Hydropneumatic Tank

Hello Moustafa, 

This situation is seen due to check valve slam, associated with the pump and since hydropneumatics tank is downstream of the check valve, which is a common modeling situation.

HGL at the hydro tank drops after some time eventually due to pump shut down event. As the check valve slams, downsurge pressure wave travels back to the pump, which causes this negative pressure that you are seeing and vapor pocket formation is there at D2A element.

Please make sure that all the input is correct, please go through below article about modeling pump shutdown event, if not gone through already.

Modeling pump shut down event

You can think about modeling check valve element separately downstream of the pump and be sure about selection of check valve type.

Modeling refernce-check valve

You can find few papers on the check valve slam situation (check valve slam from pump shutdown with hydropneumatic tank) which may help further.

Here is one link that was posted earlier for another similar forum post by Jesse,

https://www.youtube.com/watch?v=m2qMXafuT0I

To add to Sushma's reply - the drop in minimum pressure only occurs on the suction side of the pumps, and is due to the check valve closure from the tank, as mentioned by Sushma. If you have a special type of check valve at your pumps to help prevent this "slam" behavior in such situations, you can model it by way of the parameters found in the check valve node element (see the link in Sushm'a reply).

You may also want to try a smaller calculation timestep since the suction side pipe is only 5 m long. See: Understanding length/wave speed adjustments and their impact on results

Dear Sushma,

Thank you for your reply. But, when I replaced the check valve built in pump by a check valve node or check valve accompanied with pipe, the negative pressure in suction pipe increases and there is a negative pressure in discharge line between the pump and check valve. Also, when I decrease the time step (from 0.01 to 0.003) the negative pressure decreases (in the two scenarios). But, again, after installing the tank, the negative pressure increases than without tank. How to minimize the negative pressure in suction pipe? Is there any practical strategy (device, method, settings,……)? Or there is no problem if it reaches in suction for example to -5 m H2O).

Please find the reattached model.

https://drive.google.com/file/d/1hoZlWJamt02RLcXZnxejq0hFDIdDUoTV/view

Dear Jesse,

Thank you for your reply. Exactly, when I decrease the time step (from 0.01 to 0.003) the negative pressure decreases (in the two scenarios). But, again, after installing the tank, the negative pressure increases than without tank.  I do not find a solution to the high suction negative pressure. How to minimize the negative pressure in suction pipe? Is there any practical strategy (device, method, settings,……)? Or there is no problem if it reaches in suction for example to -5 m H2O).

Please find the reattached model to to Sushma's reply or the link below (because its size more than 21 megabyte).

https://drive.google.com/open?id=1hoZlWJamt02RLcXZnxejq0hFDIdDUoTV

Moustafa,

It is up to your judgment to decide how to address a low pressure problem. The model is predicting that the pressure from the tank causes the check valve to close quickly, causing transient problems on the suction line. You will need to decide if the magnitude of that problem (-5 m pressure) is acceptable to you, or if something may need to be done to address this. HAMMER can help you determine strategies for mitigating transient problems, through the ability to set up multiple scenarios for each trial strategy, and the ability to easily compare the effectiveness in the Transient Results Viewer.

Regarding the timestep, in general a smaller timestep will yield more accurate results, and it will impact the effective pipe length (or wave speed) that the transient solver uses (more on that here).

As mentioned in earlier replies, hydropneumatic tanks can often cause this check valve slam problem and you can find many papers on the subject. See the video that Sushma linked to in her previous reply.

For the check valve node element, you could try adjusting the parameters to match the specifications of the check valve that you feel will be best for this situation. Currently you have the open and close time set to zero, which will essentially do the same thing as the pump and pipe check valve. See more on check valves here.

There may be other strategies for addressing this type of situation (bypass around the pump station? Different location for the tank? Different type of tank? , but again it is up to you as the engineer to decide what is feasible for your particular project and standards, then use HAMMER to perform the simulation to show you a prediction of its effectiveness. Again, check some scholarly articles on the subject, and try some simulations in HAMMER.

If the check valve closes soon after the start of the transient simulation, you can save time by reducing the transient simulation duration to just after the time of closure, so you can quickly assess the immediate impact of any strategy. You can later change the duration back to the original value.

The initial pressure at the pump suction side is already quite low, so it would seem to already be at risk for transient problems or cavitation. You may want to check the NPSH and consider if it is possible to address the low pressure in the initial conditions.