Hydraulic Calcs - Effect of Roughness of Aged Pipe

[kdt]

1) Does anyone have a good reference regarding the effect of increased pipe roughness due to corrosion and what should be looked at in the hydraulic calculations.

We are looking at adding approximately 5km to an existing piping system and are using 5x roughness for the pipe.

2) Does corrosion (increased pipe roughness) affect the pressure drop across fittings? Can this be calculated or is this handled by an additional design factor? (fittings account for 20% of the system pressure drop)

3) I have been calculating the pressure drop using K values for fittings. Is it more appropriate to use equivalent length method when modifying the roughness value?

 

[sterl]

Pressure Drop in fittings at normal design velocities is predominantly an idealization of the Change in Momentum converted to Dynamic Head.

Roughness as highly tuberculated in small diameter pipe could effect fitting Delta-P but more common to find the Section Modifiers accumulating loose debris if the velocities are low; or in contrast, eroded where debris is travelling at high velocities.

In a large diameter, relatively "clean" fluid system, I would treat fittings as smooth-as-new, and the straight pipe as roughened by most forms of corrosion.

 

[BRIS]

There is plenty of literature on roughness of aged pipes. If you are considering ductile iron pipe have a look at the DIPRA guides as below.

http://www.dipra.org/pdf/cement-mortar-linings8page.pdf

This shows results of friction loss calculations on real pipelines (in terms of C).

Generally hydraulic design of pipelines needs to be carried out using 3 different roughness values: highest expected (future aged), lowest expected and predicted. You use the highest values to make sure your pumps are going to meet the required flow and any end control valves have adequate capacity etc.. lowest values to make sure your pumps don't run off the end of the curve, for worst case transient pressure calculations and for cavitation calculations at any end control valves. Expected average values for best efficiency points etc.

The use of K values for fittings is standard practice. The loss in fittings is due to the fitting causing a local velocity increase due to asymmetric flow and the subsequent loss of energy in the dissipation of the velocity (derived from momentum principles) not from roughness.

Brian

 

[Mech85]

The web page referenced below gives a table with multipliers for the "Increase in Friction Loss Due to Aging of Pipe".The web page does state that the values are largely theoretical and should be used for estimation purposes only.

http://pump.net/frictiondata/friclossagepipe.htm