Study on nitrogen barrier protection of an airend oil-free compressor bearings in H2 compression

The oil free compressors were specially designed for air compression. The National Research and Development Institute for Gas Turbines COMOTI gained a great deal of experience in producing/designing certified oil injection screw compressors for the natural gas field and for several years it has been focusing its research on the use of "dry" (oil-free) compressors in natural gas compression and more recently in hydrogen compression. Working with an explosive gas, one of an idea was to use a nitrogen barrier in oil bearing sealing, which are open source of gases in the atmosphere for such compressors. Worldwide, on-site nitrogen generators have been developed for a purity range of 95...99.5%, and that nitrogen can be supplied in any environment conditions. The present paper will address nitrogen flow with low percentage of oxygen for bearing sealing at the working pressure, the nitrogen consumption, ideas for H2 re-injection and the influence over the global efficiency of the process. Due to the Energy Strategy worldwide, and the studies regarding production, transport and storage of hydrogen in natural gas network, COMOTI has involved researches in developing such possibilities and to express a point of view in existing research in the newly created industry.


Introduction
Based on its experience in the development of screw-type rotary machines with oil injected screw compressor units, COMOTI has begun researching a new type of compressor package with oil-free screw compressor units, as part of a National Research Project. The proposed solutions to the research objectives for these types of compression assemblies with oil-free compressor units will allow COMOTI to demonstrate the functional advantages for the requirements of the beneficiaries, for the parameters of gas type diversity such as pressure, chemical aggression, composition, etc. A compression oil-free unit type CD14D [1] was purchased from the company GHH Ingersol-RAND for the realization of the compression assembly designed by National Research and Development Institute for Gas Turbines COMOTI. The fundamental research of new gas compression solutions has led to innovative solutions in the development of oil-free helical compressors. Within the TURBO 2020 Core Program, a fundamental research is conducted, aligning to the general objectives of the institute regarding the development of oil-free rotary machines, which is based on the knowledge attained on oil-injected screw compressors. The operating principle of oil-free compressors is identical to that the oil-injected compressors with the specification that the compression is not isothermal but adiabatically, with a high increasing temperature in the compression process. This is the reason why high-compression ratios between the inlet pressure and output pressure are not possible. The advantage of the oil-free compressors is that the contamination of the fluid is reduced to a minimum. In the next chapter, how to operate the compressor will not be detailed and the focus will be set to its constructive features.

Constructive features
The oil-free compressor is a volumetric compressor. Rotor 1 and 2 have the outer surface helically profiled after conjugated profiles. The number of lobes of the rotors differs, usually the pair of teeth is z1/z2 = 5/6 or 4/6 and it is established by the designer according to the functional parameters required for the compressor. According to figure 1, the rotors are widened with bearings: the driving rotor 1 with the radial bearings 3, 4 and with the bearing in 4 axial points, 5 and the driven rotor, 2 is enlarged with the radial bearings 6, 7 and the axial bearing 8. The lubrication circuit with bearings oil is separated from the gas circulation. Sealing boxes are used to seal the active part of the bearing rotors. Thus, the driving rotor is sealed by the boxes 15 and 18 and the rotor driven by the boxes 16 and 17. A characteristic of this compressor is the fact that between the discs with gaskets, "O"-ring, labyrinths and sealing boxes, there are channels that correspond to the outside, according to the arrows represented on the drawing, which can be connected to the suction of the compressor or to the atmosphere (basket pipe). This is essential for the operation of the gas compressor with explosive potential. In this hypothesis, if gas leaks occur from the active area of the rotors through the sealing boxes, the gases are collected in the afore mentioned channels and can be discharged to the chimney or to the inlet of the compressor where the pressure is lower. In order to protect the compressor rotors which are covered on the surface by anti-corrosion and anti-thermal treatments, the pair of gears 9 and 10 synchronizes the movement of the rotors. The clearance between the sidewalls of the wheel teeth is smaller than the clearance between the sidewalls of the rotors.
The actual compression process differs from the theoretical one in that there are internal gas leaks through the gaps between the compressor working members, pressure losses due to the gas discharge through the discharge port, and losses due to friction.
The compression process must be related to the thermodynamic process with variable mass. If this cavity is a compression cavity, the change of mass is done on account of the existence of leaks through the gaps between them and the neighbouring cavities. If the cavity is discharged, the change of mass occurs both on account of gas leakage through the gaps and on the seed of pushing the gas through the discharging port.

COMOTI Oil free compressor research
One of research branches of the INCDT COMOTI on the development of oil-free rotary machines is based on the high experience in oil-injected screw compressor packages for natural gas compression. For the purpose of developing a new research branch, an oil-free compression unit type CD14D1 [4,5] was purchased from GHH RAND, Germany ( Figure  2). The following table shows the different compressor stages and their application in the respective compressor blocks [4], and the used ones are bolded: The cells with bolded frame in the left in Table 1 are the units composing the double stage CD14D1 compressor. In Table 2 [4] are the nominal parameters of the compressor stages. In the paper, the values from the columns with normal frame were used. Sealing is an essential factor in the construction of oil free gas compressors. A nitrogen seal is required to work in potentially explosive environments. The contact between the compressed gas and bearings' oil is not allowed. According to [5], it is recommended for compressors working with neutral gas. The functional parameters of the oil-free compressor also play an important role in choosing the constructive variants of the compression unit components.
The compressor stages in the CD series are equipped with an efficient sealing system. The oil-lubricated bearing area is sealed toward the compressor chamber while the majority of the incoming oil quantity is initially rejected and discharged by a co-rotational splash ring. The oil-retaining ring, which consists of a combination of labyrinth seal ring and re-feeding thread, is attached. The compression chamber is sealed toward the bearing area by a sequence of floating metal seal rings that are placed with minimal clearance on the coated rotor shaft. The oil-side and compression-side sealing areas are separated by an intermediate space that is ventilated outward via a drill hole, thus approximating atmospheric pressure. This intermediate space and associated drill hole are referred to as a sealing vent, see figure 3. Apart from the suction-side sealing vents of the low-pressure compressor stages, a compression medium leakage flow unavoidably escapes through all sealing vents. This leakage flow can be up to 2% of the suction volumetric flow rate depending on the operating conditions and compressor type. However, it is already calculated out of the guaranteed delivery quantity of the compressor module. Some types of CDB compressor stages have a double sealing vents arrangement. In other words, there are 2 sealing vents arranged in succession in the sealing area that are separated by an additional sealing ring. For certain applications, this double arrangement can be used to create a sealing medium. For applications in which compression medium loss should be avoided as far as possible, it is possible for the leakage loss from the sealing vents to return to the compressor module suction area, see the figure 4 below.

Fig. 4. Connection diagram adapted for H2 compression
As a rule, these applications require keeping leakages of the medium to an absolute minimum. The medium must also be protected against ambient air contamination. The compressor modules from the acquired compressor (see the figure 2 above) are approved for the compression of nitrogen. Given that a minimal difference in the pressure between the vents and the oil system cannot be averted (and is actually intentional for the purpose of sealing), it is impossible to prevent a (minimal) amount of nitrogen from escaping into the oil tank and from there into the atmosphere. Moreover, the CD compressor stages are not tested for 100% leak-free performance when used with gas. Consequently, minimal direct leaks from the compression room into the atmosphere cannot be ruled out completely either. Figure 4 from above depicts how the vents should be connected to make the grade of the special requirements of hydrogen operation. To achieve this, the vents of the low and highpressure stage are guided to a collecting control tank. The container is set to a pressure that is just slightly higher than the ambient pressure. Leaked gas H2 is guided to the suction side vents of the low-pressure stage from this container. The slight excess pressure warrants that no oil is sucked-in by this vent on the one hand, and that no air can penetrate the process through the suction side vents, on the other hand. The remaining leaked gas H2 is guided to the suction side of the low-pressure stage. The necessity of separation between H2 and N2 and other impurities, before H2 re-injection in the suction pipe, will be not adressed in this paper.

N2 production and O2 % calculation for ATEX proof
The proven INMATEC membrane technology, see the figure 5.a below, allows the continual supply of nitrogen with the purity between 95% to 99.999%. The capacity of these generators is between 0.40 Nm³/h and 10,000 Nm³/h. In figure 5.b it is given an example of such installation [6]. In the present paper, an installation with N2 purity generation of 99.5% has been considered. In the membrane process the water vapour, oxygen, nitrogen, noble gases and carbon dioxide diffuse through the hollow fibre membrane at different speeds depending on their molecular structure. Oxygen and nitrogen are separated in the membrane based on their different diffusion grade. The pressure and quality determine the purity of the product [6].
From the data given by an INGERSOL-RAND software for oil-free applications with CD14D compressor unit (see the table 1 from above), there is calculated for nitrogen application at the maximum pressure, the maximum flow in m 3 /h at the given inlet conditions. In figure 6 below, the printed data is shown. In the H2 compression, it is assumed equally limited flow, but the mass flow will be decreased significantly because the molar mass of H2 is H2= 2. The N2/H2 molar mass ratio is 14, and one can estimate the mass flow of the H2 to be: The leakage flow of the H2 escaping from the vents is about 2% of the total flow in the compressor. The Leakage flow will be: From the ATEX point of view, the lower flammable limit for O2 concentration in H2 and pure O2 mixture is about 4.65% in pure oxygen [7], and in air about 4.0% [7,8]. In the literature, flammability limits and burning velocities of gaseous mixtures containing hydrogen have been measured to reach temperatures up to 250°C and pressures up to 4 MPa [9]. The methodology of EN 60079 (SR EN IEC 60079-0:2018) [10] provides for an estimate of the hypothetical volume over which the mean concentration of the flammable gas will be 0.25 times the lower explosive limit. Because the oxygen is mixed with nitrogen, one can consider an air mixture with the concentration of 0.05% oxygen, in conformity with nitrogen delivered by an INMATEC membrane technology [6].
The nitrogen flow for sealing from the figure 3, should be at the same hydrogen leakage flow, that is Mleakage H2 = 0.000714 kg/s. To calculate the volume of the oxygen in the N2/O2 mixture, we should calculate the density of the mixture. We consider the volume of a kmol = 22.4 m 3 . The density of the mixture is: assuming that the Mmixture is the mass of a kmol.
The density of the H2 in normal conditions (20°C and atmospheric conditions) is ρH2 = 0.08375 kg/m 3 . The volume flow of the hydrogen will be: The O2/H2 volume ratio is: The result is 0.034% concentration of O2 in the H2 flow, which is a negligible quantity of oxygen in the H2/O2 mixture, to create an explosion area, much lower than 4% low limit for air or pure oxygen atmosphere.
The volume of N2 consumed in the process wil be calculated same as O2 volume, and from equation 6, we will use the 0.995 concentration of N2. We will have: ̇= 0.995 • 0.000714 = 7.1043 • 10 −4 / (10) The volume flow of the nitrogen in the N2/O2 mixture is: The volume is calculated at 20°C and 1.0 bar pressure. The normal volume will be: The volume is calculated at 0°C and 1.0 bar pressure [11]. The generator IMT-PN 1150 can be chosenfrom the catalogue [12], with a capacity of 2.6 Nm 3 /h. The power consumption of the installation is just 150 W. The specific mechanical work for compression in two stages with intercooling is [13,14]: where πcST1 is the compression ratio in the first stage and this calculated according to the values from figure 6: The polytropic coefficient for each stage was calculated using the formulas [15]: assuming that compression efficiency ηcompression = 75%, and the total compression power will be: From figure 6, 42% in kW is the cooling power need for cooling the oil and gas intercooling. Also, 0.4% from the shaft power is used for oil injection. The total power consumption is: From the total power used for H2 compression, the power consumed for N2 production, of 150 W = 0.15 kW, about 0.057% is negligible, and the total efficiency of the compressing process it is not influenced.

CONCLUSIONS
The sealing of the shafts of the compressors with neutral gases is a relatively new idea, research studies have been conducted conducted successfully beginning with 1990s. Using neutral gases as a barrier sealing is also improving the protection against harmful or potentially explosive emissions, or the formation of inflammable or explosive gas/gas or gas/liquid mixtures. Many research studies and industrial achievements have been performed and obtained in in the field of turbomachines.
One of the ideas from the actual paper was to create the sealing neutral gas, in our case nitrogen, directly from the environmental air, through a membrane technology, see the figure 5, such technology being a new but mature one. While using traditional gas supply methods such as liquid or bottled hydrogen, users pay higher costs because of the rental fees, refill, delivery order processing charges, as well as environmental fees. Generating nitrogen inhouse is a cost-effective and reliable alternative to the other methods of nitrogen supply [16].
The present work does not take into consideration the warm gases in the H2 compression that can be observed in figure 6 to be about 222.4°C in stage 1 and 174.6°C in the 2nd stage of compression. The limit of the hydrogen autoignition at atmospheric conditions is between 500÷577°C. Studies have been performed for different pressures and temperatures and the increase of the temperature can slowly modify the lower limit of H2/O2 concentration. The given value RO2/H2=0.034% from the equation 9, for the concentration of O2 in the H2 flow showed that the concentration of O2 in the N2/O2 mixture for barrier sealing is negligible and the method proposed can be used in safe conditions conditions. The power consumption for N2 generation compared with the power need for H2 compression also give a negligible value, with insignificant over the global efficiency.
The presented solution with nitrogen barrier for bearing sealing of oil-free compressors for their use in potentially explosive environments is reliable, and can be implemented in the existing transport gas grid, thanks to the ability of the gas grid to integrate admixtures of hydrogen. With a view to 2050, Hydrogen Europe considers that through a hydrogen and gas integration, a significant cost-effective decarbonisation can be achieved. Furthermore, the gas infrastructure can transport larger volumes over longer distances at a fraction of the costs when compared to the electricity grid. With the switch to renewable and low-carbon hydrogen, among other sustainable gases, gas grids can be adapted and used for a costeffective decarbonisation across Europe and worldwide. In addition, the conversion of gas grids to hydrogen is the fastest way to fully decarbonise as it can be achieved via central and decentralised actions [17].