The highest operating pressure allowed for the tank.
This parameter must not be lower than the constant operating pressure in the heating/water supply system. However, the specific values depend on the type of system and the type of tank (see above). So, for a water pressure tank, the maximum operating pressure must be higher than the shutdown pressure of the pump. In general, rather high-pressure values are found in water supply systems, so most modern water pressure tanks fall into one of the following categories:
6 bar,
8 bar,
10 bar and
more than 10 bar(although there are models with more modest values).
In turn, the expansion tank must withstand the pressure at which the safety valve in the heating system is activated. Most of these tanks are designed for
3 bar,
4 bar or
5 bar — heating systems rarely use higher pressures (although exceptions are possible here).
In general, when choosing this parameter, the rule “the more the better” works quite well: a good pressure margin will provide an additional guarantee in case of emergencies. The reverse side of this reliability can only be called a slightly increased price.
The material from which the internal elastic membrane of the tank is made.
Recall that in water pressure tanks (see "Type") this membrane has the form of a kind of balloon, which is filled with water so that the liquid does not come into contact with the metal walls of the tank. In turn, elastic transverse partitions are installed in the expansion tanks. However, in both cases, one of three materials is usually used for the membrane:
EPDM (ethylene propylene diene rubber),
Butyl (butyl rubber) or
SBR (styrene-butadiene rubber). Here is a more detailed description of each of these options:
— EPDM (ethylene propylene diene rubber). The most popular material for membranes nowadays is both in water pressure tanks and in expansion tanks. This prevalence is due to a combination of reliability, practicality, safety and affordability. Thus, EPDM has high elasticity and a wide operating temperature range, tolerates an average of up to 100K cycles of stretching/compression within acceptable limits, does not react with oxygen and industrial alcohols and can be used in drinking water supply systems. However, in terms of overall strength and resistance to diffusion, this material is somewhat inferior to butyl rubber (see below), but it is also much cheaper.
— Butyl (butyl rubber). High-quality material used primarily in water pressure tanks for cold wat
...er systems. One of the key advantages of butyl rubber is excellent gas tightness — several times higher than that of EPDM; simply put, much less air penetrates through such a membrane, which allows the tank to work longer without special maintenance. Also, this material is characterized by high strength, solvent resistance and hygiene; the latter allows the use of butyl rubber even in drinking water systems. The main disadvantage of this type of membrane is its high cost.
— SBR (styrene-butadiene rubber). A material primarily used in expansion tank membranes. This specialization is because SBR does not tolerate frequent tension compression very well and is better suited for static loads, which are just typical for such containers. At the same time, this material is reliable, elastic, resistant to water hammer, and very slightly permeable to air. Its unambiguous disadvantages include unsuitability for systems with drinking water.
