What are they, how do they work and what is the scope of hydropneumatic or domestic systems?

Surely many have been presented with situations in which the need to carry out water movements arises to satisfy any of these circumstances:

• Irrigation and irrigation of green areas, orchards and gardens
• Lawn Sprinkler Operation
• Extraction of water from wells, rivers, ponds, streams or even rainwater
• Service water supply
• Supply of a household with clean non-potable water

These requirements point particularly to the use of what is often called hydropneumatic systems or domestic water systems, although they are not only applicable to a home, as we could infer from its name, but also to a wide range of uses that include, among others, small industries, shops, workshops, schools, hotels, laundries, greenhouses, sports institutions and, especially rural buildings.

While a hydropneumatic system It can be fairly complex in terms of installation, the main components consist of a few that we can summarize in the following list:

1. A Water Pump
2. A pressure tank pre-charged with compressed air
3. Control elements
4. Accessories

The type of pump can be submersible (for the specific case of extraction of groundwater or wells) or surface (for any application), and it can be a centrifugal pump or a jet type pump (for pressure). For its part, the tank can have a vertical or horizontal design, and its capacity can vary between about 8 and 200 liters. The control elements are basically two: a pressure gauge and a pressure switch. As for accessories, there are check valves, pressure relief valves, pipes, filters, hoses, couplings, etc. It is always important to know how to choose which type of pump is right for the job at hand.

The bomb and the pre-filled tank can be purchased individually, or in a single, very compact unit that also includes the control elements. In that case, the device that the market offers is the one that we will address in this article, that is, a system consisting of a surface pump mounted on a tank of 20 liters (in the most common models), unit that some manufacturers designate simply as household pump.

How important is the pre-filled tank?

Beyond how this is constituted hydropneumatic system, it is clear that the function of the pump is to supply the water from the source, for example, a well, pond, cistern, etc.

What is not so obvious is the function of the tank. In short, the tank allows the system to work “request”. What does this mean? That we can use water at a good flow rate (between 3500 and 4500 liters per hour) without bomb It must necessarily be started every time we open a tap, as in the case of conventional pressurizing pumps.

Let’s see in more detail what a pre-charged tank consists of and how it works.

Depending on the manufacturer and model, the tank shell is constructed of stainless steel, cast aluminum, or fiberglass, and may or may not have anti-corrosion treatment. Inside the tank there is a physical separation between the water and the air contained therein, which differentiates it from conventional pressure tanks, where water and air are in contact with each other and, due to the inconveniences that this causes , added to the large size of the tank, are falling into disuse.

This physical separation between air and water can be of two types: by means of a diaphragm acting as a dividing wall, or incorporating a bag vinyl inside the tank, which is intended to house the water.

In the most economical models of hydropneumatic systems, the separation is of the latter type, that is, by means of a bag, and the rest of the tank is occupied by air at a certain factory pressure, that is, it is basically compressed air. This is another characteristic that differentiates pre-filled tanks from conventional ones, because having compressed air they can perform the same work as these with a reduced size. This immediately translates into higher performance, since, for example, a 20-liter pre-filled tank yields the same amount of usable water or the same emptying capacity as a conventional 40-liter pressure tank.

In the figure below we present a diagram of a pre-filled tank with a bag, also known as membrane pre-charged tank.

The parameters that define a hydropneumatic system and that are configured at the factory are three, and their magnitudes vary in the ranges mentioned below, depending on the manufacturer and model:

Precharge pressure: it is the one that the equipment is when it leaves the factory and corresponds to the air inside the tank. It varies between 0.10 and 0.30 MPa.

Cutting pressure (controlled by the pressure switch): it is the maximum pressure exerted by the water bag against the air inside the tank, and it is the pressure at which the pump stops working. It varies between 0.28 and 0.42 MPa.

Start pressure (controlled by the pressure switch): it is the minimum value of pressure exerted by the water bag against the air inside the tank, and it is the pressure at which the pump starts to work. It varies between 0.11 and 0.28 MPa. As a general rule of thumb, the starting pressure value is always 2 psi (approximately 0.014 MPa) greater than the precharge pressure value. This is to ensure that when opening the taps and lowering the pressure in the installation there is no remaining pressure capable of preventing the system from reaching the starting pressure.

Each tank has a unique value of these three parameters. For example, hydropneumatic equipment Most widely used and accessible models have a preload of 0.14 MPa, a cut-off pressure of 0.15 MPa and a starting pressure of 0.30 MPa.

Now that we know how a pre-filled tank, let’s see how it works in conjunction with a bomb and when the system is connected and has been properly primed. This operation can be described in a series of stages represented by the following figure:

Operation of a hydropneumatic system

From the opening of a tap until the bomb it starts working again is when we use the tank as a reserve. East useful volume (VU), which is another system parameter, prevents the bomb must start and stop with small consumptions. Therefore, the bomb it does not come on every time a tap is opened, but only when the pressure equals the value of the system starting pressure. It is clear that the higher the VU of the tank, the fewer on / off cycles the pump will have.

From this it follows that hydropneumatic equipment have a series of obvious advantages, because:

• They mean energy savings.
• They extend and protect the life of the bomb by reducing the number of on / off cycles.
• They store water and deliver it under pressure only when it is needed.
• They reduce system maintenance.
• They maintain a reservoir of pressurized water (VU), allowing the pump to meet the total demand of the system to ensure a constant flow of water.

Use and maintenance of hydropneumatic systems

The figure below represents a hydropneumatic system or household pump with its main parts.

Hydraulic System or Domestic Pump

1. Supply connection (discharge line)
2. Water drain screw
3. Pressure gauge
4. Connection to the water intake (suction line)
5. Screw for water filling
6. Pressure switch
7. Electric switch

In order to use this useful device, we must first make the corresponding connections. The following figure represents a schematic of a typical installation of a hydropneumatic system.

Installation of Hydropneumatic Systems

All these connections, as well as the installation of the required valves and the preliminary priming of the bomb They are made following the instructions of each manufacturer. Here we will provide some guidance to get the most out of our hydropneumatic equipment.

Tips for installation

• Locate the equipment on a solid and level surface that is as close as possible to the water supply source and is protected from the elements. For example, a good location is a basement or a pump box.
• Make sure the bomb have adequate ventilation and that the surrounding temperature does not exceed 40 ºC, as the motor could automatically cut out due to overload.
• To avoid long priming periods and unnecessary damage to the bomb As a result of stones, sand and other solid foreign bodies, it is advisable to use a filter and a kit provided with hose, cage and check valve in the suction line.
• Take care of the arrangement of the suction line, since if it is higher than the bomb it would delay the escape of air bubbles and impede the priming process. Hose unions or connectors can be installed near the pump to facilitate removal when the pump needs to be stored or undergo maintenance or repair.
• The piping can be copper, steel, rigid PVC, or flexible polyethylene, although flexible tubing is not recommended for the suction line. Instead, a rubber hose in the supply line will reduce the noise that is transmitted to the house, for example.
• The recommended pipe diameter is ¾ inch, although ½ inch can be used. However, in the latter case, it must be taken into account that the flow decreases with smaller diameters and that long horizontal pipe runs and the use of numerous adapters and connectors decrease the water pressure due to friction loss.
• The tubes must be clean, free of rust and scale. Use a sealant for the pipe connections on the male threads of the metal pipes, and use sealing tape with the plastic threads. All connections must be watertight to ensure normal operation.

Tips for maintenance

• The correct operation of the tank depends on the air precharge. Therefore, the tank should be inspected at least once a year to ensure that the precharge pressure is equal to that indicated by the manufacturer with a variation of ± 2%. To verify this, the following steps are followed: a) disconnect the electrical supply to the equipment, b) completely empty the tank by means of the drain screw 2 (see figure of equipment components), c) check the preload on the pressure gauge 3 using a tire air gauge, and d) if necessary, adjust the air precharge to 2 psi below the pump switch start pressure.
• For optimal external tank protection, the outside should be cleaned periodically with soap and water only.

Tips for storage

• If the equipment is not going to be used for a long time, or during the winter months, it should be disassembled and put away. To do this, it must be completely emptied and allowed to dry. Never allow the water inside to freeze.
• The equipment and all accessories should be stored in their original packaging, and in a dark and dry place kept above freezing, ideally between 5 and 30 ºC.
• After long interruptions and before a new use, make sure that the rotor rotates correctly by turning the power on and off. bomb for short periods.

Respecting the manufacturer’s instructions and following these recommendations we will obtain a long useful life from our hydropneumatic system, which we can use for a long time for the intended purpose.