Pressure through a Pipeline

[buddabeast]

Hi guys, I have a question about water pressure through a 1.5" PVC pipeline.

The supply pressure is around 700kPa (from a reservior on a hill). The water line runs about 10m to a pressure regulator which is currently set to 200kPa. This water is then used to power a venturi to suck up powdered carbon. The doseline then runs about 30 m with 12 bends and rises about 6 m high. (maybe 400kPa friction and lift pressure here?)

My supervisor is telling me the doseline needs replacing as there is too much backpressure on the venturi - however I don't think so.
I can increase the pressure to around 500kPa with the PRV and decrease it right down also.

But I don't know how to explain to him that the doseline is OK..

Thanks for any help.
Heath

 

[racookpe1978]

What is the actual length and number of bends and exact change in height from the suction of the carbon tip to the discharge point? Vertical 6 meters? That's very close to the maximum you can lift anything (water) at atmospheric pressure.

What altitude is the assembly.

 

[bimr]

Injectors are tricky to size because of back pressure. To properly size an injector you have to have the pressure of your water line the injector is going to be in and the flow rate at the end of the line your injector is in.

You have not stated the flow rate or the application.

I would not expect this to work at 200 kPa. It will probably take at least 400 kPa.

Not sure what the application is, but it is usually easier to transfer carbon as a wetted slurry instead of sucking up carbon.

 

[buddabeast]

Hi - the flow rate is around 10m3/hr. The bends is x12 and height is 6m vertical after the venturi, at sea level.
The water at pressure is supplied to the venture which takes in the carbon powder and coveys it into the raw water as a slurry. This is for odour, colour, organics removal of the raw river water for drinking water.
I think using a headloss calculator I could say the headloss would be 400 KPa after the venture based on length-bends-height. But would I explain how this works when I have a 700Kpa supply pressure and a PRV that can adjust venturi pressure.. I don't even know how to ask the question haha.

 

[bimr]

Look at the flow rating if the venturi. Assume that you are discharging to atmosphere (above a mixing tank or clarifier).

If you have this installed, turn it on and test it.

 

[buddabeast]

Yeah the venturi is working fine now with good flow once I found the PRV was blocked and had it replaced. But my boss still thinks the pipework is fouled causing a restriction. he says the pressure on the gauge shouldn't be zero as there is headlosses in the pipework but it shouldn't be 200kPa so might be blocked up. I have opened up the pipe in two areas, there was no fouling at all.

I might do some research into venturis

 

[LittleInch]

Please sketch your system in profile noting where the pressures and gauges are.

I can't work out how your OP says inlet pressure I 200kPa, but you somehow have calculated 400kPa friction. something wrong there.

"he says the pressure on the gauge shouldn't be zero" Which guage? where is it? - see my first point.

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

 

[georgeverghese]

Are you sometimes operating this venturi at low HP water flow to suit the raw water flowrate? At low motive medium ( HP water) flow, the carbon particles may be settling down at the low point of the vertical riser and other elbows also.

If so, try to keep the venturi running at high motive medium (HP water supply) flow always - some change in the control scheme may be required to save on carbon consumption at low demand.

This may be easier to modify than to raise the entire venturi to a higher elevation so that the water-carbon solids-air stream flows continously downslope into the raw water line to avoid this 6m riser.

 

[buddabeast]

Hi guys, sorry for late reply. Hope everyone had a good Xmas.

I have attached a pdf diagram of the carbon system. What I basically wish to do is explain why the pressure at the 200kpa gauge is 200kpa instead of zero.
My supervisor is stating the pressure at the 200kpa gauge should be near on zero as the system is open ended, I want to be able to explain to him why not properly.
Thanks,

 

[Latexman]

I expect if you wrote a mechanical energy balance/Bernoulli equation between the point at the pressure gauge and the pipe outfall which takes in account any friction, elevation, and velocity changes, if any, AND added a correction for the venture operation/solids addition (i.e. momentum balance) you'll carry the day. For example, just the elevation lift of 6 m will require at least 60 kPag at the pressure gauge. So, I got you about 30% of the way there.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

I don't know a huge amount about venturis when used like this, but to get flow into it like you've shown you'll need to have a pressure drop inside of over 2bar/200kPa. My reading shows that leads to a pressure loss across the venturi of between 10 -20% of lowest pressure. It might also entrain some air in the system.

So your 200kPa = 20m approx head.
6m is vertical lift, another 4m loss across the venturi, maybe 2m exit loss so looks plausible that 20m will be about right as you have a lot of bends and quite a high velocity. When you do your friction losses in the pipe remember to use internal diameter. 1 1/2" pipe is not clear if this is OD or ID.

Having air in the system will add pressure losses which is difficult to quantify, but I can't see otherwise how the carbon and water will be drawn in unless you adjust the flow rate just right.

Is this a working system?



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

 

[buddabeast]

Thanks for feedback guys, yea this is a working system. There is a minimum flow rate/velocity for the venture to work. If I close the supply valve off too much to the venturi the cone will simply fill with water and carbon. So I try to find the minimal flow for the system to work reliably.

 

[buddabeast]

I think I have found the issue here. The venturi can work at a lower pressure than 200Kpa, the reason I need 200kpa+ is because of the wetting jet on the cone. This jet needs a 200kpa+ pressure to put out a jet with enough velocity to get around the cone and wet it entirely. If the wetting jet wasn't required the system could work at a lower pressure. When I have time I will turn the wetting jet off and see how low I can put the pressure before the venturi fails to work and the cone fills up.

 

[LittleInch]

Therefore you will know the min pressure that makes it work and with an open end the friction losses are what they are. What is that pressure?

What's the problem??

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

 

[buddabeast]

Problem is my supervisor wants to replace 30m of PVC pipework as he thinks the line must be blocked as we are running such a high pressure.
But I have opened the pipework up at a couple of areas and the inside wall was clean. I also believe a gritty (sand like) carbon slurry is unlikely to foul a pipe..
I could be wrong but I will test the lowest pressure it will run anyway.

 

[Latexman]

So, the pressure is 200 kPa? What's bad about that? Why does supervisor think 200 kPa is too high? Was a broke gauge replaced recently? I say, "If it ain't broke (i.e. still working), don't fix it!" I think you are on the right track calculating what the pressure should be.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[BigInch]

PVC might not be the best material to use for running a venturi with gritty fluid, which might either get sandblasted if moved too fast, or set up quickly, if it ever slows down enough.

 

[georgeverghese]

Rough pressure drop calcs with plain water (single phase liquid) with 30m of 40mm id pvc pipe at 10m3/hr, 12 SR elbows, 1 swing check valve oriented on a horizontal pipe, 6m rise, gives 70kpa frictional loss, 60kpa static = 130kpa total loss at 2.2m/sec, excluding dp across the venturi.

But your sketch shows this check valve on a vertical riser. Even with this on the horizontal, the min required rho-v2 is probably just about adequate to get good lift. But not with a swing check on a vertical riser. Suggest relocating this (no matter what type it is).

 

[buddabeast]

That's what I think Latexman - it aint broke - it used to have issues but they have been resolved but he still thought 200kpa was too high for headlosses. However I have a better understanding now with the wetting jet being the pressure user so I can explain it to him.

You might be on the button there georgeverghese, I turned off the wetting jet and decreased the pressure on the PRV until the flow stopped and filled the cone. It seemed to be the 130kpa mark where it would stop sucking at the venturi and at 140kPa run OK.

The non-return is a ball type. i.e. the water is just pushing up a plastic ball which then seats on an o-ring when the flow stops. If this valve type were put on the horizontal it would then need a spring to push the ball for seating. This spring way require more pressure to overcome than the current setup. I think the ball valve was chosen over a swing as they are more reliable.

I now have the pressure up around 400kpa to get awesome cone wetting and reduced the venturi flow down until just above the minimum before it starts to fail.
I think that's optimizing it well enough.

Thanks for all your help people.

 

[LittleInch]

buddabeast,

Just remember that georges friction loss calculation is only valid for 10m3/hr.

as you throttle back on flowrate through the venture, then these losses will reduce, but also the venture will add pressure loss itself.

If you can keep the cone so it always has a little bit of water in the bottom then that is the best otherwise you will be pulling air into the system. Now you've got the pressure down a bit perhaps your supervisor will be happier?

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