I need technical assistance for the attached cooling water model
KSP-Cooling Water System.wtg
Process Brief: The cold water from the elevated tank (actually a cooling tower) is pumped (fixed speed pump) through heat exchanger (chiller) and back to the tank. There are 7 cooling towers. 7 pumps and multiple heat exchangers all connected through common header arrangement. The model is attached. Now the task is to evaluate the head developed by the pump for this process.
Please provide your inputs for following:
Thanks
Pratik
This model require wtg,sqlite to open! can you provide it??
There is an error while uploading the sqlite file, it says contact admin
Did you try to upload using link provided earlier? Sharing Hydraulic Model Files
Or you could directly upload here in your reply if the model files are not confidential or you can hover over name and send a private message with model files.
Regards,
Sushma Choure
Bentley Technical Suppport
I have uploaded the file in Sharing Hyraulic Model Files
Hello Pratik,
In your pump definition, you have entered a design point where the flow is 14300 gpm and the head is 35 meters. Since your pump flows are all right around 14300 gpm, this is the reason that the pump head is around 35 meters.
If you are saying that the pump head needs to be lower, you would need to adjust the pump definition. If the pump definition is accurate, the head added by the pump is accurate as well.
Scott
I tried udpating the pump definition, by decreasing the had to 22 m. But then the error i receive is that the FCV's cannot achive the desired flow of 14300 gpm and also the pump flow drops below 11,000 gpm (less than the design requirement of 14300 pm)
As Jesse noted in his response above, this might be a calibration issue. However, before looking into that, I would recommend reviewing the pump information you have. Are this existing pumps or are you trying to decide on the correct pump? If they are existing pumps, I would recommend getting the actual pump definition to use.
Otherwise this may be a calibration issue, as Jesse noted above. You have demands on the tank elements, however this will not impact the pump flow directly. Instead, the demand will come directly from the tank. The pump flow will be based on the pump definition, headlosses in the system (from pipes and valves), and the initial hydraulic grade of the tanks. Confirming that the tank elevations are accurate is the first step, but you will also want to make sure that you have pump data that is as accurate as possible. Then you can try adjusting valve headloss and pipe roughness coefficients to see if you get a better match with expected results.
This link has information on model calibration: Water model calibration tips. If you are using WaterCAD, you will not have access to Darwin Calibfrator, but the link includes other tips for model calibration as well.
See also: Why can’t I specify both head and flow at a pump?
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Hello Scott,
Please see the model attached
KSP Cooling System.zip
It is a new plant (not existing), hence i do not know what is the devloped head in the new piping system. A Sushma higlighted that i can still know the pump head by deleting the pump and noting the difference in the junction HL. Please note roughness factor is specific to the pipe material and it needs to be higher considering the pipe aging. And lowering the pipe roughness does not impact the piping frictional loss significantly.It is a relatively a simple process where in the elevated tank is at 7.68 m and the inlet pipe to the tank is at 16.93 m. Therefore the static head to be worked by the pump is 16.93-7.68 = 9.25 m. Considering the fixed pressur loss across the heat exchangers and diffusers the pressure drop is 1.22+2.23+2.59+1.22 = 7.26 m. Therefore the pump head should be 7.26+9.25+piping frictional loss = 16.51m+piping frictional loss. Now the pupm head derived by th emodel is approx 35 m.
I have checked the model umpteen times, i could'nt find my way through or understand why is the pump developing so much high head.
Also, here in SNC Abu Dhabi office, we had being using Pipe-flo as process hyraulic software. However Since SNC Global had purchased the Bentley Global licence, we have been asked to migrate to WaterGems and provide feedback on our process parameter. Hence for our new project I started using WaterGems. Comapring the same parameters/piping aranagment/model between WaterGems and Piep-flo, I get two different results.
WaterGems --> Pump Head developed @ 14300 gpm is 35 m
Pipeflow --> Pump Head developed @ 14300 gpm is 22 m
This a huge difference and directly impacts our project. Hence would appreciate if you can help me find the error in the watergems model attached.
Regards
Pratik,
It is helpful to look at the system in profile view to understand where the losses occur and where the boundary hydraulic grades are:
You mentioned that the tank inlet is at 16.93 m but from the above profile (and from looking at the tank properties) you can see that it is set to 24.56 m.
I think there may be some confusion regarding the tank "Level (inlet invert)" field. - you have entered 16.93 m there, but that is a level, above the base elevation as the datum. So, you have modeled the inlet invert as 7.63+16.93=24.56 m. If you want to model the elevation of the inlet as 16.93 m, then the "Level (inlet invert)" should be set to 16.93-7.63= 9.3 m (for all tanks)
See related article: Error or bad results when using top filling tank option
With this fixed (for all tanks), the profile will show the expected hydraulic grade on the downstream side:
However, your pump head will still be about 35 m, to overcome the static and dynamic lift. In the above updated profile view, you can see some frictional loss across the pipes, but most of the losses are at the valves. You cited fixed headloss values, which you have achieved in the GPV elements, but the headloss at the FCV elements is variable. For example FCV-21 has a headloss of 10.3 m after fixing the tank inlet issue. This is because the FCV throttles automatically to induce a headloss necessary for the system to balance at the desired flow. Meaning, in order for the flow at FCV-21 to be limited to 14,300 gal/min, it needs to induce 10.3 m of headloss so that the pump shifts it's operating point to produce the flow. This is explained more here: Why do I see such a large headloss through my FCV, PSV or PRV?
If we then look at the pump definition, we can see that at a flow of 14,300 gal/min, the corresponding head on the pump curve is 35 m. This is why you see a pump head of 35. See more here: How are pumps handled in each OpenFlows product?
You mentioned that another software seeing a head of 22 m at a flow of 14,300 gal/min but that would be impossible with the pump curves set the way you have them.
Setting the pump definition's design head to 22 m results in a flow loss than 14,300 because the pump shifts its operating point, adding 25 m at a flow of 10,800 gal/min, due to varying headloss.
If you want to model a specific, fixed headloss across the "chillers", then you should use the PBV (pressure breaker valve) element instead of the GPV, because the GPV requires a curve and can shift on that curve. A PBV enables you to enter a specific head or pressure loss that applies to any flow. You can use the batch morph tool after selecting all GPVs, to convert them to PBVs. Set the PBV type to "hydraulic grade". See: Modeling a Constant Headloss
With the tank inlet elevation fixed, the pump design head set to 22 m, the GPVs converted to PBVs and the FCV disabled (status set to inactive), the pump head is much closer to your expected. In my revision I am seeing a pump head of about 21.5 m. The slight difference from 22 m may be due to differences in pipe frictional losses. You may want to take a closer look at the assumed pipe diameter, roughness and length.
Here is a copy of my revision:
KSP Cooling System - BentleyMod.zip