11 replies to this topic

[Afshin445]

Dear Members,

I am try to sizing an open top (roof) storage tank for waste water.Do you have any referenc for sizing of this type of tank,especially for high high level (overflow) and low low level setting. Also if we want to consider any agitator inside the tank is any special design should be considered.

I am waiting for your good comments.

Afshin

Edited by Afshin, 23 March 2012 - 04:30 AM.

[kkala]

In a phosphoric acid open tank, dia~12m and Height~25 m, max possible liquid level (HHLL) was set at 1 m from the top. Strong wind created waves on the liquid surface (when the tank was quite full) and the acid was "pushed" out of the tank. I would be more conservative in case of a tank full of water (having less viscosity than phosphoric acid). I assume this depends on tank diameter and max wind velocity locally. Probably there is a relevant code that I am not aware of.
Concerning LLLL, this can be specified considering:
(α) NPSH of any suction pump
(β) Avoidance of air intrusion through vortex formation (if the tank liquid is discharged to a pump suction). Look at http://www.cheresou...r-suction-pipe .
Level required can decrease by placing an anti vortex device, if risk of plugging is judged to be low (but waste water may have this possibility).
(γ) For agitators in open tanks, see http://www.cheresour...ric-acid-import '> http://www.cheresour...ric-acid-import .

Edited by kkala, 23 March 2012 - 10:48 AM.

[Afshin445]

Dear Kkala,

Thank you for your useful information and sorry for my dely in reply.I went through all of your links,For submerge depth I am clear and I will choose a minimum level so can't be any risk for air ingression to pump.
For 2nd part of my question, I am still unclear, I don't how how I can size tank with agitator and which type of mixer should I use.In 500 m3 equalisation tank I try to size two different kind of waste water(different COD) will be mixed and mixture pH will be manipulated by injection of acid/caustic in tank.The mixer is necessary to mixed different wastewater/acid/caustic and avoiding any sedimentation in tank. Actually my first problem is selection of mixer type, I need to know which type of mixer is sutibale for my application.I can select between air blower, side mixer and vertical mixer, how I can select one of them?
As I read in the links for vertical mixer normally L/D ratio is more than 1, but as per client std. there is limitation of L/D ratio which in high L/D ratio tank will be unstable agianst wind, for some certain L/D ratio anchor bolt should be used in tank foundation.Actually I need to design tank/mixer so can use existing foundation,foundation is available because equalisation tank will be install in place of existing 1800 m3 tank (available foundation dia. is 15m). In my case as I have available 15m dia. foundation is not better I use low L/D ratio and side mixer? I am waiting to know about your good comments.
Afshin

Edited by Afshin, 27 March 2012 - 05:33 AM.

[Technical Bard]

A short, large diameter tank cannot be mixed by a single side mixer - you can't circulate enough of the tank. I would recommend pumping out of the tank and back in the far side to ensure good mixing. Perforated baffles in the tank can improve this.

If you want to use mixers, there are a number of good reference books on mixers (google is your friend) on how many mixers are needed based on tank dimensions.

[Afshin445]

If you want to use mixers, there are a number of good reference books on mixers (google is your friend) on how many mixers are needed based on tank dimensions.

Technical Bard,

At the first I need to select mixer type, I have option for side mixer, vertical mixer and air blower. As we inject caustic/acid in tank I think we need to have internal mixer and external mixing as you propsed may can't be helpful.
My main concern in tank design is any relation exist between tank size and type of mixer, I mean if I want to use vertical type mixer or side mixer there is any specific L/D ratio I need to use. As I explain this new tank should be installed in existing tank foundation with about 15m dia. and there is no any area limitation, what type of mixer you recommended for my specific case.

Afshin

Edited by Afshin, 27 March 2012 - 11:56 PM.

[breizh]

AFSHIN ,
You may also consider this technology ( submersible mixer)
http://www.flygtus.com/115917.asp

Breizh

[Afshin445]

AFSHIN ,
You may also consider this technology ( submersible mixer)
http://www.flygtus.com/115917.asp

Breizh

Thank you breizh for your link.actually client need such a mixer (vertical type) for that tank.But If we using vertical type mixer is it any limitation on diameter of tank.Can you or anybody with that experince advise me about this issue.

Thanks
Afshin

Edited by Afshin, 29 March 2012 - 09:10 AM.

[kkala]

So the tank will have about 12 m diameter and 4.4 m height. "Avoiding any sedimentation" means particles to be held in suspension, apart from homogenization of liquids for neutralization. Agitator selection can be also affected by these solid particles.
1. A vertical central agitator is not judged feasible for such flat tank. Multiple vertical agitators would be effective (as in sigle phosphoric acid reactor tank, containing CaSO4*2H2O), but this looks too complex for a waste water tank.
Once we had tried a vertical agitator on a 400 m3 phosphoric acid tank (approx 5 m height, 10 m dia). It did the job, though the solution seemed far from optimum (agitator too big for the tank).
2. In local refineries three side agitators are placed in petroleum product tanks (near tank bottom, distance 120 deg from each other) for satisfactory mixing of stored liquid.
These probably perform better today than in the past, so these can be a way out.
3. Another means in local refineries is tank recirculation through a pump, where liquid is reintroduced through a jet mixer, similar to http://www.transvac...._Jet_Mixers.pdf '> http://www.transvac...._Jet_Mixers.pdf . This can be another option, as already mentioned by Technical Bard.
4. Compressed air is another option. But I do not have real experience on Nos 2, 3, 4. I have only heard of satisfactory operation of 2, 3 concerning fuels (not waste water). Compressed air in phosphoric acid tanks was an emergency way out, to clean "dead areas" of settled solids. Air was directed there manually through a pipe.

Actually I cannot say which of all above options is preferrable. One agitator supplier may be able to advise, after knowing all process data (including nature of solid particles, size and specific gravity).

[Afshin445]

So the tank will have about 12 m diameter and 4.4 m height. "Avoiding any sedimentation" means particles to be held in suspension, apart from homogenization of liquids for neutralization. Agitator selection can be also affected by these solid particles.
1. A vertical central agitator is not judged feasible for such flat tank. Multiple vertical agitators would be effective (as in sigle phosphoric acid reactor tank, containing CaSO4*2H2O), but this looks too complex for a waste water tank.
Once we had tried a vertical agitator on a 400 m3 phosphoric acid tank (approx 5 m height, 10 m dia). It did the job, though the solution seemed far from optimum (agitator too big for the tank).
2. In local refineries three side agitators are placed in petroleum product tanks (near tank bottom, distance 120 deg from each other) for satisfactory mixing of stored liquid.
These probably perform better today than in the past, so these can be a way out.
3. Another means in local refineries is tank recirculation through a pump, where liquid is reintroduced through a jet mixer, similar to http://www.transvac...Jet_Mixers.pdf . This can be another option, as already mentioned by Technical Bard.
4. Compressed air is another option. But I do not have real experience on Nos 2, 3, 4. I have only heard of satisfactory operation of 2, 3 concerning fuels (not waste water). Compressed air in phosphoric acid tanks was an emergency way out, to clean "dead areas" of settled solids. Air was directed there manually through a pipe.

Actually I cannot say which of all above options is preferrable. One agitator supplier may be able to advise, after knowing all process data (including nature of solid particles, size and specific gravity).

kkala,

Thank you for your comperhensive information in this issue.As I explained before client selection is vertical type mixer as they have good experince in using such a mixer in their waste water treatment unit.
In this regard, I think your proposed dimension is based on assumption of gross volume of 500 m3, but actually 500 m3 should be considred as a net volum, but it's not very important in this stage.
you mentioned for a 400 m3 phosphoric acid tank (10 m dia. and 5 m hight) you using a vertical agitator but it was too big for tank.My imagine was different, I thought for such low L/D ratio this type of mixer should be small, and only can be used in tanks with L/D ratio above 1.Please correct me if I am wrong.

Afshin

Edited by Afshin, 30 March 2012 - 08:57 AM.

[kkala]

1. Links mentioned in previous posts report the http://www.cheresou...eatment-plant/ , where post No 3 mentions a series of articles on agitation, published in Chemical Engineering magazine. These could give guidance on vertical agitators design, but it is not easy to find them now (the series must have also been published separately as a booklet).
2. Yes, total volume of 500 m3 was considered, not very important at this stage. The big diameter of 12 m counts.
3. I think that ideally L/D has to be around 1 for vertical agitators aiming to keep solids in suspension. In long tanks, multiple agitator blades are placed on same vertical shaft (at distance H=D from each other), if solids have to be kept in suspension along the whole tank.
High rpm make the agitator impeller smaller for the same effect.
In the mentioned case of 400 m3 tank, we used a ready agitator from another bigger tank. Ambient air should not touch agitator blades, because then agitator can be destroyed. So rotational speed of said agitator was slow, but it was adequate because of its big diameter. I used mentioned articles of Chemical Engineering to specify speed (RPM). However it was a rather huge construction (compared to the tank) and with independent support structure (if I remember well). Of course an agitator supplier could offer a more elegant solution, and much progress has been made in the last years.
Although above refers to application of suspended solids, mentioned Chemical Engineering articles (used in ~1979, not at hand now) gave guidelines for other applications too.

Edited by kkala, 30 March 2012 - 11:51 AM.

[Afshin445]

Sorry for late reponse.Actually we don't expect to much solid in this tank as waste water passed receiver pit and tank and releated pumps before that. Vendor recommended flange mounted side mixer with minimum low liquid level of 1750 mm.As per my preliminary design tank dimension was 10m(Dia.) x 10m (H), I need to check the level setting in tank based on new vendor data. For that tank we need 6.4m height between low and high and 0.1 m between low and low low and now question is how I can fix the heigh between high and overflow as tank is exposed to rainfall and evaporation.I appriciate if you or anybody can advise me in this regard.

Edited by Afshin, 13 April 2012 - 11:15 PM.

[kkala]

I saw Afshin's post No11 just now by chance, sorry that it has escaped. Following are a few notes (probably useful) on the height difference (H) between HLL (in this case LLL+6.4 m) and overflow out take (lowest part), assumed attached to the tank wall.
1. In theory H should cover at least the rainfall of the strongest tempest to occur every N years. Civil engineers with knowledge of meteorology cover these estimates, at least here, applying Porter's theory of extreme values. Meteorologists can also make these estimates, http://www.nawcinc.com/am.html. N declares frequency of overflow due to precipitation, depending on how serious this event can be. In a local refinery N=20 for sewers overflowing to the sea. Waste water tanks surrounded by dikes can have much lower N.
2. But H can cover the 20 min inflow between HLL and HHLL to be a little lower than overflow.
3. I think that conditions of para 2 predominate in most cases. Suppose 50 m3/h max flow into tank (dia=10 m), which means 210 mm between HLL and HHLL. But the average annual rainfall here is 402 mm, and it is assumed that 210 mm can practically cover the strongest local tempest concerning the 10 m diameter open tank.
4. Method of extreme values is used to estimate max run offs and consequential flooding, sewers capacity etc, mainly for new projects. Apparently it can seldom be predominant to specify H, though it is useful to be aware of mentioned max precipitation for the local conditions.
5. Matter will have settled now, above just gives a proposed reply to the query.

Edited by kkala, 18 July 2012 - 04:21 AM.