Calculating available flow at a hydrant

Product(s): WaterGEMS, WaterCAD, HAMMER
Version(s): CONNECT Edition, V8i
Area:  Modeling


How can you calculate the actual flow available when opening a hydrant nozzle, accounting for losses based on the orifice diameter, etc? (for example to be able to show the available flow at each hydrant location)


First it is important to mention that there are two ways of interpreting this:

1. How much water comes out if you discharge to atmosphere. One way to get this is to measure the flow when you do your main flushing. Another way is to specify an emitter coefficient for each hydrant and run the model of each scenario. This is tedious but will work. The discharge to atmosphere, however, is a pretty useless number as how often is the grass in front of the hydrant burning but some people think it has value.

You can simply enter an emitter coefficient at the junction or hydrant element corresponding to the actual hydrant size. The correct emitter for your hydrant size will account for losses through the nozzle and the outflow will be dependent on the pressure available in the system. There is a discussion of this in Advanced Water Distribution Modeling and Management, pages 451-453. Typical values for a 2.5 in outlet are on the order of 165 to 185 gpm/sqrt(psi). With this, you can see the predicted hydrant outflow based on the pressure vs. outflow relationship modeled by the emitter coefficient.

You can also take a look at Conventional Flushing in the Flushing tool, which allows you to essentially see results in batch for multiple, separate hydrants.

If the hydrant has been placed in-line with the distribution main pipes (lateral pipes not explicitly modeled), you can account for any losses through the lateral pipes using the fields available in the properties of the hydrant node. See: Calculating headloss through a hydrant lateral

2. A better number is the distribution capacity (available fire flow-AFF). You can get this for each hydrant in one step with the fire flow analysis in WaterGEMS/CAD. See:

How does the Automated Fire Flow analysis work?

Understanding Automated Fire Flow Results

However, this is the water you can deliver to the hydrant at a 20 psi residual (the typical value for the residual pressure limit in the United States), not the amount that will come out. Consider a 48 in. pipe, the available fire flow may be 50,000 gpm but you can only get out about 2000 gpm because of the hydrant. A suggestion is to pick a maximum flow from a hydrant (usually 1500 to 2000 gpm). Any hydrant with an AFF above that value will be credited with 1500 to 2000 gpm. AFF values below the max will be color coded based on the AFF. See: Available Fireflow vs. Upper Limit

See Also

Options for modeling an outflow that varies with pressure

(Blog) One Hydrant vs. Two Hydrant Flow Test