7/29/2019 Compressor Recycle.pdf
2/4Reprinted from the January 26, 1998 edition ofOIL & GAS
JOURNAL - Copyright 1998 by PennWell
TECHNOLOGY
Compressor-recycle valve
design meets severe noise
and operating specsPaul A. Smirl, Herbert L. Miller CCI Rancho
Santa Margarita, Calif.
Helmut H. Dietz Chevron U.S.A. Products Co. El Segundo,
Calif.
The design of the newcompressor recycle valve at theChevron El
Segundo renerysalkylation unit took into account
four main considerations: noise,fugitive emissions,
rapidstroking, and re protection.
CCI, Rancho Santa Margarita,Calif., designed a 16in. x 24in.
valve which met or exceededdesign specications. Since thevalve went
on-line on Nov. 15,1996, the compressor recyclesystem has
functioned withoutdifculty.
Fig. 1 shows a schematic of thecompressor recycle valve.
Thecompressor inlet comes from
a distillation column, and thecompressor outlet goes to areactor
in the alkylation unit viaseveral heat exchangers.
The alkylation unit processesa mixture of propylene andbutylene
to increase the octanerating of low-octane stock forgasoline
blending.
Compressor recycle serviceRegardless of design or
application, all centrifugalcompressors have one
characteristic in common. Atlow capacities, orderly gas
owthrough the compressor isimpossible because gas ows aretoo low to
be converted to therequired pressure in the dischargeline.
When compressor dischargepressure exceeds the impelleroutlet
pressure, backow occursuntil the compressor dischargepressure is
less than the impelleroutlet pressure. At this pint, owreversal
again occurs, and the
compressor operation becomesunstable, which results inexcessive
vibration and probablydamage to the compressor itself.
This phenomenon is calledstall and produces a surge inthe
system. The gas owcorresponding to this surge pointis xed at a
constant speed, butchanges at varying speeds (Fig.
2).
Valve purposeCompressor recycle valves are
used to recycle gas ow fromthe compressor discharge to
thesuction and prevent operationbelow the surge point.
The valve protects thecompressor from the destructiveeffects of
low-capacity operationwhen the surges of ow reversalcan actually
destroy thecompressor. Since unexpected
low-capacity operation can occurquickly and without warning,the
valve must be able to openquickly. The design of the fullstroke
operation of 20 in. (510mm) was specied at 1.5 sec.
Table 1 shows the requiredoperating conditions of the
newvalve.
NoiseBecause the valve is located
adjacent to a large residentialneighborhood, one strict
designconstraint for the compressorvalve was maximum
permissiblenoise level. Because the designrequired the new valve to
beelevated about 40 ft (12 m)above the ground, noise controlwas
more challenging.
The new valve uses a multistagepressure-reduction design,
whichlimits trim exit-velocity head
to below 70 psi (480 kPa), tominimize noise and vibration.
Inpressure-reducing control valvesfor gas or steam
applicationvelocity head (v2/2, where idensity and v is velocity)
ratherthan pure velocity is the accepteddesign criteria.
Fig. 3 shows the sectional viewof the large compressor
recycle
valve. Two design elements othe trim are important.First, the
pressure-reducing
element itself is made of a stackof tortuous path disks
havingmultiple right-angle turns (Fig4). This stack of disks
surroundthe plug throughout its strokeAlso, the disk stack
incorporatea pressure-equalizing ring (PER)on the inside diameter
to ensureequal pressure acting radially onthe valve plug at all
times; thisfeature eliminates vibration tha
Chevrons El Segundos alkylation unit is one ofthe largest in the
world with a capacity of 26,500 b/d.The new 16 in. x 24 in.
compressor recycle valve, located
40 feet above the ground, is on the bypass of a 15,000
hprefrigeration compressor.
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TECHNOLOGY
could occur as a result of rapidplug radial movement.
Second, relatively low differentialpressures permit the use of
asoft extruded Teon seat, whichensures nearly zero leakage whenthe
valve is closed, exceeding therequirements of leakage specied
Reprinted from the January 26, 1998 edition ofOIL & GAS
JOURNAL - Copyright 1998 by PennWell
by ANSI (American Nationalstandards Institute). This seatdesign
also permits the use of asmaller actuator, which increasesactuator
stroking speed.
The recycle valve was designed forless than 85 dbA and has
functionedbetter than this since start-up.
Figutive emissions
The valve had to meet all currentand possible future
fugitiveemissions requirements for valvestems. To comply, the
compressorrecycle valve is tted with a specialinert packing system.
This systemis made of alternate chevron-
shaped rings of uorocarbonmaterial and uoroelastomematerial plus
a exible graphitering for re safety.
The packing system is ratedfor a
maximum-continous-usetemperature of 500F (260C.) andhas
successfully passed the AP
ANTISURGESYSTEMSCHEMATIC
RECYCLEVALVEBODYSECTIONALVIEW
VALVEOPENINGCONDITIONS SPECIFICATIONS
VARIABLESPEEDCOMPRESSORSURGECURVE
MULTISTAGEDISKSTACK
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TECHNOLOGY
Reprinted from the January 26, 1998 edition ofOIL & GAS
JOURNAL - Copyright 1998 by PennWell
607, Third Edition, re test. Thepacking also decreased
fugitiveemissions in volatile organiccompounds (VOCs) to levelsfar
below those required bycurrent local, state, and
federalregulations, in particular, theClean Air Act. The leakage
hasbeen tested to less than 5 ppm.
Rapid strokingBecause very rapid ow changes
can occur in the compressor output,the recylce valve has to be
capableof reacting quickly. To accomplishthis, the pneumatic
actuators 20in. stroke was undamped, andmultiple boosters and
exhausterswere provided on the top and thebottom of the actuator to
movethe air in and out very quickly. In
service, the stroke only took 1.3sec, exceeding the specied
1.5sec stroking time. The controsystem is shown in Fig. 5.
Fire protectionFire protection was provided to
ensure reliable operation duringa hydrocarbon re environmentThe
pnuematic actuator and itcontrol system are completelyencased in a
re protection blanket
Fig. 6 hsows the blanket location.During a re, the re
blankeprotects the valve from heaby the expansion of
chemicallyinert, high temperature insulationbers, which tightly
seals all aigaps and reduces heat penetrationof the protective
envelope.
Oil & Gas Journal - 1998
CONTROL SYSTEM SCHEMATIC
RECYCLEVALVEELEVATION
THE AUTHORS
