I am seeking help in Hammer V8 XM.
The issues i want to confirm are:
- How do you model a positive displacement pump? Note: that the way the system works there are no demands (therefore can't make it seek a flow rate), as it pumps to either a high level reservoir or a low level reservoir.
- How do you correctly model an altitude valve (i.e. i have an altitude valve to one of the reservoirs. I have set it to open over a set period of time, and entered a discharge coefficient). The main problem is that the results are showing no pressure change during the closing of the valve, until it reaches 100% closed, then it spikes. Even if i extend the closure period i am getting also the exact same results. I also can not plot the reletive closure of time for the valve (is this because it is a transient?). i need to know if the valve is working correctly.
Your help would be much appreciated
Hopefully you have also sent these questions to the Tech Support Group, as they can typically provide a faster response than you might get over the forums...however, if you are still needing help:
- The Pump element in HAMMER represents centrifugal pumps. Since positive displacement pumps behave quite differently to centrifugal pumps, it is not advisable to model a positive displacement pump using the Pump element. A work-around for positive displacement pumps that many people have found satisfactory is to represent the pump as two disconnected nodes - one represents the pump suction, the other represents the pump discharge. Then you can add a 'demand' on the suction node and an 'inflow' on the discharge node (where 'demand' = 'inflow' = flow through the positive displacement pump). Of course this is just a workaround, and we will look into ways to more accurately model positive displacement pump behaviour in future (by the way, if you or anyone knows of any research work on positive displacement pump behaviour during transients, please let me know).
- Typically if you do not see any reaction during valve closure until the very end of the closure operation then you should check the valve discharge coefficient. If the discharge coefficient is too high (i.e. there is very little headloss through the valve) when the valve is open, then there will also be very little headloss through the partially closed valve (so it will appear as though the valve closure is not working). Then when the valve becomes fully closed you will see a big 'spike' in pressure on the upstream side of the valve (as the column of water flowing through the valve is stopped abruptly).
Hope that helps.