It would be wrong to assume that a water storage system was just a static collection of tanks, ponds, or cisterns without moving parts. Though the physical structure of a water storage tank is dominated by both the containment walls of the tank and its foundation/structural members, it could not function without a series of mechanical systems that monitor and operate the tank’s filling and discharge functions. In fact, every water storage facility is an integral part of an overall water supply system featuring active mechanical features.
These mechanical systems include all of the appurtenances that make sure that the water is properly stored and distributed. These include level controls and overflow alarms, discharge risers, drains and inlets, pumps along with their monitoring systems and controls, bypass and overflow piping, automatic and manual shut-off valves, water sight glass viewing ports, bubblers and air blowers, pipe couplings and fittings, pressure gauges, dosing equipment, proper cathodic protection of the tank wall surfaces, and remote control SCADA (Supervisory Control and Data Acquisition) monitoring systems.
These mechanical systems perform three broad functions: they maintain a proper balance between water storage levels and air pressure inside of a water storage tank, ensure that the system maintains a constant rate of water flow, and apply the proper amount of pressure to the water distribution system.
WATER DEMAND, SUPPLY, AND STORAGE BASICS
Each water supply system consists of several basic elements and subsystems. The first element is the actual source of the potable water. Water can be acquired from several different types of sources: well fields extracting groundwater from local aquifers, natural bodies of water such as lakes and ponds, and man-made bodies of water, such as lakes created by the damming river flows and other reservoirs. Many of these man-made sources are also lined with low-permeability compacted soil layers to prevent loss of the water through infiltration. The second element includes the means to extract this water source for use and transport it to the next stage in the water supply systems. These elements include: groundwater extraction wells and associated pumping systems, water supply pipelines with flows generated by applied pressure heads, and aqueducts or canals that rely on gravity flow to transport water. The third element is a facility for treating the acquired water and removing impurities and contaminants so that it is fit for human consumption, industrial uses, or agricultural applications. Fourth is a means of storing and regulating water flows to ensure consistent matching rates of consumption and supply. This would be a system of above-ground or buried water storage structures whose volume capacity is such that it can provide additional flows for periods of peak usage.
Variations in water usage are why storage facilities are needed in the first place. The average individual American uses between 80–100 gallons of water per day for personal use. However, water usage rates are not steady or consistent. They can vary significantly per hour as a result of: toilet flushing (1.5–3 gallons depending on type), showers (5 gallons per minute standard to 2 gallons per minute for low-flow shower heads), grooming (shaving, teeth brushing, and washing—each less than 1 gallon), dishwasher operation (6–16 gallons per cycle), clothes washing (40 gallons to 25 gallons per load), and drinking (about half of a gallon per day). The rate of personal water consumption varies greatly during the day, with peak usage occurring in late morning and early evening and minimum usage coinciding with early morning hours. Personal usage rates can also vary with seasons, summertime being a period of higher per capita water consumption than wintertime. This is due to both increased personal rates of drinking and the additional seasonal activities of watering a lawn or garden.
The average daily overall per capita water usage in the US (including residential, commercial, industrial, and agricultural consumption) is approximately 150 gallons per day. This represents an additional 50% increase over daily consumption for personal use of 100 gallons per day. Industrial and commercial use remains fairly consistent, coinciding with hours of business operation and the amount of water required for consistent industrial production cycles. Agricultural water use naturally follows seasonal patterns of irrigation, weather and climate patterns, and individual crop species requirements. Certain areas of the country require greater amounts of water as a result of hot and arid climates (Nevada’s per capita daily water use is 189 gallons vs. 51 gallons for Maine). Population density also plays an indirect role. Oddly enough, consumers in rural areas will tend to have higher overall water consumption rates due to extensive irrigation and agricultural requirements than those living in high-density urban areas. It is this variation in peak daily and seasonal water usage rates that requires a means of storing water produced during periods of low demand so that it can be used later to meet the needs of periods of peak demand.
Water storage can take many forms depending on the overall demand rate. Not every water storage facility consists of a system of large water storage tanks or towers linked to an extensive water distribution and service system. These are typically required only for larger urban areas or industrial facilities that require large amounts of process water. Individual water consumers can utilize rain collection and storage devices. Modern upgrades of the traditional rain barrel can range from 65-gallon drums to rain harvesting systems with storage capacities between 1,000 gallons and 60,000 gallons provided by above-ground and below-ground tanks made from plastic, fiberglass, concrete, or steel. Smaller rural communities and individual neighborhoods can be serviced by traditional water towers or other storage systems whose size is tailored to their needs.
No matter what their size, water storage facilities need to be resistant to both internal corrosion and external weather. Weather impacts are not just limited to external rusting and wind damage. Temperature extremes can have both a physical and biochemical effect on water storage systems. While wind gusts, snow, and rain can erode exterior protective coverings and structural supports, temperature changes can promote algae growth and require additional chlorine treatments. In addition to preventing algae formation, water storage facilities must internally filter and screen out impurities such as disease vectors (and organic materials that can be breeding grounds for disease) and debris of any kind. These not only render the water unfit for human consumption, but they can also cause internal damage and biological clogging. In addition to algae growth, temperature-induced strains can cause expansion and contraction of key joints and fitting, causing long-term structural damage and need to be compensated for in both the design of the tank and its fitting and the choices of materials used to construct the tanks.
In addition to overheating, there is the problem of freezing temperatures. Ice expands and can cause internal structural damage to both service components and to the tank body itself. These can include pressure damage to bolts, rivets, and welds used to hold the tank wall seams together. And though it is rare that the entire mass of water gets frozen solid, the freezing of just the surface of the water in the storage tanks is usually sufficient to interfere with normal pumping and discharge operations.
Storage tank materials must be both aesthetically pleasing and reduce the need for maintenance and repair. Lining and coating systems are provided to achieve both of these goals for large-scale concrete and steel water tanks while meeting safety and cathodic protection standards of the National Sanitation Foundation (NSF) and the American National Standards Institute (ANSI) which sets the standards for manufacturing and industrial operations in the US. In particular, ANSI part 61 sets the requirements for water treatment and storage. It sets very stringent standards and guidelines for equipment that either comes directly into contact with potable water, or materials, chemicals, and equipment that are part of the potable water production and supply system. In order to ensure long-term performance, these coatings must have strong bonding and adherence characteristics.
WATER STORAGE STRUCTURES AND MECHANICAL APPURTENANCES
The most common means of storing water for later use is in elevated water storage towers. These can be seen everywhere across the US. Their prominent elevations are a result of designs that allow for sufficient elevation head pressure that will provide efficient distribution of water through the town’s water supply piping system. Gravity flow has significant advantages over pumped distribution. Gravity drainage is unaffected by electrical power outages, meaning that critical supply needs (such as firefighting) are never disrupted. Pumps are used to push water up into the elevated storage tank at the top of the tower and refill it as needed, but the actual feeding of water to the service system does not require mechanical methods. The actual size of the tank is designed to ensure sufficient storage capacity to meet most emergency and peak daily use conditions. Separate water towers are used for non-potable water supply systems (irrigation, firefighting, and industrial processes).
Not seen from the ground are the tank’s internal mechanical systems, fixtures, piping, and appurtenances. These are highly specialized and often customized for individual water towers. This customization is both an engineering design problem and a mechanical challenge. These tank and water supply system appurtenances and fixtures include the following, in addition to standard piping, conduits, fittings, control valves, etc.:
- Level Controls. The tank level controller is designed to control the level of water in a tank between maximum and minimum levels. They are also integrated into a system of controls that ensure that there is proper water turnover. A poorly designed tank may leave residual water in the bottom of the tank that never leaves the tank unless it is completely emptied. As a result, the tank may experience corrosion, accumulation of chemical contaminants and organic materials, nitrification, and associated pathogen contamination. Level controls can be as simple as floater switches that halt the infeed of water once it reaches a set level within the tank.
- Overflow Alarms. Working in conjunction with level controls, overflow alarms send a visual or audio signal to warn the operator that design levels in the tank have been exceeded.
- Water Sight Glass. This is a leftover from the analog age that is still used alongside digital instrumentation that allows for direct viewing of liquid levels and liquid quality. A transparent tube that is attached to the bottom and top of a tank or boiler, it allows the operator to directly see water levels and coloration. Being hydraulically connected to the tank, the level of water in the tube is also the level of water in the tank. It also allows the operator to see any contamination or turbidity that affects the water quality.
- Air Blow-Off Valves. Also known as a scour valve, this can be automatic or manual, but in either case is designed to flush water pipes and release entrained air. Typically located at low points, blow-off valves flush out the pipeline in conjunction with air release/vacuum valves located at high points along the pipeline. The act of flushing removes stagnant water where debris and rust particles can accumulate.
- Aerators/Bubblers. These provide aeration treatment for stored water to lower BOD and reduce odors and bad tastes, as well as remove methane, radon, carbon dioxide, and industrial contaminants of volatile organic compounds (VOCs). By raising the pH of the water, aeration makes it less acidic and aids in precipitation treatments to remove metals such as iron and manganese. They are designed as either a column surrounded by packing material that diffuses as it passes into the surrounding water, or as perforated pipes set in the floor of that tank whose bubbles rise through the water to the surface.
- Pumps. Pumps in general are mechanical devices that utilize force to raise, transfer, deliver, or compress fluids (liquids or gases). They do so by utilizing applied suction or pressure. Pumps can be designed for simple delivery of clean water under moderate operating conditions, or be designed for harsh operational environments involving extreme pressures, high flow rates and flow velocities, liquids impacted by gravel and waste that require grinding as part of the pump operation, and pumps that deal with non-water liquids like oil or industrial chemicals. Pumps come in two basic types: centrifugal pumps that use rapidly spinning impellor vanes to fling water out and up, and positive displacement pumps that use piston action (which can also be a pulsating diaphragm or membrane, or even peristaltic tubing that squeezes water forward) to drive water forward. Pumps are designed to operate either submerged under the water surface or non-submersible locations above or outside the water reservoir.
- Sensors. These are categorized by the type of signal that they monitor and the physical characteristic they are designed to measure. A discrete (or digital) sensor is a binary measuring device that only reads on and off situations without being able to measure values in between. In contrast, an analog sensor can measure a range of values. Specific sensors are available to measure a wide variety of characteristics associated with water storage operations: temperature, humidity, water level, water flow, power failure, etc.
The Level Control Panel with Touch Panel (LCP-TP) interface designed by Singer Valve was created to accomplish optimum level control feasibility and flexibility. This Level Controller is designed to complement a single solenoid operated/override control valve and 4-20mA level sensor or high/low level switches. This combination package is ideal for filling any kind of tank with water that requires filling to a level setpoint and then drawing down the level of the tank to a secondary setpoint before activating the fill cycle again, thus ensuring tank turnover. The LCP-TP is quick and easy to configure to read and compare the level 4-20 mA signal to the desired setpoint. The setpoints can be set locally via an interactive button display screen or remotely via either SCADA Modbus or hardwired 4-20 mA remote setpoint signals. If a high/low level switch system is preferred, the LCP-TP can switch configuration to allow for level switch inputs and regulate the control valve accordingly. Data logging is also a useful feature to log sensor feedback and setpoint data with a time stamp, allowing for system analysis.
The biggest value of this automated level controller is flexibility and readability of control. For the user who wants to set up a full communication network that has access to all storage tanks and controls them remotely, it’s easy to do with Modbus and remote 4-20mA communication options. For the user that has a remote site, but wants to be able to data-log and analyse the tank turnover, the LCP-TP offers the data logging feature. Either option offers lots of feedback and traceability of the system operation. Based on this information, the tank level setpoints can be adjusted to match the needs/demands of the system with simple interaction to ensure that the system can function at its optimum. This ability to take all the information and then easily adjust the tank setpoints is a huge benefit. Although the LCP-TP itself does not eliminate tank turnover issues, as the system still needs to be operated correctly, having access to all the feedback information should allow insight into setting the system to run correctly.
Ferguson Waterworks provides solutions for the water, sanitary sewer, and stormwater management industries for public and private water sewer authorities, utility contractors, public works/line contractors, and heavy highway contractors. Their metering and automation group provides automated metering solutions for advanced metering infrastructure solutions utilizing proprietary web-based metering software. Their automated meter-reading technology allows for drive-by metering and manual touch read metering.
In addition to its main line of pumping systems, Grundfos Pumps also provides a wide variety of water storage tanks. Their GT tanks are used for vertical installations and are available in sizes from 2 gallons to 800 gallons. The GT-H tanks have a non-toxic butyl rubber diaphragm, dividing the tank chamber into two compartments. The upper compartment contains compressed nitrogen. The lower compartment has a liner in polypropylene (PP) and is filled with water from the pump. The GT-D tanks take this design further with a double diaphragm. A variation of this design is available with the GT-U tanks which have a replaceable, nontoxic butyl rubber bladder surrounded by compressed nitrogen. In all cases, the bladder is maintenance-free and is the only component in contact with the liquid, and is corrosive-free, ensuring high quality. Approved for use with potable water, their GT tank series can be used in domestic water supply systems, booster systems, expansion in heating and air-conditioning systems, irrigation systems, industrial systems, degassing tanks, and high-quality pressure vessels.
Containment Solutions is in the business of providing tanks for long-term fluid storage handling from fuels and lubricants to water and wastewater. CSI also offers custom fabrication to accept components such as pump platforms and rails for pumps. These customized designs allow for easy utilization of storage tank mechanical systems. Their Flowtite tanks can be used in water storage systems to store potable drinking water, wastewater, graywater, rainwater, and storm runoff. They also provide components to filter out contaminants like oil so the collected water can be used as part of a system for landscaping and other applications. Their fiberglass tanks won’t rust, corrode, or degrade. This makes them preferable for underground installations. These types of installation are suitable for rainwater harvesting, and a properly engineered water tank system can be designed to collect up to 100% of surface area rainwater. Above-ground installation is available for vertical water tanks that meet nationwide standards while allowing for customization to meet customer needs. Their aboveground tanks range in size from 1,250 to 20,000 gallons. On a larger scale, these tanks can be used by communities to manage excess water from stormwater runoff. Other uses include fire protection, commercial applications, industrial uses, or residential consumption.