Shaft-end sealing for sCO2 turbomachinery is more challenging than typical applications in centrifugal compressors for Oil & Gas. Due to strong influence of specific heat capacity near the critical point and low dynamic viscosity of sCO2 on machine efficiency and cycle performance, the lowest leakage rates are crucial for the technical feasibility of many applications. These requirements mandate the use of contactless sliding face seals (Dry Gas Seals – DGS). One of the most demanding applications is within radial expanders for power cycles, where process gas temperatures and pressures exceed 500 °C and 80 bar, with tangential speeds of rotating ring higher than 180 m/s. Current designs use a thermal management zone to limit the temperature on DGS (especially for secondary sealing elements of polymers or elastomers) to 200 °C. Such thermal zones utilize hot and cold flows to create an optimal thermal gradient on the shaft while not resulting in excessive thermal stress. A high-temperature resistant DGS with low leakage rates as a replacement for the thermal management zone would increase machine thermal efficiency, simplify the gas inlet and outlet flows, reduce the axial length required for shaft-end sealing (making possible the addition of new machine stages or shortening machine shaft) and reduce thermal stresses on machine shaft.

This paper presents the preliminary results of the development of a prototype DGS for turbomachinery in sCO2 applications with leakage rates no higher than 1,5 Nl/min for each bar sealing pressure operating with sealing gas at 510 °C and 89 bar and rotating ring tangential speed of 192 m/s. The machine shaft diameter is 110 mm.

The first project stage comprises the development of one DGS component: a high-temperature resistant balance sealing element, which seals the radial gap between stationary ring and balancing sleeve. Subjected to a dynamic reciprocating motion it is one of the most critical components in any DGS. The prototypes were tested with air/helium at room temperature and with CO2 at material design temperature (600 °C). The second project stage consists of upgrading two DGS (one tandem and one single with identical core parts) to a high-temperature resistant design. This was done by replacing usual temperature limited components by their respective high-temperature versions and doing the required design adaptations each at a time. After every upgrade each DGS was statically and dynamically tested to the operating conditions with air/helium mixture supplied at room temperature – the highest recorded temperature inside DGS was about 220 °C. Tests allowed the evaluation of equipment performance by means of leakage rates and leakage stability after changing operating conditions (speed and pressure). Visual inspection of internal parts after each test were performed. The test bench for CO2 supplied at 510 °C is still being commissioned to the time of this publication.

It was verified during the dynamic tests with air/helium supplied at room temperature that the leakage rates remained stable and around 1 to 1,3 Nl/min/bar for operating conditions. Design is still to be validated with dynamic tests with CO2 supplied at design temperature.