COMPRESSOR HANDBOOK phần 10 pps

Fire detection system shutdownFuel level lowLube oil pressure lowLubrication flow lowLubrication oil consumptionLubricator flow lowMetal in oil particle detectionMotor bearing temperatur

Fire detection system shutdownFuel level (low)

Lube oil pressure (low)Lubrication flow (low)Lubrication oil consumptionLubricator flow (low)Metal (in oil) particle detectionMotor bearing temperatureMotor bearing vibrationMotor overload

Motor power failureMotor purge fan failureMotor vibration

Motor winding failureMotor winding temperatureOil cooler temperature in and out (high)Oil filter Differential (high)

Oil level–compressor (low)Oil level—driver (low)Oil level—lubricator (low)Scrubber level (high)Scrubber level (low)Suction, discharge, interstage pressure (high)Suction, discharge, interstage pressure (low)Suction, discharge, interstage temperatureVibration—compressor (high)

Vibration—cooler (high)Vibration—driver (high)Water cooler temperature in and out (high)Water pump differential pressure (low)

The compressor industry has created a demand for special control systems andsensors for problems unique to compressors Among these are:

COMPRESSOR CONTROL SYSTEMS 21.19

FIGURE 21.10 matic compressor packing case purge control panel to control packing leakage.

Pneu-(Courtesy of Autocator Products Division, T F.

Hudgins, Incorporated ).

Packing case purge controlRod drop measurement and alarmsVibration monitoring

Vibration sensorsMounting vibration sensorsMetal particle detection in lubrication oilLubrication flow sensors

Sensors for pulsating compression pressuresEutectic temperature sensors

Energy managementCylinder pressure measurement

21.15.1 Packing Case Purge Controls

Control systems used to maintain purge gas on packing cases designed to preventleakage of process gas along the rod to atmosphere have proven effective Mea-surement of vent gases is used as a diagnostic tool to determine the rod packingwear and replacement schedule

FIGURE 21.11 Flow meter connection for measuring purge and packing leakage flow.

21.15.3 Compressor Rod Drop

Measuring rod drop is an effective tool in determining when rider rings should bereplaced and can minimize cylinder wear Rod drop occurs in a horizontal com-pressor cylinder as piston or rider rings wear, allowing the piston to descend Theintent of a compressor rod drop measuring system is to alarm this event prior tothe piston contacting the cylinder

Three measurement systems are in use today These are:

1 Pneumatic eutectic sensor is a bracket mounted sensor located beneath the rod.

As the rod rubs on the sensor, the friction causes the eutectic solder to melt,allowing the pneumatic control system to vent and alarm or shut down themachine

2 Proximity rod drop measurement without the crank angle monitoring system is

an electronic equivalent of the pneumatic system alarming when the rod reaches

a set point The operator can also observe a read-out and trends from the systemdisplay

3 Proximity rod drop sensor in conjunction with a crank angle detection system

which measures rod position at exactly the same point within the rod stroke.The proximity probe is an eddy current sensor The output varies in response

to rod position Measuring a specific crank angle allows the operator to beassured of the piston position at the time of measurement

COMPRESSOR CONTROL SYSTEMS 21.21

FIGURE 21.12 Metal particle detector.

op-21.15.5 Vibration Sensors

1 Inertia sensors are available as pneumatic valves and electric switches These

are intended to respond to vibration on low speed equipment and in ranges from

0 to 3000 cpm to 0 to 12000 cpm, at vibration sensing ranges 0 to 5g’s and 0

to 10 g’s These devices are sensitive to vibration parallel to the axis of thesensor mechanism

2 Eddy current sensors measure machinery motion and are frequently used to

monitor moving shafts because direct contact is not necessary Position, dynamicmotion, and wear are now measurable using these sensors

3 Accelerometer based sensors sense impact events and are available with and

without conditioning Conditioning, either in the sensor or in the control system,converts the accelerometer signal to velocity or displacement signals propor-tional to vibration The conditioned accelerometers convert the low poweredsignal from the sensor into a 4 to 20mA or 1 to 5 volt DC signal, and can beconnected to electronic monitors that are not especially designed for monitoringvibration Unconditioned sensors have low output signal and are intended to beconnected to an electronic monitor that includes the conditioning and vibrationmonitoring system

4 There is a wide selection of read-out devices and systems that provide

indica-tions and alarms, computerized systems data storage, and trending of the lowing data This is a very specialized technology and there are manufacturers

fol-FIGURE 21.13 Metal particle detection diagram.

COMPRESSOR CONTROL SYSTEMS 21.23

providing this service worldwide Typical displays, either in velocity or ment, would be:

displace-Running speed vibrationCrank case deflectionCompressor crosshead vibrationCompressor cylinder head vibration

5 Compressor valve covers can be monitored temporarily by accelerometers as

part of a portable compressor analysis diagnostic system to determine valvecondition and operation

Selection of vibration sensors requires careful analysis of machine speed, sensorlocation, and vibration amplitude based on experience or manufacturer’s data Vi-bration on low speed machines (120 to 1,500 rpm) is commonly measured in mils

or mm displacement High speed machine (360 to 90,000 rpm) vibration is sured in inches per second or mm / s velocity

mea-21.15.6 Mounting Vibration Sensor

‘‘Bearings take the load during vibration’’ and are the desired vibration monitoringpoints The restraint provided by the base can limit vertical movement, therefore,the sensor must be mounted where the most movement is predicted

21.15.7 Metal Particle Detectors

Metal particles in oil are an indication of:

A Contamination

B Machinery wear

Metal particles from bearings, pump wear, cylinder wear, and other failures havebeen detected by these sensors prior to the build up of temperature or vibration.These systems provide a constant monitoring for metal particles in the lubricatingfluid

The application requires a small oil side stream from the dirty side of the filter

to flow through a perforated printed circuit card within a suitable housing As theparticles complete the circuit, the alarm circuit is energized These detectors re-spond to all conductive particles

Sensors for ferrous only particles are also available

21.15.8 Lubrication Low Flow Sensors

Compressor lubrication system failure can be detected by monitoring switches orvalves which remain in a ‘‘healthy’’ condition with flow As lubrication system flowdiminishes, or stops, the sensor trips

21.15.9 Pressure Sensors

The need to measure engine firing pressures and compression pressures has causedseveral very rugged sensor types to be developed to withstand the pulsation andhigh temperatures The rugged construction is necessary because these sensors aremounted on the cylinder for measuring internal pulsating cylinder pressures.The sensors have piezo-electric and strain gage elements and are proving valu-able for controlling operating conditions by computer and evaluating the machineperformance These are available with and without the requirement for water cool-ing

Temperature sensors can be protected by suitable thermowells

Static pressure sensors can be protected by suitable gage isolators where essary These are sealed diaphragm devices especially designed for this purpose

nec-21.16 TEMPERATURE CONTROL (OIL AND WATER)

Controlling oil and water temperatures is essential in order to maintain properrunning clearances In addition, proper temperatures minimize wear, maintain clear-ances and proper lubricant viscosity, and prevent condensation in the crankcase.Temperatures are controlled by either self-contained thermostatic valves or tem-perature control valves with controllers

21.16.1 Self Contained Thermostatic Valves

Special self contained three way valves using internal copper impregnated ing wax elements were developed for the compressor and engine industries Thesevalves are located in the oil and water piping

expand-During a cold start-up, the fluid is recirculated back to the heat source (bypassposition) As the machine warms up, and approaches the element set point, within

COMPRESSOR CONTROL SYSTEMS 21.25

5⬚F to 7⬚F, the element begins to close the bypass port and send fluid to the cooler.The valve will modulate flow to bypass and cooler in order to maintain the settemperature maintaining a constant flow volume through the machine

Sizes are selected for a pressure drop of 2 to 7 psi in order to be certain ofproper velocities and to maintain heat transfer to the element for proper response.These valves are available with a variety of body materials, trim, and seals.Material compatibility and codes must be considered As an example, API rec-ommends that piping systems containing hydrocarbons be of steel construction.Certain lubricants are not compatible with standard o-rings and the thermostaticvalves must be specified accordingly Commercially available sizes range from

1 / 2 inch to 8 inch flanges Available materials are cast iron, steel, ductile iron,bronze, and stainless steel

21.16.2 Cooling Water and Lube Oil

FIGURE 21.14 Cooling water and lube oil systems.

Three way temperature control valves have been designed for use with pneumaticcylinder and electric motor operators The need to provide very low pressure dif-ferential, yet maintain control, were considerations External actuators allow theuse of remote sensing elements and controller to suit the application.

Pneumatic and electronic industrial controllers can be used with these valvesand operators to regulate temperatures Another option is to use the central plantprocess control system The pneumatic controller allows the valve to be placed in

a hazardous area such as a gas pipeline compressor station

Proportional controls which vary the output according to the input, proportional,integral, derivative (PID) controls provide a more precise control regulation byeliminating the offset between the desired and actual temperatures The derivativeaction attempts to anticipate temperature changes based on the cyclic rates ofchange experienced by the system I to P converters are used to interface an elec-tronic controller to a pneumatic operator Commercially available in sizes rangefrom 2 inch to 16 inch flanges Industrial control valves can be used in this service,but do operate at higher differential pressures

Energy management systems provide dramatic cost savings where multiple chines are used in industries such as foundries, automotive or process plants, aswell as pipeline and process industries

ma-Careful analysis and control of machine run time, along with capacity control,will optimize power consumption An added benefit is reduced machine wear andmaintenance

21.19 SPECIFICATIONS, CODES, AND STANDARDS

Refer to the following publication for suggested specifications, codes, and dards

stan-Instrument Society of America—ISAS5.1 Instrumentation Symbols and IdentificationS5.4 Instrument Loop Diagrams

S5.5 Graphic Symbols for Process DisplaysRP12.6 Installation of Intrinsically Safe Systems for Hazardous (Classified) Lo-cations

COMPRESSOR CONTROL SYSTEMS 21.27

Standards and Practices for Instrumentation

National Electrical Manufacturers Association—NEMA

ICS 3–1978 Industrial Systems

ICS 6–1978 Enclosures for Industrial Controls and Systems

American Petroleum Institute—API

API 670–Vibration, Axial-Position, and Bearing-Temperature Monitoring tems

Sys-API 618–Recommended Practice for Compressor Emissions MonitoringAPI RP–550 Manual on Installation of Refinery Instruments and ControlsAPI RP–520 Design and Installation of Refinery Instruments and ControlsAPI RP–521 Guide for Pressure Relief and Depressuring Systems

API RP–14F Recommended Practice for Design and Installation of ElectricalSystems for Offshore Production Platforms

Underwriter’s Laboratories, Inc.—UL

Standards for Safety

Factory Mutual System—FM

Approved Standards and Data Sheets

National Fire Protection Association—NFPA

National Fire Codes Volumes 1 through 16

Robert L Rowan, Jr.

Robert L Rowan & Associates, Inc.

The key to rotating and reciprocating machinery reliability is the foundation One

of the main functions of foundations is to support the machines at a precise vation, thus allowing the original precision alignment to be maintained over thelife of the machine

ele-Besides the critical task of maintaining the alignment of the machine, the dation must supply enough mass to absorb the unbalanced forces that the operatingmachine produces Good engineering input from the manufacturer of the machine

foun-is essential to the designer of the foundation, but equally as important foun-is a technical analysis of the soil on which the foundation will rest

geo-22.1.1 Types of Foundations

Reciprocating and centrifugal compressors can be packaged or unitized on a ricated skid (Fig 22.1), block mounted (Fig 22.2), or set on a pile cap foundation(Fig 22.3) Large centrifugal machines are also sometimes set on ‘‘table top’’foundations, which are shaped much like a kitchen table with multiple legs (Fig.22.4) This style is popular for larger machines and allows the space underneath

fab-to be used for long radius, large diameter piping and auxiliary equipment

The above types represent practices in the United States No review, though,would be complete without mentioning a new option that is starting to be seen inthe United States because of successful installations in Europe This option in thetype of compressor foundations, is the use of spring supports The advantages ofthis option include good isolation of the dynamic forces, good definition of supportproperties and additional possibilities for future modifications or corrections Springsupport systems for compressor applications will typically have vertical naturalfrequencies in the range of 3 to 5 Hz Horizontal frequencies usually are slightlyless than the vertical frequency As these values are less than comparable frequen-

FIGURE 22.3 Pile cap foundation (Illustration courtesy of Robt L Rowan & Assoc., Inc.)

22.4 CHAPTER TWENTY-TWO

FIGURE 22.4 Table top foundation (Illustration courtesy of Robt L Rowan & Assoc., Inc.)

cies for soil or pile supported systems, the spring system typically provides betterisolation of the dynamic forces of the compressor The springs themselves, usuallysteel coil designs, provide well defined stiffnesses both horizontally and vertically.This advantage simplifies the dynamic analysis of the foundation eliminating theneed to incorporate a range of soil properties in this analysis By including viscousdampers in the design, the complete dynamic system can be put together with greatconfidence Finally, the discrete nature of the spring support system permits easyreplacement of the elements if a change to the stiffness or damping characteristicsbecomes necessary Similarly, misalignment from settlement and similar sourcescan be corrected at the spring support level

22.1.2 Design

The detailed design of any of the above foundations is beyond the scope of thischapter Unfortunately, there are no established building codes at this time (1996),but under the auspices of the American Concrete Institute, a committee is working

to develop a report that could eventually become a foundation design guide ument Major engineering firms, operating companies, and equipment manufactur-

doc-much needed data on both dynamic and thermal stresses in foundations is beingdeveloped With such input data, along with the work of ACI, the design of foun-dations in the future can be more precise.

22.1.3 Soil Frequency and Vibration

While there have been many technical articles written on theories of foundationdesign and vibration, a very comprehensive reference is the work of Prakash and

Puri, Foundations for Machines: Analysis and Design.1 With a good background

in geotechnical engineering, the authors tie together very well the interaction tween the cyclic vibrations caused by the machine with the natural frequency ofthe soil-foundation system Foundations must be designed to avoid the dreadfulconsequences of harmonic resonance, which occurs when the frequency of thevibrating machine matches the natural frequency of the foundation (block and soil).Prakash and Puri teach that by applying the principles of soil engineering and soildynamics with theories of vibration, low tuned or high tuned foundations can bedesigned so as to avoid resonance Their work leads the way to designing foun-dations for dynamic machines which will have acceptable levels of vibration Goodengineering at this stage will pay off with a smoother running machine, bettermaintenance of alignment, and lower maintenance costs for the replacement of wearparts (bearing, seals, etc.)

be-22.1.4 Collection of Data for the Design Step*

While readers of this handbook may not ever be called on to design a foundation,they may very well be asked to supply data to an engineering design firm workingunder its direction

While the work of ACI committee is incomplete at this stage, probable mended data collection steps will be as follows:

recom-1 Data gathering

a) Design goal b) Site factors c) Sub-soil data d) Machine data

2 Design criteria

a) Static loads

*Based on preliminary draft of ACI 351-2 Sub-Committee

22.6 CHAPTER TWENTY-TWO

b) Dynamic loads

3 Concrete strength / stresses

a) Compressive b) Flexural c) Tension d) Bearing e) Fatigue

4 Concrete deflection / deformation

5 Soil strength / stresses

6 Soil deformation / settlement

7 Vibration limits

8 Psychological factors

22.1.5 Materials of Construction

Portland Cement Concrete: Reinforced portland cement concrete is the usual

material of construction for either the foundation proper, or for the mat under afabricated steel skid A mix design, based on locally available ingredients, can bedeveloped that yields a compressive strength of 4,000 psi in 28 days The amount

of steel will depend on the tensile and bending loads, as well as thermal stresses.Many foundations designed over the past 30 years have been under-reinforced, asevidenced by cracking Cracking can lead to deterioration of the alignment con-dition and even catastrophic failure Extra steel, to increase flexural and tensilestrength is very prudent Steel, put in initially, does not cost very much, but afoundation repair later, because of an under-reinforced foundation, is very costly.Figures 22.5 and 22.6 show a modern design with extra rebar vs a design donetwenty years ago

Polymer Modified Concrete: While reinforced portland cement concrete is

al-most universally used today, many older foundations have been repaired using amore technologically advanced material called ‘‘Polymer Modified Concrete.’’ Sub-stituting a polymer for the usual water in portland cement concrete, produces animproved concrete The polymer, along with fiber reinforcing, produces a very

dense product with low heat of hydration, stronger physical properties in tensile

and flexure, and cures in 24 hours2

22.1.6 Anchor Bolts

Anchor bolts are a vital link between the compressor and the foundation tunately, designers often overlook important points concerning anchor bolts, such

Unfor-as how long and strong they should be and the amount of preload Anchor bolts,

as well as other parts of the support system, such as sole plates and chocks (to be

FIGURE 22.5 Typical perimeter steel reinforcing—era 1960s.

FIGURE 22.6 Dense steel reinforcing based on current design practices.

discussed in section 22.1.7), can be one of the principle points of failure on newconstruction projects Failure usually occurs during the first year of operation.While the number and size of the anchor bolts are set by the equipment man-ufacturer, their length, configuration, and material of construction are in the hands

of the foundation designer Figure 22.7 shows good and bad designs

22.8 CHAPTER TWENTY-TWO

FIGURE 22.7 Evolution of anchorbolt designs (Illustration courtesy of Robt L Rowan

& Assoc., Inc.)

Length: Short anchor bolts have historically caused problems in compressor

foundations Horizontal cracks in the foundation often result The best practicetoday is to make them as long as possible, terminating them in the concrete matunder the concrete foundation In this manner, they do not contribute to horizontalcracking and have the added benefit of adding a post-tensioning effect

Material: Anchor bolts for any dynamic machine cannot be too strong Today,

anchor bolts made from steel, conforming to ASTM A-193 with a yield strength

of 105,000 psi, are not much more expensive than steel half as strong As the needfor high clamping forces for compressors is being recognized, alloy steel bolts toASTM A-193 provide the necessary capacity without going to a larger anchor bolt

Preload: While some compressor manufacturers will specify an initial torque

value for the initial installation, often field experience will show a much higher(maybe two to three times) clamping force will be required to lower framemovement / vibration Unless the anchor bolts put into the foundation to start withhave extra capacity, the machine will not perform as it should, or a costly retrofitwill have to be done

22.1.7 Support Systems

Figure 22.8 shows a range of options on how to support a gas compressor fromthe older method of full bed grouting, to the latest technology of adjustable support

FIGURE 22.8 Types of compressor frame support systems (Illustrations courtesy of Robt L Rowan & Assoc., Inc.)

systems Adjustable supports are the system of choice today, because they eliminate

a potential problem of poor initial alignment which happens from time to time withfull bed grouting Adjustable systems also allow the optimum hot running condition

to be achieved as the frame can be re-aligned to correct for the alignment changesthat occur as the machine heats up during its first 100 hours of operation

22.1.8 Grout

Since the introduction of epoxy grouts for gas compressor grouting in 1957, theuse of cementitious grouts mixed with water has virtually stopped Epoxy groutsare stronger, resist oil and many chemicals, and perform well in dynamically loadedsituations

While grout need not be stronger in compressive strength than the concreteunderneath, a good grout will be tough enough to take impact and cyclical loadsfrom the dynamic machine it supports For that reason, compressive strengths above5,000 psi and tensile strength above 1,000 psi are all that are required Highercompressive strengths are not necessarily better if the product is brittle and cracksexcessively in service Almost all good machinery grouts can crack, so expansionjoints are required The expansion joints should be strategically placed so crackswill not develop in the prime load transfer area adjacent to the anchor bolts

What to use for the post-tensioned repair described above is extremely important.

If the job schedule will allow 21 to 28 days, portland cement concrete is the bestchoice If a 24-hour curing product is needed, then a polymer modified concreteshould be used Either product will have a modulus of elasticity of at least4,000,000 psi, and will have negligible creep at typical compressor foundation

temperatures What should not be used as a deep pour repair material to replace

the removed concrete is the epoxy grout material that is used as the final cap ontop of the foundation Epoxy grouts are just that—a grout designed to be used in

2 to 4 inch thicknesses Epoxy grouts, as a class of material, have a modulus ofelasticity ranging from under 1,000,000 psi up to 2,500,000 psi, with the lowerrange being the most prevalent This means epoxy grout will compress under load,two to five times more than concrete Additionally, some epoxy grouts creep enough

at typical foundation temperatures to cause equipment misalignment There have

Besides up-grading the anchor bolts, an adjustable support system is also added

to allow easier realignment Figure 22.9 shows a typical foundation repair design

22.2 REFERENCES

1 Prakask, Shamsher, and Vijay K Puri, Foundations for Machines: Analysis & Design,

Wiley Series in Geotechnical Engineering.

2 Rowan, Robert L & Associates, Inc., Re-Grouting Reciprocating Gas Compressors, 5 Year Repairs vs 20 Year Reliability Criteria, 1:12 Grouting Technology Newsletter.

Energy Industries, Inc.

Compressor packages can be used for a variety of applications including gas ing, gas gathering, gas lifting, gas injection, gas turbine compression, vapor recov-ery, landfill, and digester gas compression, or propane / butane refrigeration com-pression Two basic types of positive displacement compressor packages will bediscussed in this section: reciprocating gas compressors, and rotary screw gas com-pressors

boost-Reciprocating gas compressors can handle from 60 horsepower to 7,200 power (using a 6-throw compressor frame and an electric motor driver) Althoughreciprocating units are the most common due to their ability to handle large horse-power requirements, high pressure, and varying conditions, rotary screw units areadvantageous when limited space or limited package weight specifications apply.Rotary screw units are ideal for large volume / low suction pressure applicationsand have fewer maintenance and vibration problems

The driver can be a gas engine, gas turbine, or electric motor depending on theend user’s specific requirements Although the primary components of the packagedgas compressor unit are the compressor frame and its driver, numerous other partsare essential to the efficient operation of the unit

of the entire package Bases can be designed as structural steel only when thepackage must be kept within certain weight limitations However, the preferredbase design is a steel frame filled with reinforced concrete This design is portableand eliminates the need for field-poured foundations.

The base can be designed with or without the drain and vent piping inside theskid frame An ecology rail should be designed to keep any oil or water leakageonto the skid contained into a drain system This rail is usually made of 2 ⫻2 or

3 ⫻ 3 angle around the perimeter of the base and skid drains located at four ormore locations at the skid edge

Lifting eyes, designed with a 4:1 safety factor that can be added to the skid, ordraw bars are located at the edge of the skid to aid in pulling the package up onto

in Fig 23.2

Scrubbers are designed to remove solids and liquids from the gas before they reachthe compressor cylinder This is necessary because the tolerances in a compressorare such that any foreign matter can damage the internal parts of the compressor.Scrubbers should be placed before each stage of compression On a multi-stageunit, the interstage scrubbers are required to remove any liquids formed by con-densation during the cooling process

Mist pad scrubbers are the most common scrubber design They are verticalvessels which have a wire mesh mist pad, typically 4 to 6 inches thick with 9 to

12 lb / cft bulk density (Gas Processors Suppliers Association’s Engineering DataBook, Volume I, 10th Edition1 contains a method for sizing mist pad scrubbers.)Scrubbers are sized based on the critical or terminal gas velocity required forparticles to drop or settle out of gas Equation 23.1 defines the maximum gasvelocity entering the mist pad scrubber as a function of the gas density and theliquid density

0.5

pl ⫺ pg

PACKAGING COMPRESSORS 23.3

FIGURE 23.1 Typical skid design prior to concrete fill.

where Vt ⫽ terminal gas velocity (ft / sec)

K⫽ empirical constant for mist pad scrubber sizing (ft / sec), (Fig 23.3)1

pl ⫽ liquid phase density, droplet or particle (lb / cft) This is assumed to

be water and is equal to 62.4 lb / cft

pg ⫽ gas phase density (lb / cft)

Equation (23.1) is an empirical equation based on Stoke’s Law The inside area, A(sq ft ) of the mist pad scrubber can then be found by:

Qa

Vt where: Qa⫽ actual gas flow rate (cft / sec)

From the inside area of the mist pad scrubber, we can then determine the minimumrequired inside diameter of the mist pad scrubber to ensure all liquids are removedfrom the gas of a maximum specified flow rate

Figure 23.4 shows a typical design of a mist pad scrubber and the tation normally used on scrubbers The gas enters the scrubber and is directeddownward toward an impingement or baffle plate The liquids and solids collected

instrumen-on this plate are forced through holes that allow the liquids and solids to drain into

FIGURE 23.2 Typical skid design with concrete fill.

an accumulation chamber The change in direction, decrease in velocity and itational force further help the dropout of the liquids and solids The gas then passesthrough a mist extractor which allows the fine droplets of liquid to be collecteduntil they grow heavy enough to fall to the impingement plate Liquid collected inthe accumulation chamber rises to a level sensed by a liquid level controller andthen is dumped from the scrubber through a diaphragm-operated dump valve Ifthe liquid flow is greater than the dump valve can handle, a high liquid levelshutdown switch is added to prevent liquid overflow into the cylinders A liquidlevel sight gauge allows visual monitoring of the scrubber liquid level A manualdrain is supplied to completely drain the scrubber

grav-Vane scrubbers are much more efficient than mist pad scrubbers and therefore

it is possible to use a smaller diameter vessel with a vane scrubber than is required

by a mist pad scrubber Vane scrubbers use a vane pack made up of several rugated plates with liquid drainage traps Manufacturers of vane scrubbers should

cor-be contacted on the details of the design of their vessels since vane sizing designsare proprietary

PACKAGING COMPRESSORS 23.5

FIGURE 23.3 Typical K & C factors for sizing woven wire demisters (Reprinted

with permission from Gas Processor Suppliers Association Engineering Data Book, Volume I, Section 7, Fig 7-9, p 7-7, GPSA, 1987).

d ⫽ pipe inside diameter (in)

L m⫽ length of pipe (mi)

T b ⫽ temperature at base conditions (⬚R), (ANSI 2530 specification: T b⫽

520⬚R)

T avg ⫽ average temperature (⬚R), [T avg⫽ 1 / 2 (Tin⫹ Tout)]

FIGURE 23.4 Typical scrubber dump instrumentation.

PACKAGING COMPRESSORS 23.7

P b ⫽ base absolute pressure (psia) (ANSI 2530 specification: P b⫽ 14.73psia)

E ⫽ pipeline efficiency factor, (usually assumed to be 1.00)

P1 ⫽ inlet pressure (psia)

P2 ⫽ outlet pressure (psia)

Z avg ⫽ average compressibility factor

S ⫽ specific gravity of the gas with respect to air (where S ⫽ 1)

Then velocity can be confirmed by using V ⫽ Q / A, where A ⫽ line flow area,making sure to keep the units uniform

Gas piping on a compressor package includes, but is not limited to, process gaspiping, a valved bypass line from the aftercooler outlet to the first stage scrubberfor unloaded starting, a vent line or valve for purging the machine, scrubber dumppiping, compressor cylinder packing and distance piece vents and drains, and en-gine gas start and fuel system piping All lines should be hydrotested to the max-imum allowable working pressure of the flange class Flange ratings should be inaccordance with ANSI B16.53 Piping should also be x-rayed in accordance withANSI B31.34

23.5 PULSATION BOTTLE DESIGN

On reciprocating gas compressors, some form of vibration control is required out the pulsation bottle or some other device to change the frequency of gas pul-sations caused by the movement of the piston, the harmonic rhythm of their fre-quency would cause a progressive increase in vibration and eventual damage to theunit

With-On small horsepower units (less than 350 horsepower), pulsation bottles are notrequired to eliminate vibration However, orifice plates are required in the suctionand discharge lines on each stage of compression These plates can be welded intothe lines or placed between flanges to allow flexibility when pressure and flowconditions change

On units greater than 350 horsepower, pulsation bottles are required There aretwo different approaches to pulsation bottle design One approach, adequate formany compressor applications, is to design a volume bottle only This approach,however, does not guarantee elimination of harmful vibration To design a volumebottle, the following empirical relation can be used

Bottle volume⫽ 10⫻ swept volume of cylinder(Swept volume⫽ cylinder area⫻stroke)

This gives the volume required to determine the diameter and seam-to-seam mension of the bottle Bottles should be mounted as close as possible to the cylinderand the nozzles should be located in the center of the bottle to reduce unbalancedforces Nozzle flanges should be designed to meet the required pressure and tem-perature operating conditions (refer to ANSI 16.53for flange ratings)

di-can be performed on a particular machine to further verify the guarantee of noharmful pulsations Design of these pulsation bottles takes into account the rota-tional speed of the compressor, the gas composition, and the operating conditions.Chokes should be designed for about 5% pressure drop, but not more than 1%.See API Standard 618, Reciprocating Compressors for General Refinery Services5,for a more detailed discussion on recommended design approaches for pulsationbottles All pressure vessels, bottles and scrubbers, should be designed to meetSection VIII of ASME Code6 and be of sufficient pressure rating to cover alloperating conditions.

23.6 PRESSURE RELIEF VALVE SIZING

Pressure relief valves or pressure safety valves are required on the discharge bottles

or lines of the compressor These valves are designed to open when the operatingpressure exceeds the set pressure of the relief valve The set pressure of these valvesshould be determined by the lowest working pressure of the compressor package

of the system the valve is designed to protect Figure 23.5 is a typical Piping andInstrumentation Diagram This diagram is a schematic that follows the flow of thecompressor package The table on the top of the diagram shows the maximumallowable working pressure (MAWP) of all of the vessels and coolers This diagramalso shows all the valves and instrumentation on the compressor package By usingthis diagram, the relief valve set pressure can be determined

There are two types of relief valves: spring-operated, and pilot-operated operated relief valves are useful when the operating pressure is closer than 15% ofthe relief valve set pressure The spring-operated relief valves tend to pop frequentlywhen the operating pressure is close to the set pressure Pilot-operated valves areless sensitive and will not relieve the pressure until the exact set pressure is reached.The orifice sizes required in the relief valves can be sized by the manufacturer or

Pilot-by formulas found in the ASME Boiler and Pressure Vessel Code, Appendix 11,Section VIII, Division 16

.5

where: W ⫽ weight flow of gas that valve is relieving (lb / hr)

K⫽ flow coefficient for gas [see UG-131(d) and (e)]6

A⫽ actual discharge flow area of the safety valve or orifice (sq in)

P⫽ (set pressure ⫻ 1.1 plus atmospheric pressure*) (psia)(* Relief valves are typically rated at 10% over set pressure.)

M ⫽ molecular weight

T⫽ absolute temperature at inlet (⬚F⫹ 460)

C⫽ constant for gas or vapor which is a function of ratio of specific heat,

Cp/ Cv

FIGURE 23.5 Typical piping and instrumentation diagram.

of the relief valve, and then pick a relief valve that has a standard orifice size larger

than the A value calculated.

23.7 COOLER DESIGN

Governed by the basic gas law, the gas temperature increases in proportion to theincrease in pressure so that each stage of compression requires cooling prior toentering the next stage of compression The cooler also acts as a radiator to coolthe engine and compressor frame The cooler should be fitted with a surge tank tovent the system of all entrained air and also serve as a fill point for the coolingsystem It is outside the scope of the packager to design the cooler, however, it isthe responsibility of the packager to ensure the cooler manufacturer has the correctrequirements in order to do a proper cooler design The packager must furnish thecooler manufacturer with the performance run which gives the pressures, temper-atures, flow conditions, elevation and ambient temperature The packager must alsoprovide any additional information required to correctly design the cooler, such ascustomer specifications It is important that the packager ensure that the pass ar-rangement designed by the cooler manufacturer provides the most efficient pipingarrangement available

Packagers normally use air-cooled heat exchangers These units use ambient air

to cool fluids and gases Typically, the design ambient air used is 100⬚F for summerconditions Manual or automatic louvers are used over the gas sections to ensurethe cooler does not overly cool in the winter months

Heatload information provided by the engine manufacturer is used to determinethe proper sizing of the engine jacket water sections The gas coil sections aregenerally designed to take the discharge temperature from the cylinders and cool

it down to either 120⬚F or 130⬚F on the interstage sections and 120⬚F on theaftercooler section

The basic formula that cooler manufacturers use to determine the heatload quired for the gas sections is7:

com-23.9 CONTROL PANEL & INSTRUMENTATION

The control panel is the brain of the compressor package It operates in conjunctionwith numerous safety switches to monitor the compressor package and protect it

FIGURE 23.7 Typical electric panel display (Reprinted with permission from Altronic Controls, Inc.)

PACKAGING COMPRESSORS 23.13

FIGURE 23.8 Typical electric panel.

from major failures The panel may be electric or pneumatic Although tation will vary from package to package, some of the most common safetyswitches on the panel monitor low engine lube oil pressure / level, high enginejacket-water temperature, engine overspeed, low jacket-water level, low compressorcrankcase lube oil pressure / level, lubricator no flow, cooler and compressor vibra-tion, high / low inlet pressure, high / low interstage pressure, high / low discharge

instrumen-FIGURE 23.9 Typical pneumatic panel.

pressure (each stage), high discharge temperature (each cylinder), high liquid level(inlet and interstage scrubbers), and automatic fuel shutoff valve

An electric control panel uses power generated by the magneto to perform thesafety shutdown functions of the unit Under normal operating conditions, the elec-tric current flows from the magneto to the ignition system The magneto is alsowired to the control panel which is wired to the safety switches When a safetyswitch is tripped and goes to ground, the electric current is diverted from the