Gas Compressors.ppt

Gas Compressors

Compression Requirements

Compressor Stations Location Five types of compressor stations are generally utilized in the natural gas production industry: 1. Field gas-gathering stations to gather gas from wells in which pressure is insufficient to produce at a desired rate of flow into a transmission or distribution system. These stations generally handle suction pressures from below atmospheric pressure to 750 psig andvolumes from a few thousand to many million cubic feet per day. 2. Relay or main line stations to boost pressure in transmission lines. They compress generally large volumes of gas at a pressure range between 200 and 1,300 psig.

Compressor Stations Location 3. Repressuring or recycling stations to provide gas pressures as high as 6,000 psig for processing or secondary oil recovery projects. 4. Storage field stations to compress trunk line gas for injection into storage wells at pressures up to 4,000 psig. 5. Distribution plant stations to pump gas from holder supply to medium- or high-pressure distribution lines at about 20 to 100 psig, or pump into bottle storage up to 2,500 psig.

Compressors?  Compressors used to increase the pressure of a gas

(compressible fluid)  Examples  Compressors increase the pressure for instrument air systems

(to get control valves to operate),  transport gases such as hydrogen, nitrogen, fuel gas, etc. in a chemical plant

Types of Compressors

Types of Compressors (On the Basis of Application)  Positive Displacement (PD) : Operate by trapping a specific

volume of gas and forcing it into a smaller volume

 Two Basic Designs for PD Compressors  Rotary

 Reciprocating  Centrifugal : Operate by accelerating the gas and converting

the energy to pressure

 Two Basic Designs for Centrifugal Compressors  Centrifugal  Axial

Positive Displacement Compressors: Rotary Design  Rotary compressors (get their name from the rotating

motion of the transfer element) compress gases with lobes, screws, and vanes into smaller volumes.  4 Primary Types of Rotary Compressors:    

Rotary Screw SlidingVane Lobe Liquid Ring

 Commonly used in industry.

Rotary Screw Compressors  It operates with 2 helical rotors that rotate toward each

other, causing the teeth to mesh.  As the left rotor turns clockwise, the right rotor rotates counterclockwise. This forces the gases to become trapped in the central cavity.  The 2 rotors are attached to a drive shaft and drive that provide energy to operate the compressor.  Have an inlet suction line and outlet discharge port.

Rotary Screw Compressors

Sliding Vane Compressors  Uses a slightly off-center rotor with sliding vanes to

compress gas.  Inlet gas flows into the vanes when they are fully extended and form the largest pocket. As the vanes turn toward the discharge port, the gases are compressed.  As the volume decreases, the pressure increases until maximum compression is achieved. Then the gas is discharged out the compressor.

Sliding Vane Compressor

Lobe Compressors  Characterized by 2 kidney-bean shaped impellers used

to trap and transfer gases.  The 2 impellers move in opposite directions on parallel mounted shafts as the lobes sweep across the suction port.  Compressed gases are released into the discharge line.  The lobes do not touch each other. A few thousands of an inch clearing exists between the casing and lobes.

Lobe Compressors  Designed to have constant volume discharge pressures

and constant speed drivers.  Lobe Compressors can be used as compressors or vacuum pumps.

Lobe Compressor

Liquid Ring Compressors  It has one moving transfer element and a casing that is

filled with water or seal liquid.  As the rotor turns, the fluid is centrifugally forced to the outer wall of the elliptical casing. An air pocket is formed in the center of the casing.  As the liquid ring compressor rotates, a small % of the liquid escapes out the discharge port. Make up water or seal liquid is added to the compressor during operation. The liquid helps cool the compressed gases.

Liquid Ring Compressors

 Used to compress hazardous and toxic gases as

well as hot gases.

Positive Displacement: Reciprocating Compressors  Most common type of compressors.  Work by trapping and compressing specific volumes of gas

between a piston and a cylinder wall.  The back and forth motion incorporated by a reciprocating compressor pulls gas in on the suction (or intake) stroke and discharges it on the other.  Spring-loaded suction and discharge valves open/close automatically as the piston moves up and down in the cylinder chamber.

Positive Displacement Reciprocating Compressors  Basic Parts of are:        

Piston Connecting Rod Crankshaft Diver Piston Rings Suction Line Discharge Line Spring -Loaded Suction and DischargeValves

Positive Displacement:Reciprocating Compressors

 Can have 1 to 4 cylinders. One shown only has one cylinder.

Multistage Compressors

 Discharge from Stage 1 is suction for Stage 2

Centrifugal Compressors  Centrifugal compressors accelerates the velocity of the gases

(increases kinetic energy) which is then converted into pressure as the gas flow leaves the volute and enters the discharge pipe.  Usually operate at speeds > 3,000 rpm.  Deliver much higher flow rates than positive displacement compressors.

Centrifugal Compressors  Two Types of Centrifugal Compressors  Single- Stage : Compress the gas once  Used for high gas flow rates, low discharge pressures

 Multi- Stage : Take the discharge of one stage and pass

it to the suction of another stage

 Used for high gas flow rates, high discharge pressures

Centrifugal Compressors  Basic Components  Impellers, Vanes, Volutes, Suction Eyes, Discharge lines, Diffuser

Plates, Seals, Shaft, Casing  Suction Vane Tips = Part of the impeller vane that comes into contact with gas first.  Discharge Vane Tips = Part of the impeller vane that comes into contact with gas last

Axial Compressor

Centrifugal Compressor

Centrifugal Compressor: Axial Design  Composed of a rotor that has rows of fanlike blades.

 In industry, axial compressors are used alot high flows and

pressures are needed.  Gas flow is moves along the shaft.  Rotating blades attached to a shaft push gases over stationary blades called stators.  Stator blades are attached to the casing.

Centrifugal Compressor: Axial Design  As the gas velocity is increased by the rotating blades, the

stator blades slow it down. As the gas slows, kinetic energy is converted into pressure.  Gas velocity increases as it moves from stage to stage until it reaches the discharge.  Multi-Stage axial compressors can generate very high flow rates and discharge pressures.  Axial compressors are usually limited to 16 stages (due to temperature/material limitations)  Pound for pound, axial compressors are lighter, more efficient, and smaller than centrifugal compressors.

Ejectors  Ejectors are “thermal” compressors that use a high velocity

gas or steam jet to entrain the inflowing gas, then convert the velocity of the mixture to pressure in a diffuser.  They use a jet of high-pressure gas as an operating medium to entrain a low-pressure gas, mix the two, and discharge at an intermediate pressure. Gases can be steam, air, propane, and others.

Ejectors

Compressor Selection

The Advantages of a Centrifugal Compressor Over a Reciprocating Machine  Lower installed first cost where pressure and volume

    

conditions are favorable, Lower maintenance expense, Greater continuity of service and dependability, Less operating attention, Greater volume capacity per unit of plot area, Adaptability to high-speed low-maintenance-cost drivers.

The Advantages of a Reciprocating Compressor Over a Centrifugal Machine  Greater flexibility in capacity and pressure range,  Higher compressor efficiency and lower power cost,

 Capability of delivering higher pressures,  Capability of handling smaller volumes,  Less sensitive to changes in gas composition and density.

Reciprocating Compressors: Sizing

Reciprocating Compressors Rating and Pressures Ratings Reciprocating compressor ratings vary from fractional to more than 40,000 hp per unit. Pressures Pressures range from low vacuum at suction to 30,000 psi and higher at discharge for special process compressors.

Reciprocating Compressors Number of Stages  Reciprocating compressors are furnished either single-stage or

multi-stage.  The number of stages is determined by the overall compression ratio.  The compression ratio per stage (and valve life) is generally limited by the discharge temperature and usually does not exceed 4, although small-sized units (intermittent duty) are furnished with a compression ratio as high as 8. Multi Stage machines have intercoolers to reduce the temperature developed because of gas compression

Reciprocating Compressors Codes and Standards Reciprocating compressors are typically designed to one of the following industry standard specifications:  API Standard 618 "Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services."  API Specification 11P "Specification for Packaged Reciprocating Compressors for Oil & Gas Production Services."

Comparison of API 11P and 618 Compressors

Thermodynamics of gas compression The actual compression process is often compared to one of two ideal processes:  isothermal compression (when the temperature is kept constant during the compression process)  isentropic compression (no heat is added to or removed from the gas during compression and the process is frictionless. With these assumptions, the entropy of the gas does not change during the compression process.)  polytropic compression (described as an infinite number of isentropic steps; each interrupted by isobaric heat transfer. This heat addition allows the process to yield the same discharge temperature as the actual compression process.)

Thermodynamics of gas compression

Isentropic Model  For an isentropic compression, the discharge temperature is

determined by the pressure ratio as

where k (isentropic exponent) is ratio of the heat capacities of gas at constant pressure and temperature (k = Cp/Cv), P1 is suction pressure, and P2 is discharge pressure.

Isentropic Model  The performance of a compressor can be assessed by

comparing the actual head (which directly relates to the amount of required compression power) with the calculated head for an ideal, isentropic compression. This defines the isentropic efficiency (ns) as follows:  For ideal gases, the actual head can be calculated from

 and further, the actual discharge temperature (T2) becomes

Polytropic model  Since polytropic compression is similar to adiabatic

compression, we can easily calculate the discharge gas temperature in polytropic compression by substituting polytropic exponent (n) for “k” :  The polytropic efficiency (hp) is constant for any

infinitesimally small compression step, which then allows us to write where dTs is elemental temperature rise for isentropic compression, and dT is elemental temperature rise for actual compression.

Polytropic model  Relation between n and k:  The polytropic head (∆hp) can be calculated from

In the above equation, Zave is the average compressibility factor for the gas.  Then the actual head of compression is  the isentropic and polytropic efficiencies are related by

Hydraulic Horse Power As far as power calculations are concerned, either the approach using a polytropic head and efficiency, or using isentropic head and efficiency will lead to the same result:

Brake Horse Power Per Stage  The detailed calculation of BHP depends upon the choice of

type of compressor and number of stages. The BHP per stage can be determined from:

where BHP is brake horsepower per stage; Zave is average compressibility factor; QG,SC is standard volumetric flow rate of gas, MMSCFD; T1 is suction temperature, oR; p1 and p2 are pressure at suction and discharge flanges, respectively, psia; E is parasitic efficiency (for high-speed reciprocating units use 0.72–0.82; for low-speed reciprocating units use 0.72–0.85; and for centrifugal units use 0.99); and n is compression efficiency (1.0 for reciprocating and 0.80–0.87 for centrifugal units).

Typical Compressor System

Other equipment needed in a process system.

Typical Compressor System  Safety valves and pressure relief valves used to remove

excess pressure that could damage equipment and people.  Silencers are mounted on the inlet and outlet of a compressor to ‘reduce’ the noise. Compressors are very noisy. Exxon had one for a refinery light ends stream nicknamed “Old Snort” by the technicians.

Typical Compressor System  Demister removes moisture (liquid) from the gas stream.

The liquid falls to the bottom of the demister and is removed.The clean gases goes out the top of the demister.  Dryer sometimes used on the compressor discharge line to remove any liquids (moisture). Silica gel and molecular sieves (3A mole sieve) often used.

Typical Compressor Start Up Procedures  Check valve line up on the compressor and associated

equipment.  Check compressor oil levels and bearing cooling water systems.  Be sure all the compressor controls are set correctly.  Turn on the compressor.  Monitor equipment and process until conditions ‘steady’ out.

Calculations from Natural Gas Engineering Handbook Chapter 9 Example 9.1 and 9.2

Selection of Reciprocating Compressors Volumetric Efficiency Stage Compression

Calculate the Outputs for Centrifugal Compressor using the Inputs Given in the Table Below Input Mw k,cp/cv Zsuction Psuction, psia

3rd Stage Original Design

3rd Stage 3rd Stage 3rd Stage 3rd Stage Original Original Original Original Design Design Design Design 59.01 27.72 44.00 27.72 44.00 1.08 1.40 1.40 1.40 1.40 0.97 1.00 1.00 1.00 1.00

28.00 1.25 1.00

28.00 1.25 1.00

19.50

14.70

14.70

14.70

14.70

225.00

225.00

138.00

140.00

100.00

140.00

100.00

100.00

100.00

66.40

115.00

115.00

65.00

65.00

650.00

651.00

Zdischarge

0.94

1.00

1.00

1.00

1.00

1.00

1.00

Isoenthalpic,eff

0.75

0.75

0.75

0.75

0.75

0.75

0.75

Flow Rate,MMSCFD Output

240.00

32.86

3.11

32.86

3.11

500.00

500.00

Density, suction lb/ft³ Ts,°R

0.18 597.70

0.06 599.70

0.11 559.70

0.06 599.70

0.11 559.70

1.05 559.70

1.05 559.70

Tdisch, Pred °F

215.55

779.61

696.95

563.12

494.90

276.39

276.67

25,915.68 1,554,941 3.41

1,666.61 99,997 7.82

250.40 15,024 7.82

1,666.61 99,997 4.42

250.40 15,024 4.42

25,618.50 1,537,110 2.89

25,618.50 1,537,110 2.89

140,539.43

26,318.24

2,325.00

26,318.24

2,325.00

24,421.27

24,421.27

MMSCFD Head

240.00 19,198.76

32.86 93,579.40

3.11 55,022.72

32.86 61,906.07

3.11 36,399.47

500.00 36,498.59

500.00 36,557.30

Gas Horsepower

20,102.98

6,301.42

556.68

4,168.61

368.26

37,779.37

37,840.14

Tsuction,°F Pdischarge, psia

W,Lb/min W, Lb/Hr r (Pd/Ps) ACFM

Calculate the Outputs for Reciprocating Compressor using the Inputs Given in the Table Below 3rd Stage Original Input Mw Displace (ft³/min) Clearance vol/Piston Vol Valve Losses % k,cp/cv Zsuction Psuction Tsuction,°F Pdischarge, psia Tdisch,°F (estimated) Zdischarge Iso,eff Area Piston , in² Area Rod, in²

100 WNGL

Design

Relief Case

required

59.01 804.50 19.80 4.00 1.082 0.973 19.50 138.00 66.40 200.00 0.938 0.750 153.94 3.14

42.74 804.50 19.80 4.00 1.123 0.978 30.00 143.00 75.00 200.00 0.960 0.750 153.94 3.14

42.74 804.50 19.80 4.00 1.123 0.978 25.80 143.00 75.00 200.00 0.960 0.750 153.94 3.14

0.184 597.70 72.19 4331.18 3.405 48.66 391.47 0.67 19198.76 56.00 215.55 7281.05 7011.18

0.203 602.70 110.32 6619.15 2.500 67.66 544.28 1.41 20345.25 90.69 227.58 7021.55 6691.68

0.174 602.70 85.10 5105.97 2.907 60.68 488.21 1.09 23893.61 82.15 242.33 7654.90 7338.23

Output Density, suction lb/ft³ Ts,°R W,Lb/min W, Lb/Hr r (Pd/Ps) VE ACFM MMSCFD Head Gas Horsepower Tdisch, Pred °F Load in Compression Load in Tension

Reference  “ The Process Technology Handbook”, by Charles E.

Thomas, UHAI Publishing, Berne, NY, 1997.  Natural Gas Engineering Handbook