Reducing drainage time

[ChemStel]

I have read the other threads regarding drainage on eng-tips but couldn't seem to find the answers I need.

I have a vertical, cylindrical, dished end vessel filled with water. Inside diameter is 394mm. Working height is 1313mm. Working volume is 160 litres. Inside diameter of outlet nozzle, located on bottom of vessel is 27mm. Drain line inside diameter is 18.4mm and is roughly 50mm below nozzle. Drain line is about 7m long. Drain line is a straight run on the floor.

The drainage time has been measured on site to be about 16 mins, which is causing a bottle-neck in the process. Are there any ways in which I can reduce the drainage time?

Will increasing the drain line diameter help even though the nozzle diameter is fixed? Does height have any impact on drainage time?

I have tried calculating drainage time using this formula (From Crowe, Elger & Roberson - Engineering Fluid Mechanics) : t = 2AT/(SQRT(2/g).A1).(h0^1/2 – h^1/2) and the answer I get is 2 mins, so I have come to the conclusion that above formula is only valid for direct discharge to atmosphere and does not take discharge pipework into account.

I then tried calculating discharge using the Crane manual (E.g 4-19), which is for discharge flow from a reservoir with constant head through a piping system (Q = 0.2087 x d^2 x SQRT(hl / K). My real-life application, however doesn't have a constant head, so I assumed the head to be half of the starting height. This gave me an answer very close to the actual measured value. Does this sound like the correct way of calculating this, or is there a way I can account for the changing level, or will the change in answer just be negligible. This example takes into account the head. Will raising my vessel higher reduce the drainage time, I can't see how?

 

[Latexman]

The Crane equation will work. Since the drain line is a straight run, the only knob you got is pipe diameter. You'll have to put the K of the larger pipe in terms of the smaller diameter of the nozzle. You could also add a pump or pressure it out.

Good luck,
Latexman

 

[ChemStel]

Thanks Latexman,

If the drain line is made to slope, i.e. by raising the vessels, will this have any impact? If yes, how so?

Also, I have calculated the drainage time, assuming a drain line double the size of the existing and the result is encouraging. Is the Crane equation subject to any assumptions e.g. the pipe needs to be fully flooded? I don't want to risk proposing a bigger drain line saying that it will quarter the drain time, unless I am 100% sure.

Thanks in advance

 

[Latexman]

The time to drain the tank is probably significantly longer than the time to drain just the line, and I don't know how important it is for you to drain the line quickly after the vessel is empty. What's important to you?

And, yes, the line has to run full for the Crane equation to be valid. The smaller nozzle could restrict the flow so the larger line is not full.

Good luck,
Latexman

 

[ChemStel]

Sorry, I may not have made myself clear.

The drainage time of the line is not important, only the tank. I was just wondering if I raise the tank, which increases the height relative to the drain line outlet, will this aid draining the tank any quicker?

Is there any way of calculating whether the smaller nozzle will restrict fully flooded flow in the drain line?

Thanks

 

[Homayun]

The only way to lower the drainage time is by increasing the pressure difference in the drum and the outlet. Raising the vessel would not help because you are probably moving the bottleneck (drain valve) too. You've got to somehow increase the pressure inside the drum. Put nitrogen or air into it and blow the water out..Or if you prefer the gravity flow, then the only choice is enlarging the drain valve and line...

 

[bimr]

The options are:

1. Pressure the tank and force the material out.
2. Put a suction on the tank and pull the material out. You can use a pump or eductor pump.
3. Use a larger diameter drain.

Raising the tank is not going to make much difference.

 

[SNORGY]

What is the fluid service (sour water or just plain water)? The reason I ask is that when you drain the vessel can you also vent it or is it under a gas blanket or inert blanket?

I'd be inclined to isolate it and open a high point vent before draining if the service is benign, especially if it drains to the open floor anyway, unless there are corrosion or other issues that make this undesirable.

A draining time of 16 minutes for a vessel this size would no doubt be irritating.

Regards,

SNORGY.

 

[SNORGY]

Apologies...in my previous post I assumed that the statement:

"Drain line is a straight run on the floor."

meant that the drain was open to atmosphere at floor level, which of course may not be the case at all.

My apologies.


Regards,

SNORGY.

 

[ChemStel]

Thank you all for your useful contributions. You have provided the possible solutions I was looking for to reduce drainage time.

Snorgy,

The service is wash water, that it used to wash down the acid from the previous process step. The acid is pumped out for re-use, while the water is gravity-drained out along 7m of straight (not sloping) drainage pipework. During draining the vent is opened.

I am however, still trying to understand, purely from an academic point of view, why raising the vessel has no impact? Logic tells me it shouldn't make a difference, but Bernoulli tells me that z1-z2 is greater. If you drain a tank with a hole in the bottom, I understand that it makes no difference whether the tank is on the ground or 100m in the air. If you however, have a 1" pipe attached to that hole, does that mean you can apply the Bernoulli equation and effectively have 100m differential head?

 

[SNORGY]

For a given drain line size, you would have to raise the tank high enough to offset the 1-1/2 velocity heads you lose for entrance and exit losses plus dynamic losses through the pipe. You might be facing a "diminishing or marginal returns" scenario if the change in rgh is almost equally offset by the change in (1.5+fl/d)u^2/2g.

I am really struggling with visualizing 16 minutes of draining time for a vessel of this size vented at the top and open at the bottom. Are there internals or packing or something else that might be holding things up?

Regards,

SNORGY.

 

[Latexman]

If the drain time of only the vessel is important, then z1-z2 is fixed by the elevation of the liquid level and the bottom nozzle. No matter how high you raise the vessel, the elevation difference, z1-z2, remains somewhat constant as long as the initial liquid level is about the same every time.

Good luck,
Latexman

 

[Compositepro]

The siphon effect of the drain pipe will be significant.
You have a narrow and tall tank. Vortexing at the outlet is probably slowing your drain time. See what happens when you drain a 2-liter soda bottle with the water swirling versus not (don't forget to poke a vent hole in it). Anti-swirl baffles may be all you need.

 

[ChemStel]

Snorgy,

Correct. There are internals. A frame holding 12 containers, which in turn hold the items being washed.

I was incorrect in saying the vessel is open-vented. The vent line is fitted with a pressure-vacuum relief valve. The vacuum setting is -15 mBar.

Looking at the vessel in isolation, the calculated drain time is 2 minutes. So therefore, I assume, it is the 7m of 18.4mm diameter pipework that is causing the bottleneck.

I am now looking at ways of pumping to drain.

 

[SNORGY]

Hi ChemStel.

Not sure if the PVRV is vented to atmosphere or vent header or flare, but regardless, you could probably put a 3/4" or 1" ball valve bypass around it with a padlock on it to lock it closed until such time as you drain it. Being simple wash water, that fix would probably withstand Risk Assessment or PHA scrutiny. If the drain line is clean and obstruction free, the ability to fully vent might be the only help you need. If threads and screwed unions are allowed in the vent line, a pipe fitter could do that in an afternoon for a few hundred dollars in parts and labour. You might need to do a couple of socket welds somewhere.

Regards,

SNORGY.

 

[Japher]

While there is a vacuum relief device I would be cautious pumping out of a non-vented tank as you can cause a vacuum and damage to the tank. Venting or pressurizing would be best, or increase the discharge line size to above a self-venting size for the flow at self-venting, this will allow the solution to "burp".