Gravity Flow

I have a filter press than needs to gravity drain filtrate to a 0.15 psig holding tank. Per the vendor, "The filtrate is effectively a gravity drain.  If the pipe is set up so that it builds pressure back to the filtrate discharge, a check valve should be installed.

When there is pressure build up at the filtrate discharge, the slurry feed pumps will move the filtrate at a much lower pressure than what is seen at the head of the press.  This would require a pump with a much higher output pressure than the filter press would require.  This can have a detrimental effect filter cake as well."

How important is it that I meet the criteria in Hills paper of Froude < 0.3 in the vertical sections (see attached)? The filter press has a flow of nearly 400 gpm. This results in a a 10" sch 10 pipe (for me in the chemicals industry, this is huge even 4" is big). Obviously, if we need a 10" pipe, we will run a 10" pipe, but I don't want to overkill this because I am meeting an unnecessary criteria. The filtrate should only be draining for less than 10 minutes every 3 hours.

According to the International Plumbing Code equation (attached), Qw = 27.8*rs*D^(8/3), assuming 1/3 full for rs according to "Selecting the Optimum Pipe Size"  by  Randall W. Whitesides (even though I think he might have messed up transposing the equation), I only need a 4" sch 10 pipe. 

I don't see how the IPC equation can allow so much more flow or which one I should use (Hills vs IPC). I don't necessarily trust the IPC equation since it has a very poor explanation of how to use the equation and what the terms are, but since it's supposedly used by plumbers worldwide, I would think it's semi accurate.

If you have any questions for me or the vendor, please ask me . 

Hi ,

There is a typo in your formula ,   rs ^ 5/3 not rs ^1.

good luck

Breizh

Edited by breizh, 25 July 2019 - 09:31 PM.

0.15Psi is a very small pressure. Is the tank actually just an atmospheric tank?

I think the question should not be about the drain pipe. I think you have jumped the gun in assuming the filter press setup is correct and started worrying about the drain pipe flow instead.

I'm suspicious about whether the setup of the filter press and the filtrate is correct. There is a hydraulic break between the feed and filtrate, therefore there should be no link between the feed pressure and the filtrate pressure.

The filtrate always free drains into open collecting pans.Therefore the feed pressure has no impact on the filtrate since the filtrate is just free flowing on an open collection pan below the press.

Perhaps you should clarify what type of filter press you are using. A simple sketch of your setup would help others to understand.

Edited by thorium90, 26 July 2019 - 12:11 AM.

hi,

As suggested a sketch or a simple PFD will be helpful together with a description of the filtration unit.

I've attached a example of PF , underneath you should have a pan or 2 gutters to collect the liquid .

thanks

Breizh

Edited by breizh, 26 July 2019 - 02:59 AM.

The filter press is by MW Watermark as shown in this video: https://www.youtube....h?v=_FghCcAZfPw . The PFD for the filter press is attached. The streams in red are the feed and filtrate line. The filtrate line is going to our atmospheric tank. The pan or gutter collection is a separate line.

I believe the equation by Whitesides has an error. In my original post, I copied the pages from the IPC where I believe he obtained his equation.

Edited by ryn376, 26 July 2019 - 07:08 AM.

There are a few items that need considering.

1. 400 gpm at 4 ft/s requires a pipe ID of 6.4".  Using 4" pipes sounds totally wrong.  What size are the slurry feed pipes?

2. If there is nowhere for gas to escape there is no point in using self venting flow.  Your PFD shows no vents.  If the drop in the drain lines from the filter to the tank will not induce sufficient vacuum to cause boiling I would not worry about self venting flow.

3. The filtrate tanks pressure of 0.15 psig is equivalent to only 4" of water column.  Is the vendor aware how small this is?

4. If the pressure in the filtrate tank is because of nitrogen blanketing etc then you need some way to prevent the nitrogen (or other gas) from escaping up the filtrate drain lines when the filter is off line.  A simple U seal (with a drain/cleaning point) is probably all you need.

Katmar,

1. They are feeding the press with slurry at ~12 ft/s with 4" sanitary tubing. So 4" does seem way too small. Maybe the IPC messed up the equation or I used it incorrectly?

2. Sorry, that I didn't make it clear. The vent is actually the drip tray line that is tied in. The question is, what criteria or sizing equations should I use instead of self venting flow?

3. That backpressure is not the issue, that was just put in for completeness.

4. There are actually an on/off valves that are not shown in the line, but are needed to switch filtrate flow from the holding tank to the feed vessel and a line that branches off and goes to the feed vessel (for re circulation to build the cake).

Edited by ryn376, 26 July 2019 - 08:48 AM.

Hills paper of Froude < 0.3 in the vertical sections - This one is correct for "Self Venting Gravity flow"

Regarding points 2 and 3 - if the drain line is vented then the filtrate will have to back up in the line by the 4" required to overcome the pressure into the tank, but this should not be a problem.  I am not convinced that your valve system will prevent the pressure in the filtrate tank blowing back up to the vent - but that could be just because I have not fully understood your system. 

The question of whether you need self venting flow or not really hinges around what is the impact of air being entrained into the filtrate tank. If a bit of air being pulled into the filtrate tank does not matter then you don't need self venting flow.  If it is important to keep the air out of the tank then sticking to Hills' guideline of Fr < 0.3 is your best bet.

Looking further into the IPC equation, I found the original paper (freely available online) and it looks like Whitesides is correct and the IPC has an error, see page 19 of the attached. In my case, since entrained air will not cause a problem, I believe I can use the Whiteside equation and at 1/3 full, this only requires a 6" pipe.

For gravity flow where I don't care about entertainment in horizontal sections, should I still be using the horizontal flow equations by Hills? 

Edited by ryn376, 29 July 2019 - 08:50 AM.