WaterCAD generates nice system head curve if the model has (reservoir -pump - reservoir) configuration. I have other case which common force main; two pumps joining one force main as shown in the screenshot below, the question is: why when one pump is operating (PMP-1) I have correct static head (25 m) but when two pumps operating the static head of (PMP-1) does change?
I trust the result but the presentation needs elaboration because if I attach the system head curve resulted from the model to the design report; it could be somebody will ask about the static head, in theory the static head should be fixed even other pump contributed with the force main.
I had this situation many times and was hard to convince others by the output in terms of how the static head change to satisfy the pump and operation and always they expect the static head reflect the real life.
I am looking to understand why that is happening? and how to explain that to others?
PS: Why I am not able to see my post at the Home tab??, I have to go to Forum tab to follow up, previously when add new post it does appear in the Home tab, Please support.
Regards........Mohamad
Hello,
Here are just a few related forum threads:
http://communities.bentley.com/products/hydraulics___hydrology/f/5925/t/60048.aspx
http://communities.bentley.com/products/hydraulics___hydrology/f/5925/t/57815.aspx
In short, when you look at a system head curve for a particular pump (right click on pump > system head curve), it considers the hydraulics of the rest of the system that you have set up, including the status of the other pumps.. So, if the other pump is on, that will change the hydraulics of the system and thus affect the system head curve. You should probably use the combination pump curve tool, as long as the pipes down to the common junction (J-3) have insignificant headloss.
Regarding viewing posts on the "home tab" - are you referring to the "Site Activity" widget, which shows your activity on be Communities? Are you also subscribed to get email alerts for the Hydraulics and Hydrology forum? (see "Email Subscribe" or "Email unsubscribe" link on the right side of the forum list (under "options".)
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Thanks Jesse,
I already participated in those posts; and as I said I trust the results and know more flow is more resistance and needs more head to overcome that, which will lead to changes in the hydraulics of the system head curve. but here come the confusing issue why the changes in the system head curve by increasing the static, i.e. why is not moving it to the left or wright (or change the shape) with keeping the static head at (Q=0) fixed? that is main challenge in elaborating this discussion.
Thanks..........Mohamad
The y-intercept is only equal to 'static head' in the simple case where you have one pump in the system. This case also happens to be the one all engineers learned about in school because it is easy to solve by hand, and so this causes a lot of confusion. But your model below is different to this simple case because you have 2 pumps.
In your example, the value where Q=0 is the minimum head that a pump must provide before it can deliver any water into the pipeline. This is dependent on what the second pump in your system is doing.
Try looking at it this way.
Q. In your example below, when PMP-1 and PMP-2 are both off (and assuming there are no demands in the system) what is the HGL elevation at node J-1 (the discharge side of the pump)?
A. It's 30 m (because that's the level of water in R-2).
Q. What is the minimum head that pump PMP-1 must provide before it can deliver any water?
A. 25 m (because that's the difference between the HGL on the discharge side of the pump, 30m , and the suction side of the pump, 5m). That's what we know as 'static head'. So far so good.
Q. If PMP-2 is turned on and PMP-1 is still off, what's the HGL elevation at node J-1 (the discharge side of the pump)?
A. You need to do a calculation here, but it looks to me like it is about 45m
Q. So what's the minimum head that PMP-1 must provide before it can deliver any water?
A. ~40m (because that's the difference between the HGL on the discharge side, ~45m, and the HGL on the suction side, 5m). That's why the system head curve shows a value of ~40m when flow is zero. You could say that pump PMP-1 must overcome the static head PLUS the dynamic head of PMP-2 before it can deliver any water.
If you are designing PMP-1, you must pick a pump that works well when PMP-2 is either on or off, so you need to look as system head curves for both cases.
Mal
Thank you Mal, that is perfect answer and it is really; exactly what I am looking for.
One more thing, I know that you estimated the HGL(@J-1) = 45m when PMP-2 is one and PMP-1 is off because I provided the system head curve and the elevation at the R-1, how to calculate the HGL at J-1 when PMP-2 is on and PMP-1 is off? how is the calculation would be (J-1 is not in the flow path)
Thanks..............Mohamad
I would probably just use WaterGEMS to compute the HGL at J-1 when PMP-2 is on and PMP-1 is off...
But you should also find that the HGL at J-3 is the same as the HGL at J-1. That's because there is no flow from J-1 to J-3 (so no headloss)...but they are still connected hydraulically so J-1 'sees' the HGL from J-3.