Dimensioning method for precharged membrane expansion vessels for pressure maintenance of hydronic systems

Abstract

Effective pressure maintenance is essential to ensure fault-free and energy-efficient operation of hydronic systems. Precharged membrane expansion vessels (MEV) are widely used for this purpose. A membrane within these MEVs separates the liquid connected to the circuit from the fixed amount of gas. Several standards provide dimensioning methods. However, these methods have two main disadvantages. First, pressure measurement uncertainties are not considered. Second, changes in the MEV temperature are neglected. The resulting uncertainties in dimensioning, commissioning, and maintenance can lead to malfunction and damage. Based on the extremal states of both the circuit and the MEV, an improved and generalized dimensioning method was derived. The change in the gas temperature within the MEV is accounted for. The measurement uncertainties of preset- and system pressure, as well as average MEV and circuit temperature, are considered. Furthermore, this method accounts for temporary process volumes that can occur, for example, during vacuum degassing or the stagnation of solar thermal circuits. By applying the new method, operational failures owing to incorrectly dimensioned MEVs, or faulty maintenance can be avoided. For simple commissioning and maintenance, corresponding software is provided as a free, open-source tool.