Water expansion tank. What are pressure tanks for? Membrane instrument

18.10.2019

When heated, any coolant expands and increases in size. As a result, the pressure in the closed heating system gradually increases and reaches a critical level. The membrane expansion tank of the heating system is designed to prevent the destruction of components and pipelines due to the expansion of the coolant.

The main function of the expansion tank is to optimize the operating pressure in the heating system. Closed heating systems cannot work properly if a membrane heating tank is not connected to them.

Membrane expansion tank device

Although membrane expansion tanks may differ depending on the manufacturer and purpose, some details remain unchanged in any model purchased. Namely:

Metal body - a prerequisite for the production of tanks is the ability to withstand extreme loads without breaking the tightness.
Membrane - must be highly elastic and able to respond to changing pressure associated with heating of the coolant. At the same time, high demands are placed on the membrane in terms of strength. Typically, rubber is used in the manufacture of the membrane.

Device membrane tank heating for closed heating systems involves the use of tanks with replaceable and non-replaceable diaphragms. Each design has both its advantages and disadvantages.

How a diaphragm expansion tank works

The operating principle of a membrane expansion tank is based on the use of physical laws. After heating the coolant, the following occurs:

Water or antifreeze begins to expand, as a result, its volume in the system increases.
The design of the membrane-type expansion tank implies its filling with gas.
The membrane is a kind of interlayer between the gas and the coolant.
When heated, the liquid, expanding and creating pressure, enters the tank and displaces air or gas.
After the coolant pressure drops, the gas pushes the coolant out of the tank using a membrane.
The operation of the safety valve in a heating system with a membrane tank is to relieve excess gas pressure in the event of a large expansion of the coolant. The pressure relief valve ensures the safety of the system in case of overheating of the liquid or antifreeze.

For normal operation of heating in the membrane tank, there must be a pressure corresponding to the height of the upper point. If the tank is installed in two-storey house and the maximum height from the boiler on the ground floor to the radiator on the top is 7 meters, then we take 0.7 into the calculations and add 0.5 to it. We get the initial pressure when the coolant is supplied to the system. The resulting coefficient for the tank should be lower by 0.2. It turns out that the pressure rate in the expansion tank of the membrane type in this case is 1 atm.

Like all heating equipment, the membrane tank needs service. It must be maintained at the correct working pressure and refilled with gas or air from time to time.

Types of expansion tanks of the heating system

Each manufacturer introduces innovations into the design of a closed expansion tank. But basically, all modifications can be divided into several groups depending on the membrane used. Namely:

The membrane of the expansion tank in the form of a diaphragm. Such a device is more like a barrel, separated by a movable rubber partition. Entering its department, the liquid fills the tank, and then, under pressure, it begins to compress the gas, gradually moving the membrane. This device is not always effective for houses with a small heated area.
Round membrane tanks of balloon type. In this case, the air chamber is located around the perimeter of the entire tank. It surrounds the water chamber. As the pressure increases, this chamber begins to expand like an inflated rubber balloon. The uniqueness of such a device lies in the fact that with its help it is possible to more accurately control the pressure of the coolant, even in closed systems with a small volume of liquid in the pipeline.
Removable membrane. The diaphragm is attached around the entire perimeter. Non-removable membranes are intended for use in private heating systems and for heating cottages. Limited use and installation in small industrial facilities is allowed.
Tank with replaceable membrane. They are hollow pear. Removable membranes are able to work effectively in systems with a high intensity of heating of the coolant and high atmospheric pressure. The advantage of such a device is the ability to replace the diaphragm. The disadvantage is that high demands are placed on the performance of membrane replacement work. It is not allowed to distort the membrane during its installation.

The role of the expansion tank in the heating system is not limited solely to the depreciation of excess pressure. Before choosing a suitable device, it is necessary to determine for what purpose it is planned to be used.

How to calculate the volume of a membrane-type expansion tank

When choosing a tank, you need to pay attention to the following several indicators:

Temperature range considered operating for the device.
Membrane elasticity.
diffusion stability.
Dynamic indicators.

In addition to these four criteria, it is important to calculate the pressure in heating system with membrane tank. Pressure data will help you choose the most suitable model tank. Requirements for performing calculations in complex systems closed type are presented high. Correct calculations can be made using the following formula:

V=(V sys ×K)÷D

Expansion tank volume for closed system heating is, according to this formula, the product of the volume of the system V sys and the coefficient of increase in the coolant K (it is 4%) divided by the efficiency of the tank itself.

D=(Pmax-P initial)÷(Pmax+1)

P - in this case is an abbreviation for maximum and initial pressure. Using these two formulas, you can easily perform the calculation and select the required model.

In addition to the standard round device, a rectangular membrane-type expansion tank can be purchased, it is more convenient to operate and has an attractive appearance.

How to install a membrane-type expansion tank

Installing an expansion tank in a closed heating system is quite simple. The only condition for connection is an understanding of the basic principles of work. Installation can be carried out following the instructions below:

It is better to install an expansion tank before, and not after, the circulation pump, this will help to avoid surges in pressure. There are no other restrictions regarding the installation location.
After installation, it is necessary to check whether the working pressure of the device corresponds to that which is required. You can make a check quite simply if you install a pressure sensor in the tank when connecting. The sensor that measures the pressure in the tank is installed directly at the inlet. If the existing indicators do not correspond to the required ones, it is necessary to bleed the air and bleed the device again until the pressure of the diaphragm matches the required one.
When the heating system is closed, the expansion tank is correctly mounted so that the inlet valve (water connection) points downwards. This will drain the coolant, even if the membrane fails. Some models have a coolant level indicator that allows you to determine whether the fluid has been completely drained from the system.

The installation of a membrane tank is a prerequisite for the installation of a closed-type heating circuit. Some boilers are already equipped with such a device, in which case, if necessary, it is possible to install an additional tank.

In an autonomous heating system, there must be an expansion tank for heating, or a compensator. Its function is to compensate for the excess pressure that occurs in the system when the coolant expands due to heating. With a rapid increase in temperature, the heat transfer fluid expands and a pressure surge occurs, the so-called water hammer. It can destroy elements of the pipeline and connecting fittings. Other names for the expansion device: hydraulic accumulator, expandomat.

The device and principle of operation of expansion tanks for heating

Heating systems are open and closed. Accordingly, there are open and closed heating expansion tanks.

Open type tanks

An open expansion tank for heating is a parallelepiped-shaped container made of stainless steel. Such a tank is placed at the highest point of an open heating system, usually in the attic.

Pipes are connected to the tank:

main;
circulation;
alarm, with a locking device.

In this type of heating system, the coolant (water) circulates naturally, without pumps. Despite the comparative cheapness and simplicity of such heating, it is gradually becoming a thing of the past due to numerous shortcomings.

In an open tank, the coolant constantly evaporates, so you need to control the water level and top up as necessary. For the same reason, it is problematic to use another coolant, such as antifreeze - it evaporates even faster.
It is possible to overflow water from the tank, so it is necessary to provide for its removal to the sewer or drainage.
An open expansion tank requires good thermal insulation so that the water does not freeze in very coldy.
For installation in the attic, additional pipes and connecting elements will be required.
Air entering the system from the expansion device provokes corrosion of the pipeline and radiators, and also leads to the appearance of air locks.

The system with an open compensator is suitable for heating small one-story houses. Larger houses are heated by closed systems.

Closed type tanks

The closed or membrane expansion tank of the heating system contains an elastic membrane inside, which divides the internal volume of the expansion tank into two compartments, gas and liquid. The gas part contains pressurized air (in some models, nitrogen or an inert gas), and excess coolant enters the liquid part when heated.

Closed type tank (membrane)

The higher the temperature, the more the liquid part of the accumulator is filled. At the same time, the gas part is reduced and the pressure in it increases. When the threshold value is reached, safety valve, excess pressure is released. And when the heating system cools down, the reverse process occurs, and the coolant returns from the tank to the pipeline.

The principle of operation of the membrane expansion tank

There are two types of membrane compensators.

With a diaphragm type membrane. These are small tanks. The diaphragm membrane in them is non-removable and cannot be replaced: if it breaks, you will have to completely change the device.
With a balloon (pear-shaped) membrane. It can be changed when worn, it is used in large thousand-liter tanks.

The volume of expansion tanks for heating can vary widely from two to several thousand liters. The shape of a closed accumulator is flat or cylindrical. In a flat expansion tank, the membrane-diaphragm is located vertically, in a cylindrical one, horizontally.

It is worth paying attention: the membrane compensator is sometimes mistakenly called a vacuum expansion tank for heating. However, vacuum is not used in this device. The heating system may have a vacuum deaerator to remove air microbubbles from the water.

Installation of a membrane expansion tank

Unlike an open one, a diaphragm accumulator can be installed directly into the heating point, next to the boiler. Usually it is placed on a straight section in front of circulation pump, preferably so that water (or other coolant) enters the compensator from above. It must be equipped with a pressure gauge, safety valve and connected to the return line.

Hydraulic accumulators up to 30 liters are mounted on the wall, larger ones are installed on the floor. When mounted on a wall, the tank must be securely fastened, as its weight increases dramatically when filled with water.

Several membrane tanks in a substation

Important performance data and compensator volume calculation

When selecting an expansion tank, take into account the maximum operating temperature and pressure. For example, the heating medium can be heated up to +120°C, and the peak pressure in the heating expansion tank can reach 6-10 bar (usual average value is 2-4 bar). Therefore, the characteristics of the membrane are important, its durability, heat resistance, compliance sanitary standards.

The volume of the compensator depends on the volume of the coolant as a whole in the system. It is not necessary to calculate the volume mathematically exactly, a simplified method is often used: a tank with a capacity equal to 10% of full volume coolant. And if this volume is unknown, then they proceed from the power of the boiler and the type of heating devices. The ratios are as follows: for heating batteries they take - 11 l / kW, for underfloor heating - 17.5 l / kW, for wall-floor heaters - 7.5 l / kW.

If the capacity of the selected compensator is insufficient, the relief valve will release pressure too frequently. In this case, it is enough to purchase and connect another expansion tank in parallel.

It is rather difficult to take into account all the nuances, especially since in each house the heating system necessarily has its own characteristics. In order not to make a mistake when choosing and installing a device, it is better to contact a specialized company.

Video: expansion tank installation

Expansion tank

Both types of devices used in autonomous systems have a similar design and work on the same principle. Consider the main elements of products using the example of an expansion tank for a heating system:

The main purpose of the tank is to compensate for the expansion of the coolant. When heated, water increases in volume, and quite strongly (+0.3% for every 10 degrees Celsius). In this case, the liquid practically does not shrink, so that the heated coolant will exert significant pressure on the pipe walls, junctions and stop valves.
To compensate for this pressure, as well as to minimize the effects of water hammer, an additional tank is built into the system - an expansion tank. The first tanks had a leaky design, but today pneumatic-hydraulic models are almost universally used.
Inside such a tank is placed from an elastic material. Since the membrane is in contact with a heated coolant, it is made of polymers that are resistant to high temperatures– EPDM, SBR, butyl and nitrile rubbers.
The membrane divides the tank into two cavities - the working one (the coolant enters it) and the air one. With an increase in pressure in the system, the air chamber decreases in volume (due to air compression), and this compensates for the load on the shut-off valves. Approximately the same thing happens with water hammer - but here the process goes at a higher speed.
When the temperature of the coolant decreases, the volume of water decreases, and the air, exerting pressure on the membrane, displaces the additional volume hot water into the pipes of the heating system.

An example of an expansion tank is a 35 liter capacity, great for heating systems in a large house or public building.

Hydraulic accumulator

The accumulator, at first glance, practically does not differ in design from the expansion tank:

The base is the same container made of corrosion-resistant steel, only painted blue.
There is also a membrane inside the tank - however, it is somewhat different in shape from the membrane of the expansion tank.
The internal volume is also divided into two chambers, only in hydraulic accumulators the water chamber is located inside the membrane, i.e. liquid contact with the metal walls of the tank is completely excluded.

Yes, and the design functions according to a similar principle, although they use it for a different purpose:

When the pump is turned on or water is supplied through a centralized water supply system, the chamber is filled with liquid under a certain pressure.
If the pressure drops for some reason, the air chamber increases in volume, and water from the working chamber enters the system. Thanks to this, the pressure in the pipes is stabilized, and the equipment (washing machines, dishwashers, etc.) works without failures.
The second aspect of the operation of the accumulator is protection against frequent switching on. As long as it is possible to compensate for the withdrawal of water from the system due to the reserve in the tank, the pressure switch will not work and the pump will not start pumping water. Thus, the equipment will turn on less often, which means it will work longer.
A large accumulator (for 50, 100 or more liters) is also a supply of water. Yes, you won’t last long on such a supply, but with economical spending it is quite possible to survive a water supply accident or a power outage that will make the pump impossible to work.
In addition, the accumulator, like the expansion tank, compensates for water hammer.

An example of a hydraulic accumulator in the catalog of the Alfatep online store is a 24 liter horizontal model.

In modern heating systems and water supply systems, special membrane-type expansion water tanks are installed as protection equipment. Often such membrane tanks are called pressure tanks, and sometimes even "hydropneumatic tanks" and "hydraulic accumulators". In the water supply system, they are designed to perform three important functions.

The main function of membrane tanks is obvious - this is a reservation, or accumulation water resource, as well as maintaining water pressure in the pipes. The pressure tank in this case is a reserve tank that accumulates water reserves. When the tap is turned on and the water intake begins, the pressure of the water entering the system comes from such a tank. When the water reserves in the "hydraulic accumulator" decrease and the pressure level inside the pipes drops below the permissible limit, the pump starts. Further, if the tap is turned off, the pump does not stop working, but continues to pump water into the membrane tank for a certain time. Over time, the tank is completely filled, the upper pressure limit in it is reached, and the automation turns off the pump. It is logical that the use of a membrane tank increases the life of not only the pump, but the entire system as a whole. And all because in its absence and a high frequency of turning on / off the taps of the water system, the pump would have to constantly switch from one state to another.

The second important function of the tanks is to prevent water hammer. Water hammer is such a pressure jump in the system when the energy of movement of a sharply stopped water flow in the pipeline is converted into energy that destroys the walls of the pipeline. The risk of water hammer is high when the valve suddenly closes. Such a blow is no less likely during the shutdown of various household electrical appliances, such as, for example, washing machine. "Hydroaccumulator". In this case, it does not allow a sharp change in pressure and plays the role of a silencer for such shocks.

Well, the third function is, of course, the creation of a supply of water in case of an incident: whether it be a temporary shutdown of a pumping station, or something like that. With a membrane tank in the house there will always be a spare vessel with water.

It is necessary to distinguish between membrane tanks for water supply and heating systems, although outwardly they are very similar to each other. In most cases, they look like spherical metal vessels. Unlike a pressure tank, the main function of a membrane tank for a heating system is to reserve excess water resulting from its thermal expansion for subsequent return to the system. Like pressure tanks, they act as a dampener for hydraulic shocks in the system. Both types of tanks have a special elastic membrane inside, made of heat-resistant rubber. This membrane divides the vessel into two parts (for air and for liquid) and is able to stretch, which allows the system to be in equilibrium with pressure drops. Our

Wester WRV-8 is a membrane tank for heating systems.

An expansion membrane tank is an element of a closed heating system designed to compensate for the thermal expansion of the coolant and maintain the required pressure.

Note! In addition to being used in heating systems, membrane tanks are also used in water supply systems. They "soften" the water hammer that occurs when turning on / off pumping stations and maintain a constant pressure in the system.

Membrane tank design

The expansion membrane tank for heating is a sealed steel case cylindrical shape, covered with red epoxy varnish (there are also tanks covered with blue varnish, but they are intended for cold water). There are 2 chambers in the body: gas and water, which are separated from each other by a movable gas-tight membrane (diaphragm) made of butyl rubber. Thanks to this material, the membrane is able to function stably under various temperatures(from -10 to +100°C) and perform up to 100,000 cycles.

The membrane almost completely eliminates the interaction of the coolant and gas. The absence of such interaction allows the pre-pressure to be maintained in the gas chamber longer, which has a positive effect on the service life of the tank.

Note! Modern high-quality membranes are not simply pulled out under the pressure of an expanding coolant, but seem to “stick” to the tank walls. This principle of operation allows to increase the service life of the membrane.

Cross section of the Reflex tank.

Both chambers have the same pressure, which allows you to maintain the tightness of this section of the heating system. The air chamber is filled with a nitrogen-containing mixture. When the coolant expands, nitrogen is “compressed”, allowing the coolant to “enter” into water chamber.

Most modern membrane heating tanks have a nipple built into the body (similar to a conventional car), with which you can “pump up” the air chamber by increasing the pressure in it. This can be done independently at home using a pump or compressor. However, it should be remembered that it is recommended to pump nitrogen, not air. The fact is that the oxygen contained in the air will cause accelerated corrosion of the walls of the tank body, which will inevitably shorten the life of the device. Nitrogen is neutral and does not contribute to corrosion.

Beam for tank and safety group. Producer: ROSTerm North-West LLC, St. Petersburg.

Expansion tank membrane type Imera.

The tank body has an outlet with an external threaded connection, which simplifies the installation process. Depending on the model, the thread can be:

At the tanks low pressure(from 0.5 to 1.5 bar) - 3/4″ or 1″;
For medium pressure tanks (1.5 bar) - 1 ″;
At the tanks high pressure(from 3 bar and above) – from 1″ to flange connection DN 100;

The principle of operation of the membrane tank

When the heating system is started, the coolant heats up and increases in volume. This excess volume moves to the water chamber of the expansion tank. After the coolant cools, the pressure in the air chamber squeezes out the membrane, thereby displacing the coolant from the water chamber back into the heating circuit.

In addition, as noted above, the membrane tank maintains the required pressure in the entire heating system. So, for example, if an insignificant leakage of the coolant occurred somewhere, then the pressure in the entire system should drop, but this does not happen, because. the pressure in the air chamber will push the membrane, and with it the coolant, back into the system, thereby creating a limited make-up.

Membrane tank with safety group.

The membrane may be damaged as a result of incorrect operation:

There is a possibility of membrane rupture if, when filling the water chamber with coolant, the necessary pressure in the air chamber was not created;
Before venting gas from the air chamber, it is necessary to shut off and drain the coolant from the water chamber.

Tank calculation

Heating for every 10°C gives an increase in the volume of the coolant by an average of 0.3-0.4%. Based on these data, the required volume of the tank is calculated.

The percentage of expansion of the coolant (water) depending on the heating temperature:

Important! Any membrane tank for heating is equipped with a ball valve with a drain, which allows you to block the flow of coolant to the tank. This is necessary for quick, convenient replacement of the tank in case of failure.

Expansion tank open type

At the moment, this type of expansion tanks is practically not used, because. has the following disadvantages:

Open expansion tank.

The coolant is in constant contact with air, which leads to airing of the system and the appearance of air pockets. Therefore, it is necessary to regularly remove air or is required. Otherwise, air can lead to corrosion of individual elements of the heating system, as well as to a decrease in the heat transfer of heating devices;
Due to the constant presence of the coolant in contact with air, it evaporates. Have to regularly add coolant to the system;
Air microbubbles circulating through the heating system create unpleasant noises in pipes and radiators, and also lead to premature wear of parts. In addition, microbubbles “degrade” the circulation pump;
Unlike a membrane tank, which can be installed at any point in the system (next to the boiler, in the basement, ...), an open-type expansion tank is installed only at the highest point. This leads to an increase in the cost of the system, because. it is necessary to use additional pipes and fittings to mount the tank at the top point.