ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 2 pps

[Courtesy of Dresser-Rand Company,} Vane compressors tend to be limited to low pressure service, generallyless than 100 to 200 psi discharge.. Hg, and three-stage compressors can develop

2 Crosshead guides are cast integrally with engine frame provided with O-rings.

3 He toil compressor valves for any service selected from the 14, water jackets are provided with removable cover plates for unmatched Dresser-Rand line, including famous gas-cushioned inspection.

Channel Valves r 1 ) Pistons are precision-ground for a perfect fit in a honed cylinder

4 Clearance pockets and other types of capacity control devices are liner bore Long-skirt, lightweight piston reduces wear.

available to suit any application 1o, For sustained low oil consumption, narrow, deep-groove piston rings

5 Compressor cylinders (dry or water-cooled) of cast iron, nodular conform easily to liner walls Top compression ring is chrome-plated iron, or forged steel are engineered to suit required pressures and to condition liners during break-in.

capacities 17, Fuel gas headers, one for each bank of power cylinders, are

6 Full-floating packing adjusts itself in operation and assures best seal controlled by common automatic valve and safer/ devices Orifice with minimum wear Packing is pressure-lubricated and vented plates equalize the distribution of gas to each of the cylinders.

7 Oil wiper rings remove excess oil from piston rod and seal the Individual adjustments are eliminated.

frame 13 Simple fuel injection valves are operated from single camshaft.

8 Crossheads, running in bored guides, have shim-adjusted babbitted 19 Long-life special alloy valves have chromium-plated stems, hardened shoes at top and bottom and either full-floating or fixed crosshead shrink-f it valve seats, and replaceable guides.

pins Suitable for addition of balance weights 20 Common air inlet manifold conducts air from turbochargers to each

9 Simple, low-cost foundation, made possible by the smaller size, power cylinder.

lighter weight, and smooth running blance of the KVSR 21 Large covers give easy access to valve gear, exclude dust and dirt,

10 Large frame openings give ample and unrestricted access to 22 Water-jacketed exhaust eliminate expansion strains and crankcase keep engine room temperature down.

11 Flywheel-mounted ring gear for starting motors permits cranking 23 Fitted with a reliable Altronic II CPU solid slate tow tension

with either air or gas between 150 psi and 225 psi supply pressure breakerless ignition system,

12 alloy-iron frame and top are well reinforced with cast-in ribs, 24 The engine can be fitted with either hydraulic or electronic governor Integrally cast bulkheads hold main bearings on both sides of every systems to control engine speed

crankthrow Keys, double-bolting, ami Me rods are used to secure 25 in recent years, each power head is fitted with a bolt-in

pre-tfie frame ami frame top together as a solid structure combustion chamber, which allows the engine to burn a very lean

13 To assure oil-tight joints between power cylinders and frame top, the mixture, resulting in very bw exhaust emissions

Figure 10-5 Integral engine compressor (Courtesy of Cooper Industries Energy

It should be obvious that one of these large integrals would require a very large and expensive foundation and would have to be field erected Often, even the compressor cylinders must be shipped separate from the frame due to weight and size limitations Large integrals are also much more expensive than either high-speeds or centrifugals.

For this reason, even though they are the most fuel efficient choice for large horsepower needs, large integrals are not often installed in oil and gas fields They are more common in plants and pipeline booster service where their fuel efficiency, long life, and steady performance outweigh their much higher cost.

There are some low horsepower (140 to 360) integrals that are

normal-ly skid mounted as shown in Figure 10-6 and used extensivenormal-ly in small oil fields for flash gas or gas-lift compressor service In these units the power cylinders and compressor cylinders are both mounted horizontally

cylin-The major characteristics of low-speed reciprocating compressors are:

Size

• Some one and two power cylinder field gas compressors rated forL40 hp to 360 hp.

»Numerous sizes from 2,000 hp to 4,000 hp

• Large sizes 2,000 hp increments to 12,000 hp

• 2 to 10 compressor cylinders common.

Ac^antages

• High fuel efficiency (6-8,000 Btu/bhp-hr)

• High efficiency compression over a wide range of conditions

• Long operating life

• Low operation and maintenance cost when compared to high speeds

Disadvantages

• Usually must be field erected except for very small sizes

• Requires heavy foundation

• High installation cost

• Slow speed requires high degree of vibration and pulsation suppression

Vane-Type Rotary Compressors

Rotary compressors are positive-displacement machines Figure 10-7shows a typical vane compressor The operation is similar to that of avane pump shown schematically in Figure 10-10 of Volume 1, 2nd Edi-tion (Figure 10-9 in 1st Edition) A number of vanes, typically from 8 to

20, fit into slots in a rotating shaft The vanes slide into and out of theslots as the shaft rotates and the volume contained between two adjacentvanes and the wall of the compressor cylinder decreases Vanes can becloth impregnated with a phenolic resin, bronze, or aluminum The morevanes the compressor has, the smaller the pressure differential across thevanes Thus, high-ratio vane compressors tend to have more vanes thanlow-ratio compressors

A relatively large quantity of oil is injected into the flow stream tolubricate the vanes This is normally captured by a discharge cooler andafter-scrubber and recycled to the inlet

Compressors 265

Figure 10-7 Vane-type rotary compressor [Courtesy of Dresser-Rand Company,}

Vane compressors tend to be limited to low pressure service, generallyless than 100 to 200 psi discharge They are used extensively as vaporrecovery compressors and vacuum pumps Single-stage vane compressors

can develop 27 in Hg vacuums, two-stage compressors can develop 29,9

in Hg, and three-stage compressors can develop even higher vacuums.The major characteristics of vane compressors are:

Size

• Common sizes up to 250 bhp, but mostly used for applications under125bhp

• Available in sizes to 500 bhp

• Discharge pressures to 400 psig

• Single- or two-stage in tandem on same shaft

• Must have clean air or gas

«Takes 5 to 20% more horsepower than reciprocating

* Uses ten times the oil of a reciprocating Usually install after-coolerand separator to recycle oil

Helical-Lobe (Screw) Rotary Compressors

Screw compressors are rotary positive displacement machines Twohelical rotors are rotated by a series of timing gears as shown in Figure10-8 so that gas trapped in the space between them is transported fromthe suction to the discharge piping In low-pressure air service, non-lubri-cated screw compressors can deliver a clean, oil-free air In hydrocarbonservice most screw compressors require that liquid be injected to helpprovide a seal After-coolers and separators are required to separate theseal oil and recirculate it to suction

Screw compressors can handle moderate amounts of liquid They canalso handle dirty gases because there is no metallic contact within thecasing

Figure 10-8 Screw-type rotary compressor {Courtesy of Dresser-Rand Company.)

Compressors 267

It tends to be limited to 250 psig discharge pressures and a maximum of

400 hp in hydrocarbon service, although machines up to 6,000 hp are able in other service Screw compressors are not as good as vane compres-sors in developing a vacuum, although they are used in vacuum service,Non-lubricated screw compressors have very close clearances and thusthey are designed for limited ranges of discharge temperature, tempera-ture rise, compression ratio, etc., all of which can cause changes in theseclearances Lubricated compressors have a somewhat broader tolerance

avail-to changes in operating conditions, but they are still more limited thanreciprocating compressors

The major characteristics of screw compressors are:

Size

« Up to 6,000 hp in air service, but more common below 800 hp,

• Up to 400 hp in hydrocarbon service,

• Discharge pressures to 250 psig

« Single- or two-stage in tandern on same shaft

Advantages

«Available as non-lubricated especially for air service

• Can handle dirty gas

• Can handle moderate amounts of liquids, but no slugs

Figure TO-9 Centrifugal compressor {Courtesy of Dresser-Rand Company,!

head to the gas This is then converted to pressure head as the gas isslowed in the compressor case They are either turbine or electric motordriven and range in size from 1,000 hp to over 20,000 hp Most largercompressors (greater than 4,000 hp) tend to be turbine-driven centrifugalcompressors because there is such a first cost advantage in that size rangeover integrals Centrifugal compressors have high ratios of horsepowerper unit of space and weight, which makes them very popular for off-shore applications

As shown in Figure 10-10 they can be either horizontally split case orvertically split case (barrel) To develop the required gas velocities andhead they must rotate at very high speeds (20,000 to 30,000 rprn), makingthe design of driver, gear, and compressor extremely important Turbinedrives are also high speed and a natural match for centrifugal compressors.There is a disadvantage in centrifugal machines in that they are lowefficiency This means it requires more brake horsepower (bhp) to com-press the same flow rate than would be required for a reciprocating com-pressor If the compressor is driven with a turbine, there is even a greaterdisadvantage because the turbines are low in fuel efficiency The netresult is that turbine-driven centrifugal machines do not use fuel very

Comp res so rs 269

Figure 10-10 Horizontally split centrifugal compressor (top) and vertically split

centrifugal compressor, barrel (bottom) {Courtesy of Dresser-Rand Company.)

efficiently This fuel penalty can be overcome if process heat is needed Waste heat can be recovered from the turbine exhaust, decreasing or eliminating the need to burn gas to create process heat.

As with electric motor and engine-driven high-speeds, turbine and tric motor-driven centrifugals can be easily packaged for use in oil and gas fields They are very common in booster compressor service (high volume, low ratio) and for very high flow rate gas-lift service Centrifugal compressors cannot be used for high ratio, low-volume applications The major characteristics of centrifugal compressors are:

elec-Size

« Starts about 500 hp.

* 1,000 hp increments to 20,000 hp.

Advantages

* High horsepower per unit of space and weight.

* Turbine drive easily adapted to waste-heat recovery for high fuel

efficiency.

* Easily automated for remote operations.

* Can be skid mounted, self-contained.

* Low initial cost.

» Low maintenance and operating cost.

* High availability factor.

» Large capacity available per unit.

Disadvantages

* Lower compressor efficiency.

* Limited flexibility for capacity.

* Turbine drives have higher fuel rate than reciprocating units.

* Large horsepower units mean that outage has large effect on process

or pipeline capabilities.

SPECIFYING A COMPRESSOR

In specifying a compressor it is necessary to choose the basic type, the number of stages of compression, and the horsepower required In order

where R = ratio per stage

n = number of stages

P(j = discharge pressure, psia

Ps = suction pressure, psia

A first approximation for horsepower can be made from Figure 10-11 or from the following equation:

where BHP = approximate brake horsepower

R = ratio per stage

n = number of stages

F = an allowance for interstage pressure drop

= LOO for single-stage compression 1.08 for two-stage compression 1.10 for three-stage compression

Qg = flow rate, MMscfd

Once the required horsepower and number of stages are estimated, a choice of compressor type can be made from the considerations included earlier Some example selections are included in Table 10-1 The selec- tions listed in this table are meant as common types that would normally

be specified for the given conditions It must be emphasized that these are

not recommendations that should be accepted without consideration of the

advantages and disadvantages listed earlier In addition, local foundation conditions, type of drivers available, cost of fuel, availability of spare

Figure 10-11 Curve for estimating compression horsepower (Reprinted wirfi

permission from GPSA Engineering Data Book, Wth Ed.)

parts and personnel familiar with operating and maintenance, waste heatrequirements, etc., could influence the selection for a specific installation

Procedure for More Accurate Determination of Horsepower and Number of Stages

There are economic and operational reasons for considering an tional stage of compression The addition of a stage of compressionrequires an additional scrubber, additional cylinder or case, and morecomplex piping and controls In addition, there are some horsepowerlosses due to additional mechanical friction of the cylinder or rotatingelement and the increased pressure drop in the piping This horsepowerloss and additional equipment cost may be more than offset by theincreased efficiency of compression

addi-Comp res so rs 2 73

Table 10-1 Example Compressor Type Selections

100

2240.11.02.0

R

2.02.02.72,72.72.02.02.04.03.03.0

n 1

!33.3122122

Approx.

bhp

4,4004409803,92019,602881903809143286

Most Likely

CentrifugalHigh SpeedHigh SpeedCentrifugalCentrifugalScrewHigh SpeedHigh SpeedVaneScrewHigh Speed

Selection Alternate

Integral(onshore only)

Integral(onshore only)High SpeedScrew

ScrewVaneScrew

Figure 10-12 shows the pressure-volume curve for both single stage compression and two stage compression (neglecting interstage losses).

By adding the second stage and cooling the gas from A to D before beginning the compression cycle in the second stage, the area under the curve is reduced by an amount equal to A-B-C-D This represents the power saved by adding the second stage.

It is often even more important to add an additional stage in order to limit the discharge temperature of any one stage It is clear from Figure 10-12 that because of the cooling that occured in the interstage (A to D) the gas at C is cooler than it would have been at point B.

The discharge temperature for any single stage of compression can be calculated from:

where Td = stage discharge temperature, °R

Ts = stage suction temperature, °R

Pd = stage discharge pressure, psia

P = stage suction pressure, psia

Figure 10-12 Horsepower reduction by multistaging (neglects interstage losses).

k = ratio of gas specific heats, Cp/Cv

T| = polytropic efficiency

= 1.0 for reciprocating, 0.8 for centrifugal

It is desirable to limit discharge temperatures to below 250°F to 275°F to ensure adequate packing life for reciprocating compressors and to avoid lube oil degradation At temperatures above 300°F eventual lube oil degradation is likely, and if oxygen is present ignition is even possible Under no circumstances should the discharge temperature be allowed to exceed 350°F.

The discharge temperature can be lowered by cooling the suction gas and reducing the value of P</PS, that is, by adding more stages of com- pression.

The brake horsepower per stage can be determined from: