A storage tank is the stage that separates two incompatible regimes in time: a water treatment unit runs at a steady design flow, while the consumer draws water unevenly and in peaks. The tank takes that difference upon itself, and for that reason it determines not only the reserve of water but the size of the equipment ahead of it.
Principle
A storage tank is an unpressurised vessel working at atmospheric pressure. It has an inlet, an outlet, an overflow, a bottom drain for emptying, a vent connection and a hatch for inspection and sanitising; the level is watched by float or hydrostatic sensors. The diaphragm pressure vessel often called by the same word «tank» does a different job — it smooths the pressure and cuts the number of pump starts — and is not a storage tank.
The main work of the tank is to separate supply and draw-off in time. The treatment unit starts on the low level, fills the tank and stops on the high level; the consumer takes water whenever it is needed. Hence the sizing rule: the unit is selected for the flow that suffices to restore the volume within the time available, not for the peak draw. Without a tank a membrane unit or a filter has to be sized for the peak, and that is a different size and a different price.
The second purpose is time and source yield for the equipment's own needs. Backwashing a filter, regenerating a softener and flushing membranes call for both water and a pause in supply. While such an operation runs the consumer is fed from the tank, and the source is not obliged to cover the wash and the draw-off at once.
Pressure downstream of the tank is created anew. The water enters the vessel through an air gap and loses the head of the source, so a booster set is installed after the tank — a pump with a pressure vessel or with variable-speed control. The head is calculated from that set to the highest and most distant point of use; the pressure at the source no longer enters the calculation.
Inside the tank the water stands still, and that is not a free operation. Residence time is as much a property of the stage as flow, and it is what determines what happens to the water between the inlet and the outlet. Sediment gathers at the bottom, the walls and internal surfaces grow biofilm, and in the light algae are added. The tank is therefore treated not as an empty vessel but as a part of the scheme with its own regime and its own maintenance routine.
Principle
Water in a tank is not preserved but changed, and usually for the worse. Ultraviolet disinfection leaves no residual action at all: the lamp acts on the flow at the moment it passes, and water that has entered the tank after it is colonised again from the walls and the pipework. Permeate from reverse osmosis makes matters worse — it is almost free of salts and weakly buffered, and surfaces downstream of the membrane grow biofilm faster than they would on the raw water. Hence a rule that rearranges the scheme: disinfection goes after the tank rather than before it, and the tank itself must allow sanitising — with a hatch, access to it and a bottom drain.
A tank «with a margin» makes the water worse. A volume taken twice as large as needed doubles the residence time: at the same draw-off the water is renewed half as often and has time to stagnate. It looks like a failure of the water treatment — the analysis at the tank inlet is within limits, the analysis at the consumer is worse — although not a single stage ahead of the tank is faulty. The volume is therefore worked out from the daily draw-off profile and the refill time, and any margin is kept within limits that preserve the turnover of the water.
A tank is a decision about sizing rather than about reserve. Its real effect is seen not in the volume of water but in the specification of what stands ahead of it: with a tank the unit is calculated for the average flow over the filling period, without a tank for the peak draw, which is usually several times higher. The difference shows up in the size of the membrane unit or the cross-sectional area of the filter, in the water spent on washing and in the demands placed on the yield of the source. The cost of a tank is to be compared with that difference, not with the price of an empty vessel.
Practice
The treatment unit is selected for the flow over the refill period rather than for the peak draw-off. On sites with pronounced morning and evening peaks this directly reduces the size required and the water spent on the equipment's own needs.
The equipment's own needs cease to be a break in supply. Backwashing and regeneration are carried out in the hours of low draw-off while the consumer is fed from the tank; in many schemes the need for a second vessel working alternately disappears.
A limited source yield stops limiting the flow. A borehole that gives less than the peak draw but covers the daily demand serves the site through a tank — provided there is enough time between peaks to restore the volume.
A finite emergency reserve appears — for planned maintenance, a power cut or an interruption in supply. Its size is set deliberately, because the same figure increases the residence time of the water: reserve and freshness pull in opposite directions here.
A tank is a standard product and we do not manufacture it: our part of the work is calculating the volume from the draw-off profile, choosing the material and the version, tying it into the scheme with its pipework, level control, overflow and disinfection downstream, supply, commissioning and subsequent service.
Limits
Practice
The cases are typical examples, not site reports.
Scheme
A tank is never a stage on its own: it stands between whatever prepares the water and whoever consumes it, and its place in the scheme is set by one rule — disinfection goes after the tank. Ultraviolet leaves no residual action, so a lamp ahead of the tank protects the contents of the vessel from what arrives with the water, but not the water that leaves it.
The pipework of a tank is more than an inlet. The inlet is made with an air gap so that nothing is drawn back into the outside network; the outlet is taken above the bottom so that the settled sediment is not pulled in; the overflow is sized for more than the inlet flow and is led out through an air gap; a drain at the lowest point allows the vessel to be emptied completely. The vent connection carries a filter: the tank breathes through it while filling and emptying, and whatever is in that air comes in with it.
A booster set is installed after the tank, and the head is calculated from that set to the highest and most distant point of use. This is a separate part of the scheme with its own equipment: its own capacity, its own margin of head and its own dry-run protection from the low level in the tank.
The failure case is provided for in advance. A stuck filling valve or a failed level sensor means that the whole contents of the vessel, and whatever the source delivers afterwards, ends up on the floor: the room needs a gully or a drip tray, and the controls need an independent high-level alarm sensor that cuts off the supply regardless of the working one.
Scheme
Operation
Energy
A storage tank consumes nothing of its own: there are no moving parts and no drives in it, and the level is watched by low-power sensors. The energy in this part of the scheme is spent downstream of the tank, by the booster set, and its consumption follows from the head required and the actual draw-off rather than from the volume of the vessel. Heating appears as a separate item where the tank stands in an unheated room: here the demand depends on the volume, the insulation and the temperature around it, and is calculated for the particular site rather than taken as a typical figure.
Consumables
What goes to drain
A tank produces no waste of its own: it removes nothing from the water. Three flows go to drain, and all three are provided for in advance. The first is the water of sanitising with the remains of the disinfecting agent: it is discharged according to the instructions for that agent, not as ordinary effluent. The second is the water of emptying before maintenance, together with the sediment from the bottom; this is the whole contents of the vessel in a short time, and the receiving point has to take it. The third is the overflow, which in normal service never runs at all but is sized for a failure of the controls, when the entire supply from the source goes through it.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
There are two flows required here, and they differ: the daily average and the peak in the morning hours. Without a tank the equipment has to cover the second one, because the consumer wants water at the moment the tap is opened. With a tank the unit is calculated for the flow that suffices to restore the volume within the time available, and that is usually a different size — with different demands on the yield of the source and a different amount of water spent on its own needs. A tank is therefore compared not with «a vessel kept in reserve» but with the difference in the specification of everything ahead of it.
The one that covers the peak draw less the supply arriving at the same time, and refills before the next peak. It follows from the daily draw-off profile, not from the number of residents and not from the daily consumption: two sites with the same daily figure need different volumes if one draws evenly and the other in two short peaks. Bigger is not better: an excessive volume increases the residence time of the water and makes it worse.
Because between the unit and the consumer the water stands still. In that time the sediment settles, the walls of the vessel and the pipework beyond it grow biofilm, and water that has passed through ultraviolet carries no disinfecting action — the lamp acts only on the flow at the moment of exposure. With permeate from reverse osmosis this is more pronounced: it is almost free of salts and weakly buffered, and surfaces downstream of the membrane are colonised faster than they would be on the raw water. Hence two compulsory measures — disinfection after the tank, and a routine for sanitising the vessel itself.
A lamp ahead of the tank protects the contents of the vessel from what arrives with the water, but it does not disinfect the water leaving the tank: ultraviolet has no residual action, and colonisation proceeds from the walls and from the pipework beyond them. Where there is a single disinfection stage in the scheme, its place is after the tank. Lamps both before and after make sense where the raw water is known to be microbiologically contaminated, but that is two stages rather than one moved around.
No: these are different devices with different jobs. A storage tank works at atmospheric pressure and holds a volume; a diaphragm pressure vessel works under pressure, holds an incomparably smaller volume and exists so that the pump does not start for every short draw. In a scheme with a storage tank a pressure vessel is usually present as well — as part of the booster set downstream of the tank, not instead of it.
Water expands as it freezes, and it is not only the vessel that is destroyed but the pipework as well — valves, sensors and the sections of pipe within the freezing zone. The question is settled at the scheme stage: a place in a heated room, insulation with heating and temperature monitoring, or emptying the tank for the winter if the site is seasonal. The thermal calculation is done for the particular room, because it depends on the volume, the insulation and the temperature around it.
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