I am reviewing engineering standards for my company's irrigation systems.  I am looking at design and maximum fluid velocites in our mains and transite lines.  They are C-900 or C-905 PVC typically ranging 6"-18".  We use reclaimed water.  I am concerned that 5 fps may present excessive thrust forces at directional changes (I am looking for conservative specs).  Also, I am not sure about a minimum velocity needed to prevent buildup of sediment, etc.  Can anybody recommend any good resources on this and related topics.

5 f/s (1.5 m/s) is pretty quick, but not unreasonable.  Good thrust blocks will ensure that the pipe and appurtenances will remain in place.  Hammering may preseeent a bigger problem if the line is too long without  subsiduary looping to diffuse pressures, or nearby reservoirs.

KRS Services
www.krs-services.com

As a rough rule of thumb, velocities above 0.75m/s are often specified to prevent sedimentation in dirty water lines. The exact figure would be dependent on the size and density distribution og entrained solids. Pumped line velocities up to 3m/s, and gravity lines around 1m/s are usually reckoned to be suitable from the POV of headloss (subject to confirmation by hydraulic calcs). Thrust blocks etc are then usually designed around the resulting forces.  

Sorry about the metric. Funny how you call imperial units British, but we dont use them any more...

Seán Moran

www.expertise-limited.co.uk

I agree with KRSServices...5 fps is not considered excessive for pressure pipelines.  Often velocities up to 8-10 fps are considered acceptable.  Velocity is generally considered for two reasons...1) erosion of the pipe material and 2) energy loss from greater frictional forces.  If you're concerned about thrust forces, you should concentrate on thrust restraint requirements  because thrust is a function of system pressure and cross-sectional area, not system velocity.  That means you could have a 18" pipe with velocities at 0.1 fps (or even zero) and still have trouble with thrust at bends, dead ends, reducers, etc.  

If you post your fax #, I will send you the pages you need out of the book Buried Pipe Design, Moser. You really should not have sediment build on reclaimed water, unless the reclamation process is not very thorough. And if that's the case, I wouldn't use it. If you can see solids in the stream, then it's the stuff you don't see that's scarry.

One thing I forgot to mention, in pressure systems, unlike gravity, flow characteristics in the pipe tend to lend themselves to siltation or accumulation of material on the bottom of a pipe.  I'm not referring to scaling.  If you have ever noticed, many municipalities, particularly those with surface reservoirs such as ponds or lakes, flush their system to remove this material.  I have had to implement many such programs due to the buildups.  With the velocities you mentioned, and considering that the water is likely from a surface source, flushing (at least annually) will be required to remove the buildup.

KRS Services
www.krs-services.com

Velocities for water systems should average 5 fps and not exceed 7.5 fps when fire demand plus peak demands are flowing.  If you are over this then you should strongly consider upsizing or looping the mains.  For flushing, 11 fps or greater is recommended as the debris are not siltated (in which case 2 fps would be fine).

BobPE

BobPE,

It is always a pleasure to read your posts. Nice work.

Keith   

The cost benefit analysis needs to be done as to cost of energy over the life of the pipeline versus the cost of constructing and maintaining the pipeline. Long distance pipelines would work out at 5 fps or lower depending upon the cost of energy/costruction labour/materials etc.

When choosing the Pressure Class of pipe consider working pressure.  C900 has a surge allowance of 2 fps built in.  If you expect recurring surges above that velocity you should de-rate the pressure rating respectively.  

If the surges are not recurring and are only occasional take a look at the C905 standard and how the working pressure is calculated.  

Thanks to everybody for the posts.  They have been helpful.  

GTRAINOR: my fax is 323-551-5070.  Please mark it attn: J. Arden

I'll get this out tomorrow.