Groove shape effects on the performance of dry gas seals in supercritical CO 2 centrifugal compressors

Purpose This study aims to study the gas film stiffness of the spiral groove dry gas seal. Design/methodology/approach The present study represents the first attempt to calculate gas film stiffness in consideration of the slipping effect by using the new test technology for dry gas seals. First, a theoretical model of modified generalized Reynolds equation is derived with slipping effect of a micro gap for spiral groove gas seal. Second, the test technology examines micro-scale gas film vibration and stationary ring vibration to determine gas film stiffness by establishing a dynamic test system. Findings An optimum value of the spiral angle and groove depth for improved gas film stiffness is clearly seen: the spiral angle is 1.34 rad (76.8º) and the groove depth is 1 × 10–5 m. Moreover, it can be observed that optimal structural parameters can obtain higher gas film stiffness in the experiment. The average error between experiment and theory is less than 20%. Originality/value The present study represents the first attempt to calculate gas film stiffness in consideration of the slipping effect by using the new test technology for dry gas seals.

U.S. Department of Energy (DOE) has recently sponsored research programs to develop megawatt scale supercritical CO2 (sCO2) turbine for use in concentrated solar power (CSP) and fossil based applications. To achieve the CSP goal of power at $0.06/kW-hr LCOE and energy conversion efficiency > 50%, the sCO2 turbine relies critically on extremely low leakage film riding seals like dry gas seal (DGS). Although DGS technology has been used in other applications before. making it successful for stringent conditions of an sCO2 turbo-expander is challenging. This paper presents results from a multi-scale coupled physics model that predicts the performance of DGS under a typical sCO2 turbine mission cycle and addresses some of the risks specific to operation in sCO2. Real gas equations of state are incorporated in the models to capture large discontinuities in fluid properties close to the critical point. A novel experimental setup is developed to observe and characterize transition of CO2 through liquid-vapor and supercritical phases. Coupled fluid-structure-thermal interaction model investigates the effect of aerodynamic and thermal perturbations on the structural and rotordynamic instabilities. Dynamic instabilities arising from sonic transition in thin sCO2 film of DGS pose additional challenges while the large surface roughness changes due to sCO2 corrosion warrant further design considerations. Effectiveness of features like spiral grooves in converting fluid momentum into pressure rise in the thin film and also in achieving local flow reversals is investigated. Effect of various design features on the optimal performance is quantified and insights for a successful DGS operation in a sCO2 turbomachine are provided.

The supercritical CO2 closed loop Brayton cycle operates at high pressure to achieve higher energy conversion efficiency. One of the important components in turbomachinery of the power cycle plant is the dry gas seals. Dry gas seals are gas-lubricated, mechanical, non-contacting, end-face seals, consisting of a mating (rotating) ring and a primary (stationary) ring. Low leakage dry gas seals are considered as a key enabling technology for achieving the improved thermodynamic cycle efficiency in the supercritical CO2 power cycle. Alternate seal types, for example, labyrinth seals, suffers from high leakage. Even so there is a growing interest and importance of applying the small length scale dry gas seal in the small to medium scale supercritical closed loop Brayton cycle (1-20 MWe), there are still uncertainties for their operation at supercritical CO2 conditions. These include the real gas effects near the critical points and the methods of minimizing the deformation of the supercritical CO2 dry gas seal. In the supercritical region in the vicinity of the critical point (304 K, 7.4 MPa), CO2 behaves as a real-gas, exhibiting significant and abrupt nonlinear changes in fluid and transport properties and high densities.Comprehensive analysis is performed to simulate the supercritical CO2 dry gas seal. First, an isothermal simulation assuming rigid sealing ring walls in the gas film is performed using ANSYS Fluent to study the influences of real gas effect on performances of the dry gas seal. Then, conjugate heat transfer simulation is used to optimize the face geometry for a small to median scale supercritical closed loop Brayton cycle (1-20 MWe) using ANSYS Fluent. Finally, the pressure and the thermal outputs from the conjugate heat transfer analysis are used as boundary conditions for one way coupling fluid-structure-thermal simulations using ANSYS Static Structural to study the effect of deformation of the sealing rings under applied pressure-loads, thermal-loads, and centrifugal effect.Finding from the simulation results shows, close to critical point the real gas effect is significant, whereas far from the critical point the supercritical fluid resembles an ideal gas. The centrifugal effect is enhanced by the higher density due to the real gas effects, causing a reduction of average pressure in the dam region hence reduces the opening force, and seal leakage.Increasing the groove radius decreases the opening force whereas increasing the spiral angle, increases the opening force. However, the variation is small for all tested cases studies with variation up to 6%. Increasing the groove radius decreases the leakage rate whereas increasing the spiral angle, increases the leakage rate. The variation in changing groove radius is more significant than the spiral angles. The variation in leakage is up to 29.2% for all tested cases. In term of the film stiffness, there is no clear pattern when changing the groove radius but the impact of the spiral angle is significant, up to 46.6% for all tested cases studies. For a small diameter seal, groove radius, rg = 17 mm and a spiral angle α = 30o are recommended as it gives the optimum seal performances, owing to its lowest leakage rate and high film stiffness respectively. Results from one-way coupled analysis to explore sealing ring deformation, show that the thermal-induced deformation has more pronounced effects than the pressure-induced or centrifugal-induced deformations on the net deformation of the dry gas seal. It is shown that the larger thermal deformation is caused by the presence of a radial temperature gradient, which arises due to the much better heat transfer experienced at the seal outer diameter (to the sealed fluid), compared to heat transfer at the inner diameter of the rings (to fluid at ambient conditions). Reducing convection area, the portion of sealing ring exposed to high heat transfer is explored as a method to minimise this. It is shown that this has some benefit. However, overall deformation and coning remain large. Consequently finding a way to control heat transfer and temperature profiles of the rings is critical.The numerical models are validated with the previous computed data in term of the pressure, opening profiles and friction heat and a reasonable agreement have been achieved. The velocity profiles near the wall region are verified with the empirical formulas and reasonable agreement has been achieved.This project provides some new insights on how to design a seal for supercritical CO2 that reveal new flow physics and seal distortions management.

The emergence of dry gas seals has revolutionized the form of fluid sealing. The traditional research and analysis of dry gas seals is carried out by considering the lubricating medium gas as an ideal gas, but at this stage, the sealing application environment is complicated, so it is necessary to consider the real gas effect of the lubricating medium gas to expand and break through the design system of dry gas seals. We choose seven common lubricating media in dry gas seal applications and screen the optimal density expression of the real gas using different real gas equations of state. Then, we study the extent to which the compression factors of different lubricating gases deviate from the ideal gas and analyze the errors of different real gas equations of state. These results can provide an optimal expression to clarify the mechanism by which the real gas effect affects the dry gas seal performance, which helps to grasp the nature of dry gas seals, predict the dry gas seal behavior, and guide the dry gas seal application.

The dry gas seal is a promising sealing technology to control the leakage flow through the clearance between the stationary and rotational components of Supercritical Carbon Dioxide (SCO2) turbomachinery. The dry gas seal is firstly designed for the SCO2 compressor shaft end of the GE’s 450MWe Brayton cycle power generation system. Then the effects of the spiral angle and gas film thickness on the designed dry gas seal performance are numerically investigated using the three-dimensional Reynolds-Averaged Navier-Stokes (RANS) and SST turbulence model. The accuracy of the numerical method is validated by comparison of the previous research data done by Gabriel et al. with air as the working fluid. The Current study analyzed the sealing performance parameters of the designed dry gas seal for SCO2 compressor shaft end at five gas film thicknesses and four spiral angles. These parameters include: opening force, leakage rate, stiffness, and opening force leakage ratio. Also, the impacts of the spiral angle on flow direction in the fluid film are analyzed. The obtained results show that the designed dry gas seal meets the requirement of the leakage flow rate of the SCO2 compressor shaft end. The dry gas seal with a spiral angle of 15° is the best solution due to its low leakage rate and its’ best comprehensive sealing performance. On some occasions where high stability is required, the dry gas seal with a spiral angle of 30° can be selected due to its’ highest film stiffness. The present work provides the reference of the dry gas seal design for the SCO2 compressor shaft end.

Objective Shaft sealing prevent uncontrolled escape of process gas from the gas compressor casing. During shaft sealing failure, apart from loss of production, consequences due to loss of containment of hazardous and toxic process fluid may extend to multiple fatalities and major asset damage. Due to this high risk, the operation team is highly focused in improving reliability of compressor seals. Over the years, traditional oil film seals are replaced by dry running gas seals. Most of the process gas centrifugal compressors manufactured today are equipped with dry gas seals. Dry gas seals are available in a variety of configuration; the most common one for process gas application is tandem seals. This report describes a centrifugal compressor dry gas seal (tandem seal) failure investigation in an oil and gas field and the recommendations to enhance gas seal reliability. Dry gas seals in a medium pressure, hydrocarbon gas compressors had experienced frequent failures, to be precise, five dry gas seals failed in a span of seven months. These frequent failures become a serious challenge to sustain gas production. Analysis Data for analysis was gathered during site visit, in addition, discussion with operations & maintenance team was held to understand equipment performance and maintenance status. Real time operational data was obtained from data server for selected parameters. Defect elimination is the method used for analysis, this include identifying all possible causes of failure. The trends on key operating parameters that have the potential impact on seal failure and the variation of these parameters during the event are analyzed. The report describes the details of analysis performed. Results and conclusions An evidence based approach is used either to qualify or eliminate the cause of failure. The report indicates the most likely cause of seal failure based on the method of analysis. This report describes the modifications in the system and operating requirements to prevent future dry gas seal failures. Modification includes additional instrumentation for monitoring to increase safety of existing equipment and systems. Site utility conditions, compressor operating conditions, control and monitoring of key parameters can significantly influence dry gas seal reliability and requires special care in the design of seal auxiliary systems. By providing proper training to operators and maintenance team the seal reliability can be enhanced. Dry gas seals in critical rotating equipment are very common in the oil and gas industry. Seals can fail for a variety of reasons, hence improving reliability of gas seals is important for the operations to achieve hydrocarbon production targets.

The supercritical carbon dioxide cycle is a Brayton cycle with great application prospects. As a key equipment in this cycle, the turbine machinery usually adopts a dry gas seal as the sealing method between the cylinder and sliding bearing to reduce the leakage of carbon dioxide. In this paper, the numerical model of supercritical carbon dioxide turbine rotor cooling is established, and the grid independence is verified. The effects of inlet temperature and flow rate of dry gas seal and leakage flow rate from cylinder to dry gas seal at the high-temperature inlet side of a turbine upon rotor cooling are studied. The effects of inlet temperature T in and flow rate Q v of sealing gas in a dry gas seal and leakage mass flow rate Q m from a cylinder to dry gas seal on pressure loss, outlet flow distribution, exhaust temperature, and rotor temperature distribution are analyzed. As a result, it can be found that with the increase of the inlet flow rate of dry gas seal gas and the leakage flow rate from cylinder to dry gas seal, the pressure difference between the inlet and outlet of each seal gas increases. When the inlet flow rate of dry gas seal gas ranges from 300 N m3/h to 900 N m3/h, with the leakage flow from cylinder to dry gas seal increasing from 1.3 kg/s to 2.08 kg/s, the pressure difference between inlet and outlet of each seal gas increases by 7.9% to 13.4%. The pressure difference between the inlet and outlet of each seal gas decreases with the increase of the inlet temperature of dry gas seal gas. When the inlet flow rate of the seal gas of the dry gas seal is 300 N m3/h and the leakage flow rate from cylinder to dry gas seal is 2.08 kg/s, the inlet temperature of seal gas increases from 100 to 150 °C, and the flow distribution at the outlet is basically unchanged. The research provides theoretical reference for rotor cooling design of a supercritical carbon dioxide turbine.

The performance of supercritical CO2 (SCO2) dry gas seal (DGS) with different deep spiral groove is investigated with the thermal-fluid-solid coupling method. The performance parameters of DGSs with five different kinds of grooves are obtained. The influence of inlet temperature, inlet pressure, velocity and film thickness on performance is analyzed compared with air DGS. The average film pressure, open force and leakage decrease while the average face temperature and flow velocity increase as the spiral groove number increases. The average film pressure, average face temperature, open force and leakage of DGS with radial different deep groove are higher than those of DGS with circumferential different deep groove respectively under the same spiral groove number while the average flow velocity is the opposite. SCO2 DGS can generate larger average film pressure, open force and leakage with lower average face temperature than air DGS. SCO2 DGS could maintain better sealing performance despite larger leakage with the variations of inlet temperature, inlet pressure, velocity and film thickness. The variables hold a more remarkable influence on SCO2 DGS compared with air DGS.

The use of dry gas seals in process gas centrifugal compressors has increased dramatically over the last twenty years, replacing traditional oil film seals in most applications. Over 80% of centrifugal gas compressors manufactured today are equipped with dry gas seals. As dry gas seals have gained acceptance with users and centrifugal compressor original equipment manufacturers (OEMs), the operating envelope is continually being redefined. Ever greater demands are being placed on dry gas seals and their support systems, requiring continual improvements in the design of the dry gas seal environment, both internal and external to the compressor proper. Contamination is a leading cause of dry gas seal degradation and reduced reliability. This paper will examine the experiences of one centrifugal compressor OEM in this regard. Several potential sources of dry gas seal contamination will be analyzed, drawing from actual field experience, and various means of increasing dry gas seal reliability will be discussed.

Commercially available dry gas seals (DGS) are the seal-of-choice for shaft-end sealing locations in compressors used in the oil and gas industry, and supercritical carbon dioxide (sCO2) power cycles. Typically, a DGS operates with a very thin gas/supercritical fluid film (typically 2 to 7 microns thin). DGS reliability is tied to how well this ultra-thin film is sustained under varying pressure, thermal, and speed conditions. Recent sCO2 turbomachinery development efforts have experienced a couple of catastrophic DGS failures at the compressor shaft-end locations. A suspected root cause of these DGS failures is thermal deformations caused by the excessive windage heating expected with the supercritical CO2 working fluid when the compressor operates at high pressures and high rotational speeds. In this paper, the thermal behavior of a DGS operating in a typical sCO2 compressor is investigated. Specifically, test data are presented for a specially instrumented, commercially available DGS operating in the GE-SwRI sCO2 compressor. The test data include temperatures measured on the seal stationary ring and DGS housing locations using several embedded metal thermocouples to monitor the thermal behavior of the DGS during typical sCO2 compressor missions. This paper describes how an existing GE-SwRI sCO2 compressor housing was modified to accommodate and route temperature sensors to the instrumented DGS. Test data shows higher than expected temperatures (about 180 to 190 °C) in the cavities surrounding the DGS, which provides useful insights to turbomachinery designers for designing seal cavities. The main reason for these high temperatures is the low sCO2 flow purging the seal cavity. Furthermore, the measured temperatures are compared with the predictions of a steady-state thermal model of the GE-SwRI sCO2 compressor. The thermal model uses a 1D flow advection network (accounting for cavity swirl, windage and heat transfer coefficients for sCO2 flow, air flow and bearing oil flow) exchanging heat with the surrounding structure, along with heat transfer through the structure. The thermal model uses CFD-based flows to account for the sCO2 flow past the DGS. The predictions of the thermal model match reasonably well with the measured temperature data, thereby providing validation to the modeling assumptions. The high structural temperatures in the regions around the DGS that are predicted by the model as well as measured during the tests point to the importance of devising cooling strategies for reliable operation of the DGS.

This paper provides a numerical technique to determine the dynamic characteristics of a gas film inside a dry gas seal gap based on steady-state and transient CFD calculations. Two dry gas seals with spiral grooves were studied. Steady-state calculation results were compared with the experimental data. The transient dry gas seal model was created to obtain the dynamic characteristics of a gas film. This model is able to take into account the mutual influence of stator and rotor rings on a gas film. This is made by adding the mesh deformation within very small range (from 2 to 10 microns) along with adding for the mesh the ability to move freely within a big range (up to 500 microns). FSI simulation was added to solve Reynolds- averaged Navier-Stokes equations and dynamic equations of rigid body motion together. Using steady-state and transient gas film models, dynamic and static opening forces were obtained for the different gap values. Gas film stiffness was calculated. In order to determine the oscillation frequency of a gas film, Fourier transform was applied.

The spiral groove dry gas seal is sealed by only a 3–5 µm gas film thickness between the faces of rotating and stationary rings. Considering the complexity and difficulty of measuring the gas film stiffness and system damping of a spiral groove dry gas seal, a test system is designed to study the stability of the dry gas seal. Vibration amplitude of the stationary ring and gas film is measured in this study by writing a LabVIEW test program, adopting a high-precision improved eddy current micro-sensor, obtaining a series of anti-interference measures, and so on. The velocity diagram of the stationary ring and frequency spectrum are processed. According to the data, the variation of gas film stiffness and system damping is analyzed under different operating conditions. The measurement results of gas film stiffness and system damping for the dry gas seal show that at the rotating speed of 1000–3000 r/min and a medium pressure of 0.2–0.6 MPa, the range of gas film stiffness values is 4.8×108−1.50×109 N/m and the range of system damping values is 7.2×104−2.15×105 N.s/m. Both variations of gas film stiffness and system damping are nonlinear. The gas film stiffness value decreases with an increase in rotating speed and increases with an increase in pressure. The system damping value remains stable despite a change in pressure and decreases with an increase in rotating speed. The results have solved the problem that the exact solution of system damping cannot be obtained through theoretical calculation, and such findings will be beneficial to further studies on bifurcation, and chaos of dry gas seals. The optimization of groove design in the future can be guided by the results.

Within the energy transition framework, it is believed that supercritical carbon dioxide (sCO2) power cycles can significantly contribute to climate change goals. Compared to other energy conversion technologies, higher efficiency can be achieved thanks to the minimized compression work, driven by the peculiar properties of sCO2. Footprint reduction coupled with good operational flexibility and reduced water consumption are additional advantages. The present work discusses design challenges and solutions studied for an sCO2 power system object of the CO2OLHEAT project, “Supercritical CO2 power cycles demonstration in Operational environment Locally valorizing industrial Waste Heat”, European Union Horizon 2020 funded program (grant agreement # 101022831). The project aims at demonstrating the first MW-scale waste heat recovery system employing sCO2 in the EU; the demo cycle will be installed in an existing industrial plant. Object of the authors’ work includes the design and testing of a turboexpander driving the cycle compression phase. In this paper, the main challenges faced in the conceptual design phase of turbomachinery will be assessed. The centrifugal compressor, whose inlet conditions are in proximity to CO2 critical point to get the highest cycle efficiency, has been designed using specific equilibrium models and equation of state to both properly consider phase-change regions and strong gradients of thermodynamic properties. Regarding the expander, the paper will describe how the inlet conditions (pressure and temperature) represented specific challenges in particular for mechanical configuration and safe operation of crucial components such as the Dry Gas Seals (DGS), for which dedicated thermal analyses were conducted. The paper concludes with considerations on design choices for turbomachinery made to improve the operability of the complete cycle even in off-design conditions such as possible variations in minimum and maximum CO2 temperatures due to external factors.

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Performance analysis of supercritical carbon dioxide Brayton cycle with leakage reinjection system