reciprocating natural gas compressors lubrication

7/27/2019 Reciprocating Natural Gas Compressors Lubrication

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Reciprocating Natural Gas Compressors

Large, double-acting (Figure 1), multicylinder, multistage crosshead design reciprocatingcompressors are used to generate low-pressure plant air, move natural gas in the natural

gas production industry, supply high-pressure gas for oil well drilling, and for variousapplications in manufacturing or chemical processing that require medium- to high-pressure air. Because positive displacement screw compressors (Figure 2) are most

common to industrial plant air requirements, this article focuses on gas transmissionpipeline applications.

However, many of the issues pertaining to lubrication of reciprocating compressors for thepipeline application would also pertain to the effective lubrication of machines for in-plantservice.

Gas transmission applications are typically powered by a reciprocating engine fueled by

natural gas. These compressors may be located in the field at a remote station or at the

inlet to a gas plant where raw, wet (containing droplets of water or hydrocarbon) andpossibly sour (containing hydrogen sulfide, H2S) natural gas is compressed.

Figure 1. High-pressure Service, Forged-steel, Double-acting Cyclinder

The packing box is water-cooled. The packing may be oil- or water-cooled by a forced

circulation system for maximum heat dissipation, depending on operating pressures. Tiebolts which run perpendicular to the cylinder base pre-stress high-pressure forgings,

decreasing maximum tensile stress induced by gas pressure.(Source: Dresser-Rand, Painted Post, NY)

They may also be located at the downstream end of a gas plant where totally clean and drysales gas is compressed and moved into a pipeline system. In addition, reciprocating

compressors are used for high-pressure gas reinjection into downhole reservoirs to enhance

crude oil recovery. Centrifugal compressors (not discussed here) are typically used onnatural gas pipeline systems to move large volumes of gas at lower pressures.

Compressor crankcase lubrication is relatively simple. Crankcase lubrication typically uses

an oil system that is entirely separate from the compressor cylinder and rod-packing lubesystem. Natural gas engine oil is typically used because it is readily available if the drive,

that powers the compressor, is a natural gas engine. A diesel engine could also be used.

The difference between a natural gas engine oil and diesel engine oil is primarily in theamount of antiwear additive and the amount and types of detergent additives used. A rust


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and oxidation (R&O)-inhibited oil or an antiwear (zinc containing) oil could also be used. The

oil viscosity used in the compressor crankcase is typically an SAE 30 (ISO 100) or SAE 40(ISO 150) grade.

Figure 2. Pair of Rotors from a Screw Compressor(Source: Aerzen USA, Coatesville, PA)

The compressor cylinders and rod pressure-packing oil is the main focus of this section. Theoil required in these units is injected into the cylinders, packings and, in some instances,into the incoming gas upstream of the suction valves.

The Lubrication SystemTwo main types of oil feed pumping systems are available for the cylinders and packings.

Pump-to-point systems have multiple positive displacement pumps that pump oil from asmall reservoir. Each pump feeds oil to one lubrication point on the compressor. Distributionblock systems use one or two larger pumps to force oil through sized and sequenced valveswhich distribute the oil to the various lubrication points. Distribution block systems must be

designed specifically for a given compressor. Once installed, only the overall flow throughthe entire system can be readily adjusted by the on-site operators and maintenance staff.This is a once-through application in which the oil is not recovered or recirculated.

Lubricant CompositionLubrication of the cylinders and packings when compressing dry sales gas or pipeline gas istypically accomplished with a natural gas engine oil (NGEO) having a viscosity of an SAE 40

grade (or a heavy SAE 30), as used in the crankcase. For pressures less than 1,000 psi

(7,500 kPa) this is adequate. At pressures greater than 1,200 psi (8,273 kPa), a higherviscosity oil is required and typically a special cylinder oil is used, as described below.


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If the gas being compressed is a wet or sour natural gas or a solvent gas (propane and

CO2), a special cylinder oil is recommended. If in doubt about the composition of the gas, itis best to assume that it is wet and use one of the special cylinder oils available.

Most compressor cylinder oils are formulated with mineral base oils and a synthetic additive

or component to prevent the oil being washed-off the cylinder wall by the solvent-like action

of the gas. Older technology products were compounded oils which included animal and/orvegetable fatty additives such as tallow, lard or castor oil. These older technologies were

susceptible to forming deposits in the tanks and flow lines of the lubrication system,especially if stored for a long time and/or exposed to cold temperatures.

The viscosity of the oil required for the cylinders and packings in raw field or solvent gasservice depends upon the specific gases being compressed and the highest discharge

pressure encountered. It is common for some of these compressors to handle two or three

different gases at one time in different cylinders. The cylinder with the most severe serviceon the compressor dictates the oil to be used, as most compressors will have only one oil

injection system. For the compression of natural gas below 2,500 psi (17,000 kPa), which iscommon, an ISO 220 viscosity grade product or an oil with a viscosity of 250 to 280 cSt at

40C is recommended. If the gas being compressed has significant carbon dioxide content,then a heavier ISO 320 or 460 viscosity grade may be required to compensate for theviscosity-thinning effect of the CO2 gas.

As discharge pressures increase, the cylinder oils viscosity must also increase. To compress

natural gas at pressures from 2,500 to 4,000 psi (17,000 to 28,000 kPa), an ISO 320 or460 grade of oil is required. At pressures greater than 4,000 psi (28,000 kPa), as used indownhole gas reinjection systems, an ISO 460 (mineral oil) product may not be sufficient

and an ISO 150 or 220 synthetic polyalkyleneglycol (also known as polyglycol, PAG) may be

required. These polyglycol lubricants do not absorb any of the hydrocarbon gases andtherefore do not thin out or get washed off of the cylinder walls by the high-pressure gas.

Polyalkyleneglycol oils are incompatible with mineral oils and therefore cannot be mixedwith mineral oil-based products, including those that are typically used in the crankcase.

There is some risk of the cylinder/rod lubricant mixing with the crankcase lubricant in

crosshead-type systems. These machines are designed with a wiper that pushes packing rodlubricant down the rod toward and into the crankcase. It is imperative to periodically test forcontamination if incompatible crank and cylinder lubricants are used.

The low-temperature properties of the oil selected are also important as the oil must be able

to flow, under gravity, from the (typically elevated) day storage tank to the oil injection

pumps on the compressor at ambient temperatures at or below 0C. Other importantproperties include the ability of the oil to resist coking due to heat on the valves and rodsand, in sour service, the ability of the lubricant to protect against H2S corrosion.

If propane is being compressed in any cylinder on the compressor, to be used as arefrigerant within the plant, an oil with good low-temperature properties is required. This isnecessary to allow the oil to spread on the cylinder walls at the coldest propane suctiontemperature encountered. Propane also dissolves into the oil on the cylinder walls and

lowers the oil viscosity. Therefore, the oil selected must be heavy enough to protect all thecylinders at the most severe conditions encountered throughout the compressor.

Oil Injection RatesDetermining the oil feed rates to each cylinder and rod pressure-packing set is an important


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but inexact science. There are numerous methods for calculating or determining the oil feed

rates to each cylinder and each packing box. Most are rough calculations that are intended

to provide only a starting point for setting the oil injection rates. It is important for theuser/engineer to understand the intended purpose of the specific formula being applied.

Some are intended for use during the break-in period. Others assume some degree of watersaturation of the gas, while others are minimum injection rates. The old method of counting

drops of oil in the pump site-glass to determine actual feed rates is inaccurate. These site-glasses are useful only as a quick visual check to see if any oil is moving through thesystem.

Early formulas used to quantify feed rates were based on the amount of lubricant necessary

to provide an oil film sufficient to cover the cylinder - piston ring contact area. One method

suggests one U.S. pint per two million square feet of swept surface area. This is a simpleformula to estimate feed rates (in U.S. pints per 24 hours) and could still be useful for low-

pressure, plant air or dry gas applications (this formula is not intended to supercede more

sophisticated methods for calculating feed rates for natural gas, CO2, high-pressure air, orsimilar process applications) as follows:

[ (Bore Diameter (inches) * * 2 * Stroke * RPM * 1440) / 144 ] / 2,000,000, where:

Bore Diameter = Cylinder internal diameter or rod external diameter (for low Ps) ininches Stroke = Cylinder stroke or travel in inches

RPM = Motor speed (piston cycle speed assuming no speed reduction or increase from themotor)

A simplified formula was devised by Ingersoll Rand (now Dresser-Rand), and is as follows:

Bore Diameter * Stroke * Speed * / 31,800 = Pints per 24 hours

More recently, some original equipment manufacturers (OEMs) have developed plots and

mathematical calculations that take more factors into account, including discharge pressure,gas composition and degree of liquid saturation.

The system engineer begins to estimate feed volumes by calculating feed quantities for all

cylinders and piston rods. The oil feed rates to the main rod packings are calculated using

the same formula as used for the cylinders except that the diameter of the rod is usedinstead of the bore of the cylinder. Auxiliary rod packings are usually supplied at half of therate of the main packings.

Therefore, very roughly, a typical cylinder and packing consumption rate for an entire four-

cylinder, multistage unit will be approximately 12 to 30 U.S. pints (5 to 15 liters) per 24-hour operating period.


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Once the rate per cylinder or rod packing is estimated, then the next step is to divide the

quantity required into the actual number of injection pumps or ports available in a cylinderor packing assembly.

During the break-in period, the oil volume should be increased, either by using a formula

designed for break-in periods or by injecting 1.5 to 3 times the normal operating calculated

value.

Underlubrication will result in heat and wear. Overlubrication may result in valve depositsand breakage problems plus hydrocarbon contamination of processes downstream of the

compressor and premature failure of the packings.

Final InspectionThere is obviously more that could and should be considered, such as including coolant

temperatures, temperature differentials between the coolant and the suction gas, gas

filtration quality, risk for recompression due to discharge value plugging or malfunction, andoutput temperatures.

Individual system variations require the system engineer to follow up hard calculations byperiodically shutting down the unit, pulling suction or discharge valves and inspecting the oilfilm on the cylinder wall and buildup of oil in pockets or buildup of deposits on dischargevalve surfaces.

A triple layer of cigarette wrapping paper provides a useful guideline when whipped acrossthe lubricated cylinder wall. If oil soaks through the three layers, then the feed rate isconsidered to be high. If the oil fails to soak through the first to the second layer then the

rate is low. The OEM can and should provide assistance to the user in making a finaldetermination.

reciprocating natural gas compressors lubrication

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