Drain rate of declined safety valve outlet line

[novis76]

Hello,

I want to get a estimate on how fast the outlet line of a safety/relief valve is drained. The outlet line is rather long and the fear is that the bellow can get damaged if the valve experience a subsequent opening. The medium is water (cold)

Can anyone point me in the right direction about drain rates of sloping pipes or in other way help?

Thanks

 

[BigInch]

Run this in Google "Gravity flow of water in a pipe"

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[novis76]

Ok, I am not sure I am getting the same result on my google search as you (Google search result is nowadays personal/dependant on past search queries). Can you specify what I should look for?

Meanwhile, what I expect is this.

1) at t=0 the pipe is filled with water. The upstream end is a dead-end (closed safety valve).
2) for t>0: Air "bubbles" up at the top part of the pipe towards the closed safety valve. At the same time the water drains in the lower part of the pipe. At some point in time the air reaches the closed valve. At that point the pipe is almost completly filled with air.

This is a non-steady behaviour. Is there more simple equations that describes this event or is some kind of code required that handles two-phase flow?

 

[BigInch]

Oh OK. I was thinking water through a culvert type problem. Without an inlet air vent, you will drain very slowly. Pressure will reduce by flowing through the drain valve. You must get a Cv coefficient for the valve at full open and calculate the flow across that with a starting pressure in the pipe upstream and atmospheric pressure at the outlet pressure. Do it in small timesteps, calculating volume out and the corresponding pressure reduction. It won't take much volume of water to drain before pressure reduces considerably and to continue without an inlet air vent, you will basically have to reach water vapor pressure to drain more, so your driving pressure at the top of the water's surface will only be 0.25 psia or so. The water will drain, due to pressure at the outlet being 0.25 psia + the weight of the water column above it. At small timesteps, that will slowly decrease each timestep and the next step's outflow will be a little less. When the total outlet pressure equals atmospheric, 14.73 psia, drainage will pretty much come to a halt, until you can figure out how to get a vent on it, because not much water will flow out, when air is trying to flow in. Try it with a clear plastic Coke bottle first, turned upside down and you'll see what I mean. It will drain a Coke bottle quickly, but not a large pipe.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[novis76]

I forgot to post my pretty picture in the previous post. Here it is.

BigInch, I agree that the depressurization rate should be very fast and not important to the total time it takes to drain the outlet. I would guess that the outflow from the pipe would be less intermittent than that from a coke bottle.

Anyhow, if anybody has a way to guesstimate this value I would appreciate it. Due to a small (gas) overpressure in the outlet line a drain next to the outlet of the valve (air intake) is not preferred.

 

[BigInch]

Not without a "vent" for air ingress. Relatively speaking, its probably worse than a Coke bottle. I have just barely enough patience to drain a Coke bottle without knocking a hole in the bottom.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso