HDPE pipe thickness 4

[robertsaba]

Hello all,

I am working on a water project of which the design does not make sense to me, I need a technical justification if some could please help. The problem is that the spring feeding the plant is located in a mountain about 400 meters above, and 2.4 Km away as in the drawing, and the designer from the catchment to the plant put six Pressure reducing valves in series to the plant, the pipe is HDPE 110mm PE 100 pipe, the flow is about 7 l/s. At around every drop of 70 meters he has a PRV. This does not make sense, since the other side will be a plant and some water will be used or we could put a surge vessel on the end. Why is the designer taking that the bottom of the line will be at 40 bars and he needs all these prv's why not install a pressure relief valve on the end and dispose of all the prv's?

Your help is highly valuable since the final design will be finalized soon. The cost of the this design is quite big for the budget of the community.

best regards,

 

[LittleInch]

Robert,

You don't say what SDR the pipe is (i.e. wall thickness) It makes quite a difference, but judging by your designer I would hazard a guess the max working pressure of the pipe is about 10 bar ( PE100 has a maximum of 16 bar at its thickest rating )

By PRV I assume you mean Pressure Regulating Valve?? (It can also mean Pressure relief valve). Also that what the valve is doing is limiting the pressure in the downstream pipe to something like 3 bar??

Your designer is working on the basis that at some point someone at the base of the pipe could either turn it off or reduce the flow rate from the maximum flow. In either of those circumstances the pressure in the pipe would rise to well above the pressure rating of the pipe which could cause it to bulge or break.

Yes you can eliminate them, but then your only flow control or on/off needs to be at the top, you need to have NO possibility of blockages, i.e. NONE EVER?? and NO valves whatsoever on the pipeline until it empties into an open pond. If you can do this / guarantee this then yes, remove the Pressure regulating valves.

The problem is you build it that way, go away and then someone who doesn't understand this fits a valve at the end because he doesn't want to walk / cycle / ride / drive to the start point 400m up. He then turns it off or throttles it back and then 15 minutes later has water flowing out of a hole in the ground and blames you for building a shoddy pipe.

Pressure regulating valves are not that expensive and most can work on pilots and springs needing no external power. You could always compare the cost to a steel pipe or flexible pipe with a higher pressure rating?

A relief valve at the base is uselsss as the flow would need to be the same as if the end valve was fully open, ditto P Relief VS along the line would simply open up full bore until you could turn off the start valve.

does that help?



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[LittleInch]

Looking at the profile, why don't they just build an open channel - apart from one small section which goes uphill, it seems to be a continuous drop.?? You could dig a small channel and line it with PVC or similar?

That would solve all your problems - just put a weir at the top and flow it like a small stream?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

Looking at your profile and flow rate I would walk away from the HDPE and the PRV's and for about the same price run 4-inch (DN 100) reinforced thermoplastic pipe (RTP, brands I like are Soluforce and Fiberspar, but there are several other good ones) rated to 40 bar. Then you can put your control at the bottom and not worry about the pressure control valves that only work while the line is open on the bottom (shut it off and leakage past the PRV will bust the pipe pretty quickly).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[bimr]

There is a tradeoff between the number of PRV's and the PE wall thickness.

For maintenance purposes, it makes more sense to reduce the number of PRV's and increase the pipe wall thickness. You are correct, it may be possible to eliminate all of the PRV's except one.

There should not be a large difference in project cost for either alternative.

 

[LittleInch]

File won't download BI.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Because it works similar to this spreadsheet model. Pressures can build as elevation decreases. They can build to higher levels than the inlet pressure of each pipe segment. This is not an exact simulation (made up friction factors and units), but rather a black box type, input, output simulation, so you can learn the idea without getting confused with the numbers. As pressure drop increases across the valve, flow increases or decreases by the square root of that differential pressure. That can affect your system flow rate, as when flow resistance becomes greater than the pressure available to drive it. In that case it will tell you to reduce the flow rate. Your valves may be fitted up with a set pressure designed to control the maximum downstream pressure. I deleted the original post, since I have now made it possible to simulate downstream Pressure Control Valves. If you want to control pressure at the downstream outlet of a valve, simply enter the set pressure at that valve and the valve will close to a new percent open when necessary. Otherwise change the set pressures to a value higher than the system design pressure and they will not activate. Please treat the spreadsheet as "Experimental".

 

[BigInch]

I don't know why the spreadsheet won't download. Second time now. I will report the problem to the site manager.

 

[robertsaba]

Thank you all for the input, it is quite refreshing to look at something from different point of view. The cost is big since these PRV's are going to be made of stainless steel 316L according to the designer and they do cost a bit, few thousand bucks each. The HDPE proposed is PE100 SDR 11, my proposal on the bottom side to use SDR 7.4 with a pressure rating of 25 bar. Putting the collection pit about 100 meters from the top also will eliminate about 10 bars and just putting one PRV in the middle of the left over distance. Using a tank at the bottom with a pressure relief valve. I need to size the tank accordingly, since the flow in the pipe should be to a maximum without any air inside. Steel pipe was not an option, only HDPE was approved.

So thanks again for all the inputs God bless

 

[rconner]

Another thing perhaps to keep in mind is that with that small DR/thickness of pipe the clear flow area will be much less than 100-110mm or 4" (instead it will be closer to 80mm or 3", without consideration of inner "beads"?).
As for a given design delivery/flow quantity involved for the pipeline the flow velocity is proportional to the inverse of the inside diameter squared, this means a MUCH higher required flow velocity for the proposed pipe (and with any concomitant effects therefrom).