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  • #76
    Originally posted by MidwestCamper View Post
    It seems this issue occurring only during a rapid drain from a full tank. If so, maybe just to use the water via the pump when full would prevent the the partial tank collapse. A valve on the overflow would prevent pumping from sloshing during travel and a high vent would help to prevent tank collapse when we forget to open the overflow valve for tank use.

    Jim
    This just got me to thinkin , I know bad news......

    What if after we fill the fresh tank until water is seen from the over flow pipe. Then we stop the fill and then turn on the pump and run some water in a sink inside , or spray port outside. I wonder if this would pull a small amount of vacuum thru the over flow pipe enough to stop the siphoning effect? I think when the weather warms and I can get the RV out in the drive I will have to do some experimenting.

    Brian
    Brian & Michelle
    2018 Reflection 29RS
    2022 Chevy 3500HD

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    • #77
      Country Campers Brian, let us know how that goes. I think you will find that the tank is going to be siphoning and supplying water to the pump at the same time resulting in a collapsed tank.
      Joseph
      Tow
      Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
      Coach: 303RLS Delivered March 5, 2021
      South of Houston Texas

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      • #78
        Originally posted by Country Campers View Post

        This just got me to thinkin , I know bad news......

        What if after we fill the fresh tank until water is seen from the over flow pipe. Then we stop the fill and then turn on the pump and run some water in a sink inside , or spray port outside. I wonder if this would pull a small amount of vacuum thru the over flow pipe enough to stop the siphoning effect? I think when the weather warms and I can get the RV out in the drive I will have to do some experimenting.

        Brian
        Brian,

        That could work and so would shutting off the overflow valve to a drip. Lately I've been leaving the valve this way since I'm concerned with the cold water expanding and putting too much stress on the tank during travel while expanding. Last trip to your state PA, I did this and lost very little water. Once the high vent is added before the overflow shut off, I'll leave the overflow valve turned off 100%. Even when sanitizing, I use the pump to flush plenty of water though the plumbing. The high mounted loop and Keith's algri valve should also work great. All the ideas in alleviating expansion would help with the overflow closed. So would a baffled tank. We baffle tanks in the auto industry to prevent sloshing but also to reduce noise from sloshing at stops.

        Jim

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        • #79
          Cate&Rob ; Country Campers Guest DISCLAIMER - PLEASE EXCUSE THE FULL GEEK BUT THE ENGINEER IS COMING OUT.

          So I started the sanitation of the plumbing system by dumping all of the water put in the holding tank during delivery to ensure there was a clean fresh supply during the process and to ensure that the tank would have enough room to accept the required solution.

          The more I think about the statements by others and the setup of the system, I feel that the tank is experiencing "plastic" failure and it starts with the dumping of water through the valve. "Plastic" failure is a method of failure where the material fails past returning to its original state. In "plastic" failure breakage may not occur. From materials knowledge (not my forte), ferrous and non ferrous materials experience different property changes along the way. Most metals experience "strain" hardening where the material actually gets a touch stronger just before failure. (Look at ammo, brass work hardens to the point of becoming brittle, thus the annealing stage to soften the open end to prolong life.) Once failure occurs in metals, the ultimate strength of the material is retained. Plastic materials I do not think experience the same mechanical changes. Thus once failure occurs in the plastic, it looses strength. Once the plastic looses strength it takes less for the deformation to occur next time. Also once plastic deformation occurs in the plastic of the tank, the material stretches and cannot change to its original form compounding the failure mode as its already in the less than ideal shape.

          Now, I think failure of the tank starts with the mass exodus of liquid out the gate valve on the bottom with insufficient venting. I.E. the tank vent is so small that it allows for a huge vacuum to occur in the tank. Once the vacuum in the tank gets large enough, flexture of the walls occur which is an in implosion of the tank. ( https://academy.paulmueller.com/top-...sions-in-tanks ) While its not a instantaneous implosion, it does occur. All of the venting through the existing penetration with the opening below the top of the tank will not prevent this from happening as the intake is not large enough to allow for enough equalization to occur. Adding lines to the existing vent do not correct the issue. Installing an additional shorter vent line exiting low like the original does not correct the issue either.
          Joseph
          Tow
          Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
          Coach: 303RLS Delivered March 5, 2021
          South of Houston Texas

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          • #80
            Your plastic failure is called yield. Why would this yield not occur when the tank is overfilled and balloning the tank? I could see lots of these filling and emptying cycles causing oil-canning possibly leading to fatigue. That would lead to cracking at the corners.
            i would not expect the defection would be great enough to reach plastic deformation. I could be wrong. You could get near 0.5 psi difference.
            Ted
            2021 Reflection 310RLS
            2020 F350 PS,CC,LB,SRW

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            • #81
              TedS since its not metal and I'm not up on polymer design, yield is probably the correct term for failure, but the deformation is plastic meaning its more permanent.

              Most (not all) metals and plastics can take large tensile loads (pulling) but not the same on compression. When over filling the walls are put into tension based expansion thus the tank really just wants to become a sphere. Under Vacuum (negative pressure) the walls are pushed in. While 0.5 psi does not sound like much, lets put that in perspective. 1 sq ft = 144 psi thus 0.5 psi = 72 lbs force per square foot. Put this on the top of the tank that measures approximately 44" x 59" (3.67 ft x 4.9fy) or 18 sq ft that is like putting about 1,300 lbs spread across the top of the tank. And its plastic. For comparison for a 4' span 1/8" plate has no rated load, 3/16" plate 47 lb/ sq ft and 1/4" plate only 83 lb/ sq ft.

              Positive pressure pushing out is not the issue here, I think its the negative pressure causing the atmosphere to push in that is the issue. Positive pressure may initiate some of the failure modes.

              For an experiment, take a new unused plastic fuel container such as https://www.tractorsupply.com/tsc/pr...1?cm_vc=-10005 but replace the spout with https://www.tractorsupply.com/tsc/pr...r?cm_vc=-10005, fill the tank with water, place the spout on to it, then flip the can such that the opening is straight down. Observe the tank side walls, they collapse inward until there is enough elastic flex in the tank to overcome the vacuum created by the gravity drain. I have several cans in my possession that were stored with fuel and the caps replaced during the summer heat wave and found the sides sucked in, corners permanently deformed (without intervention of heat and pressure) when the internal pressure dropped due to thermal contraction of the gas vapors in the can.

              Even this tank as small as it is has a vent https://www.amazon.com/Reliance-Prod...5579510&sr=8-3 . I have one of these also and the sides of it will suck in if you forget to open the vent.
              Joseph
              Tow
              Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
              Coach: 303RLS Delivered March 5, 2021
              South of Houston Texas

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              • #82
                As to your valid draining scenario, I think the vent needs to be at least the size of the tank drain and terminating higher than the full tank water level. That would elimnate ballooning and collapsing of the tank.

                I can relate to pressure changes. A previous trailer had sewer hose storage that allowed the hose to remain attached to the drain connection. When I left home at 7000 ft el and arrived closer to sea level, the hose was collapsed into the carrier. The low pressure trapped between the tank valves and hose cap 'sucked' the hose in. Opening the empty grey tank valve vented the hose so I could pull it out.
                Ted
                2021 Reflection 310RLS
                2020 F350 PS,CC,LB,SRW

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                • #83
                  Jlawles2

                  "Plastic" deformation would mean that the tank retained a different shape when empty, after this happened. That does not seem to be the case. I have heard of tanks ballooned so far (on filling) that they lifted the floor, but returned to normal. Previously in this discussions were comments about tanks severely drawn in . . . again, without permanent deformation. So . . . we can probably all agree that the tanks are rather "flimsy" (there's an engineering word LOL!) . . . but they do not seem to suffer permanent (plastic) deformation from being stressed/stretched in either direction.

                  Rob
                  Cate & Rob
                  (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
                  2015 Reflection 303RLS
                  2022 F350 Diesel CC SB SRW Lariat
                  Bayham, Ontario, Canada

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                  • #84
                    I'm not aware of the tanks suffering from creep since they tend to retain their original shape when empty as Rob has stated. But the top and bottom of the tanks are flimsy (x2 engineering term) and would benefit greatly with baffling to support the tank as well as to prevent sloshing. Ted S, the vent can be much smaller than the water valve opening since the inflow of air has much less density than the out-flowing water. But the vent must also always be above the water level which may not be occurring as the top of the tank is collapsing.

                    Back to fluid mechanics on mass flow rate, this is m-dot Air = m-dot water.

                    For a sedative:
                    https://theengineeringmindset.com/ma...explained-kgs/

                    Jim
                    Last edited by Guest; 03-12-2021, 10:10 PM.

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                    • #85
                      Cate&Rob TedS Guest I think we all agree that there is something fishy going on here. Again, plastic mechanics is different than metal mechanics.

                      I think the tank vent is suspect in design and needs to be evaluated. I do like the tank drain, but the reality of the situation is venting of the tank is not adequate. Volume of air = volume water released and the current venting arrangement cannot do this.
                      Joseph
                      Tow
                      Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
                      Coach: 303RLS Delivered March 5, 2021
                      South of Houston Texas

                      Comment


                      • #86
                        Originally posted by Jlawles2 View Post
                        Cate&Rob TedS Guest Volume of air = volume water released and the current venting arrangement cannot do this.
                        Given that none of us is likely to go back through the fluid mechanics referenced by Guest Jim in post 84 . . . yikes ! . . . lets take a real world example. When you pour out a 5 gallon fuel can through a 1" diameter opening, the makeup air is supplied through a 1/8" vent hole to allow full liquid flow without compressing the tank. The same principle will hold true for the water tank in an RV with a 1/2" vent opening more than adequate to supply the makeup air for a 1 1/2" water outflow opening. If this were not the case, the tank drain would be "burping" as the water drains out. I have never seen this on mine nor heard of anyone reporting this as a problem.

                        Rob

                        Cate & Rob
                        (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
                        2015 Reflection 303RLS
                        2022 F350 Diesel CC SB SRW Lariat
                        Bayham, Ontario, Canada

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                        • #87
                          The real world example is better Rob where link I provided was to help anyone with insomnia.

                          Jim

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                          • #88
                            Certainly not mass flow rate equality. At these low pressures air flow in essentially incompressible flow.
                            Ted
                            2021 Reflection 310RLS
                            2020 F350 PS,CC,LB,SRW

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                            • #89
                              Here is a fitting I just came across for adding a connection to a tank.
                              Ted
                              2021 Reflection 310RLS
                              2020 F350 PS,CC,LB,SRW

                              Comment


                              • #90
                                The reason you do not hear girguling is because the tank flexes inward. once the tank reaches a point where it can resist the vacuum, then it would girgule. Since it appears the tank is flexing to the point of collapse, I would suspect the vortex around the draining occurs before the necessary resistance occurs thus air is introduced up through the center of the vortex.
                                Joseph
                                Tow
                                Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
                                Coach: 303RLS Delivered March 5, 2021
                                South of Houston Texas

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