How does the Automated Fire Flow analysis work? 

The automated fireflow analysis routine will perform a series of independent steady state calculations, with various extra demands applied to your fireflow nodes. It first runs a steady state analysis on the scenario in question, with the "needed" fireflow applied only to one of your fireflow nodes. It checks the resulting pressure at that node, at other nodes in the same zone and (if desired) pressure at all other nodes in the model and velocities in the pipes. If these are within the constraints specified in your fireflow alternative, it will add a little bit more demand to that node, check the pressures, and repeat, until the pressure constraints are violated or the demand reaches the "upper limit" that you specify. In then reports this "available" fireflow and moves on to the next fireflow node, performing the same steps. 

The hydraulic results seen in my junction/pipe table and element properties do not seem to account for fire flow. Why? 

After the fireflow calculations are complete, WaterCAD/GEMS computes an additional steady state simulation, without any fire flow demands. This establishes baseline results for the model and is what you see in the element properties and flextables. To see results for fireflow calculations, you must check in your Fire Flow report (flextable) or in the "Results (fire flow)" section of the properties. To see hydraulic results for elements other than the fire flow nodes themselves, you must use the Fire Flow Results browser.

Why do I see "N/A" for all my nodes in the fireflow report (flextable)? 

You may not have your calculation option set to perform the automated fireflow calculations. Go to Analysis > Calculation options. Double click the calculation option set associated with the scenario in question (designated by the red check mark) and change the "calculation type" to "fire flow". Also ensure that you are not encountering any red user notifications, which indicate a critical data-entry problem that prevents the model from being calculated. Note that "N/A" entries can also be caused by omission from the fireflow selection set. In your fireflow alternative, make sure that all the nodes you'd like to study are included in the selection set selected for "Fire flow nodes". The fireflow routine will only analyze and provide results for nodes in this selection set.

How do I remove the "N/A" entries in the fireflow report for nodes not included in my fireflow nodes selection set?

In cases where a selection set is chosen for the "fire flow nodes" field in your fireflow alternative, results will not be computed for nodes not included. In this case, you can filter the fireflow report/flextable using one of the two methods:

1) Open the selection set manager, double click the selection set used in your fireflow alternative, right click anywhere in the drawing pane, choose "edit group" and open the fireflow report.

2) Open the fireflow report/flextable, right click any column header > filter > Custom. Filter like this: Fire flow iterations > 0

Why do I get zero available fireflow for one or more nodes in my fireflow report? 

This is likely due to the pressure or velocity constraints being violated, even with the baseline demands. Check the "calculated residual pressure", "calculated minimum zone pressure", "calculated minimum system pressure" (if using this constraint) and "velocity of maximum pipe" (if using velocity constraint.)

My fireflow report shows fireflow nodes failing due to low pressure at the suction side of a pump. What can I do about this? 

If you are not concerned about this, you can create a special "low pressure" zone and select that as the zone for the node in question. This way, it will not be considered in the automated fireflow analysis (unless you select to have a minimum system pressure constraint.) However, you may want to make sure the reservoir elevation upstream of the pump is correct and that the pump intake pressure meets the minimum NPSH requirements. 

What is the Fire flow results browser used for? 

The fire flow results browser is used to check results for others elements in your model, during individual fire flow runs. Say for example you wanted to see the exact pressure distribution, valve status and pipe velocities when fire flow was applied to Hydrant #1. Say for example Hydrant #1 had a total available fireflow of 1000gpm. Selecting Hydrant #1 in the fire flow results browser would switch the calculated results shown in element properties (and flextables) to show the results of the model when the consumption (demand) at Hydrant #1 was 1000gpm. You could then color code pressure, check velocities in the pipe report, etc. You could also easily check other fire flow nodes' "auxiliary" results by simply clicking them on the list. Before using this tool, ensure that the fire flow auxiliary results type and output selection set are chosen, in your fire flow alternative.  

What if I want to see how much pressure is available exactly at the total needed fire flow? Do I need to set the upper limit equal to the needed fire flow?

No, you don't. Click the yellow edit button at the top of the fire flow flextable and add the fields "Pressure (Calculated residual @ total flow needed)" and "Pressure (Calculated zone lower limit @ total flow needed)". These will show you the residual pressures exactly at the total needed fire flow, instead of showing at the total available. 

Is there an easier way to check residual pressures at a hydrant, for a range of flows? 

Yes. With normal base demands entered for your model nodes, simply right click on the hydrant/junction and choose "Hydrany flow curve". This will generate a curve of flow versus pressure. Basically it will run several steady state simulations with various demands on that node, reporting the pressure at the node for each one. For example, if you wanted to see how much flow could pass at 20psi, you could simply look up the flow for 20psi on the hydrant flow curve. 

See Also

Product TechNotes and FAQs

Haestad Methods Product Tech Notes And FAQs

[[General WaterGEMS V8 FAQ|General WaterGEMS V8 FAQ]]  

WaterGEMS V8 Modeling FAQ 

[[WaterGEMS for ArcGIS FAQ|General WaterGEMS for ArcGIS FAQ]] 

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server