REFRIGERATION SCREW

COMPRESSOR PACKAGE

“WHAT FEATURES DO I NEED

FOR MY GAS PROCESSING

FACILITY?”

April 2019

Introduction

• Most midstream gas dew point or liquid recovery facilities require

mechanical refrigeration equipment.

• Compression is a key component of any mechanical refrigeration

plant.

• Since the 1980’s, oil flooded screw compressors have been the

compressor of choice in refrigeration systems.

• Despite their wide use, screw compressor drivetrain specifications

are not clear or uniform throughout the industry.

Presentation Outline

• Oil flooded screw compressors;

• Types of compressor drivetrains;

• Compressor lubrication system;

• Other parts of the drivetrain;

• Package specifications; and

• Case study.

Oil Flooded Screw

Compressors

Oil Flooded Screw CompressorsGeneral overview

• Positive displacement machines;

• Off-the-shelf design with standardized rotor diameters and typically 3-4

various rotor lengths for each rotor diameter;

• Approximately 10,000 ACFM (maximum) at suction per machine;

• Driven by 2 or 4 motor drives or gas engines, up to 10,000 BHP;

• Lube oil is used for bearing and seal lubrications, sealing of meshing

rotors sealing, and cooling of the compressed gas;

• Slide valve capacity control is set to 15-20% of its maximum capacity;

• Oil carryover is less than 10 or 1 parts per million on weight basis.

*Illustration courtesy of Howden

Oil Flooded Screw CompressorCompressor internals

*Illustration courtesy of Howden

Oil Flooded Screw CompressorSectional view of standard design

*Illustration courtesy of Howden

Oil Flooded Screw CompressorSectional view of standard design

*Illustration courtesy of Howden

Oil Flooded Screw CompressorCompressor discharge ports

*Illustration courtesy of Howden

Oil Flooded Screw CompressorCompressor standard design features

Standard features include:

• Cast or ductile iron compressor casing;

• Machined steel or ductile iron rotors;

• Forged steel design is preferred with larger diameters.

• Sleeve or anti-friction radial bearings;

• Angular contact thrust bearings;

• Single mechanical shaft seal;

• Slide valve capacity control;

• Vi radial discharge port size adjustment (axial port size fixed); and

• Single side port for economizing of the refrigeration cycle.

Oil Flooded Screw CompressorCompressor options

Common options include:

• Steel compressor casing;

• Forged steel small diameter rotors;

• Tilting pad thrust bearings;

• Dual wet/dry mechanical shaft seal with seal support system; and

• Male/female rotors proximity position probes.

Other options (not common):

• Radial bearing RTD probes;

• Dual or tandem wet/wet mechanical shaft seal;

• Dry gas shaft seal; and

• Flanged compressor casing connections.

Oil Flooded Screw CompressorCompressor – sectional view with dual shaft seal

and tilting pad thrust bearings

*Illustration courtesy of Howden

Oil Flooded Screw CompressorJournal and Thrust bearings, shaft seal details

*Illustrations courtesy of Howden

Oil Flooded Screw CompressorDual/Wet shaft seal support system

• API 75 plan

collects oil from

the seal in the

accumulator.

• Optional API 72

plan provides a

buffer gas to

cavity between

the seals.

Compressor Drivetrains

Compressor DrivetrainsDriver types

• Screw compressors can be driven by an electric motor or a gas

engine.

• Majority of the electric motor drivetrains are direct coupled and

operate at two-pole motor speed (3550 or 2950 rpm for 60 or 50

Hertz power, respectively).

• Typical midstream refrigeration compressor power is between 1500

and 4000 HP per train.

• Engine operating speeds range between 1400 and 1000 rpm for the

above horsepower range and engine drives typically require a

gearbox to speed the compressor above the engine speed.

• Gearboxes can be stand alone, bolted to the engine, or built into the

compressor. The two latter options are typical for horsepower below

2000.

Compressor DrivetrainsElectric motor drivetrain

Lube oil separator

Electric motor Compressor

Compressor discharge piping

Package

discharge piping

Compressor/package

suction piping

Compressor

sideport piping

Compressor DrivetrainsEngine drive with a compressor internal gearbox

Compressor with a gearbox

Gas engine

Utility water cooler

Engine exhaust

and silencer

Lube oil separator

Compressor/package

suction piping

Package

discharge piping

Compressor sideport piping

Compressor DrivetrainsEngine drive with external gearbox

Gas engine

Compressor

External gearbox

Engine exhaust

Engine air intake

Lube oil separator

Lube oil cooler

Compressor/package

suction piping

Package

discharge piping

Compressor Lubrication

Compressor LubricationTypical components

• Lube oil separator

• Lube oil filter(s)

• Instrumentation and controls

• Lube oil pump(s) (if required)

• Lube oil cooler(s) (if required)

• Pipes, valves, etc.

Compressor LubricationOil Cooling methods

Basic lube oil cooling methods include:

• Separate dedicated aerial lube oil cooler:

• Oil cooled directly;

• Oil cooled by glycol/water solution (glycol cooled in the air cooler);

• Pros – proven and reliable; and

• Cons – additional cost, additional motor and a fan, cooler mounted externally.

• Thermosyphon lube oil cooler:

• Oil cooled by condensed refrigerant;

• Pros – proven, reliable, utilizing excess condenser surface for oil cooling duty; and

• Cons – careful refrigerant piping required, condenser elevation requires careful

evaluation.

Compressor LubricationOil Cooling methods

Basic lube oil cooling methods (cont’d):

• Liquid Injection oil cooling:

• Liquid refrigerant injected into the compressor chamber keeping the discharge

gas/oil mix temperature below maximum allowable.

• Pros – lube oil cooler not required.

• Cons:

• Liquid refrigerant injected into compression chamber via a high quality

control valve;

• Compressor capacity decreases and power draw increases due to additional

volume of gas compressed by the machine for the same refrigeration duty;

• Expected capacity decrease is 3-5%;

• Expected power increase is 3-5%;

• May not be an issue if compressor frame has excess capacity.

• Sudden changes in compressor condensing/discharge pressure could cause

foaming of the oil due to increased refrigerant dilution in lube oil.

Compressor LubricationNote on oil carryover and recovery

• Typical oil carryover for

a compressor package

is less than 10 ppmw.

• This typically is not an

issue in the refrigeration

systems because oil

can be recovered from

the system evaporators

and returned to the

compressor.

• If liquid refrigerant is fed to brazed aluminum exchangers, a secondary

high efficiency coalescing oil separator reducing the oil carryover to less

than 1 ppmw would be recommended.

Other parts of the drivetrain

Other parts of the drivetrain

Other parts of the system consist

of:

• Structural steel baseplates;

• Piping and valves; and

• Control systems.

Items to consider also include:

• The role of economizer port;

• Volume ratio (discharge port)

adjustment; and

• A need of suction scrubber.

For details refer to the 2019

Convention paper submitted to GPA

Midstream

Package Specifications

Package Specifications

• Reciprocating compressors can be specified as manufacturer

standard, in accordance to API 11P - ISO 13631 (Specification for

Packaged Reciprocating Compressors for Oil and Gas Production

Services) or to API 618 (Reciprocating Compressors for Petroleum,

Chemical, and Gas Industry Services).

• Midstream industry very rarely requires reciprocating compressors

built to API 618.

• Screw compressors can be supplied to manufacturer standard or

API 619 (Rotary-Type Positive-Displacement Compressors for

Petroleum, Petrochemical, and Natural Gas Industries).

• There is not a specification that would be equivalent to API 11P for

the screw compressor packages.

Package SpecificationsAPI versus non-API - compressor

Design features API 619 style unit Midstream standard unit

Compressor design

Casing material Cast Steel for flammable gases Cast Iron for small compressors

Nod Iron for large compressors

Rotors Forged steel Ductile iron, Steel bar or forged steel

based on suppliers’ manufacturing

process

Mechanical seal Double mechanical type with seal

support systems

Single mechanical

Lube oil connections for

compressor casing

NPT or optional flanged with stub pipe NPT

Condition monitoring X-Y casing vibration transmitters,

male/female rotor axial position

proximity probes, optional radial

bearing RTD’s

Single casing vibration transmitters

Compressor testing Witnessed or Unwitnessed

API 619 4 hr performance test

Casing hydraulic test

Rotor balance test

Air under water leak test

Additional optional tests

Helium under water leak test

Casing RT

Casing MPI

UT of casting and rotor stock

Noise test

Vibration spectrum

Unwitnessed

Vendor standard mechanical run test

Casing hydraulic test

Rotor balance test

Air under water leak pressure test

Studies Torsional and lateral study

Gas correlation or performance test

None (except for special drive train

configuration)

Package SpecificationsAPI versus non-API - package

Design features API 619 style unit Midstream standard unit

Package design

Baseplate Multi-runner heavy duty structural skid

with deckplate and drip rim

Multi-runner structural steel skid with

optional deckplate and drip rim.

Process Piping B31.3 piping with minimum 5% RT, 1/8”

corrosion allowance, API style isolation

valves

B31.3 piping 5% RT, 1/16” corrosion

allowance, Refrigeration or API style

isolation valves.

Oil separator design 2-minute lube oil retention time

minimum

Packager standard lube oil retention

time, typically in a range between

30 seconds and 1 minute.

Lube oil pumps Dual (2x100%)

Steel casing (option)

None or Single

Cast iron casing

Lube oil coolers Dual (2x100%) Single or Liquid Injection

Lube oil filters Dual Single or dual

Lube oil piping Carbon steel piping upstream of the

filters, stainless steel piping

downstream of the filters. Optional

stainless steel piping upstream of the

filters. Stainless or carbon steel valves.

Carbon steel piping upstream of the

filters, carbon steel or optional stainless

steel piping downstream of filters with

carbon steel valves.

Coupling API 671 Non API, flexible disc

Relief Valves API 520/521, optional dual API 520/521, single

Valves API valves API or refrigeration valves

Control Instruments and

wiring

Separate Instruments for control and

shutdown (SIS), both having SIL 2/SIL 3

rating, SMART, Class 1, Div.2, Group C

&D (or IEC Zone 2, Group IIB)

Single instruments for control and

shutdown, SMART Class 1, Div.2, Group C

&D (or IEC Zone 2, Group IIB) for most

cases, optional blind non-rated

Control Panel PLC or DCS control panel. Separate

panel for shutdown (SIS) functionality

Single PLC control panel. Optional

microprocessor panels with blind

instruments.

Case Study

Case StudyRefrigeration Compression for a typical cryo plant

• A typical 200-300

mmscfd cryo plant

utilizes multiple

compressors driven by

2000-3500 HP motors,

depending on the plant

design and inlet gas

composition.

• We decided to look at a

3500 HP compressor

drivetrain, and compare the typical design

versus a unit following API 619 guidelines.

Case StudyTypical midstream compressor package

Basic design consists of: • Non-API motor;

• Cast iron compressor case;

• Single compressor mechanical shaft

seal;

• Single lube oil pump;

• Dual lube oil filters;

• ASME vessels and piping;

• Air cooled lube oil cooler;

• 1/16” corrosion allowance;

• 5% RT;

• API gate and globe isolation;

• all carbon steel;

• PLC control system with SMART

instrumentation;

• common devices for shutdown and

control; and

• rotor axial position probes.

Case StudyCompressor package following API 619 guidelines

API design consists of: • API motor;

• Cast steel compressor case;

• Dual compressor mechanical shaft seal

with API 72/75 seal plan;

• Dual API lube oil pumps and dual lube oil

filters;

• ASME vessels and piping;

• API air cooled lube oil cooler;

• 1/8” corrosion allowance;

• 100% RT;

• Ball isolation valves;

• All carbon steel;

• PLC control system with SMART

instrumentation including separate

instruments for shutdown and control; and

• Extensive Machine Monitoring

instruments.

Case StudyHow do units compare?

• No rating difference;

• Both packages have the

same refrigeration capacity,

efficiency, and motor power.

• Typical package would be about

40% less expensive.

• Typical package delivery would be

about 3 months shorter.

• Removing external lube oil cooler

would reduce both package costs

by about 10%, decrease unit

capacity by 3% and increase

power draw by 4%.

Case StudyConclusion

• Careful features considerations

are recommended.

• Some API 619 features may be

worthwhile depending on the plant

operating philosophy, number of

units employed, etc.

• Simply requiring “API 619

compliance” should be properly

evaluated.

• There is not a right or wrong

design.

Thank you

Contact:

pdanilewicz@enerflex.com

281-345-5070