3 replies to this topic

[swapnilchemengg]

While deciding the diameter of pipe based on flow rate we generally consider, as a rule of thumb velocity in pipe to be 2-4 m/s and find the diameter of pipe. Is there any theoretical basis of this thumb rule ? If i have higher head pump such that it can bear head losses for higher velocities say 7-8 m/s can i reduce the diameter of pipe as it will result in less pipe line cost /

[ankur2061]

Swapnil,

Refer section 6.2 (6.2.1 & 6.2.2) of Norsok standard P-001 edition 5. The section is self-explanatory and dispels the old thumb rule of 2-4 m/s for liquid lines. It correlates MOC of pipe with the type of liquid to be pumped. As rightly mentioned if head loss is not a big deal then higher velocities can certainly be tolerated within the boundaries of erosional velocity.

Following is the link for a free download:

http://www.standard....er.exe?id=13264

Hope this helps.

Regards,
Ankur.

[rxnarang]

The "thumbrule" is actually a optimum velocity based on:

a) Capital cost of pipe
b ) Operating cost of pumping fluid
c) Density of fluid
d) Mass flow rate of fluid
e) Viscosity of fluid

a) and b ) have equal weightage and is less than c) and d)
e) effect is very minor.

Note that the length of pipe is not a variable.

So, if you want to MINIMISE capital cost, the solutions are different.

If you want to OPTIMISE COSTS ( Capital + Operating) then the solutions are different.

We have carried out senstivity analysis of the above problem, specially in view of fluctuating commodity prices.

Our conclusions are

Below 10" ( typical CS pipe, STD Schedule), use of the traditional thumb rules will not lead you astray.

Above 10" it pays to do an analysis.

Concentration on density, mass flowrate and hours of operation should influence you more than costs.

If your Objective function is to MINIMIZE Capital Cost, I suggest you keep this to yourself. As a client, I would be very unhappy if a consultant shortchaged me and gave me a pipe which is going to cost me for the next 10 years of plant life!

Regards

[JoeWong]

rxnarang, good say...

Besides, just wanna to add a bit more...

i) Erosion and corrosion aspect
ii) vibration aspect
iii) noise aspect
iv) Likelihodd of failure due to flow induced vibration
v) piping class management aspect
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