mdpe water pipe

[Whizarc]

Does anyone use 20mm mdpe water pipe for compressed air piping as it seems to be rated at 12 bar and i want to use some at 8 bar and the fittings seem to be easily available too.

 

[Anguz]

I do between workshops

 

[dan.taylor.1]

You need to make sure its rated for uv exposure, it's fine with air in it but I've seen thw stuff that's meant to be buried crack after being in the sun for a few years.

 

[Whizarc]

You need to make sure its rated for uv exposure, it's fine with air in it but I've seen thw stuff that's meant to be buried crack after being in the sun for a few years.

Not worried about uv as rats will chew through it first so will be replaced often and it is cheap so i can. Going to use 20mm stuff for now and 25 later on .

 

[Whizarc]

I do between workshops

20 or 25mm pipe and for what i am doing at the mo 20mm and its fittings are perfect.

 

[Anguz]

Think mines 25mm.. it was what i had available... about 15 m

 

[123hotchef]

I scored some free 20mm and 25mm i will run that up the length of my garden between the work shop and garage just use a reducer to join it somewhere

 

[angellonewolf]

could you cover it in pipe insulation would that help ?

 

[mike os]

Had a length threaded through a palisade fence for getting on 20 years without problems, the water pipe is a different matter

 

[Bravheart]

Believe black is available for above ground and uv stable.
25 mm will give you a bigger store of air

 

[Agroshield]

could you cover it in pipe insulation would that help ?

Any covering at all that prevents the UV getting to the pipe would do. Most pipe insulation itself is not UV-rated though. The most common one that is UV rated is Armorflex HT and is relatively pricey.

As Malky says, the black water pipe is specifically rated for exposed use.

With regards to the pressure rating, have a little read up on 'water hammer' and consider if the same thing could happen in your set up and the consequences if it does.

 

[Chris Snow]

have a little read up on 'water hammer' and consider if the same thing could happen in your set up and the consequences if it does.

Not a problem with air as it's compressible and also low mass. Certainly not at the flow rates involved anyway.

 

[pedrobedro]

could you cover it in pipe insulation would that help ?

A layer of gaffer tape would do it cheaper than insulation.

 

[timgunn1962]

It works fine. Things to watch out for:

Take a good look at where it is running and where the condensate will collect. Incorporate some means of draining it off at the low point(s).

Make sure you have sufficient provision for cooling the air before it gets to the MDPE pipe. The pressure capability drops off pretty quickly as the temperature goes up. I tend to think in terms of anything I wouldn't be happy to lick as being too hot.

MDPE has a surprisingly high Thermal Expansion Coefficient. Be aware of this and give it room to move. Also consider what will happen if a fitting fails: a long length of MDPE pipe flailing around can be pretty scary.

Water Hammer is not generally a problem with compressed air systems because air is compressible, but if you get slugs of condensed water moving around, things can get pretty exciting.

A lot of 32mm MDPE gets used to distribute air to down-hole pneumatic pumps on landfill sites, which is where I have come across it. The large diameter keeps air velocity and pressure loss down (some of the runs are hundreds of metres) and there is usually either an air fuse or a simple restriction (often just a 1/4" reduced-bore ball valve) to limit the flowrate into the MDPE and keep the risk of flailing lines down. Usually, air is compressed to at least 8 bar, refrigerant-dried and regulated down to 6 bar before it enters the MDPE. Condensate is always a problem, particularly in winter. Frozen condensate is a problem in cold weather.

 

[Agroshield]

Water Hammer is not generally a problem with compressed air systems because air is compressible

The reason I suggested he think about it is if he has 8 bar of air at full flow through the line (maybe he is blasting) and then closes the valve, will the pressure spike come uncomfortably close to the 12 bar rating of the pipe?

The situation you describe has had some engineering input to it to limit the pressure and flow rate: without those precautions, how close to its limit is he asking the pipe to work?

Thanks.

 

[timgunn1962]

Water hammer is basically the result of a fast-moving column of water suddenly hitting a wall (the closed valve). The momentum of the column depends on how fast the water was moving just before the valve closed and on the length of the column. Water is incompressible, so there is nothing to cushion things and the effect is a lot like a hammer blow.

With air, the density is lower (by a factor of "about" 100), and the air is compressible, so provides a substantial cushioning effect. On an 8 bar line pressure, however fast the valve is closed, the local pressure is not going to approach 10 bar, let alone 12 bar.

 

[Chris Snow]

The reason I suggested he think about it is if he has 8 bar of air at full flow through the line (maybe he is blasting) and then closes the valve, will the pressure spike come uncomfortably close to the 12 bar rating of the pipe?

Kinetic energy is Ek = 1/2 mv2
Mass of water is 997kg/M3

Air is more complicated as it's compressible but ballpark 13kg/M3 at the pressure and temperature involved


So the energy involved is 76 times less for the same velocity.

I need to think about the maths for pressure gain but it's going to be complicated!

 

[Dev]

I have a pile of Speedfit pipe taking up space - would using it for compressed air be practical?
Not huge runs - 20m with 2 or 3 drops.

I woulder if you could even get the right fittings to terminate it.

 

[cheb]

That's what I have between my house and shed. 15mm with inserts into JG black plastic fittings. I don't leave it pressurised all the time, just when I need it.

 

[Dev]

well theres a weekend project then. What fitting did you use to take it back to brass for the tool connection?