5% and 85% of pipeline pressure (mentioned in the modeling reference wiki article), but this might be very system-dependent. Depending on the system's initial pressure and the full size of the tank, a different preset pressure will be required to achieve a certain ratio of gas to liquid.

If you have tried a range of values for the preset pressure and have found that a certain number (like 0.5 bars) works well, then that might be OK for your system. You could check with the manufacture to confirm that there are no problems with a certain preset pressure. If the manufacture only allows certain preset pressures, you could try those in HAMMER along with adjusting other parameters (like the tank full size and orifice)

thank you jesses dringoli