Hydraulic accumulators and expansion tanks look similar but perform different functions. They have different requirements for the membrane, temperature, pressure, and contact with water.
Are a hydraulic accumulator and a membrane tank the same thing?
In a broad sense, a hydraulic accumulator is a type of membrane tank: an elastic partition separates the liquid from compressed air or nitrogen. In a pumping system, a hydraulic accumulator for water creates a reserve under pressure and reduces the number of starts. In heating, a membrane expansion tank compensates for the increase in the volume of the coolant.
They cannot be replaced because of the same volume and thread. The heating membrane may not be approved for drinking water, and the tank parameters are different. Therefore hydraulic accumulators, tanks for heating and sanitary water are produced in separate series.
The principle of operation of a membrane (expansion) tank
The membrane divides the body into water and air chambers. Pre-pressure is created through the nipple. The water deforms the membrane and compresses the gas, which, after reducing the pressure, expands and pushes it back into the system.
In water supply, this allows water to be supplied without immediately starting the pump. At the lower pressure limit, the relay turns the pump on, and at the upper pressure limit, it turns it off. This reduces the number of short motor cycles.
In heating, the excess volume of heated coolant enters the tank, and after cooling, it returns to the circuit. The tank stabilizes the pressure, but does not replace the safety valve.
Is a hydrophore another name for the same device?
A booster pump is a booster system consisting of a pump, accumulator, pressure switch, pressure gauge, valves, and sometimes automatic protection. Such sets are also called pumping stations.
In everyday life, the accumulator itself is also called a hydrophore, but this is a simplification. In catalogs, you can also find the variants "hydrophore" and "hydrotank".
Design differences: bulb, cylinder, replaceable and fixed membrane
The membrane can be pear-shaped or cylinder-shaped. Water flows inside and almost does not come into contact with the steel walls. In many models, the part is replaceable: the flange for the accumulator is removed and the membrane is taken out. A new pear and flange are selected according to the model.
A fixed diaphragm divides the body into two parts. When damaged, the entire tank is usually replaced. In some sanitary series, this design reduces stagnant zones. If water comes into contact with the body, its internal coating is important.
The query "hydraulic accumulator without a pear" often means a model with a fixed diaphragm, that is, there is a membrane. On the other hand, a hydraulic accumulator without a membrane is a traditional design with direct contact of water and air. Since air dissolves in water, its supply must be replenished.
The type of membrane also affects maintenance. If water comes out of the pre-charge fitting instead of air, this may indicate damage to the elastic element. In the model with a replaceable bulb, a compatible spare part is installed, while the tank with a fixed diaphragm is usually replaced completely. Diagnostics is performed after disconnecting from the system and releasing the pressure.
Where is a hydraulic accumulator used, and where is a membrane tank used?
The choice determines the purpose of the system. Vertical or horizontal installation is allowed for the corresponding models. The horizontal tank often serves as a support for the pump, and the vertical one is placed on the floor. The pipes should not hold the filled container.
Water supply systems
In water supply, a hydraulic tank works with a borehole or surface pump. It:
- creates a water supply;
- supports pressure range;
- reduces the frequency of engine starts.
The usable water supply is less than the geometric volume and depends on the pre-pressure and relay settings. Therefore, a 50 liter tank does not deliver 50 liters between starts.
The required volume is selected taking into account the pump flow and the allowed number of starts per hour. A small accumulator empties quickly and does not protect the pump from frequent short cycles. Increasing the tank increases the pauses between starts, but requires additional space and does not increase the maximum pressure that the pump can create.
Near the boiler, a membrane water tank absorbs the thermal expansion of sanitary water. In terms of function and design, it is not a storage tank for a pumping station.
Heating systems
In heating, the tank must be suitable for the temperature and compatible with water or glycol. The volume is determined by the following indicators:
- the amount of coolant,
- temperature range,
- static height,
- initial and maximum pressure.
A tank that is too small will fill up during heating, the pressure will increase, and the safety valve may trip. A tank that is too large will cost more and take up space. The calculation is performed according to the manufacturer's method.
During the calculation, not only the nominal displacement of the boiler is taken into account, but also the entire coolant in the pipes, radiators, collectors and heat exchangers. The proportion of antifreeze is also important, since the thermal expansion of the mixture may differ from water. The pre-charge is coordinated with the static pressure at the tank installation site.
Tank Color: Why It's Not Just Aesthetics
In many series, blue tanks are designed for water supply, white for sanitary water, and red for heating. However, there is no single mandatory standard: the water tank can be green, gray, or made of stainless steel.
Color does not prove suitability. The passport checks the environment, temperature, pressure, membrane material and sanitary approvals. The heating tank is not connected to drinking water only through the appropriate thread.
Pressure and volume: how the parameters differ depending on the purpose
The answer to the question "what pressure should be in the accumulator" depends on the pump control. For a system with a relay, many manufacturers recommend a pre-pressure of 0.2 bar below the start point, others - about 0.9 from the switch-on pressure. The equipment passport has priority.
The pre-pressure is checked through the nipple after the pump has been switched off, the tank has been isolated and the water has been completely drained. In a filled system, the pressure gauge will show the water pressure, not the air chamber charge.
The factory charge does not always correspond to the settings of a particular station, so it is checked before starting and subsequently at the intervals specified by the manufacturer. If the air chamber has lost pressure, the pump starts to turn on more often, and the water supply decreases. It is impossible to increase the charge above the permissible value on your own: excess pressure also reduces the useful volume.
In heating, charge is related to static pressure, and working volume is related to the expansion of the coolant. The rule "10% from the system" is only a rough estimate.
Material and anti-corrosion coating of the housing
Most tanks are made of carbon steel and painted on the outside. If water comes into contact with the shell, an internal anti-corrosion coating, such as epoxy, is required. Moisture and condensation can cause external corrosion.
The flange is made of galvanized or stainless steel. Stainless steel resists corrosion better, but one detail does not determine the durability of the product. Fully stainless tanks are more expensive.
How not to make a mistake when choosing: a hydraulic accumulator or a membrane tank
For a pumping station, the accumulator is calculated based on the pump flow, relay, and allowable number of starts. For heating, the tank is selected based on the volume and expansion of the coolant, and for a boiler, based on the parameters of hot sanitary water.
Check the medium, temperature, maximum pressure, factory charge, useful volume, membrane, mounting position and connection. For the repaired model, the accessibility of the bulb and flange is important.
