drainage trench slope inside a building

[gmorin]

I have a project (Flue gas desulfurization) where we'll have slurry pumps and lines as part of the process. We're planning to have a trench system draining to a sump to collect line flushes, drain flows, etc.

The slurries will be a 30 wt% limestone, 20 wt% (mostly) gypsum, and we'll have some other lines that will range from water to 5 wt% solids.

Containing the flow is not an issue - a reasonably sized and sloped trench will contain 5x what we think our maximum flow will be. The dilemma I have is how much to slope the trench to keep the slurries moving and eliminating buildup / solidification in the trenches.

Our first shot has been engineering by doing what the last job did - always trouble. A prior project used 1/2" /ft for the heavier slurries - 1/8"/ft for water and the very light slurries - but they were pretty conservative and that particular project had much shorter trenches. I'd be looking at a 4-5' deep trench at the end if I were to follow that criterion.

I was wondering if there are any studies / references or opinions out there as to what a good minimum trench slope would be.

Thanks in advance

Greg

 

[pardal]

maybe you can make a "powered trench" , just add some water flow at the start point , so to dilute the slurry.


Pardal

 

[dicksewerrat]

Most pipes inside bldgs. are sloped 1/4"/ft. The velocity for self-cleaning is 2.5-3 ft/sec. Use a trench with a round bottom. And like stated before, add some water when the heavy stuff is being drained.

 

[gmorin]

Thanks to you both for the input.

I understand the rationale, but I'll have to see if the rounded bottom idea flies with the structural engineers - there might be some resistance there due to the added construction cost. I'm also concerned that if I did get a little buildup in the bottom of a rounded trench, it would make a dam and mess up my slope, creating more deposits.

I am planning on having flush water at critical locations - though I can't overdo it. Any water I use will end up back in the process. I'm trying to sell the project team on an actuated flush valve, timed to go off before and after I flush a line & send slurry to the trench.

Just wondering where your 2.5-3 ft/sec criterion came from - a rule of thumb???

 

[dicksewerrat]

It is a rule of thumb, perfected by watching sewer pipes for 40 years. what happens is that at that vel. solids don't settle out. The reason I mentioned a rounded bottom is if you have small quantities to flush, it will all ride in the rounded part and be easier to flush down. In a flat bottom the same thing will happen, if you get settling. the added cost would be to saw some pipe in half and use as the bottom of the form. Leave in place and you have a smoother bottom.