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Thread: AWC Salt Adding Ideas?

  1. #1
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    AWC Salt Adding Ideas?

    What can automatically add salt mix to an AWC reservoir?

    After an AWC RO water is added to the reservoir. I'd like to then add salt until a conductivity level is reached. The programming is easy enough ... But what can I use to move salt mix from the bucket to the reservoir?

  2. #2
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    This is very hard to do and keep the salt dry. It don't take much moisture to clump the salt up into a rock.


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    Yep ... That's why am looking for suggestions!

  4. #4
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    Industrially there are rotary valves that act like a gum ball machine. But I am sure they are much more expensive than what a hobbyist would want to spend.

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    It's just not practical. This will be a multi step process.

    First, you need to store the bulk salt in a hopper that has an active air drying system. You cannot count on an air tight system because you have to get the salt from the dry environment to a water vessel somehow, which means you have a penetration.

    Once you figure out the storage issue, for distribution, you will want to use a screw type pump for transfer from bulk to an intermediate container...sitting on a scale. You will need a way to measure the salt transferred by weight...volume will not be reliable as bulk density will vary greatly. Once your scale hits limit, you will want it to trigger a switch which starts the next step.

    Mix and move the salt from the scale to the storage container. My thinking on this is the intermediate container is an aquarium with a fill line and two drain lines, all with actuated valves. You cannot just drip RO/DI water into this as the slow addition of water will result in an unpredictable quality of salt water...many of the parameters will precipitate out due to hyper salinity levels. So you will need to add the water rapidly. So figure having a large vessel of RO/DI water on standby for this. Once the water is added to a high level float switch, start up a rapid mixing pump or two, Aqueon propellor pumps would be perfect. Add heaters then monitor temperature and salinity. Set the float switch on the low side so you make the water too saline. This way you can slowly add RO/DI to hit the exact conductivity you want. If you overshoot the initial water, you will have no reliable way to add "a little more salt".

    Once you hit the right volume and salinity and temperature, you can then drain the tank from the mixing tank to your AWC. You will want the drain inlet to be about an inch of the bottom so you don't drain the crud (various metal precipitates). Once the tank has drained, you will then add a flush volume of RO/DI, operate your mix pumps a bit then operate a lower drain that fully drains the tank. This will wash out any remaining salt water and crud. You might need a second rinse.

    Once this is done, operate a blower to fully dry the tank so you are ready to accurately measure your next batch of salt. and you will need to reset the tare weight on your scale somehow.

    Sensors:
    Scale Trip
    Initial Water Fill
    High Water Overflow
    Service Water Drain
    Low Water Drain

    Probes:
    Temperature & Conductivity

    Outlets:
    Salt Hopper Screw Pump
    RO/DI Water Transfer
    RO/DI Water Doser
    Internal Mixing Pumps
    Transfer Drain Pump
    Rinse Drain Pump
    Dryer Blower/Fan
    Scale Tare Reset



    I think I will just stick top doing my batch mixing by hand.

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    Do you see any issues with a "brine" reservoir (max salinity water)?

    After each AWC the AWC reservoir would fill with RO water and a dose of brine would be added every 15 minutes until the AWC supply reaches a conductivity target. The Apex would not initiate AWC's if the reservoir were not near display tank temp and conductivity levels.

    Thoughts?

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    You can't really over concentrate sea water without causing things to precipitate out. My concern is once you re-dillute it to NSW concentration, many elements will be missing, calcium and alkalinity.

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    Quote Originally Posted by clsanchez77 View Post
    You can't really over concentrate sea water without causing things to precipitate out. My concern is once you re-dillute it to NSW concentration, many elements will be missing, calcium and alkalinity.
    You're right. I just received the following back from Tropic Marin:

    Due to the high concentration of magnesium, calcium and hydrogen carbonates there is always the danger of precipitation of calcium or magnesium hydrogen carbonates or hydroxides when increasing the salinity above the natural level. Once there is a precipitation, it is impossible to dissolve it again at the pH given.

    Therefore we don’t recommend to prepare a brine on stock and to dilute it when necessary.

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    Quote Originally Posted by LHillman View Post
    You're right. I just received the following back from Tropic Marin:

    Due to the high concentration of magnesium, calcium and hydrogen carbonates there is always the danger of precipitation of calcium or magnesium hydrogen carbonates or hydroxides when increasing the salinity above the natural level. Once there is a precipitation, it is impossible to dissolve it again at the pH given.

    Therefore we don’t recommend to prepare a brine on stock and to dilute it when necessary.
    I cannot find the thread, but there is someone who built a dry salt hopper somewhere. It wasn't here, but on another reef board. I'm curious about its long term success given all the issues I mentioned above. But to do it correctly and account for all the steps needed to truly automated, refer to above. The biggest unknown to me is finding a scale that will do what you need, and you will need to DIY a controller that will read the weight and then update the tare weight with each use.

    You will also need to decide if you create the same amount of salt mix with each batch, or do you calculate how much you need and adjust the weight accordingly.

    The second major issue is the screw pump. They are pretty common in industry, but I have not seen one that would be hobby grade per se...and be able to handle direct contact with salt. So you are talking all plastics and high grade stainless steel.

    Lastly, don't over look the importance of rinsing and flushing the precipitate solids. Otherwise these will build up over time. The amount varies by salt mix and batch, so can't just assume yours will be minimal based on what you currently use today. I have seen it vary greatly.

    If you decide to pursue this, please keep me posted. As you can tell, I am highly interested.

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    OK, so (with Tropic Marin's response) I'm now back to the "adding salt" approach.

    I believe I can manage the weight/measurement issue by connecting putting a conductivity probe in the AWC reservoir and adding salt a little at a time (every 15 minutes or so) until the salinity is in range. That way to amount of salt added each time does not need to be really accurate.

    I'm going to look into the screw pump idea you suggested. Its also been suggested that I use a very small wet-vac. I'm open to all ideas!

  11. #11
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    Quote Originally Posted by LHillman View Post
    OK, so (with Tropic Marin's response) I'm now back to the "adding salt" approach.

    I believe I can manage the weight/measurement issue by connecting putting a conductivity probe in the AWC reservoir and adding salt a little at a time (every 15 minutes or so) until the salinity is in range. That way to amount of salt added each time does not need to be really accurate.

    I'm going to look into the screw pump idea you suggested. Its also been suggested that I use a very small wet-vac. I'm open to all ideas!
    If you use a rotary valve with a hopper above it, it will work. You would have to calculate the volume of salt not the weight. Each turn of the valve will give you a specific volume. These are used for dry powder applications in many industries. I've seen them for mixing dry sugar with water for juice & soft drinks.

    It will require maintenance. You can get them in a CIP (clean in place) configuration used in food process but it will be stainless steel.

    The valve will require a motor to operate. You will need a hopper mounted on a frame above your mix tank. There are all kinds of options like plastic curtain type shutes to plastic flexible couplings or open pipe to direct the salt from the valve outlet into the tank opening.

    I would suggest a Google search for rotary valve and check out the images and websites.

    FYI. The last one I quoted to a customer was 6" all stainless with motor and CIP. It was ~$10,000.00 so be prepared. I believe they come in sizes as small as 1.5" or 2".

    It was mentioned above - keeping the salt dry and clump free is in my mind the biggest obstacle. Also, to move powders, there is almost always some sort of vibration that needs to happen. Otherwise powders don't "flow" properly.


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  12. #12
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    Quote Originally Posted by davidewen View Post
    If you use a rotary valve with a hopper above it, it will work. You would have to calculate the volume of salt not the weight.
    I can only partially agree with that. Salts are not sugars. They are highly hydrophilic, corrosive and abrasive. Anything with metal parts outside of industrial grade stainless steel (Grade 316) will fail. Thus you will end up with PVC valves of some kind; the abbrasive nature will make quick work on PVC and resilient seats. Any actuated valve with moving parts (ball, gate, rotary, etc) will be difficult to maintain and will require constant maintenance if not replacement...so be generous with unions.

    Salts are not sugars and salinities are measured in parts per thousand, as in parts weight of salt per parts weight of volume. Water has a fixed weight per volume (and sugars are pretty predictable), bulk salts do not. The bulk density will vary and thus two or more fixed volumes of salts will have different weights. Moisture content does lend itself to using fixed volume instead of measured mass...however you then have to vibrate it to maximum bulk density to get a predictable mass for a fixed volume...and once you accomplish that, it will stick in the valve and no longer flow. Also, because salt is so highly hydrophilic, it will not flow like say sugar, so you need to drop this into a dry hopper, not a wet one. This comes back to my original suggestion that you need some kind of intermediate hopper or you must quickly add water to dry salt. Slowly adding water wont work as you will clog the feed line and dumping salt into water wont work as you will precipitate everything out.

    I am not saying David is wrong or this absolutely won't work. But salts are not sugars and for the amount of money you will have to sink into this, there will be a large amount of trial an error and failed parts.

    Also, good point on the vibration, I overlooked that part. But I was thinking about needing some kind of high torque paddle. Vibration would work better. You will want to isolate the salt hopper from the supports so it does not drive you crazy.

    Finally, I did cross a thread on Reef Central where someone claimed to pull this off with pneumatic pinch valves. These might be more appropriate for dry bulk salt applications as there are no moving parts directly exposed to the salt. However, it is a pneumatic valve that introduces the need of a compressor.

    If you are serious about pursuing this, you can drill a bulkhead into the bottom of a bucket of instant ocean salt mix, install an electric actuated ball valve (on unions) and attach an electric vibrator to the table. You will want to isolate an upper table with the bucket from the lower table and/or support legs. Use SS screws with rubber bushing between the upper table and support and use rubber bushings on both ends of the bolt, so that the bolts are not in direct contact with any part of the table. Also use SS fender washers, lock washers and bolts. Hayward makes actuated ball valves for only a few hundred dollars. You can experiment with this and resolve the workflow before you spend a few grand on the final valve.

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