## 4 Discussion  
The previous section investigates the comparison between chamber-type and semi-pneumatic surge tanks for the same structural parameters. The mathematical model for the chamber-type surge tank is a part of the formerly developed and experimentally verified software that has been used in numerous projects. With the aim of initial validation of the developed SPST model, the conditions have been accomplished by defining the equivalent diameter of the aeration orifice, i.e., the upper chamber diameter of the chamber-type surge tank is 15 m (Figure 3a) as well as the aeration orifice dimensions (\(D_{aorf}=15\) m, Figure 3b). The results of the simultaneous turbine start and load rejection for the chamber-type and semi-pneumatic surge tank almost match (Figure 4). The mathematical model of the SPST confirms that the air pressure changes remain equal to the atmospheric pressure, as expected. For the large closed surge tanks, [PERSON] [34] recommends isothermal process in the air, whereas [PERSON], [PERSON] & [PERSON] [1] suggest the adiabatic process for the pneumatic surge tanks, noting that different thermodynamic behavior should be expected depending on the size, construction material and period of mass oscillations. Considering that the ST of the HPP Pirot is relatively large with the low-frequency pressure and discharge oscillations, isothermal air behavior should be expected and defined with polytropic exponent value \(n=1\). Regarding the behavior of air in the SPST, sensitivity analyses of the polytropic exponent changes confirm negligible influence in the researched case but should be considered on the HPPs with smaller STs. The above analyses were followed by investigations of the various dimensions of the aeration orifice diameter. The STWL of the SPST for the various values of the aeration orifice diameter is presented in Figures 5 and 6 for the load rejection and simultaneous turbines starts, respectively. The implementation of the aeration orifice with a diameter lower than \(D_{aorf}=3\) m, affects the reduction of the STWL. In the cases of \(D_{aorf}=3\div 0.2\) m, reduction of the aeration orifice diameter reduces maximum and minimum STWL values. For the analysed range of orifice dimensions, the WMOs characteristics have the same  
Figure 10: STWL for the throttled chamber and optimized semi-pneumatic throttled chamber surge tank for (**a**) load rejection and (**b**) simultaneous start of both turbines. trend as the ones during the increase of the minor losses with the orifice in the standpipe. Hence, during the increase of the minor losses in the standpipe, the water hammer effect in the high-pressurized section (surge tank-penstock-turbine) may be reflected in the low-pressurized section (accumulation-tunnel-surge tank). The aeration orifice diameter range \(D_{aorf}=0.2\div 0\) m for the load rejection has a positive impact on the maximum STWL value although slightly negative for the value of minimum STWL. The fact that the relatively small diameter of the aeration orifice does not allow enough energy dissipation while the air remains highly pressurized leads to negative effects afterwards, which is in accordance with the same trend for the pneumatic surge tanks. According to the dependence of the transient processes analyses on surge tank characteristics, quick-restarts are the most extreme cases of minimum STWL. Therefore, the analyses should be thoroughly performed before the aeration orifice diameter adoption in order to completely prevent air suction [2]. Along with the correct dimensioning of the aeration orifice diameter, its vertical positioning in the surge tank has also been researched. The decrease in STWL is achieved by lowering the aeration orifice level in the surge tank.