Break pressure design

[gidza]

I'm designing a concrete break pressure tank. The pressure of the incoming pipeline at this location is 18bars (180m), which I have to break to zero.

My intention is to design two CONCENTRIC tanks such that the inlet pipes will discharge into the central tank wherein I will provide weirs for ouitward flow of the stilled water into the outer tank. The central tank will then be designed to take the force of the incoming water and turbulence.

I want to line the inner tank with a stainless steel plate to prevent potential erosive effect of the jet of water on the concrete. I wonder if my proposal is sound ?

By the way this is africa so I cannot use fancey devices like pressure reducing valves or such-like. The key is simplicity and ability to use locally available materials and ease of maintenance.

Thanks !

 

[BigInch]

If you have that much pressure and no pressure reducing valves, you'd better make a tall tank.

If you're worried about eroding the concrete, I'd also be worrying about eroding any steel you put in front of it.

If T.I.A., why line the whole tank? Put a (replaceable) steel plate at the jet inpingement. When its gone, you can easaily put a new one there.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gidza]

I was thinking of a height for the tank of say 4m, with a maximum top water level of say 3m in the tank.

Why do i need a tall tank ? If I make the tank tall then I will be building up an undesirable static head which will give me problems again downstream of this location....

In terms of lining, I'm looking at approximately 6m diameter inner tank which works out at approx 75m2 of lining to cover floor, sides and soffit of roof. I might just find out whether the cost of this is reasonable, if not I will pick up your suggestion. Other wise my reason for lining the whole tank is that I don't know where the reflected water will be directed on impact; and this direction can vary depending on what flowrate the pumps are delivering.

In terms of overall life perfomance of the stainless steel I doubt that it will need much attention when in service because the intention is that once the tank is in operation the low water level will be such as not to empty the tank, else the inlet valve at the delivery side will close down and a signal will be sent to the pumps to resume pumping. This minimum water level will provide a dampening effect but somewhat not sufficient to still the water or eliminate turbulence

 

[katmar]

Where exactly is the 18 bars measured? I would doubt that you could have 18 bars at the pipe outlet? To absorb this 18 bar as exit losses would require a velocity of 60 m/s. If this is your real situation then I understand why you are concerned about erosion.

What is your flowrate? Why not just make the last 10-20 metres of pipe big enough to bring your velocity down to something reasonable.

Without knowing your full situation it is impossible to say whether your proposed solution is the right way to go.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[gidza]

The flow rate is 162 l/s and the incoming pipe diameter is 450mm. The velocity at the terminal point is 1.02m/s.

The pressure is assumed to be 18bars at the outlet point.

The principle of the break pressure tank is not to dissipate this energy via exit losses, but rather by discharging the water into a tank with a free surface.

Using a larger pipe will not solve the problem because as soon as the pipe fills up it will become pressurised and the problem will resume.

 

[BigInch]

You have a pressure equal to 190 meters of water, so, without a pressure reducing valve, that will pretty much be the height of the free level surface. (Pressure head, velocity head, when its all standing still, its just plain head.)

Tall isn't it?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gidza]

No, because I have an outlet discharge pipe which will draw out the maximum flow at a maximum rate of 162 l/s to feed a reservoir some 10km away.

When the system is in operation inflow = outflow; there is no real storage intended at this break pressure tank save for just 1hr detention to cater for potential flowrate variations emanating from the pumps. By the way the pumps are to be rated for 162 l/s and 80m head. The problem is that after the high ground there is a very long descent...thats where the pressure build up is coming from...its like pumping from one side of a mountain to the other side...only that when on the other side its much lower. Re-alignment is not possible.

The elevation at the break pressure tank is more than sufficient to enable gravity flow to the storage reservoir, with an ultimate residual pressure of some 2bars at the discharge point ...as you can see the 180m is absolutely unwanted.

 

[BigInch]

Oh, ya. OK. Right.

Then what is it you're worried about again?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gidza]

LOL !
I need peer review on my proposed solution to dissipate 180m pressure, which is:

"to design two CONCENTRIC tanks such that the inlet pipes will discharge into the central tank wherein I will provide weirs for ouitward flow of the stilled water into the outer tank. The central tank will then be designed structurally to take the force of the incoming water and turbulence. I propose to line the inner tank with a stainless steel plate to prevent potential erosive effect of the jet of water on the concrete."

 

[zdas04]

I'm a bit confused (it is Monday after all). Is the fluid in the line concrete or water? From your original post I thought you were pumping concrete ("I'm designing a concrete break pressure tank"), but you kept not talking about the normal issues with concrete flowing.

Now I'm thinking that you are talking about dissipating the hydrostatic pressure on the back side of a hill in a Newtonian Fluid line (maybe water?) in a concrete tank. In that case, flowing into a quieting chamber and overflowing into your outflow reservoir is a pretty clever idea.

I think I'd look at the inner ring being pretty big (1 m diameter would give you a decent velocity) and I'd probably go all the way to the top of the vessel with the inner pipe (maybe 4 m) with weir holes at 2 m). Going to the top of the vessel lets any tendency to jet dissipate some in air and then hit a peice that is more easily replaceable than one down inside the vessel.

I've never tried to solve a problem exactly like this (usually I'm trying to preserve pressure instead of dumping it), but I think your approach is far better than what you could acheive with control valves (your volumes are just too big for valves to have much chance of success).

Good luck with this, let us know how it turns out.

David

 

[BigInch]

That pressure on that area is only about 250,000 lbs.

LoL.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[katmar]

You have a flowrate of 162 l/s and a retention of 1 hour. That makes the tank 583 m³ in volume. The inlet velocity is 1.02 m/s. I would simply pipe the line straight into the side of the tank, a metre or so above the base, and allow the jet to dissipate naturally. I'm afraid that I cannot see why you are concerned about jet impingement causing erosion in this situation.

Your pumps seem to pump 80 m up the hill and then the line runs down the other side, dropping a vertical distance of 180 m. So the break tank is 100 m below the pumps? I hope this understanding is correct. If you were to put a closed valve at the end of the downleg (the 180 m drop) and allow it to fill with water you could get 180 m of head (but zero flow). But if you do not have a valve at the end of the line there is no pressure there. You have not stated how long this downleg is. But if you have 180 m static head and a line size of 450 mm and a flow of 162 m/s you would have to have an enormously long pipe to use up all that head. What will happen in practice is that the line will run part full and you will need a vacuum breaker (a big one) at the top of the hill. The actual velocity will depend on the geometry and length of the downleg. But you could easily arrange an inverted gooseneck at the end to ensure the last part is full, and you get your 1.02 m/s velocity, which as I said above is hardly a problem.

Either there is no problem, or I have not understood your situation correctly.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

The velocity is not the problem. Any time the flow from that tank is reduced (a valve downstream ?) you've got a potential of 180 m head, or 18 bars of pressure there, if flow is blocked completely. If its an open tank, it will overflow, if it isn't 190 meters tall.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/