7 replies to this topic

[chempaul2014]

Dear Gentlemen,

 

We are in the process of designing a temporary pipe line of MOC poly ethylene for transferring water from point A, which is having head of 20 m to the main water pipe line of 4 m diameter

Flow of water is only by gravity and available head of 20 m

As per the line hydraulics, pipe size turn out to be 8"

Total length 4 km

Flow rate:80m3/hr

Considering to put gate valves for every 1 km for isolation, in case of any repir work?? Do any body have any guidelines for this??

Since it is flowing by gravity, is it really required to put air relief valves?

What are the other precautions to be taken in designing these pipelines?

 

Pl. find the attached drawing for the actual layout of the system.

 

I will be grateful, if someone can throw some highlights in my design.

Attached Files

•  Drawing of the pipe line layout.pdf   194.44KB   23 downloads

[Art Montemayor]

My response and comments are found in the attached workbook.

 

 4km PE Water Pipeline Rev0.xlsx   39.55KB   35 downloads

[TS1979]

You only need one block valve at the piping inlet. The second block valve at the piping outlet is not necessary if your commodity is water. You need vent valve at each highest point. You also need drain valve at each low point to drain the water when the piping needs for maintenance.

[chempaul2014]

Dear Art Montemayor,

 

Thanks a lot for your valuable inputs. Currently this 4 m main pipe line is empty so i believe the 20 m head will be sufficient for the water to flow.

 

For 80 m3/hr ,8” line of 3.5 km is given a pressure drop of .8 bar

.

 

I will surly consider all your comments in my design.

[katmar]

I agree with Art Montemayor that you need air vents at all the high points.  With a flow of 80 m³/h in an 8" pipe the velocity is about 0.7 m/s.  This gives a Froude Number (= V / √(g x D)  )  of 0.5.  This would be adequate to sweep the air along the horizontal sections, but to entrain the air down the vertical downlegs after every road crossing you need a Fr No of 1.0 or greater.  You could use 6" pipe for the vertical downlegs to ensure this, but you would have to look carefully at the pressure drops.

 

There is one piece of critical information missing.  You have not shown the elevation of the point where the 8" line joins the 20" line above the existing main.  You will get no pressure recovery in the 20" line (it is too big to run full) and so your available driving force is only 20m minus this elevation.  In your sketch it looks to be higher than the 13m high road crossings.  If this elevation is 15m and your driving force is only 5m then it cannot overcome the friction loss of 8 m.

 

If this entry point into the 20" section is actually quite low then you run the risk of having slack flow in the downlegs.  In this case you need a fully dimensioned drawing so that you can consider each section in detail.

 

In general, gravity flow is much more complex than pumped flow.  If you install a pump then you can define the pressure and flow.  With gravity flow, Nature does its own thing, which isn't always what we want.

 

 

[chempaul2014]

Dear Katmar and Art Montemayor,

 

Your suggestions are really valuable.

Based on your suggestions, I have modified our piping layout. I have also attached the new drawing and detail sizing calculation for your review.

I will be grateful, If you could verify and throw some lights.

 

This is going be a water service and planning to use MOC as HDPE is it fine?

If we route these pipe line via under ground does it have any advantage?

 

Thanks in advance for your suggestions

Attached Files

•  Revised Drawing of piping layout and hydraulic calculations.xlsx   26.56KB   16 downloads

[katmar]

In your calculation your viscosity is too low if the fluid is water, and this makes the Reynolds number too high (both by a factor of 10). But your friction factor and pressure drop are OK.

 

The fact that you have 2 bar of head available and require only 0.8 bar means that your system is unstable.  When the flow is 50 m³/h the required head is less than 0.4 bar.  You will therefore have to throttle the flow using one of the valves.

 

If I assume that the road crossings are evenly spaced and the pipe length from the last crossing to the entry point into the 4 m main is 1000 m (and the pressure in the main is atmospheric) then the pressure drop from the last crossing to the old main is about 0.1 bar.  This makes the pressure at the top of the last crossing -0.4 barg (i.e. slight vacuum). As long as the water temperature is below 80°C there will not be any boiling, but the air vent should be of the type that does not relieve vacuum in the pipe or you will draw air in.  You could avoid this complication by doing the throttling at the downstream valve to get the desired flow rate.  This will raise the pressure at the top of the last crossing (and all the others) to above atmospheric pressure.

 

Knowing your actual crossing positions you should calculate the pressures at the top of each crossing under normal and maximum flow rates to check that they are all OK.  Provided that you do not have a situation where you are drawing air into the vent valves (and therefore losing some of the pressure recovery in the downlegs) I cannot see any need to go underground.

[chempaul2014]

Dear Katmar,

 

Really appreciated for your comments, i will definitely take care during the design and incorporate in the procedure.

 

Thanks

Paul