GB2075544A

GB2075544A - Lubricants in Gas Compressors

Info

Publication number: GB2075544A
Application number: GB8113298A
Authority: GB
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1980-05-02
Filing date: 1981-04-30
Publication date: 1981-11-18
Also published as: NL190622C; NO811436L; NL8102164A; CA1181061A; NO150642C; NO150642B; NL190622B; GB2075544B; DE3016805A1; MY8500085A

Abstract

Piston compressors used in natural gas processing are lubricated with a lubricant which is a solvent, solvent component or a drying solution used in the processing of the gas downstream of the compressor or is compatible with them. A preferred class of lubricants comprises a polyglycol and/or a polyglycol ether.

Description

SPECIFICATION Lubricants for Gas Compressors The present invention relates to lubricants for gas compressors, particularly piston type gas compressors used in the processing of natural gas.

Non-lubricated compressors, with any liner-type cylinders, have been used, for compressing gases, especially in applications where the delivery pressures are not excessive and the wear and tear imposed on the working parts is not too great. One such application has been in the processing of natural gases, for example, in compressing the gases leaving the flash steps of a sweetening plant. In certain applications of this kind, it has been found that the gas from the flash steps of the sweetening plant, which still contains H2S and CO2, may contain an unusually high content of solids which will cause excessive wear in the piston supporting and packing rings and will, within a few days, require the compressor to be taken out of operation.

Various solutions to this problem are possible-and even technically feasible-but for differing reasons may be unattractive. For example, complete removal of the entrained solids by filtration, centrifugation or precipitation may be technically unfeasible because of space limitations on the physical equipment which would be necessary.

On the other hand, conversion of the compressor to conventional oil lubrication in order to control the wear problem would require the oil mist to be separated from the compressed gas and even then, complete exclusion of residual oil particles cannot be guaranteed, thereby creating a likelihood of oil contamination and foaming in subsequent processing steps. The technical problems and expense associated with this particular solution are therefore substantial.

We have now found that the operation of any liner-type compressors used in the processing of gases may be improved by using a lubricant which comprises one or more of the solvents, solvent components or drying solutions used in the processing of the gas. It is also possible to use a lubricant fluid which is compatible with the solvents and other materials used in the processing downstream of the compressor, i.e. a fluid which does not adversely react with such materials. The use of such fluids as the lubricant avoids contamination and other processing difficulties in subsequent stages of the processing plant while, at the same time, increasing the useful working life and maintenance interval of the compressor. The compatibility of the lubricant fluid with the liquids used in subsequent processing steps prevents undesired reactions between these materials.

The final choice of compressor lubricant depends, of course, on the kind of processing plant following the compressor and upon the solvents or solutions used in that plant. Suitable compressor lubricants for use in natural gas compressors therefore include the anhydrous, physically effective solvents, solvent components or drying solutions which are used alone or in combination in the gas sweetening or dehydration plants. These lubricants do not, in general, contain components which chemically react with or dissolve components of the gases which are being compressed.

The lubricant can be supplied to the compressor by feeding it in at the lubricating points provided in the compressor structure for lubrication of the cylinder. The lubricants may be used for lubricating all parts of the compressor which come into contact with the gas which is being compressed, including the cylinders, cylinder tracks, pistons, piston rings and main packing boxes and glands.

Preferred lubricants for use in natural gas processing plants are described below but the particular material used will, as stated above, be chosen according to the nature of the processing steps downstream of the compressor e.g. the dehydration or sweetening steps.

The polyglycols or polyalkylene oxides comprise a preferred class of lubricants for the present purpose. The polyglycols are essentially linear polymers having the following general formula RO#(CH2#CHR'#O)x#R" \ 2 Ix where R' is hydrogen or an alkyl group or both, in different portions of the molecule. The end groups, R .and R", are usually hydrogen or alkyl groups, generally lower alkyl, especially C1 to C4 alkyl, although aromatic groups such as phenyl may be used in certain phenyl ether polyglycols. For present purposes R and R" are preferably hydrogen or methyl and R' is preferably hydrogen. That is, the preferred polyglycols for use according to the present invention are polyethylene glycols and their dimethyl ethers.

In the formula given above, x represents the degree of polymerization of the glycol. For the present purposes, relatively low molecular weight polyglycols and polyglycol ethers are preferred, where x is from 2 to 12, although higher molecular weight materials may be used. The glycols and glycol ethers may, of course, be used in combination with one another.

Specific materials which may be used include the following: Name Formula Diethylene glycol HO-(CH2-CH2-O)2-H Triethylene glycol HO-(CH2-CH2-O)3-H Tetraethylene glycol HO-(CH2-CH2-O)4-H Name Formula Triethylene glycol-dimethyl ether CH3O-(CH2-CH2-O)3-CH3 Tetraethylene glycol-dimethyl ether CH3O-(CH2-CH2-O)4-CH3 Pentaethylene glycol-dimethyl ether CH3O-(CH2-CH2-O)#-CH3 Hexaethylene glycol-dimethyl ether CH3O-(CH2-CH2-O )6-CH3 Heptaethylene glycol-dimethyl ether CH3O-(CH2-CH2-O)7-CH3 Octaethylene glycol-dimethyl ether CH3O-(CH2-CH2-O)8-CH3 Nonaethylene glycol-dimethyl ether CH3O-(CH2-CH2-O)9-CH3 Thus, for the present purposes, polyglycols and polyglycol ethers having molecular weights from 62 up to about 450 are preferred. Other similar materials and blends of materials may also be used.

Examples of them are disclosed in the art, for instance, Synthetic Lubricants, Gunderson and Hart (Ed.), Reinhold, N.Y. 1962, Chapter 3. They are also commercially available, for example, as the UCON (trademark) fluids of Union Carbide.

Specific and preferred blends of polyglycols and polyglycol ethers are as follows: Lubricant A A blend of polyglycols which has the following properties: Density at 200C 1.1254 kg/l Molecular weight 150.2 Boiling point at 760 Torr 287.40C Thickening point at 760 Torr -7.20C Viscosity at 200C 47.8 cP.

Lubricant B A blend of seven ethylene glycol dimethyl ethers having the following properties: Density at 250C 1.031 kg/l Vapor pressure at 250C 0.001 mbar Molecular weight 280 to 310 Freezing point 22a to -290C Specific heat: at 50C 0.49 kcal.kg#1 0C-1 at 1000C 0.51 kcal.kg-' OC-' Viscosity at 250C 5.8 cP Surface tension at 250C 34.3 dyn/cm Thermal conductivity at 250C 0.16 kcal.m- hour-1 0C#1 Flash point 1510C A specific blend conforming to the requirements of Lubricant A is as follows: Lubricant A-l Component Wt. Percent Diethylene glycol 1 Triethylene glycol 98 Tetraethylene glycol 0.5 Other ingredients 0.5 A blend of seven ethers conforming to the specification for Lubricant B is as follows: Lubricant B-l Component Wt. Percent Triethylene glycol dimethyl ether 12 Tetraethylene glycol dimethyl ether 24 Pentaethylene glycol dimethyl ether 25 Hexaethylene glycol dimethyl ether 19 Heptaethylene glycol dimethyl ether 11 Octaethylene glycol dimethyl ether 6 Nonaethylene glycol dimethyl ether 3 The invention is illustrated by the following Examples.

Example 1 A strip gas piston-type compressor was lubricated with Lubricant A-l above. The compressor characteristics were as follows: Manufacturer: Klein, Schanzlin and Becker (KSB) Capacity: 600 m3 . hr Inlet pressure: 40 bar Delivery pressure: 70 bar No. of stages: 1 The compressor was supplied with the lubricant on a flow-through basis at a rate of 4 1 . her~1 for cylinder and piston rod lubrication.

Example 2 A recycle compressor in a sour natural gas sweetening plant was lubricated with Lubricant B-l above. The compressor characteristics were as follows: Manufacturer: Thomassen Capacity: 18,800 m3. hr' Inlet pressure: 1 bar Delivery pressure: 78.5 bar No. of stages: 4 The compressor was supplied with the lubricant on a flow-through basis at a rate of 1 1 . hrl for cylinder and piston rod lubrication.

Example 3 A strip gas compressor was lubricated with Lubricant A-1 above. The compressor characteristics were as follows: Manufacturer: Borsig Capacity: 9,000 m3 . hr' Inlet pressure: 59 bar Delivery pressure: 119 bar No. of stages: 1 The compressor was supplied with the lubricant on a flow-through basis at a rate of 4 . her~1 for cylinder and piston rod lubrication.

The use of lubricant fluids according to the invention avoids undesired contamination of the downstream processing steps and eliminates the problem of foam formation which would be encountered with conventional oil lubrication. At the same time, the operating life and maintenance interval of the compressors may be increased.

Claims

1. A method of lubricating a piston-type compressor used for compressing natural gas in which the lubricant is a solvent, solvent component or a drying solution used in the processing of the gas downstream of the compressor or is compatible with them.

2. A method according to claim 1 in which the lubricant is used to lubricate the cylinder tracks, pistons, piston rings and main packing boxes of the compressor.

3. A method according to claim 1 or 2 in which the lubricant comprises a polyglycol, polyglycol ether or mixture thereof.

4. A method according to claim 3 in which the lubricant comprises: Diethylene glycol 1 Triethylene glycol 98 Tetraethylene glycol 0.5 Other ingredients 0.5.

5. A method according to claim 3 in which the lubricant comprises: Triethylene glycol dimethyl ether 12 Tetraethylene glycol dimethyl ether 24 Pentaethylene glycol dimethyl ether 25 Hexaethylene glycol dimethyl ether 19 Heptaethylene glycol dimethyl ether 11 Octaethylene glycol dimethyl ether 6 Nonaethylene glycol dimethyl ether 3.