HAMMER (10.03.01.08) - Pump speed & Flow graph

Question

Dear Bentley Team, 
 After pump trips (Shutdown after time delay case) why continuous reverse flow showing on graphs, it should reach zero flow after some time, but graphs are not showing the same. Once flow is reached to zero then graph should end even though we consider any run duration time. Please refer attached graph with PINK highlighted color, continuous reverse flow showing in between -200 l/s to -300 l/s, not reaching to 0.0 l/s. 
 Same as flow, pump speed graph showing that pump is continuously running reverse, it should reach zero speed after some time, but graphs are not showing the same. Once speed is reached to zero then graph should end even though we consider any run duration time. Please refer attached graph with RED highlighted color, continuous reverse speed is showing in between -750 rpm to -1000 rpm, not reaching to 0 rpm. 
 Could you explain why theses graphs are showing like this, as client is not accepting this graphical representation. 
 
 Thank you

Yashodhan Joshi · Answer

Hello Jayakanthan, 
 Please check if you have setup a check valve downstream of the pump with a delayed closure. Due to this delayed closure there could be reverse flows in the pump. Also check the pump properties and see if you have set the "Reverse Spin Allowed?" property for the impeller to "True". 
 If you are using instant closure in the check valve, then check to see if an air or vapor pocket is forming between the location you are graphing and the check valve. If cavitation occurs, the water column can flow in reverse as the air/vapor pocket increases in size. 
 See the below article on Check Valves and their use in HAMMER for reference; 
 Modeling Reference - Check Valves in HAMMER 
 Let me know if this helps.

Jesse Dringoli · Answer

Jayakanthan, 
 Is a vapor or air pocket forming in your system? I would recommend examining a profile animation in the Transient Results Viewer to see if this occurs. if so, it could be that the pressure remains low enough with the pump shut down that a vapor or air pocket continuously increases in size, and the flow you see is the water column moving away from the pocket (and toward the pump). By default vapor and air pockets are limited to the location of formation (the liquid interface is not tracked unless using the Extended CAV option for air valves), so HAMMER does not know how long it would take to drain out and reach a zero flow condition. See related article: Assumptions and limitations of tracking air or vapor pockets in HAMMER 
 If this is not the case or if you are not sure, please provide a copy of the model and steps to reproduce: Sharing Hydraulic Model Files on the OpenFlows Forum

Yashodhan Joshi · Answer

Hello Jayakantan, 
 As Jesse suggested this is due to air pocket formation which is causing the air volume to increase without going down. As air pockets are not tracked beyond their point of formation it never reaches zero and the column separation continues to push water away from that point towards the pump. See the animated profile below; 
 
 See the air volume continuously increasing after your pump shutdown event. This shows evidence of an ever increasing air pocket.

Jesse Dringoli · Answer

Jayakanthan, 
 As was previously explained, the problem is due to a limitation of HAMMER's tracking of vapor pockets. When the pressure drops below the vapor pressure limit at the pipe end point, a vapor pocket forms and continues to expand in size since the pump remains off. Since HAMMER does not track the vapor/liquid interface and keeps it at the point of formation (the end junction) as the pipe drains out, flow continues in the reverse direction. The water column in the rising main "pushes" the water back through the pump - this can happen even when using the option to prevent reverse spin of the pump, as water can still flow through the pump in reverse with the impeller not spinning as mentioned in this article . 
 Jayakanthan said: What is the best solution for this type of systems. 
 In this situation it may be best to simply explain the limitation to the client. Certainly the continuation of reverse flow does not reflect reality but the primary purpose of the model is likely to assess the transient impact of the pump shutdown and HAMMER does model that. Meaning, it correctly handles the transient that happens when the pump first shuts down. So, although you may not necessarily be able to see the system "settle" to its zero flow point, that may not really matter since you are still able to see the immediate transient impact. 
 Note that I would also recommend using the Discharge To Atmosphere (D2A) element at the end of the system instead of a junction with demand. See more about why in item #2 of the "common application" section of this article: Modeling Reference - Discharge To Atmosphere You can use the junction demand (600 l/s) as the "flow (typical)" and the observed junction pressure (1.348 bars) as the "Pressure drop (typical)" field in the D2A properties so that the initial conditions results are equivalent to the junction demand approach. 
 If you really want to see the flow reach zero as a result of the pipe draining out, you would need to apply a workaround involving the use of the air valve element. Even though you may not have an air valve in the system, you could place one at the end of the rising main in order to use the Extended CAV feature. Extended CAV can be enabled in the calculation options and enables HAMMER to track the air-liquid interface for air pockets at air valves. See more in the "tracking of air pockets" section of this article . This has some limitations of its own such as needing to have the two adjacent nodes as junctions at a lower elevation. It also can only track the air-liquid interface in the two adjacent pipes, so you would need to merge the multiple rising main pipes into one pipe in order to see it fully drain. I tested this and was able to see the pipe drain out by about 120 seconds. Below is a screenshot showing the plan view setup - I can provide a copy of the model if needed.