Pumps n Introduction
= Pump Curves - Head versus Capacity - WSH - EBciency
u Single-Stage Centrifugal Pwmp Design - Pump Components
- Shaft SeaXing
(continued) m Reliability
Fan Laws a Hydraalics 8 Pump Control 8
r Sdessh p s r h p Selection and PerEomance r Doizble Suction, Mujti-Stage, and Sundyne funips
This is a typical pump curve. The pump curve gives information on how the pump will perform, the NPSH required by the pump, and the impeller size m g e for the casing. AH p m p manufacturer's c m e s are similar so, if you can read one nnanufiictwer's curve, you can read anybodys.
800 1200 I W Gallons Per Minme
Head-capacity curve. Once this curve is established based on the impeller diameter and speed, the pump wig1 always operate on this curve. Note how the curve rises as the Row goes down. This is a charactexistic of d l centrifugal pumps.
Single Stage Centrifagal Pump Mechanical Seal
Shaft f Sleeve
~ k l i n ~Deflector Met Ring
0 3 Levef BoHe
Single stage centrifugd pump. As the centrifugil force of the impeller throws the fluid out towards the cstsing, the velocity of the fluid goes up. -4s the fluid leaves the p m p , this velociQ energy is changed to pressure energy.
Identical Pumps Handling Liquids of Dgferent Specific
CrasoEioc. S.G. = 0.75
Waxer, S.G. = f .O
Brine, S.G = 1.2
Pump perfomance is measured in feet or meters of head. Head i s the height of the column that the pump cm move the fluid. Pump head is a function of impeller diameter and sped. It is not a function of the density or specific gravity of &e pumped fluid. Here are three identical. pumps pumping out of three identical tanks. Note that the head or column height is identical even though the specific gravity of the Ruid is different.
Galbns Per ,Minute
Each pump casing size can handle more than one size impeller. This pump casing can handle impeller diameters between 9 and J 1 inches. Also, the impeller can be trimmed to any size between 9 and 11 inches to meet the sated opemxing point. The impeller diaeter does not have to be a whale inch size.
1200 ISOO 2000 ~rl.00 CaXlons Per Minute
The pump m e also gives the NPSH required by the pump. Note how %he ,WE required curve rises with increasing flow.
Puhc Along Liquid Pit&
The fluid loses pressure in the pump before the pressure starts to rise. As the fluid enters the pump, these are entrance and friction losses. As the ffaid enters the rotating impeller, $here are turbulence and friction Iosses at the vane tips. If this pmssure drop is enough to drop the pressure of the fluid below its vapor pressure point, flashing will occur. This phenomena, called caviation, will quickly destroy an impeller and a pump. The h?SH ava3lable must be greater than the WSH required.
The NPSH avaifable is a function of the pumping sysxem. WSW available is the pressure at the pump suction minus the fluid vapor presswe. Xt is the pressure thaz can be lost in the pump inlet area before Washing or catritaeon begins. For a bubble point or vapor pressure point fluid, the &iSH avaitfabje is gained with vessel. elevation.
800 1200 1600 Galtoas Per &ate
The pump curve shows the efficiency of tfxe pump at any operating point. Note that the efficiencies rise with rising Bow to the best efficiency point (BEP), and then quickly drop off. Optimum pump operation is at or near best efficiency point.
I d -
800 lGZOO 1600 Gallons Per Minute
Pwnp curves also show the FP requirement for the pump. Do not use these curves. CAU3UX,AE W. These FP c w e s only appfy if the specific gravity of the fluid is 1.0. Also, it i s difficult to get a good, accurate reading. EEP is a simple calculation shown later in this talk.
Pump Selection Y
60 Cycle C m n t 3550rlmn
Singk Sxcion 35% r/min
Two Sage Prmss Uoubit. Suction S550 r h n
This chart shows the approximate head-capacity ranges of single stage fuX1 and half speed pumps, doable suction pumps, and two and &ti-stage pumps. Low Bow, high head applications are Sundynes.
AE API pumps today are centerline mounted. The centerline mounx allows the pump casing to p w both up and down as the casing hears up. This keeps the shaft in the horizontal plane and helps prevents seal leaks and shaft mis-alignment.
All API pumps today have closed impellers with covers or shrouds on both sides of the vanes. This gives the fluid a more defined path through the pump and raises efficiency. The flow splitter in the outlet or double volute equalizes the radial forces around the impeller and minimizes the load on the radial bearings.
Single Suetion Enclosed Impeller
Single suction enclosed impefier.
Siagle Suction impeller
Large single suction impeller. Note tee impeller vanes at the inlet md outIet, This is a half speed impeller. Full speed impellers are only allowed up to 15 inches in diameter to control tip speeds,
Suction Specific Speed
S = rpm ( g ~ r n/)frvPSItr)3J4 ~
r Can range between 3000 20000
The suction specific speed relates rpm, gpm, and NPSH required. UOP limits the suction specific speed to 11000. If a pump manufacturer w a s to reduce the NPSH required of a certain pump, he can increase the impeller eye m a to reduce fiction drop and reduce IWSW required. This increased eye area increases the internal circulation in rhe suction area of the pump. This can buitd up heat which can also flash the fluid and reduce pump reliability. This also reduces the sable operating range of the pump. As the flow is reduced, the p m p becomes less efficient and more heat is built up in the pump. At higher suction,. specific speeds this can promute cavitation.
Model 3735 High TmperatuteMigh Pressure Process Pumps Heavy Duty Design Features to Meet the Total Range of Process Indu:stries Intpetier Wearing Rings
Sealing Renewable Stuffing Box Refiability Froat Bushing
IWfficient Voiate Mechanical Casing Sealcooijng
,- Heavy Cast
Large Cooling Jacket
This is a single stage (one impeller), single suction (one entry into the impeller), overhung (impeller is cantilevered on one set of bearings) pump. This is c d k d a Process pump. The metdfwgy is as follows: Casing Impeller Shaft Wearing rings Throat bushing Throttle bushing
Carbon Steel Carbon Steel 50O0F Carbon Steel 11-1.396 Cr il-13%Cr Bronze or non-sparking m a h a 3
1 Single Stage Overhung Pump
Single stage, single suction, overhung p m p . Note the vent connection on the top of the casing.
Single Stage Pump
Single stage, single suction, overhung pump. This pump is self-venting as the dischsge is at the high point of the casing. This design is typic$.
Before there were mechanical seals, pumps were sealed by "stuffing" an absorbent material caned packing wound Ehe shaft. Since the process fluid had to lubricate &e surface between the stu%h,a and the shaft, the packing had to leak, typically a b u t 200 cchr for a new application. Over time, &e packing would become sitmated with fluid and the leakage would increase until the pump had to be shut down m d &e packing replaced. Today, UOP does not specify any pumps with packing.
Single Mechanical Seal I
Single mechanical seal. Mlost A H pumps today have single mechanical seals. The single mechanical pusher type seal has two members, a rotating member md a stationary member. The main sealing takes place due to the friction between the rotating seal face and the stationary seal face. Since the pumped fluid lubricated this seal face, the si-qle mechanical seal does leak. Typical leak rates are about 2 ccfhr or about f 00 p p of emissions in the air sumunding the se& As the seal faces wear, springs in the rotating member keep a t i a t fit between %he two seal faces. O-rings prevent X&age between the seal and the shaft and between she seal and the pump casing.
Connection B (refer io appropriate \xiliary seal piping m g e m c n t )
Connection A (refer to appropriate primary seal piping arrangement)
Single mechanical seal. Used for most non-hazardous services.
Single Mechanical Seal
Here is another view of the single mechanical seal. Note the yellow process fluid coming from rhe pump discharge to the process side seal face. The mbbing seat faces generate heat. If the pumped Ruid is at vapor pressure or bubble point and. heat is added, the fluid could Aash around the sea$and the seal faces codd b e their lubricant- Process fluid flows fron the discharge of the p m p &-ou& an usifice. The pmsswe is kept high enough momd the seal to stay above the vapor pressure point even though with the seal faces are adding heat.
Solid &.eel W v e Lags
Welded Metal Betitows
Carbon or Tungsten Carbide vs. SteIlite Sealing Faces
]Bellows seals are specified for high temperature applications, above 5S0°F. Bellows seals have two members9 a rotating member and a stationary member, similar to &e pusher type seal.
When the seal face wears on a bellows seal, the metal beljlows expands like an accordion. The o-rings bemeen the seal and the shaft do not move dong Ehe shaft as they do in a pusher type seal. Since the xing material starts to break down at higher temperatures, pusher type seak are temperame limited due to h e dynamic o-ring. Since the o-ring on the bellows seal is sattic, the bellows seal cca operate effecgvely at temperatures up to 800°F.
m ~1000 ppm (Most
Pressure (pig) FD-2OWCD-65
Typical seal leakage rates. As can be seen from the tests, leakage is primarily a function of speed and pressure. Nfolecular weight and temperature can d s o affect the leakage rate to a lesser degree.
Control System m Provjdes three main Xiunctions
IF&&-ation of buffer gas - Replation of buffer gas - Monitoring of seal performance r Ugem 1-1 indication;of fEtw amd seal performance m Design is simple $0 operate and user friendly with
minimnm maintenance requireme~&ts
The control system filters and regdates tho gas and monitor seal
Dry Gus Seal
The seal buffer gas is typically process gas from the discharge of the compressor. The process gas is filtered and coalescecl, down to one micron liquid and three microns solids. Note that it is critical to supply the seals with a clean, dry baffer gas. The gas is injected on the process side of the seal about 20 psi above the conrpessos suction pressure. Most %hethe gas goes across a labyrinth back into the compressor. Under two cfm leaks across the seal. This gas is routed to 8are,
. Type 28
Double Opposed Seal
UOP specifies tandm dry gas seals for process gas applicadons.
Single &y gas seat. The grooved silcar rotor is attached to the shaft.
Tandem dry gas seal. Most of the gas which Eeaks across the primary seal is
vented to flare. Under 0.5 cfm leaks across the secondary seal. A nitrogen separation gas prevents the process gas from migrating to the lube oil in the bearing box. The gas that escapes across b& seals is routed to atmosphere outside the compressor shelter.
If zero fugitive emissions is &sired, rhe nitrogen separation gas can be routed between the two seal faces. This will force dl of the gas that teaks across the primary seal our the vent to flare, Only inert nitrogen will leak across the secondary seal to atmosphere.
Comparison of Wet Seal vs, Dry Gas Seal
Sed oil consu&tjon Maintneance cost
Wet Oil Seals Pumps,~t:semoirs,filters traps. coofers. consoles 1-100gaXfonslday A major ex nditsue over equipment Ke SgaI R p e r loss: 10-30MP ZImt &ven pumps: 20-100 HP Gas Leakage: 25 scfm & higher
Dry Gas S d
j NO sea1 oil NegIigibIe
) Less than 2 scfm
Today dmost d1 process gas compressors axe specified with dry gas seaIs.
Dry Gas Seal Console
Dry Gas Seal Console
Compressor Dry Gas Seal System vet to
seal schematic. A 2 of 3 voting high-high pressure shutdown UOP dry is specified on each primary seal vent to shut down in the event of a primary seal leak. A low flow d m across the primary seal vent warns of a secondafy sed leak. If more gas is leaking across the secondq seal, less gas will be tmveEing out &e primary seal vent.
A Model of Sarge m Surge DelEi~ritioa Surge is self-osc~~ions of pressure and fbw, including a flow reverkaL Tlxe surge flow reversal is the only point of the curve when pressure and flow drop simulbneously.
Characteristic Curve of42 Typical C&rrtnjCugaZCompressor - BtoCin24lto50mSec - Cycle B to B at 0.33 to 3 Hertz
All centrifugal compressors can and surge. If the compressor rides up its curve due to rising discharge pressure or decreasing gas rno~ecalaxweight, the compressor will physically not be able to overcome the downsueam pressure requirements. Since gas is eompsessible, pressure and energy ~ $ 1 1 build up [email protected]
When the compressor curve reaches point B, the cornpressor will no Ionger be able to push the gas out the discharge. The gas will then reverse Bow through the compressor. Row & point C is negative. Now that the downstream presswe has been relieved through the compressor, the compressor cstn start pushing gas out the &sctnage again. When the pressure builds up and the compressor em no Xonger keep up with the demand, the gas once again goes back through the compressor to the suction. This can happen multiple .timesper second.
The Surge Phenomena 1 3
Rapid flow osciIfations
r Thmt reversals r Potentialdamage
r Rapid pressure oscilfations with smess instabilitv 4
r Rising tempmbres inside cowressar
T i m e
The consequences of surge are severe. The thrust reversds on the shaft wilI damage seals, bearings, md open up critical internal clearances. Since the same gas is passing throagh the compressor multiple t i m s a second, the temperatwe in the compressor rises rapidly.
Surge Description FXQWreverses -in20 to 50 milJiseconds Surge cycles at a rate of 113 to 3 hertz r Compressor vibrates 8
Trips may wcur I Conventioad btmrnents and hmm operators m y fail to rec~~anize surge
Some Stlrge Conseqrcences r Umbbie flow and pressure Damage h sequence with increasing severity to s 4 s , bearings, impeEEers, shaft r Increased seal c1eat.mces and leakage
r Lower energy efficiency i
Redaced compressor fife
PlatJorming Recycle Gas Circuit
This is a VOP Piatforming recycle gas circuit. Note that this i s a circulating loop circuit. The compressor discharges into the comprressor suction. There are no automatic conxols on compressor speed and no ausoxnazic control valves. Assuming no blockages occur in the exchangers or reactor, which will only happen slowly over time, there is nothing in tkis circuit &a%can put the cornpressor into sage. Therefore, UOP recycle gas circaits do not require anti-surge control.
FCC Muin Air Blower
This is a FCC Main Air Blower. There are automatic valves which if not operating properly can put the blower into surge. Therefore, anti-surge equipment is specified-
Antisurge Controller Operation S e e limit Line (SU)
The anti-surge con.trolIes measures gas pressure, temperature, and flow a minimum of 40 ~ m e sper second. E the operating point hits &e surge controf line, the anti-surge co~trolfersmds a signal. to open the spillback valve. This allows tbe gas downstream of the comp~msoran alternate path around the compressor instead of back through fhe compressor. A margin7 b2, is left between the surge con&ol Ene and the actual surge line. This ensures that the spillback valve will open in time to prevent the compressor from surging.
Antisurge Controller Operation Surge Limit Line (SLL) APc 0
Surge Control Equarion: KA PC+ bX = APo minimum
Aattslcrge ControIZer Operation S&ge
Limit Line (SLZ)
Antisurge Controller Recycle Trip Circait Operation swge ~ i n - i~t
i (SLL) e \
Recycle Trip Line (RTIL)
Surge Control Line (SCL)
Activates open loop control m Reven& surge in all but $he most severe disturbance
The surge controller is also progmmmed with a recycle trip h e . If the opera%ing point hits the surge control line, the spilgback valve opens slowly. Xt is not desirable for &e spillback valve %ogo wide open because catalyst in the circuit cadd be starved of hydrogen.
K -the opefating p i n t hits the recycle trip line, now the compressor is in danger of surging.. The spillback valve wiIl step open quickly to avoid surge.
Simple Antisurge System r Flow measured in suction
Suction and discharge pressure transmitters for pressure differential calculation r Control strategy is proportional plus integral control to maniprrEate recycle valve I Increase flow through compressor and reduce discharge resistance when required to prevent surge
h a simple amisurge system, flow and pressure measurements on the suction and pressure measuremen%on the discharge me sent to rhe antisurge contro31er7 'LTXC. The U C controls the recycle valve which is nonnaEXy closed. If surge is approached, &e ZsyC opens the recycle valve.
FCC Axial Main Air Blower with CCC Performance Controller CCC Pctiomwicc Co~twl!c
This is an actual UOP FCC Main Air Blower P & XD. Note the flow, pressure, a d ternpermre measurements transmitted to the perfommce controfler which i s contxoIliag the flow of air to the regenerator. The performance controller is adjursting the axid air blower guide vanes or the tufbirte driver speed. The pdomance controller is also sendlag d&a to the antisurge c o n ~ l l e rwhicb , is controlling the normally closed snort valve. If the surge line i s appxoached, the antisurge contro1ler will open the snort valve md uncouple the performance controUer from the Emp.
Occasionafly, in a surge event? the pedommce contrler could actually push the compressor further towards swrge by speeding up or slowing down the compressor. With the antisurge and perfommce cortrroflers '?taiking'!"'to each other, this is pzvented fiom happening. 1
Purchasing Reciprocatiag Compressors Technical Evahatiovc 1.
Scope of Supply (Per Spec and API 618)
No. of Cylinders
P~crchasingReciprocating Compressors Technical Ifvaluation 6.
Exceptions to specs and standards
Purchasing CentPz?$ugal Compressors Technical Evaluation scope of Supply
Can it meet all,operatiagpoints (N2 start)
Materials (Impeller yield strength)
3Es proposal complete
Does proposal meet APE 617 IPtT1oae'CP$O
Purchasing Centm~uggalCompressors Technical Evaluation 6.
Efficiency of various operating points
Rotor: dynamic stability (How many wheels)
Exceptions to specs and standards
Experience similar casing sizes, rotor sizes, pressufes, Bows
TRAXNlNG P R O G U M - PROBLEMS A & 3 (Complete BBP Calculation Forms) TYPE
1 Date I BY
PROBLEM A RECYCLE GAS
PROBLEM B MAXN AIR BLOWER
Subject: RECIPROCATING COMPRESSORS-BHP
Sample Problem A
and Suction Tmpemtufe
CEhmFUGAL COMPRESSORS-BHP Sample Problem B
tosses frictional Seal
Compressor Palytropic Efpclency
Compression W o
SpeciJic Gravity Cowectiun
PLATFORMXNG W T GAS - PROBLEM C
Gentxifugd compressor (condensing steam turbine driven) vs. 3-50% reciprocating compressors
(motor driven). Which is better?
Cost of 2 body compsessor, condensing steam turbine including oif console and g s seal console:
Spare compressor and tmbine rotors: $1,065,000 Cost of 2 stage (6 cylinder) reciprocating compressor: $3,327,000 Assumptions: Installation:
Centrifugal - 20% of compressor cost Reciprocating - 50% of compressor cost
Reciprocating - $35hp/yr
PROJECT N W E R
- PROBLEM A
C ~ / C ~
1.002 ft3/min Q Suction
(sfd 1 ? ~ / d ) 7 4 . 7 ) ( ~ ~ ~ 1 ~ ) (1440)(pS1520) k-1 k
BHP/mm @ 14.4 psia and Suction Temperature
sp gr Correction (Qb XI. 02 KTs ) 520
Add 5 pct if N.L.
Gear Loss 3 pct (if Gear is u s e d )
BHP Req d
( A + BXCXDXE)(F)
COMPRESSOR PROBLEM SOLUTIONS PROBLEM B
-) Centrifugal - Main Air Blower
A ft3/min =
std ft3/d 14.7 psia Ts" R x xminuteslday ps psia 520°R X zs
Aft 3/min = 34.54x106 x-14'7x555x1.0=26134fi3/min 63 Inlet 1440 14.4 520
Ib/min wt flow =
34.54 x 1D" 28.49 Ibllb mol = x 7440 379.48 std ft 3/lb mol
- Beta = 1.590
Polflropic Head = ZavgRTs B = (1.0)(1545J28.49)(555)(1.590)=47846 ft Ib f/lb m
Head x lblmin - (48376)(1800.8) = 3138 33000 x Eh (33000X0.832)
SOLUTION PROBLEM B (cont.)
Main Air Blower
Molecular Weigh1 1545 mol wi
ft3/min @ Suction (Qb Xmol wt) 546000
Iblmin wt flow
54.23 1.4 26 134 1800.8 1.o
Polytropic Efficiency k-1 k k-1
Beta Head Gas hp
0.832 0.2857 0.343
(r)M - 1
(rlM - I M
(Wt low X ~ e a d ) ( 3 3 0 0 0 ) ( ~) ~
Losses Frictional Seal
I pct of Gas hp
(If used) 3 pct of hp
Total bhp 7d
- PROBLEM C Centrifugal
Savings with centrifugal compressor - $8.78 million for first year
CFM @ Suct. QbxMW 546000
k - l/k€h
( r ) -I M
Zav R Ts Beta
wt Flow x Heaa
7% of GHP
3% of GHP
#/Min Wt. Flow
( r ) -1
33000 x Eh Losses Frict. Seal Gear
Total BHP Td
Ts( r )
CFM Suct k-l/k k-l/k
BHP/mm @ 14.4 psia & Suct T
Sp. Gr. Corr
Qb (1.02) Ts/520
Add 5% if N.L.
Gear loss 3%
(A+B) (C) (D) (E) (F) (G)