Tw T wo Phase Separator Horizontal Type Design Procedure Ali Farrokhzad
1395
Jan 2017
. = .ℎ ℎ =
A
. 2
T bl (GPSA
)
Example: Size a Horizontal separator with a mist eliminator pad to separate the following mixture .
The operating pressure is 975 psig and the holdup and surge are to be 10 min and 5 min respectively. Use a design temperature of 650°F .
Design Procedure:
Example: 1) Q v =
456
6 ×4.
=10.09 ft3/s
2) QL =
46 6 ×8.8
= 19.8 ft3/min
3) As we show in 2-phase vertical presentation: K York_mist= 0.2714
,
K GPSA = 0.2625
K theo ( μ=0.01cP, Dp=300 micron) = 0.2766 For Horizontal drum (with/without mesh pad) K shall be divided by 2.
K=
min {Ki} 2
U T = 0.39 ft/s
=
0.2625 2
= 0.1313
Design Procedure:
Example: 4) V H = TH×QL =10×19.8 = 198 ft3 5) V S = TS×QL = 5×19.8 = 99 ft3
6) P=975 psig 4 <
<6
Assume L/D = 5
D=
4 (98+99) ×.6×5
=5.01 ft.
Use D = 5.0 ft.
Design Procedure: 7.
Example: 7) HLLL = 0.5×5+7 = 9.5 in.
8) x =
HLLL/D =0.158
From Eq. y = 0.102 ALLL/A T = 0.102 ALLL =0.102×19.63 = 2 ft2.
Design Procedure:
Example: 9)
Acc. to Article: with mesh pad:
H v =max{0.2×5 , 2} = 2 ft. Acc. to modified formula:
q=
456
=0.76 = 0.85×0.76 =0.65
456+46
H v =max{0.65×5 , 2} = 3.23 ft. x = H V /D =0.65
From Eq. y = 0.68 A V /A T = 0.68 A V =0.68×19.63 = 13.4 ft2.
Liquid Inventory Length:
L
V H AT
V S
AV A LLL
H V U V
U VA
QV
L MIN
AV
U VA
L Feed
Vapor
Av
HV D
A(holdup+surge) +
ALLL
A T
Liquid Inventory Length (L) vs Separation Length (LMIN)
L
L > LMIN
Design is Acceptable
H LMIN
L
L >> LMIN
Liquid hold up is controlling
H LMIN
⪧ L
can on ly be decreased and L MIN increased if Hv is decreased.
⪧Hv
may only be decreased if it is greater than
Liquid Inventory Length (L) vs Separation Length (LMIN) L L < LMIN
H
Vapor/liquid separation is controlling ⪧ Set L= L MIN
LMIN
Continue with new L. This simply results in some extra holdup.
L
L << LMIN H
⪧ Increase Hv and repeat from step 9. LMIN
Design Procedure:
Example: 10)
L=
97.9 + 98.9
9.6 −.4 −2
.2
= 70.16 ft.
11)
=
12)
U VA =
13)
LMIN= 0.753 × 11.14 = 8.39 ft.
.29
= 11.14 s
.9 .4
= 0.753 ft./s
Example:
Design Procedure: L > 1.2 × LMIN
L >> LMIN
According to Step9:
9 )
H V =max{0.2×5 , 2} = 2 ft.
AND Acc. to modified formula:
H V =max{0.65×5 , 2} = 3.23 ft.
’
Type:
So H V shall be in the range of 2~3.23 ft.
L
Our design with 3.23 ft. shows that L is so much greater than LMIN. H LMIN
We should reduce H V to the minimum allowable value: Let H V = 2 ft. With new figure we are continuing from step 9: x =
H V /D =0.4 y = 0.374 A V /A T = 0.374 A V =7.33 ft2.
10 ) ’
L=
97.9 + 98.9 9.6 −7. −2 2
= 28.81 ft.
11 )
=
12 )
U VA =
13 )
LMIN= 1.38 × 6.9 = 9.48 ft. L >> LMIN
’
’
’
.29
= 6.9 s .9 7.
= 1.38 ft./s
But H V is the min. allowable value and we could not reduce H V .
Calculation of thickness of Head & Shell Reference: RULES OF THUMB: SUMMARY , Walas
Click to see contents:
1. Design temperature between -20°F and 650°F is 50°F above operating temperature; higher safety margins are used outside the given temperature range. If -20 oF
DT=OT + 50 oF
2. The design pressure is 10% or 10-25 psi over the maximum operating pressure, whichever is greater. The maximum operating pressure, in turn, is taken as 25 psi above the normal operation. MOP= NOP+25 psi
DP=max.{1.1×MOP , MOP+10~25 psi}
3. Design pressures of vessels operating at 0-10 psig and 600-1000°F are 40 psig.
Calculation of thickness of Head & Shell P, design pressure, psig
Typically, operating pressure+(15-30) psi or 10-15%, whichever greater.
T, design temperature , oF
Typically, if Top > 200°F, DT=operating temperature +25-50°F. If Top < 200°F, DT=250°F
Vessel diameter , in. allowable stress, psi (ASME Pressure Vessel Code, Section VIlI.
D, S ,
Division I, Table UCS-2J , p. 270 - 21771 (1986) )
joint efficiency, (0.6-1.0), 0.85 for spot examined joints, 1.0 for 100% x-ray joints t c , corrosion allowance, in, typically to 8 in. 6
E , • •
under 650°Fdoesnot reduce wall thickness if overpressure caused by boiling, should be T BP .
t,
thickness of plate , in. , larger of tS and tH (to nearest 8 in.)
Allowable Stress of Alloy Pressure Vessel Handbook-Megyesy Evaluate Design Temperature for the selected alloy and then read S from the Table.
Design Procedure: Tips: According to Walas Rules of Thumbs for Pressure Vessels Corrosion allowance: 0.35 in. for known corrosive conditions, 0.15 in. for non-corrosive streams, and 0.06 in. for steam drums and air receivers.
Example: DT = 650 oF
NOP=975 psig MOP=975+25=1000 psig DP=max{1000+25, 1.1×1000}=1100 psig
Alloy: SA 516 70 Carbon Steel From Table: 17.5 S= 17500 psi
Let C.A. tC= 1/16 = 0.06 in. &
Joint Efficiency (E) = 0.85
Head Type: D < 15 ft. & P > 100 psig 2:1 Elliptical
Design Procedure:
Example: 15) Head Thickness & Area: t H
PD
2SE 0.2 P
t C
1100 (5 12) 2 17500 0.85 0.2 1100
0.06 2.295 in.
Shell Thickness & Area: t S
PD
2SE 1.2 P
t C
Alloy plate thickness:
1100 (5 12) 2 17500 0.85 1.2 1100 t
max{t H , t S }
2.38 in.
AS DL 3.14 5 28.81 452.5 ft 2
16)
A H
1.09 D 2
1.09 (52 ) 27.3 ft 2
17) t
2 38
0.06 2.381in.
Design Procedure:
Example: D (ft.)
L (ft.)
LMIN (ft.)
Hv (ft.)
W (lb)
5
5.47
21.89
9.48 8.91 10.18 8.94
2 2 2 2
49281
4.5
28.81 21.56 40.88
5.5
46797
54589 46887
L/D
Status
5.8 Weight is not optimum. 3.9 L/D out of Range 9.1 L/D out of Range 4.0 Optimized Case.
Nozzles Orientation:
Nozzle Sizing:
Example:
Usually 10% overdesign shall be considered in Nozzle Sizing.
Inlet Nozzle:
Inlet Nozzle:
Q=Q L+Q V
1. Based on CEP method: 8.5
U =7~13 m/s or U 2
2.
4×.×.295
×
= 1000 Pa (for no inlet device)
D=
4
1,2,3 DF =
10
”
7
×7
3. No inlet device: U2 = 1000
U 2 = 1500 Pa (for half open pipe)
Note: All units in SI system.
4×.×.295
”
U=
”
=3.5 m/s D>13.5
”
(Selected)
Liquid Outlet:
Liquid Outlet:
4×.×.9
U =1~4 m/s (min 2 ). ”
Vapor Outlet: U =15~30 m/s or U2 = 3750 Pa.
×4
4×.×.9
2.3
×
”
”
Vapor Outlet: 4×.×.286
× U2 = 3750
4×.×.286
×5
75
v
4.5 < D < 6.4
=7.6 m/s D=
”
”
”
DV=6 4×.×.286
×7.6
D=9
”
Feed Nozzle 10"
6"
16"
Min 12"
Vapor outlet
12"
Hv= 610 mm
Min 12"
8
m
m High Liquid Level
H HLL =
1 05 8 mm
=
1
6
Normal Liquid Level
H NLL =
804 mm
6
Surge
D
HoldUp Low Liquid Level
H LLL =
247 mm
9"
Liquid Outlet
L=6671 mm
3"
Home Work: Design a Horizontal flash drum by considering below conditions. Feed enters at 5 barg & 40 oC. Vapor density is 8 kg/m 3 & liquid density is 780 kg/m 3. Flow rate of feed is 12,000 kg/hr. Feed is non corrosive, use your own assumption during sizing. Report required data as much as possible. Assume previous (Vertical H.W.) assumptions and compare the both design cases.
1) Unit Feed Drum with q=40% , with Mesh pad. 2) Flare K.O. Drum with q=0 , without Mesh pad 3) Unit Feed Drum with q=40% and without mesh pad. 4) Unit Feed Drum with q=80%, with mesh pad. 5) Unit Feed Drum with q=100%, with mesh pad. Extra Example: Slug catcher with q=20% and 40 m 3 slug volume as unit feed drum.
References:
1. Wayne D. Monnery and William Y. Svrcek. "Design Two-Phase Separators Within The Right Limits"; ,University o Calgary; October 1993. Chemical Engineering Progress (CEP).
2. Eugene F. Megyesy. "Pressure Vessel Handbook", 10th Edition, University o Tulsa, 1997.
3. Stanley M. Walas, "Chemical Process Equipment Selection and Design", University of Kansas, 1990.
