RE: Fire Water network

Question

Ref: http://communities.bentley.com/products/hydraulics___hydrology/f/5925/t/77570.aspx 
 Hello Jesse , 
 I modeled the hydrants as a discharge to atmosphere elements which fully opens in 1 second. the transient engine gave an error that the pump flow should be larger than the flow tolerance ! 
 So I used a very small demand ( 1% ) at a junction so that the flow from pumps is not ZERO & the system worked as expected. using a very small demand could be the most real case ,as you always advise to model what we really have , because fire systems are kept pressurized using a jokey pump which flow is very small compared to system flow. 
 Before I read this article ,I tried to model the fire network using pump type a constant speed - pump curve & an FCV before each hydrant. the hydrants are set to be initially opened. the FCVs are initially closed with an operating rule to start with a relative closure 100% & opens in 1 second after start & standstill open till the end of simulation. the system never worked properly such that the pumps never delivered the design flow & flow kept oscillating between positive & negative through the pumps & through the hydrants. the notification was that the pump is not working in its normal range. so , what is wrong with this modelling ? 
 Best, 
 Ahmed

Scott Kampa · Accepted Answer

Hello Ahmed, 
 We may need to see you model for this issue. First though, what exactly do the user notifications you are reporting say? And do they happen after computing the initial conditions? 
 One key part of transient model is that the initial conditions need to be an accurate reflection of the start of the transient event. That doesn't mean that user notifications cannot occur, but you will want to review these to make sure that they are expected. If not, some adjustment would need to be made in order to get an accurate reflection of the starting point of the model. 
 Regarding the setup of the operating rule for your FCV, your pump flow may not be accurate. One thing you could try is to start with the valve open and then *slowly* close the FCV. Allow the results to reach a new equilibrium and then reopen the FCV. This may allow you to have accurate pump flow results for the initial conditions while still seeing the results you want. It is important to both slowly close the FCV (to minimize the transients that may occur from the valve closure) and to wait for the model to reach a new equilibrium (to allow for an accurate starting point for the results when the FCV is reopened). The timing may take some trial and error, but it may yield better results. 
 However, for modeling hydrants opening, the discharge to atmosphere element is the best choice. It may just be a matter of getting the initial conditions corrected. Knowing the user notifications you are getting will be important for this. Here is a TechNote on the discharge to atmosphere element, which includes some general information for modeling the hydrants opening and a sample model. The sample model may also be at C:\Program Files (x86)\Bentley\HAMMER8\Samples\Discharge_To_Atmosphere_Example.wtg for later releases of HAMMER. 
 You can also send us a copy of the model if you need to. There are two options for sharing your model files on BE Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley. 
 communities.bentley.com/.../bentleysecurefilesupload.aspx 
 If you upload the model to Sharefile, please post here to let us know that it is available. 
 Regards, 
 Scott Kampa 
 Bentley Technical Support

Scott Kampa · Answer

Hello Ahmed, 
 In the model you provided, you have demands on the hydrants, but FCVs that are closed in the initial conditions. The initial conditions work just like WaterCAD and WaterGEMS. All demands need to be satisfied in order for the result to be accurate. However, since the FCVs are closed, accurate results are not obtained for the initial conditions so bad data will be passed to the transient solver. 
 I would refer you to what I said in an earlier post: 
 One thing you could try is to start with the valve open and then *slowly* close the FCV. Allow the results to reach a new equilibrium and then reopen the FCV. This may allow you to have accurate pump flow results for the initial conditions while still seeing the results you want. It is important to both slowly close the FCV (to minimize the transients that may occur from the valve closure) and to wait for the model to reach a new equilibrium (to allow for an accurate starting point for the results when the FCV is reopened). The timing may take some trial and error, but it may yield better results. 
 However, for modeling hydrants opening, the discharge to atmosphere element is the best choice. It may just be a matter of getting the initial conditions corrected. Knowing the user notifications you are getting will be important for this. Here is a TechNote on the discharge to atmosphere element, which includes some general information for modeling the hydrants opening and a sample model. The sample model may also be at C:\Program Files (x86)\Bentley\HAMMER8\Samples\Discharge_To_Atmosphere_Example.wtg for later releases of HAMMER. 
 Part of the issue with the FCV usage in your model is that the valves are closed, so there is no flow to the satisfy the demands that are on the hydrants. If you need to use FCVs you need to start with the FCV set to Active and adjust the Operating Rule so that the FCV is initially open, closes *slowly*, then reopens when any transients that occur from the valve closure stop, then reopen the valves again. You can then look at the results from when the valve opens to see what happens in your modeling case. This may take some trial and error to before you get to a place where you can look at the results. 
 However, using Discharge to Atmosphere elements to simulate the hydrants would be the best alternative. Please note that if you use the D2A element, you don't need to include the FCVs. Only the D2A is needed. There is a sample model showing how this is done. The path to that can be found above. There is also a D2A TechNote that gives some general information on the element itself, including the general usage. That TechNote can be found at this link. 
 The model you sent didn't have the Discharge to Atmosphere element included. As mentioned, the "unbalanced network" may be occurring if you also have the closed FCV there as well. Removing the FCV or making it inactive will hopefully help with that issue. If not, I would need to see the model with the Discharge to Atmosphere elements in it. 
 Regards, 
 Scott