8 replies to this topic

[kangas]

Hello everyone,

 

So here is my question.

I have a tank at an elevation of 10m. This tank will drain water under gravity into another tank at 0m elevation.

I calculate my velocity due to gravity which is about 14m/s.

Using this and the diameter of my pipe I can calculate the flow rate. However, as I know in piping rules of thumbs the velocity must be 1-3m/s. How can I do this? or I need to live with 14m/s? Is this going to damage my pipe?

 

Regards

[Francisco Angel]

If I remember correctly,  a recommended velocity exists as a rule of thumb because as you increase velocity the pressure differential also increases (increasing the cost of the pump), but if you use a too small velocity, the required pipe diameter gets too large.

In this case, provided that your pressure differential is provided by gravity, you are not constrained by pump cost.

I hope a more experienced engineer correct me if I'm wrong and provide better insight.

Best regards.

[Bobby Strain]

You can use a restriction orifice to achieve whatever you wish. But I don't think you have told us anything about your objective. For instance, if you need to control the flow rate, you will need a control valve to accomplish this.

 

Bobby

[cea]

As you know, governing factor for maximum flow that can achieve in any pipe is the available pressure drop. Velocity is just guiding factor to guess optimum diameter of pipe, so as to arrive at right hardware cost (pipe, driver (pump etc)) & right  operating cost.

 

One should also take into consideration erosion in pipe due to high velocity while deciding velocity value.

 

In your case, available head is about 10 m. If you try to "eat" entire available pressure drop by reducing pipe diameter, you are bound to get high velocity. Hence, suggest you to keep available pressure drop as "reference point" & check that your selected line size shall offer pressure drop below it.

 

Now your pipe being free flow pipe, you are requested to consider velocity value, that is good for gravity flow (recommended 1 m/s).

 

Another aspect, that shall be consider is "self venting" requirement. In order to ensure self venting requirement, you need to make sure that froude no. shall be less than 0.3. Accordingly, you need to adjust velocity by changing diameter.

 

Thus, once you can get all three parameters (recommended velocity, pressure drop, froude no.) matched, you can consider your pipe diameter estimation is correct.

 

Hope, this answers your query.

[kangas]

Thanks everyone for your replies.

 

cea - how can I calculate the maximum flow rate given by the maximum pressure drop. Without flow rate, I can't find diameter of pipe/

 

Andreas

[cea]

Thanks everyone for your replies.

 

cea - how can I calculate the maximum flow rate given by the maximum pressure drop. Without flow rate, I can't find diameter of pipe/

 

Andreas

Well, you need to know either of flow or time required to drain the tank or drain line size (pipe diameter). The above discussion is valid only if you have sufficient input.

[breizh]

Hi ,

you may find some interest in the spreadsheet I developed a few years ago.

 

Good luck

Breizh

[kangas]

Hi ,

you may find some interest in the spreadsheet I developed a few years ago.

 

Good luck

Breizh

Thanks a lot Breizh,

 

I have one final question. For a fluid flowing under gravity out of a tank does its flow rate depend on the diameter of the pipe that will deliver that fluid?

 

 

[breizh]

Hi ,

If you change the diameter in the spreadsheet you should observe some differences ? are they significant?

Bigger diameter means less resistance .

Hope this helps

Breizh