low velocities in the system

Hello Ahmed,

Where are you seeing the results you mention above? Generally speaking, the results you see in the element FlexTables, such as the pipe FlexTable, are for the steady state results only, and not related to the fire flow. You can view auxiliary results by setting up the appropriate fields in the fire flow Alternative. Knowing how you are viewing the results you are seeing and how you have set up your fire flow alternative would be helpful. For reference, here is a TechNote on understanding fire flow results: communities.bentley.com/.../2614.understanding-automated-fire-flow-results-tn

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Regards,
Scott

Hello, 

thanks for answer, i am aware of auxiliary results and the low velocities which i am talking about is in pipe flex table but the high velocities are in fire flow analysis table.

i have attached the model 

regards 

MODL.rar

regards 

Hello Ahmed,

As mentioned before, the results in the pipe flextable are only reflective of the steady state results, not the fire flow results. So a low flow in pipe flextable does not necessarily indicate a low flow in the fire flow run as well.

The velocity results in the fire flow report are for "Velocity of Maximum Pipe". This is paired with another results field called "Pipe w/ Maximum Velocity." With both of these added to the fire flow report, you will be able to see the highest velocity that occurs for all pipes in a given fire flow run and the pipe where that occurs. Again, this will not necessarily match the results from the pipe flextable, since the pipe flextable will only show the steady state results.

So what you are seeing is expected. The maximum velocity for all pipes for a given fire flow run is reported in the fire flow report. The velocities for a steady state run is reported in the pipe flextable.

Regards,
Scott

Hello Ahmed,

As mentioned before, the results in the pipe flextable are only reflective of the steady state results, not the fire flow results. So a low flow in pipe flextable does not necessarily indicate a low flow in the fire flow run as well.

The velocity results in the fire flow report are for "Velocity of Maximum Pipe". This is paired with another results field called "Pipe w/ Maximum Velocity." With both of these added to the fire flow report, you will be able to see the highest velocity that occurs for all pipes in a given fire flow run and the pipe where that occurs. Again, this will not necessarily match the results from the pipe flextable, since the pipe flextable will only show the steady state results.

So what you are seeing is expected. The maximum velocity for all pipes for a given fire flow run is reported in the fire flow report. The velocities for a steady state run is reported in the pipe flextable.

Regards,
Scott

Hello Scott

OK i understand the difference now between the two flex table calculated velocities.
but i am required by the client now to increase the velocities as they are below the minimum, i have tried to increase pipes or pump head with no considerable change. so i checked the model again.

regards,
Ahmed

Hello Ahmed,

So the client needs the *steady state* velocity to be higher? If so, you will want to confirm that the demands in the steady state are accurate. With a higher flow in the pipe from a higher demand, the velocities will be higher as well. Decreasing the pipe diameters would increase the velocity as well, but I am not sure that is possible in the actual system.

Regards,
Scott

If you are working with a drinking water system, there will be times when the velocity is very low, especially for situations where the pipe is sized for fire flow and the normal demand is small.

Just think about the pipe outside of a pump station. When the pump is off, the velocity is zero but there is nothing wrong with that.

The place where low velocity may cause problems is that where the water age at the end of the line is so low that the disinfectant residual is lost.

What size pipes are you using and what are the fire flow requirements? How large is the system in terms of distance from source to farthest user?

To increase velocities, you can always reduce pie diameter, but there is a tradeoff in reduced fire flows and peaking capacity.

In general, I would not worry about low velocities.