10 replies to this topic

[Sherif Morsi]

Hi,

 

I am recalculating the pressure across an existing 16" line and there is a 22m high vertical portion of it. Do I account the friction losses in this vertical segment or I can ignore it compared to the head losses?

 

I also need some mathematical/technical proof for it.

 

Regards,

Sherif

[latexman]

It depends.  Tell us more.

[katmar]

Is it up-flow or down-flow in the vertical section?

[Sherif Morsi]

It is an upward flow. 

 

It's a water injection line that is running from a filtration unit to the top of the De-aerator tower before injection.

The line is around 15 m horizontal segment with some valves and fittings then goes upward for 22 m.

[Bobby Strain]

What leads you to believe that you should not include friction loss in the vertical section? Do you think that somehow the vertical section allows frictionless flow?

 

Bobby

[Sherif Morsi]

No at all ... It's an inquiry if it's too small compared to the head loss hence could be dropped off and not considered. That's why I am asking

[Bobby Strain]

You should not select a particular pipe section to eliminate from your engineering assessment. Your work should be thorough and complete. But neither should you report values such as 10.3567. Round the values to suit the requirement. When you speak of head loss, that is not an accurate description. Head loss refers to both static and dynamic loss. The proper term to describe the pressure difference between two elevations is static head. And, what is the purpose of your exercise? Is it only a part of a broader hydraulic analysis? As a practicing engineer, if your employer doesn't have guides and standards, then you should use your engineering judgement.

You may recall an earlier post by a member puzzling over the pressure drop in a municipal water system. I repeatedly asked for elevation at the source and destination. I don't believe he ever understood that he could not reference to grade at each location, but must use elevations.

So, everything is important in engineering. Don't develop sloppy habits. It eventually leads to trouble.

 

Bobby

[katmar]

This is a question that cannot have a general answer because the friction pressure drop depends on the flow rate and the static head depends on the elevation change. You are comparing apples and oranges. Until you calculate these two pressure drops you do not know if either of them can be neglected.

 

Even if you had been doing a lot of 16" pipe pressure drop calculations recently and you knew that the velocity in this line was low and your experience told you the friction loss was negligible you should still document how you came to the design decisions that you did so that someone else working on this line at a later stage will have the full picture.

[Sherif Morsi]

Thanks for all the feedback.

 

Bobby,

This specific exercise is just reviewing the pressure losses in the line after we install a new filtration unit. The line starts at EL+114 runs for about 15 meters then takes off vertically for 22 m to the top of the Deaerator tower. The tower is operated at vacuum.

 

Sherif

[navneet2109]

Find the pressure drop for both and add, it will give you total pressure drop.. Thats it..

[sgkim]

Sherif,

 

If you are to calculate the total head loss of the water line from the suction tank to the deaerator, you should count on vertical and horizontal piping itself and also every part of piping components, such as inlet and outlet nozzles for a equipment connected, elbows, tees, reducers, valves, filter or strainer and tip nozzles if any.  

 

Vertical elevation, the only static head loss, shall be added to total head loss.  Friction thru filter or strainer and nozzle would be major portion of dynamic head loss and the rest thru all piping components .  The pressure head due to the vacuum in the deaerator might be deducted from total head loss but some head allowance in case of vacuum failure may also be essential.  

 

Bear Bobby's comment in mind, please.

 

Stefano Kim         

Edited by sgkim, 07 October 2016 - 06:20 AM.