The influence of the hydropneumatic tank on the suction side of the pumps

Question

Hi guys, 
 How is it possible that the characteristics of the hydropneumatic tank modeled on the pressure side of the pump have an influence on the pressure value in the node on the suction side of the pump during transient? 
 More precisely, in the first scenario, I defined the diameter of the pipe (branch) from the main pipeline to the hydropneumatic tank with DN 250. In comparison, I also observed the condition on the suction side of the pump because in this case a negative pressure of approx. -0.8 bar appears in the node on the suction side. 
 In the second scenario, I reduced the profile of that same branch from DN 250 to DN 150 and in that case the situation on the suction side improved drastically (the negative pressure dropped to approx. -0.2 bar). 
 Is this realistic since the activity of hydropneumatic tank on the pressure side of the pump (when the pump is shuted down after time delay) does not, as a rule, affect the suction side? Or does it? 
 Is it maybe possible that this is an error in the results due to the very small length of the pipe on the suction side (5 m from water chamber to pump), since every time I change the time step interval in the settings, the value of the negative pressure in the observed node on the suction side also changes in the final result? 
 P.S. Unfortunately, I am not trying to reduce that interval to less than 0.001 seconds, because in that case the calculation of the results takes a very long time. 
 Thanks in advance for Your help! 
 Regards!

Jesse Dringoli · Answer

Emir, 
 Yes, generally speaking it is possible for the hydraulics on the discharge/downstream side of an emergency pump shutdown to impact the transient results on the suction/upstream side. Typically a hydropneumatic tank will help prevent the discharge pressure from dropping too quickly during an emergency pump shutdown, which will change how quickly the pump speed will drop after that shutdown. Since the pump is operating at a different point on its four-quadrant pump curve, it will influence the hydraulics on both sounds of the boundary condition, which means the wave that travels in the upstream direction (pump suction side) could change, impacting the transient pressure. See also help topic "Pump and Turbine Characteristics in HAMMER". 
 You mentioned you're changing the diameter of the branch pipe connecting from the main line to the hydropneumatic tank - the performance of the tank during pump shutdown can potentially be significantly impacted by headloss through that connection pipe. Reducing from 250 mm to 150 mm may be introducing significant headloss, impacting how quickly the pressure on the discharge side is dropping during the pump shutdown, which in turn influences the hydraulics at the pump boundary condition (four quadrant curve operating point) which in turn influences the hydraulics of the transient wave that travels upstream of that pump boundary condition as mentioned above. 
 Note that you can observe this same behavior in the "Hydropneumatic_Tank_Example" model included with HAMMER (in the Samples subfolder within the HAMMER installation folder). Create a new child scenario with child physical alternative, change the diameter of the pipe connecting to the hydropneumatic tank, add the pump suction side to the report points collection and perform a batch run of both scenarios. You will see that the change in performance of the tank due to the change in headloss, will have an impact on the fast-reflecting waves on the suction side. I recommend using the Time History graph with both scenarios selected (see this article ), so you can observe not just the max/min transient pressure, but how it changes over time. You will notice the rapid-changing nature of the suction side to better understand the situation. 
 Also, indeed if the suction side pipe is very short, you may also see significant differences in transient results from changing the timestep, because as described here , HAMMER will automatically adjust pipe wave speed or length (depending on what you select) in order for a wave to travel from one end of the pipe to the other in even multiples of the timestep. With a very short pipe between two boundary conditions (reservoir and pump), a transient wave can reflect (and interact with the boundary conditions) many times in a short period of time, so the results in that suction side can tend to be quite sensitive. However, this would be independent from what I mentioned above. Meaning, changes to the hydraulics downstream of the pump can still influence the hydraulics upstream of the pump (with the timestep set the same) due to the shift in operating point of the pump four quadrant characteristic curve.