THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y. 10017 84-GT-214 The Society shall not be responsible for statements or opinions advanced in papers or in G discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. ]^L Released for general publication upon presentation. Full credit should be given to ASME, the Technical Division, and the author(s). Papers are available from ASME for nine months after the meeting. Printed in USA. Copyright 1984 by ASME SEAL OIL DEGASSING IN GAS TURBINE CENTRIFUGAL COMPRESSORS F. M. STEELE The Hilliard Corp. 653 Euclid Ave. Elmira, N.Y. 14901 INTRODUCTION Oil provides several functions in the circulating system of a gas turbine-centrifugal compressor. It cools, dampens, flushes, lubricates, resists corrosion, seals, and transmits power. Uncontrolled accumulation of contaminants in the oil can impair these functions, can cause excessive wear, deterioration, malfunction, and failure of system components, and can create safety hazards for the equipment and its environment. Compressor shaft seals receive oil from a pressurized, circulating system that may supply both the turbine and compressor components, the compressor components only, or the seals only. As oil flows continuously from a central reservoir through each seal at a typical velocity of four liters per hour, it absorbs some of the gas with which it comes in contact. Absorption increases with gas pressure, and gas-to-oil exposure area in the seal, and decreases with oil film thickness, temperature, velocity, and viscosity. In most natural gas compressors oil drains from each seal under operating pressure to a trap which discharges periodically through a built-in float valve to an atmospheric degassing tank. A pressure reduction of several atmospheres from traps to tank releases from the oil some absorbed gas components, generally in proportion to their vapor pressures. Atmospheric degassing normally removes H2, Cl, C2, and most C3, and is adequate for turbocompressors pumping sweet gas from which higher boiling hydrocarbons have been stripped, or for compressors using sweet buffer gas for the seals. Vacuum distillation is required for degassing seal oil in compressors pumping untreated, well-head gas or process gas which contains hydrocarbons C4+, hydrogen sulfide, or moisture. Sparging sometimes is used to decrease the degassing temperature and/or to increase the degassing rate. PRINCIPLES OF DEGASSING Degassing of seal oil is accomplished by atmospheric distillation, vacuum distillation, and sparging, and by combinations of these methods. Distillation separates components of an oil and gas mixture by partial vaporization of the mixture. The more volatile gaseous components are obtained in increased concentration in the vapor and the less volatile component, oil, remains a liquid. The pressure exerted by a vapor in equilibrium with its liquid is called vapor pressure. Vaporization or boiling is the change from a liquid to a vapor state. The change requires the addition of heat energy to a liquid and occurs when the vapor pressure of the liquid exceeds the absolute pressure of its environment.
U Vacuum distillation is distillation at pressures below one atmosphere. Reduced pressure permits vaporization at reduced temperature which has three advantages: it minimizes thermal decomposition of oil and additives; it reduces the energy requirement for heating and possible cooling of oil; it makes the process more efficient since the difference between component vapor pressures increases with a decrease in temperature. (1) In sparging, bubbles of an inert gas such as nitrogen rise through an oil and seal gas mixture and escape from the surface bearing a concentration of the mixture in proportion to its component partial pressures. An oil-gas mixture plus nitrogen has a greater vapor pressure than the mixture alone, and will distill at a higher pressure or lower temperature. The mols of oil-gas mixture vaporized are proportional to the mols of nitrogen used times the ratio of the partial pressures of the mixture's components to the partial pressure of nitrogen. The components of natural gas which become contaminants of seal oil have much higher vapor pressures than oil and have much higher partial pressures in oil mixtures; hence, they are removed preferentially during sparging or distillation. Turbulence in oil during degassing accelerates the release of higher vapor pressure gaseous components. An analogy is the rapid release of carbon dioxide when shaking a bottle of carbonated beverage after opening. Minor turbulence is created by a flow of nitrogen bubbles during sparging. Greater turbulence is generated during circulation of oil in a vacuum chamber by cascading, impingement, etc. DESIGN CRITERIA The design of a seal oil degassing system depends primarily on the flow rate of oil through all seals, the composition of gas used in the seals, and the amount and composition of gas absorbed in oil as it leaves the seals. In addition the design must conform to available utilities, ambient conditions, and applicable codes and standards. The basic flow rate is developed by adding together the manufacturer's design flow rate for each seal. The total basic flow rate is doubled to compensate for seal wear, and a 25% safety factor is added to establish a design flow rate. The seal gas composition indicates qualitatively what must be removed from oil. The actual amount and composition of absorbed gas determines the degassing temperature and absolute pressure, and the capacity (m 3 /s) of a vacuum pump if required. The gas content of seal oil is obtained by analysis of a 50:50 mixture from suction and discharge traps. The content may be over 30 V% with new seals and high gas pressure and under 1 V% with worn seals and low gas pressure. For a new project it is desirable to design a degassing system to handle the maximum absorbed gas content observed in previous similar operations. The composition of seal gas varies greatly from one geographic location to another. Sweet buffer gas usually contains over 99 mol % Cl-C3 and only traces of higher boiling hydrocarbons. Well-head gas may contain over 15 mol % of C4+ and 15 mol % H2S, H2O, and other non-hydrocarbons. Process gas may contain over 50 mol % H. See Table 1. TABLE 1 VARIATION IN WE L-HEAD GAS COMPOSITION COMPONENT MOL % * CH4 5.20-92.67 C H 3.00-21.68 CH 8 2.40-31.48 C4H10 0.14-10.54 C 5H12 0.04-4.40 C 6H14 0.00-3.70 C 7H16 CH 0.00-0.43-0.12 C 9H 0 0.00-0.04 0.00-0.02 C 10 22 0.00-0.02 C11H24+ H 0.00-56.75 H 2O 0.00-2.69 N. 0.00-5.00 H`"S 0.00-12.79 C620.00-9.90 * Analyses from Abu Dhabi, Iraq, Kuwait, Libya, Mexico North Sea, Russia, Suez Gulf, United Arab Republic The composition of absorbed gas is similar to that of seal gas but is somewhat richer in lower boiling, higher partial pressure fractions Cl-C3 (Henry's Law) which constitute the bulk of both seal and absorbed gas, and are removed almost completely by atmospheric degassing. Analysis of oil after atmospheric degassing indicates the amount and composition of residual gas to be removed by vacuum degassing. Seal oil viscosity and flash point, which are reduced significantly by dilution with hydrocarbons, often can be used for estimating gas content of oil before and after degassing based on experience with a given compressor and seal gas composition. ATMOSPHERIC DEGASSING SYSTEMS An atmospheric degassing tank or drum (API Standard 614, Section 2.10) is furnished as standard equipment with most gas turbine 2
driven compressors pumping natural gas. As shown in Fig. 1, oil is circulated from reservoir R by pump P through seals S of compressor C into traps T from which it drains periodically to tank degassing tank A. In some cases a single seal oil system with one atmospheric degassing tank can serve two or more compressors. VACUUM DEGASSING SYSTEMS For oil containing C4-C7 and/or H2S, an existing atmospheric degassing tank can be supplemented by vacuum degassing with no change in the seal oil flow pattern. As shown in Fig. 2 oil enters tank A from traps of one or more compressors and displaces an equal flow to the reservoir. With seal wear oil flow through tank A to reservoir may double. -2X FROM FLARE L/h TRAPS STACK -----I-- I A 4! 5XL/h2-2X L/h TO R ESE,RVOIR FIG. 1 ATMOSPHERIC DEGASSING TANK Tank A is divided into two compartments by baffle B. Gas released from oil in larger compartment 1 is vented to a flare stack FS. Relatively gas-free oil flows through a hole in baffle B into compartment 2 from which it displaces an equal volume of degassed oil through standpipe SP to reservoir R. To insure nearly complete degassing, tank A is sized to hold oil for 24 hours since the gas release rate is slow in the relatively quiescent body of oil in the tank. Heater H maintains a temperature of 75 0-8000 to release most C3. See Table 2. TABLE 2 TYPICAL DEGASSING PRESSURES & TEMPERATURES HYDROCARBON 1,MG C C 2H6 760 27 C 3H 8 760 73 C 4H10 100 71 C 5H12 40 86 C 6H14 10 90 C7H16 3 94 C 8H18 1 101 CH 1 116 C90 I 2 2 1 131 1 C12H26 163 1 192* C14H30 * Sparging recommended to reduce temperature IX L/h 7XL /h V FROM VACUUM PUMP FIG. 2 VAC. SUPPLEMENTED ATMOS. DEGAS. TANK vacuum degassing system V circulates oil from the gas-laden end to the gas-free end of tank A at a rate at least twice that of the maximum seal oil flow. This creates a constant flow from compartment 2 to compartment 1, and insures that compartment 2 always contains and delivers fully degassed oil to the reservoir. For removal of C4-C7 system V operates at an absolute pressure of 3-100 mmhg and a temperature of 71-94 C. See Table 2. Vapors which condense in the lube and seal oil of the vacuum pump are purged continuously by a flow from system V of degassed oil which returns to compartment 1 of tank A. For a new installation a combination atmospheric-vacuum degassing system may reduce the required space and cost of separate units. For removal of hydrocarbons C8-C12 which require relatively high temperatures for vacuum distillation (Table 2), it is desirable to reduce time of heating to minimize energy requirements and to avoid degradation of oil additives. This is accomplished by a vacuum 3
degassing system which heats oil only during degassing once a day or less instead of continuously as with the conventional atmospheric degassing tank. For a multiple compressor installation a central vacuum degassing system offers cost and space saving advantages in comparison to a separate degassing system on each compressor. As shown in Fig. 3 oil drains from seals and traps of four compressors to a common dirty oil collecting tank A which holds 12-24 hours flow for release of most H2 and Cl, and some C2. From tank A oil passes through vacuum degassing system V into reservoir R from which it returns to the seals. Major system components can be on different levels, which may be a spacesaving advantage on an offshore platform. (2) FS LEVEL 5. R V -_-LEVEL _ LEVEL 2 FIG. 3 CENTRAL DEGASSING SYSTEM Typical vacuum degassing equipment operates automatically on a continuous, cyclic basis. As shown in Fig. 4, pump P1 transfers a batch of gas-laden oil from the collecting tank through heater H into vacuum distillation chamber V. High level float switch FS1 stops pump P1 and starts timer T which closes solenoid valve S and starts pump P2. Oil then recirculates from chamber V through relief valve R and heater H which gradually increases oil temperature. At the same time vacuum pump Pv exhausts vapors of lower boiling oil contaminants to flare stack FS and gradually reduces the absolute pressure in chamber V enough to vaporize the highest boiling contaminant. OIL VAPOR FS I TO TO --r ---- DOT FS I OIL PI H V Pv IN.2.LL T R I FS2 0 I LIJ I L P2 SOIL "-- ELECTRIC OUT "S. VAPOR FIG. 4 AUTOMATIC CENTRAL DEGASSING PROCESS A small portion of oil from pump P2 passes through orifice 0 into the sump of vacuum pump Pv from which an equal volume overflows to the dirty oil collecting tank. Continuous purging removes condensed hydrocarbons and prevents loss of seal in and efficiency of the vacuum pump. Condensation of vapors in the vacuum pump can be minimized by means of a gas ballast valve, which admits traces of ambient air that increases vapor compression pressure and condensing temperature, and by use of a sump oil heater to maintain an oil temperature of 70-75 C. Timer T stops recirculation by opening solenoid valve S. Pump P2 then transfers degassed oil remaining in chamber V. perhaps 80% of initial charge after vapor loss and flow through vacuum pump, to the seal oil reservoir. Low level float switch FS2 stops pump P2, to end the cycle, and starts pump P1 to begin the next cycle. Total cycle time for each batch is less than the time for the batch-volume (e.g. 250 liters) of gas-laden oil to flow from eight worn seals of four compressors into the common dirty oil collection tank, including a 25% safety factor. The gas content of oil leaving seals may be 30 V%. This is reduced to 10-15 V% or less by atmospheric degassing in the dirty oil collecting tank and to 5-10 V% or less when the vacuum chamber is charged with a batch of oil. Further reduction occurs during recirculation. The time of recirculation or the number of passes of a batch of oil through the vacuum chamber depends on the efficiency of vaporization. As shown in Fig. 5 for an efficiency E of 90% or more, five passes reduce the content of residual gas (C4+) to 0.1 V% or less, enough to restore viscosity and flash point 4
U to within 5% of new oil values. Further reduction is not required for adequate contamination control. CONTAMINANT REDUCTION CHART IOO.OI. I I 1 I I I 110.0 50.0 H z Z_ 20.0 Z0 10.0 w Z0 5.0 m J_ 0 2.0 J ~ 1.0 z w 0 0.5 0.2 E=99.9% \\ E=90% E =50% E =20% 5.0 2.0 0 1.0 a cd J 0.5 D w 0.2 w 0 0.1 > M 0.05 0 I I I I I I I i 1\ I 1 1 1 0.0 2 0 4 8 12 16 20 NUMBER OF PASSES FIG. 5 CONTAMINANT REDUCTION BY RECIRCULATION When hydrocarbons Cg+ are components of seal gas, they are present only in traces, and tend to accumulate slowly in seal oil with gradual reduction of flash point. The vacuum degassing temperature, set for example for continuous removal of C8, can be increased temporarily until C9+ is removed and desirable flash point is restored, and then reduced to normal temperature. The successful use of fuel gas in sparging to reduce vacuum distillation temperature for removal of C10-C15 in an offshore North Sea installation was reported by Shell U.K. (3) A mobile vacuum degassing system may be practical for periodic use at two or more compressor stations. Each station requires dirty and clean oil tanks sized to hold oil accumulating during each cycle; for example, a 3-day cycle for three stations. If a seal oil system is common with a compressor lube system or with a combined turbine-compressor lube system, any residual gas in oil overflowing continuously from a conventional atmospheric degassing tank to the reservoir, may contaminate the entire lube system. This problem, which periodically will require a complete oil change, can be eliminated with a central vacuum degassing which isolates seal oil for degassing before returning it to the lube oil reservoir. PERFORMANCE MONITORING Degassed oil viscosity and flash point within 5% of new oil values indicate adequate performance of degassing equipment for removal of low boiling hydrocarbons. Vacuum degassing reduces moisture content below 35 PPM, the approximate solubility level at 200C, and reduces H2S content below 2 PPM. The absence of free water and the low residual H2S content maintain a low acid level in seal oil and avoid significant corrosion when a compressor uses sour seal gas. Periodic tests for viscosity, flash point, moisture, and acidity are recommended to check performance of degassing equipment. For a new installation tests should be run every 250 hours until test data indicate stable, consistent performance, and then every 2500 hours. EFFECTS OF DEGASSING ON OIL QUALITY In an accelerated test for determination of oxidation inhibitor life (4), oil is placed in an oxidation cell to cover a coiled copper wire catalyst. While the cell is held at 95 C, pure oxygen is bubbled through the oil and water is added to maintain the required level. Good inhibited turbine oil has a life of over 2000 hours with this test. Seal gas and, hence, seal oil normally contain little or no oxygen and water. A degassing system contains little or no copper catalyst. In a vacuum degassing system, which operates at a higher maximum temperature than an atmospheric degassing system, oxygen and water are removed below 95 0C. Since a degassing system operates essentially in the absence of copper, oxygen, and water, it has no significant effect on the life of oxidation-inhibited turbine oils. Since seal oil in a central vacuum degassing system is exposed to the maximum vaporizing temperature only briefly during each degassing cycle, which may occur only once every few days, thermal degradation of oil and additives is insignificant. ECONOMICS Without adequate degassing equipment it may be necessary in the worst case to discard oil after one pass through the seals. For 5
four compressors the design flow through eight seals is 32 liters per hour or 256,000 liters per 8000-hour year. At a delivered price of $0.90/L the cost of oil discarded annually could be $230,400, plus operating and disposal costs, less possible value as boiler fuel. A central vacuum degassing system, worth $200,000-$250,000 installed depending on design specifications and installation site, can pay for itself in about one year, or less if seal wear increases oil flow. SUMMARY Gas absorbed in seal oil is removed effectively by atmospheric and vacuum degassing, sometimes supported by sparging. The net result is safer, more efficient and more economical operation of gas turbinecentrifugal compressors. REFERENCES 1. Steele, F. M., "Distillation Processes for Reclaiming and Upgrading Oils", BFPR Journal, Oklahoma State University, 15, 3, pp 315-321 (1982). 2. Kelchner, J. R., and Stevens, V. S., "The Reclamation of Centrifugal Compressor Seal Oil", World Filtration Congress III, King of Prussia, PA (1982). 3. Cootes, L., "Contamination of Centrifugal Process Gas Compressor Lube Oil & Seal Oil Systems by Hydrocarbon Condensate", ASME Gas Turbine Conference, Houston, TX, March 1981. 4. ASTM D943 (IP 157) "Oxidation Characteristics of Inhibited Steam Turbine Oils". 6