chapter 4 - separator design
DESCRIPTION
Separator design guidelines for oil gas hyudrocarbonsTRANSCRIPT
4-1
CHAPTER 4
DESIGN FOR THE TWO-PHASE SEPARATOR (V-101)
4.1 Introduction
In the petrochemical production, a separator is a large drum designed to separate
production fluids into their constituent components of oil, gas and water. In the event that
water is not present, the bottom output would consist of only oil. It works on the principle
that the three components have different densities, therefore allowing them to stratify
when moving slowly with gas on top, water at the bottom and oil in the middle. Any solids
such as grit and sand will also settle at the base of the vessel.
Separators may cater to the separation of all kinds of phase combinations, whether
it be liquid-liquid, vapour-vapour and vapour-liquid, the latter being the kind that we are
designing as an example for this 1-propanol plant. Vapour-liquid separators are the most
common types of process equipment. They may be oriented either vertically or
horizontally, depending on which one is more economically feasible according to the plant
design. The operation principle is rather basic. Once the oil and other fluids have been
separated the oil will leave the vessel at the bottom through a dump valve that is
controlled by the level controller. The separated gas rises to the top, leaves through the
top and is passed through a meter run for measurement purposes.
The degree of separation between gas and liquid depends on the separator
operating pressure, the residence time of the fluid mixture and the type of fluid flow. All
three of these parameters will be accounted for in the calculations.
4-2
4.2 Process Description
The purpose of the calculations in this chapter is to size the two phase separator
V-101 that performs the separation of the incoming vapour from the first catalytic reactor
R-101 into a waste vapour stream and liquid propanal that would later enter the second
reactor, R-102. This separation therefore involves only vapour and heavy liquid. The
absence of a light liquid distinguishes this type of separator from the more conventional
three-phase one. This separator operates under high pressure but low temperature, at
1990 kPa and 10oC. Figure 2.1 below exhibits the schematic (not to scale) diagram of the
proposed two phase separator.
8
9
10
Figure 4.1: Schematic diagram of a horizontal two phase separator
4-3
4.3 Chemical Design
4.3.1 Steps Taken for Separator Design
Below are the steps taken to determine separator chemical design specification:
1. Calculate the design flow.
2. Determination of section 1 sizing.
3. Determination of section 2 sizing.
4. Vapour Liquid Separation. Check gas available area.
4.3.2 Types of Separator
A separator can be either horizontal or vertical. Spherical separators may also be
used for high pressure and high liquid hold-up systems like storage of light hydrocarbons
etc. The choice between horizontal or vertical types of separator primarily depends upon
the following process requirements:
relative liquid and vapour load,
availability of plot area,
economics,
special considerations.
Table 4.1: Selection guideline for separator types
System Characteristics Type of Separator
Large vapour, less liquid Load (by volume) Vertical
Large liquid, less vapour Load (by volume) Horizontal
Large vapour, large liquid Load (by volume) Horizontal
Liquid-liquid separation Horizontal
Liquid-solid separation Vertical
The horizontal three-phase separator is the most conventional and versatile type of
process in the three phase industry. Design procedures of this type of separator can also
be incorporated into the simpler one of the two-phase separator.
4-4
Vapour-liquid disengagement section
Liquid section
Figure 4.2: Sections in the separator
Section 1 is basically the liquid division of the separation system where heavy
liquid propanal is most prevalent at. Section 2 covers the full length of the vessel and is
where vapour and liquid disengagement occurs.
4.3.3 Design Data
4.3.3.1 Calculation for Gas Mixture Density
The critical temperatures and pressures are needed to determine the densities for
gas mixture. These critical properties as displayed in Table 4.3 are used to find the
compressibility factor Z, which can be estimated from a generalised compressibility plot.
Table 4.2: Molecular weights of each component
Component Formula Molecular weight (kg/mol)
Carbon Monoxide CO 28.0
Hydrogen H2 2.02
Propanal CH3CH2CHO 58.08
Ethylene C2H4 28.05
Ethane C2H6 30.07
4-5
Table 4.3: Critical properties for each component
Component
Critical temperature,
Tc (K)
Critical pressure,
Pc (bar)
Critical volume,
Vc (m3/mol)
Carbon Monoxide 133.2 35.0 0.089
Hydrogen 33.2 13.0 0.065
Propanal 496.5 47.6 0.223
Ethylene 282.9 50.3 0.129
Ethane 305.4 48.8 0.148
Table 4.4: Separator inlet and outlet data
Stream 8
(feed) 9
(liquid out) 10
(gas out)
Pressure (kPa) 1990 1990 1990
Temperature (oC) 10 10 10
Mass flow (kg/h) 35400 16400 19030
Mole flow (kmole/h) 1509 283.9 1225
Vapour fraction 0.562 0 1
Component mole
fractions
Carbon monoxide 0.4016 0.0036 0.4938
Hydrogen 0.3996 0.0026 0.4916
Propanal 0.1948 0.9925 0.01
Ethylene 0.002 0.0006 0.0023
Ethane 0.002 0.0008 0.0023
4-6
𝑃𝑐 ,𝑚 = 𝑃𝑐 ,𝑖𝑦𝑖
8
𝑛=1
𝑇𝑐 ,𝑚 = 𝑇𝑐 ,𝑖𝑦𝑖
8
𝑛=1
Where, Pc = critical pressure,
Tc = critical temperature,
y = mole fraction,
suffixes,
m = mixture,
i = component.
Pc,m of gas out:
𝑃𝑐 ,𝑚 = 𝑃𝑐 ,𝑖𝑦𝑖
5
𝑛=1
= (35.0 x 0.4938) + (13.0 x 0.4916) + (47.6 x 0.0100) + (50.3 x 0.0023) +
(48.8 x 0.0023)
= 18.37773 bar
Tc,m of gas out:
𝑇𝑐 ,𝑚 = 𝑇𝑐 ,𝑖𝑦𝑖
8
𝑛=1
= (133.2 x 0.4938) + (33.2 x 0.4916) + (496.5 x 0.0100) + (282.9 x 0.0023) +
(305.4 x 0.0023)
= 89.41347 K
Pr = P/Pc,m
Where, Pr = reduced pressure
Pc,m = critical pressure
Tr = T/Tc,m
Where, Tr = reduced temperature
Tc,m = critical temperature
4-7
Pr of gas out:
Pr = P/Pc,m
= 19.9 bar / 18.37773 bar
= 1.08283 bar
Tr of gas out:
Tr = T/Tc,m
= 283.5 K / 89.41347 K
= 3.17066
With Pr = 1.08283 bar and Tr = 3.17066 K, the value of the compressibility factor, Z is 1.0.
Specific volume of outlet gas:
V/n = Z (RT/P)
Where, P = absolute pressure, bar
V = volume, m3
n = moles of gas
T = absolute temperature, K
Z = compressibility factor
R = universal gas constant, 0.083 bar.m3/kmol
V/n = 1 [(0.083 bar.m3/kmol)(313.15 K)/1.5 bar]
= 17.3276 m3/kmol
Density of gas mixture going out of the separator:
Pv = A MWi,gas / (V/n)
Therefore, Pv = (57.7668 kg/kmole) / (17.3276 m3/kmol)
= 3.334 kg/m3
4-8
Using the above calculations, the densities of the other streams are also computed and
tabulated in Table 4.5 below:
Table 4.5: Stream densities
Stream 10 11 12
*Density (kg/m3) 24.12 804.1 3.334
4.3.4 Design Flow Rates
A flow rate is defined by;
Q= 𝑚
𝜌
Where, Q = Volumetric flow rate (m3/min)
𝜌 = Gas phase density (kg/m3)
𝑚 = Mass flow rate (kg/hr)
Volumetric flow rate for vapor phase,
𝑄𝑔 =𝑚 𝑔
𝜌𝑔
=19030 kg/h
3.334 kg m3 x 60 min
= 95.131 m3/min
Volumetric flow rate for liquid phase,
𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 =𝑚 𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙
𝜌𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙
=16400 kg/h
804.1 kg m3 x 60 min
= 0.3399 m3/min
Zero margins are added to separator flow or design.
4-9
So, design flows are;
𝑸𝒈 = 95.131 m3/min
𝑸𝒑𝒓𝒐𝒑𝒂𝒏𝒂𝒍 = 0.3399 m3/min
4.3.5 Assumptions
1. Vessel dished end volumes are ignored to simplify calculation and add margin.
2. No vessel margin shall be added to maximum flow rate.
3. No design margin shall be added to separator sizing.
4. Residence time for two phase separator is 5 to 30 minutes.
4.3.6 Calculation of Section 1 Sizing
4.3.6.1 Volume of Cylinder Section
The separator is required to have residence time of 30 minutes. Therefore the
required volume operating volume is:
Vpropanal = 0.339 m3/min x 30 mins = 10.17 m3 = Total Liquid Operating Volume
The vessel Normal Liquid Level (NLL) is intended to be more than 50% of the vessel
diameter; this is equivalent to 50% of the vessel volume.
Cylinder volume, Vcyl = Liquid operating volume/0.5
= 10.17 m3/ 0.5
= 20.34 m3
4.3.6.2 Diameter and Length of Vessel
In the design of a horizontal separator, the vessel diameter cannot be determined
independently of its length. The length to diameter ratio is in the range 2.5 to 5.0, the
smaller diameter at higher pressure and for liquid settling. A rough dependence on
pressure is based Table 4.6 below.
4-10
Table 4.6: L/D ratio dependence on pressure
P (kPa) 0 ≤ P ≤ 1724 1731 ≤ P ≤ 3447 3454 ≤ P
L/D 3 4 5
(Source: Sinott et al, 2005)
The suitable L/D ratio for 1990 kPa is 4
Lv / Dv = 4
Lv = 4Dv
Volume of vessel,
𝑉𝑐𝑦𝑙 =𝜋𝐷𝑣
2𝐿𝑣
4
Where, Vcyl = Cylinder volume (m3)
Dv = Vessel diameter (m)
Lv = Vessel length (m)
Subtitute Lv = 4Dv into equation above, Therefore
𝑉𝑐𝑦𝑙 = 𝜋𝐷𝑣3
Rearrange equation above. So that diameter of the vessel is
𝐷𝑣 = 𝑉𝑐𝑦𝑙
𝜋
3
𝐷𝑣 = 20.34 𝑚3
𝜋
3
= 1.8638 m
Select standard separator diameter = 2.1336 m (7 ft)
Length of the vessel,
L1 = 4Dv
= 4 x 2.1336
= 8.5344 m
4-11
Pseudo-weir Section Sizing
This section is the volume to the right of where the weir would be if this separator was a
three phase one. It is a nominal length to allow for the heavy liquid propanal outlet nozzle.
This length is typically 0.3 of the vessel diameter.
Vessel diameter, Dv = 2.1336 m
Typical weir section length, L2 = 0.3 Dv
= 0.3 (2.1336) m
= 0.7 m
Total Vessel Length = L1 + L2
= (8.5344 + 0.7) m
= 9.2344 m
4.3.6.3 New Volume Cylinder Section
Volume for selected separator size is,
Vcyl =πDv
2L1
4=
𝜋 2.13362 𝑚 × (9.2344 𝑚 )
4
= 33.016 m3
Operating volume of separator = Vcyl x 0.5
= 33.016 m3 x 0.5
= 16.508 m3
4-12
4.3.6.4 Liquid Section Level Setting
The partial volumes within the vessel are calculated using the following equation
for the area of the segment of a circle. (Perry, 1997)
Figure 4.3: Vessel cross-section
𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−1𝑟 − 𝐻
𝑟− 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2
Where, Asegment = Area of the segment (m2)
r = Radius of the vessel (m)
H = Height of the liquid above the vessel base (m)
There area of the segment can then be multiplied by the length of the section to
determine the partial volume.
From the process design philosophy, level settings should be as minimum as
specified in Table 4.7 below.
Table 4.7: Level setting in the separator
Level type Level setting
Level Alarm High High (LAHH) 30 – 60 seconds or 200 mm whichever is
greater
Level Alarm High (LAH) 30 – 60 seconds or 200 mm whichever is
greater
Normal Alarm Level (NAL) 60% of horizontal separator
Level Alarm Low (LAL) 30 – 60 seconds or 200 mm whichever is
greater
Level Alarm Low Low (LALL)
30 – 60 seconds or 200 mm whichever is
greater
Should be at least 200 mm above the vessel
bottom or maximum interface level
A H
4-13
4.3.6.5 Residence Time for Propanal
Vessel radius, r = D/2
= 2.1336 m / 2 = 1.0668 m
Section length, L = 9.2344 m
1 minute of heavy liquid propanal hold up = operating volume for propanal
= 10.17 m3
Liquid section volume = 16.508 m3
Propanal hold up = 30 min
At Normal Liquid Level (NLL)
Internal level = 0.067 m
Cumulative level = 1.067 m
𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−1𝑟 − 𝐻
𝑟− 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2
= (1.06882) cos-11.0668−1.067
1.0668 – [(1.0668-1.067)
2 × 1.0668 × 1.067 − 1.0672]
= 1.7877 m2
Cumulative volume, V = Asegment x L
= 1.7877 m2 x 9.2344 m
= 16.5083 m3
4-14
At Level Alarm Low (LAL)
Internal level = 0.200 m
Cumulative level = 1.00 m
𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−1𝑟 − 𝐻
𝑟− 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2
= (1.06682) cos-11.0668−1
1.0668 – [(1.0668-1) 2 × 1.0668 × 1 − 12 ]
= 1.6452 m2
Cumulative volume, V = Asegment x L
= 1.6452 m2 x 9.2344 m
= 15.1924 m3
Internal volume at NLL = Cumalative volume at NLL – Cumulative volume at LAL
= 16.5083 m3 - 15.1924 m3
= 1.3159 m3
Internal hold-up time for heavy liquid propanal;
t = V /1 minutes of heavy liquid propanal – up
= 1.3159 m3 / 10.17 m3
= 0.13 mins
These calculations were repeated for LAL, LALL, LIAHH, LIAH, NIL, LIAL, LIALL and
vessel bottom. Table 4.8 below displays the summary of the level calculations for the
separator.
4-15
Table 4.8: Liquid levels
Level Internal
level
(m)
Cumulative
level
(m)
Cumulative
volume
(m3)
Internal
volume
(m3)
Internal hold-up
time -propanal
(minutes)
NLL 0.067 1.067 16.5083 1.3159 0.13
LAL 0.200 1.000 15.1924 3.8874 0.38
LALL 0.200 0.800 11.3050 2.4729 0.24
LIAHH 0.150 0.600 8.8222 3.7508 0.37
LIAH 0.100 0.450 5.0714 1.5364 0.15
NIL 0.100 0.350 3.5350 1.3677 0.13
LIAL 0.100 0.250 2.1673 1.1448 0.11
LIALL 0.150 0.150 1.0225 1.0225 0.10
Vessel
Bottom 0.000 0.000 0.0000 0.0000 0.0000
Residence time for heavy liquid propanal,
tpropanal = time from Vessel Bottom to NLL
= (0.13 + 0.38 + 0.24 + 0.37 + 0.15 + 0.13 + 0.11 + 0.10) mins
= 96.6 seconds
4.3.7 Vapour-Liquid Disengagement Section
This section contains the oil high level alarm and high level trip. The volumes are
calculated in the same way as for the liquid section, but the whole vessel length can be
used.
Vessel radius, r = 𝐷
2
= 2.1336 𝑚
2
= 1.0668 m
Vessel length, L = 9.2344 m
1 min of heavy liquid propanal hold-up = operating volume for heavy liquid propanal
= 10.17 m3
4-16
At Level Alarm High High (LAHH)
Internal level = 0.202 m
Cumulative level = 1.579 m
𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−1𝑟 − 𝐻
𝑟− 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2
= (1.06682) cos-11.0668−1.579
1.0668 – [(1.0668-1.579)
2 × 1.0668 × 1.579 − 1.5792
= 2.8365 m2
Cumulative volume, V = Asegment x L
= 2.8365 m2 x 9.2344 m
= 26.1934 m3
Level % of Vessel diameter = (Cumulative level / Vessel diameter) x 100%
= (1.579 m / 2.1336 m) x 100%
= 74.00%
At Level Alarm High (LAH)
Internal level = 0.30 m
Cumulative level = 1.367 m
𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−1𝑟 − 𝐻
𝑟− 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2
= (1.06682) cos-11.0668−1.367
1.0668 – [(1.0668-1.367)
2 × 1.0668 × 1.367 − 1.3672
= 2.4192 m2
Cumulative volume, V = Asegment x L
= 2.4192 m2 x 9.2344 m
= 22.3399 m3
Level % of Vessel diameter = (Cumulative level / Vessel diameter) x 100%
= (1.367 m / 2.1336 m) x 100%
= 64.06%
4-17
Internal volume at LAHH = Cumulative vol. at LAHH – Cumulative vol. at LAH
= 26.1834 m3 – 22.3399 m3
= 3.8435 m3
Internal hold-up time for heavy liquid propanal,
t = V/1 minute of heavy liquid proanal hold-up
= 3.8435 m3 / 10.17 m3
= 0.3779 mins
These calculation steps were repeated for LAH and NLL. Table 4.9 below shows the
summary of the level calculations for the vapour section of the separator.
Table 4.9: Vapour section liquid levels
Level
Internal
level (m)
Cumulative
level (m)
Cumulative
volume (m3)
Internal volume
(m3)
Internal hold-up time –
propanal (mins)
LAHH 0.202 1.579 26.1934 3.8535 0.38
LAH 0.300 1.367 22.3399 5.8316 0.57
NLL 1.067 1.067 16.5083 0.0000 0.0000
The LAH volume is 5.83 m3 as calculated and tabulated above. Therefore, the surge
volume can be accommodated within the LAH volume.
4.3.8 Vapour Liquid Separator
Most separators that employ mist extractor are sized using equations that are derived
from gravity setting equation. The most common equation used is the critical velocity
equation:
𝑉𝑐 = 𝐾 𝜌𝑙 − 𝜌𝑔
𝜌𝑔 𝐿𝑣
10
0.56
4-18
Where, Vc = Critical gas velocity necessary for particle to drop or settle (m/s)
𝜌𝑙 = density of liquid (kg/m3)
ρg = density of vapour (kg/m3)
Lv =
Vessel length (m)
K = 0.101 (refer to table 2.10)
ρl = 804.1 kg/m3
ρg = 3.334 kg/m3
Lv = 9.2344 m
Vc = 0.101 (804.1 𝑘𝑔/𝑚3−3.334𝑘𝑔/𝑚3
3.334 𝑘𝑔/𝑚3 ) (9.2344 𝑚
10)0.56
= 1.5308 m/s
Table 4.10: Typical K factors for the sizing of wire mesh demisters
Separator type K factor (m/s)
Horizontal (with vertical pad) 0.122 to 0.152
Spherical 0.061 to 0.107
Vertical or horizontal (with horizontal pad)
At atmospheric pressure
At 2100 kPa
At 4100 kPa
At 6200 kPa
At 10300 kPa
0.055 to 0.107
0.107
0.101
0.091
0.082
0.064
Wet steam 0.076
Most vapours under vacuum 0.061
Salt and caustic evaporators 0.046
(Source: IPS-E-PR-880, 1997)
Note that the preferred orientation of the mesh pad in horizontal separators is in the
horizontal plane, and it is reported to be less efficient when installed in vertically.
4-19
4.3.8.1 Area for Vapour
4.3.8.1.1 Area Required for Vapour Flow
Vs = 1.5308 m/s
Qg = 95.131 m3/min = 1.5855 m3/s
Area required for gas flow, Ag = Qg / Vs
= (1.5855 m3/s) / (1.5308m3/s)
= 1.03573 m2
4.3.8.1.2 Vapour Height
Liquid height at liquid mixture LAHH, HLAHH = 1.579 m
Vapour height, Hv = Dv - HLAHH
= 2.1336 m – 1.579 m
= 0.555 m
4.3.8.1.3 Area Available for Vapour
Total Vessel Area, Av = 𝜋𝐷2
4 = 3.5753 m2
Area of liquid, Al = Area at LAHH
= 2.8365 m2
Area of available gas = Total Area – Liquid Area
= 3.5753 m2 – 2.8365 m2
= 0.7388 m2
Therefore, the area available for gas is acceptable.
4-20
4.3.9 Mist Extraction Section
Wire mesh pads are frequently used as entrainment separators for the removal of very
small liquid droplets and therefore a higher overall percentage removal of liquid. Most
installation will use a 150 mm thick pad with 150kg/m3 bulk density. Minimum
recommended pad thickness is 100 mm. The pad length recommended is 0.348 to be
installed0.0508 m from the roof of the vessel. (Sinnot et al, 2005)
4.3.10 Conclusion
Chemical design specifications:
Table 4.11: Summary of the chemical design for this separator
Item Value
Diameter of vessel, D 2.1336 m
Length of vessel, L 9.2344 m
Volume of vessel, V 33.016 m3
Critical velocity, Vc 1.5308 m/s
Area of vessel, Av 3.5753 m2
Area of liquid, Asegment 2.8365 m2
Area of vapour, Ag 0.7388 m2
4-21
4.4 Mechanical Design
4.4.1 Steps Taken for Separator Design
Below are the steps taken to determine mechanical design specification for a two-phase
horizontal separator:
1. Determination of separator design pressure.
2. Determination of separator design temperature.
3. Determination of suitable material for construction.
4. Determination of separator design stress.
5. Determination of cylindrical wall thickness.
6. Determination of head and closure.
7. Determination of weight loads.
8. Determination and selection of a suitable separator support.
9. Determination of nozzle size.
10. Determination of flanges.
4.4.2 Design Pressure
In order to allow for possible surges in operating, it is customary to raise the maximum
operating pressure by 10%.
Operating Pressure, Pi = 19.9 bar (absolute value)
By considering 10% safety factor for internal pressure, the design pressure, Pdesign is,
Pdesign = (10
100 × 19.9 bar) + 19.9 bar
= 21.89 bars
= 2.189 N/mm2
4-22
4.4.3 Design Temperature
T = 10oC = 50oF
Tmax = T + 50oF
= 50oF + 50oF
= 100oF = 37.78oC
4.4.4 Material of Construction
Many factors need to be considered when selecting engineering materials, but for
a chemical process plant the overriding consideration is usually the ability to resist
corrosion. The material selected must have sufficient strength and easily operated. The
most economical material that satisfies both process and mechanical requirements should
be selected; this would be the material that gives the lowest cost over the working life of
the plant, allowing for maintenance and replacement. Other factors such as product
contamination and process safety must also be considered.
Table 4.12 shows some criteria to be considered in selecting the material to be
used in constructing the separator. The melting points and corrosion resistance towards
the components in the separator are the main criteria that will affect the system.
Table 4.12: Construction material characteristics
Criteria Aluminium Stainless
steel 304
Carbon
steel Lead Copper
Melting point
(oC) 660 1371- 1399 1540 327 1084
Density
(kg/m3) 2700 8300 7900 11340 8940
Corrosion
resistance Low High High Low Low
From the criteria above, it can be concluded that Carbon Steel is the best material
to be used in constructing our separator.
4-23
4.4.5 Design Stress
The material to be used is carbon steel. The design stress for a design temperature of
37.8oC is obtainable from Table 4.13 below.
Table 4.13: Typical design stresses
Material
Tensile
Strength
Design stess at temperature oC (N/mm2)
(N/mm2) 0 to
50 100 150 200 250 300 350 400 450 500
Carbon
steel (semi-
killed or
silicon killed) 360
135 125 115 105 95 85 80 70
Carbon-
manganese
steel (semi-
killed or
silicon killed) 460 180 170 150 140 130 115 105 100
Carbon-
molybdenum
steel
0.5% Mo 450 180 170 145 140 130 120 110 110
Low alloy
steel (Ni, Cr,
Mo, V) 550 240 240 240 240 240 235 230 220 190 170
Stainless
steel
18Cr/8Ni
unstabilised
(304) 510 165 145 130 115 110 105 100 100 95 90
Stainless
steel
18Cr/8Ni
Ti stabilised
(321) 540 165 150 140 135 130 130 125 120 120 115
Stainless
steel
18Cr/8ni
Mo 21
2 %
(316) 520 175 150 135 120 115 110 105 105 100 95
(Source: Sinnott, 2005)
Design stress, f = 135 N/mm2, Tensile stress = 360 N/mm2
4-24
4.4.6 Vessel Thickness
4.4.6.1 Minimum Practical Wall Thickness
There will be a minimum wall thickness required to ensure that any vessel is sufficiently
rigid to withstand its own weight and any incidental loads. As general guide the wall
thickness of any vessel should not be less than the values given in Table 4.14 below. The
values include a corrosion allowance of 2mm.
Table 4.14: Minimum thickness according to vessel diameter
Vessel diameter (m) Minimum thickness (mm)
1 5
1.0 to 2.0 7
2.0 to 2.5 9
2.5 to 3.0 10
3.0 to 3.5 12
(Source: Sinnott, 2005)
Minimum wall thickness required is given by,
t = 𝑃𝑖𝐷𝑖
2𝑗𝑓 − 𝑃𝑖 + c
Where, t = minimum thickness required (mm)
Pi = operating pressure (N/mm2)
Di = internal diameter (mm)
f = design stress (N/mm2)
J = joint factor, (taken as 1)
c = corrosion allowance, (taken as 2 mm)
Pi = 2.189 N/mm2
Di = 2133.6 mm
f = 135 N/mm2
t = 2.189 × 2133.6
2 ×1 ×135 − 2.189 + 2
= 19.4394 mm ≈ 20 mm
4-25
The thickness is of the separator wall is ideal.
4.4.7 Design of Heads and Closure
Heads and closures are used at the end of a cylindrical vessel. The heads come in
various shapes and the principal types used are hemispherical heads, ellipsoidal heads
and torispherical heads. For this design, an ellipsoidal head design is chosen as it is the
most commonly used as end closures for high pressure vessel and as well as being
economically effective for vessels with an operating pressure above 15 bar. (Sinnott,
2005)
4.4.7.1 Ellipsoidal Heads
Most standard ellipsoidal heads are manufactured with a major and minor axis ratio of 2:1.
For this ratio, the following equation can be used to calculate the minimum thickness
required:
t = 𝑃𝑖𝐷𝑖
2𝑆𝐸−0.2𝑃𝑖
Where, S = maximum allowable stress
E = joint efficiency
4-26
Table 4.15: Weld Joint Efficiencies
Joint Acceptable Joint
Degree of Radiographic
Examination
Type Categories Full Spot None
1 A, B, C, D 1 0.85 0.7
2 A, B, C, D (See ASME Code for limitations) 0.9 0.8 0.65
3 A, B, C NA NA 0.6
4 A, B, C (See ASME Code for limitations) NA NA 0.55
5 B, C (See ASME Code for limitations) NA NA 0.5
6 A, B (See ASME Code for limitations) NA NA 0.45
Table 4.16: ASME Maximum Allowable Stress
ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW ALLOY STEEL
Spec. No Grade
Nominal Composition P-No.
Group No.
Min. Yield (ksi)
Min. Tensile
(ksi)
Carbon Steel Plates and Sheets
SA-515 55 C-Si 1 1 30 55
60 C-Si 1 1 32 60
65 C-Si 1 1 35 65
70 C-Si 1 2 38 70
SA-516 55 C-Si 1 1 30 55
60 C-Mn-Si 1 1 32 60
65 C-Mn-Si 1 1 35 65
70 C-Mn-Si 1 2 38 70
Low Alloy Steel Plates
SA-387 2 Cl.1 1/2Cr - 1/2/Mo 3 1 33 55
2 Cl.2 1/2Cr - 1/2Mo 3 2 45 70
12 Cl.1 1Cr - 1/2Mo 4 1 33 55
12 Cl.2 1Cr - 1/2Mo 4 1 40 65
11 Cl.1 1 1/4Cr - 1/2Mo-Si 4 1 35 60
11 Cl.2 1 1/4Cr - 1/2Mo-Si 4 1 45 75
22 Cl.1 2 1/4Cr - 1Mo 5 1 30 60
22 Cl.2 2 1/4Cr - 1Mo 5 1 45 75
4-27
Table 4.17: ASME Maximum Allowable Stress (cont’d)
ALLOWABLE STRESS IN TENSION FOR CARBON AND ALLOY STEEL
Maximum Allowable Stress, ksi
for Metal Temperature oF, Not Exceeding
650 700 750 800 850 900 950 1000 1050 1100 1150 1200 Spec.
No
Carbon Steel Plates and Sheets
13.8 13.3 12.1 10.2 8.4 6.5 4.5 2.5 SA-515
15 14.4 13 10.8 8.7 6.5 4.5 2.5 SA-515
16.3 15.5 13.9 11.4 9 6.5 4.5 2.5 SA-515
17.5 16.6 14.8 12 9.3 6.5 4.5 2.5 SA-515
13.8 13.3 12.1 10.2 8.4 6.5 4.5 2.5 SA-516
15 14.4 13 10.8 8.7 6.5 4.5 2.5 SA-516
16.3 15.5 13.9 11.4 9 6.5 4.5 2.5 SA-516
17.5 16.6 14.8 12 9.3 6.5 4.5 2.5 SA-516
Low Alloy Steel Plates (Cont'd)
13.8 13.8 13.8 13.8 13.8 13.3 9.2 5.9 SA-387
17.5 17.5 17.5 17.5 17.5 16.9 9.2 5.9 SA-387
13.8 13.8 13.8 13.8 13.4 12.9 11.3 7.2 4.5 2.8 1.8 1.1 SA-387
16.3 16.3 16.3 16.3 15.8 15.2 11.3 7.2 4.5 2.8 1.8 1.1 SA-387
15 15 15 15 14.6 13.7 9.3 6.3 4.2 2.8 1.9 1.2 SA-387
18.8 18.8 18.8 18.8 18.3 13.7 9.3 6.3 4.2 2.8 1.9 1.2 SA-387
15 15 15 15 14.4 13.6 10.8 8 5.7 3.8 2.4 1.4 SA-387
17.7 17.2 17.2 16.9 16.4 15.8 11.4 7.8 5.1 3.2 2 1.2 SA-387
Based on Table 2.16, the chosen type of carbon-steel plate for the separator’s
ellipsoidal head is SA-515 Gr. 60. With a design temperature of 37.78 oF (not exceeding
600oF), the maximum allowable stress, S, is 15 ksi = 15 000 psi . Based on Table 2.15,
the joint efficiency, E, is 1.
Therefore, with Pi = 2.189 N/mm2 = 473.1 psig and Di = 2.1336 m = 85.3 in;
t = 473.1 × 85.3
[ 2 ×15000 ×1 − 0.2 ×473.1 ]
= 1.35 in
= 3.38 cm ≈ 𝟑𝟒 𝐦𝐦
For convenience, the thickness of the vessel is taken to be the same as the head
thickness = 34 mm
4-28
4.4.8 Weight Loads
4.4.8.1 Weight of Shell
For preliminary calculations, the approximate weight of a cylindrical vessel with
ellipsoidal heads and uniform thickness all around, can be estimated from the equation
below:
Wv = 240CvDm(Hv + 0.8Dm)t
Where, Wv = total weight of the shell, excluding internal fittings such as plates (N)
Cv = a factor to account for the weight of nozzles, manways and internal
supports. (for separator = 1.08)
Hv = height or length of the cylindrical section (m)
Dm = mean diameter of vessel = Di + t x 10-3 (m)
t = wall thickness, (mm)
Mean diameter, Dm = Di + t × 10-3
= 2.1336 + 34 × 10-3
= 2.1676 m
Therefore,
Wv = 240(1.08)(2.1676)[9.2344 + (0.8 × 2.1676)](34)
= 209.53 kN
4.4.8.2 Weight of Insulation
Mineral wool is chosen due to its characteristics that make it a great insulator at absorbing
heat.
Mineral wool density = 130kg/m3
Thickness of insulation = 75 mm
Approximate value of insulation;
Vi = π × Dm × Hv × thickness of insulation
Vi = π × 2.1676 m × 9.2344 m × 0.075 m
= 4.72 m3
4-29
Weight of insulation;
Wi = Vi × ρ × g
= 4.72 m3 × 130kg/m3 × 9.81m/s2
= 6.02 kN
Double this value to allow for fitting, therefore W i = 12.04 kN
4.4.8.3 Weight of Demister Pad
In this separation, stainless steel pads around 100mm thick and with a nominal density of
150kg/m3 is to be used.
Demister pad density = 150 kg/m3
Demister pad thickness = 100 mm
Pad area, A = (0.348 m)2
= 0.696 m2
Weight of pad;
Wp = A × ρ × thickness× g
= 0.696 m2 × 150 kg/m3 × 0.1 m × 9.81 m/s2
= 0.11 kN
Therefore, total weight;
WT = Wv +Wp + Wi
= 209.53 kN + 0.11 kN + 6.02 kN
= 215.66 kN
4.4.9 Wind Loads
Wind loads are only important and considered when designing tall columns to be installed
outdoors. Since our separator is horizontal with a diameter of only 2.1336m, wind loads
are therefore insignificant.
4-30
4.4.10 Design of Saddle Support
The method used to support a vessel depends on the size, shape and weight of the
vessel; the design temperature and pressure; the vessel location and arrangement; and
the internal and external fittings and attachments. For a horizontal vessel, it is commonly
mounted with two saddle supports (Sinnot, 2005).
Figure 4.4: Horizontal cylindrical vessel on saddle supports
Figure 4.5: The dimensions of the saddle support
4-31
Table 4.18: The dimensions of the saddle support
Dvessel Max.
weight Dimensions (m) (mm)
(m) (kN) V Y C E J G t2 t1 Dbolt
Bolt holes
1.4 230 0.88 0.20 1.24 0.53 0.305 0.140 12 10 24 30
1.6 330 0.98 0.20 1.41 0.62 0.350 0.140 12 10 24 30
1.8 380 1.08 0.20 1.59 0.71 0.405 0.140 12 10 24 30
2.0 460 1.18 0.20 1.77 0.8 0.500 0.140 12 10 24 30
2.2 750 1.28 0.23 1.95 0.89 0.529 0.150 16 12 24 30
2.4 900 1.38 0.23 2.13 0.98 0.565 0.150 16 12 2733 33
2.6 1000 1.48 0.23 2.30 1.03 0.590 0.150 16 12 2733 33
2.8 1350 1.58 0.25 2.50 1.10 0.025 0.150 10 12 2733 33
3.0 1750 1.68 0.25 2.64 1.18 0.665 0.150 16 12 2733 33
3.2 2000 1.78 0.25 2.82 1.26 0.730 0.150 16 12 2733 33
3.6 2500 1.98 0.25 3.20 1.40 0.815 0.150 16 12 2733 33
From Table 4.18 above, the dimensions of the saddles suitable for our separator
are extracted and displayed in Table 4.19 below. The diameter used to obtain the
dimensions the dimensions is 2.2 m (diameter of the vessel). The saddle’s material is
concrete.
Table 4.19: Selected dimensions for the saddle supports
4.4.11 Nozzle Sizing
The sizing of nozzles shall be based on the maximum flow rates, including the
appropriate design margin. Nozzles shall be sized according to the following criteria
(PTS,2002).
Dvessel Max.
weight Dimensions (m) (mm)
(m) (kN) V Y C E J G t2 t1 Dbolt
Bolt holes
2.134 750 1.28 0.225 1.95 0.89 0.520 0.510 16 12 24 30
4-32
For inlet
No inlet device: ρV2 < 1400.0 kg/ms2
Half pipe inlet device: ρV2 < 2100.0 kg/ms2
Inlet vane: ρV2 < 8000.0 kg/ms2
For outlet
Gas outlet: ρV2 < 2100.0 kg/ms2
Liquid outlet V2 < 2.0 m/s
4.4.11.1 Inlet Nozzle Sizing
The volumetric flow for all;
Qg = 95.131 m3/min
Qpropanal = 0.3399 m3/min
Qtotal = Qg + Qpropanal
= 95.131 m3/min + 0.3399 m3/min
= 95.4709 m3/min
= 1.5912 m3/s
The density,
ρg = 3.334 kg/m3
ρpropanal = 804.1 kg/m3
ρmixture = 𝜌𝑔𝑄𝑔 + 𝜌𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙
𝑄𝑔 + 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙
= 11.748 kg/m3
Assume inlet vane pack, therefore;
Allowable ρV2 = 8000.0 kg/ms2
Allowable velocity, v = 𝜌𝑉2/𝜌
= 8000𝑘𝑔
𝑚𝑠 2 /11.748𝑘𝑔
𝑚3
= 680.967
= 26.095 m/s
4-33
So, the nozzle area, A = Qtotal / v
= 1.5912 m3/s
26.095 m/s
= 0.061 m2
Required nozzle diameter, dnozzle-in = 4𝐴/𝜋
= 4 0.061
𝜋
= 0.28m = 280 mm
4.4.11.2 Vapour Outlet Nozzle Sizing
The volumetric flow for gas outlet;
Qg = 95.131 m3/min = 1.586 m3/s
Gas outlet density;
ρg = 3.334kg/m3
Allowable ρV2 = 1500 kg/ms2
Allowable velocity, v = 𝜌𝑉2/𝜌
= 1500
3.334
= 449.91 m/s
So, the nozzle area, A = Qg/v
= 1.586 𝑚3/𝑠
449.91 𝑚/𝑠
= 0.0035 m2
Required nozzle diameter, dnozzle-out = 4𝐴/𝜋
= 4 0.0035
𝜋
= 0.067 m = 66.76 mm
4-34
4.4.11.3 Heavy Liquid Propanal Outlet Nozzle Sizing
The volumetric flow for heavy liquid propanal outlet;
Qpropanal = 0.3399 m3/min = 0.0057 m3/s
Heavy liquid propanal outlet density;
ρpropanal = 804.1 kg/m3
Allowable velocity, v = 2 m/s
So, the nozzle area, A = Qpropanal/v
= 0.0057 𝑚3/𝑠
2 𝑚/𝑠
= 0.0029 m2
Required nozzle diameter, dnozzle-propanal = 4𝐴/𝜋
= 4(0.0029)/𝜋
= 0.0061 = 61 mm
4.4.12 Standard Flanges
Flanged joints are used for connecting pipes and instruments to vessels, for manhole
covers and for removable vessel heads when ease of access is required. Figure 4.6 below
shows the typical standard flange design (Sinnott, 2005).
Figure 4.6: Standard flange design dimensions
4-35
Table 4.20: Standard flange design specifications
Nom. Pipe Flange Raised face Drilling Neck
Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r
d1 ≈
200 219.1 340 24 62 268 3 M20 8 22 295 235 16 10
250 273 395 26 68 320 3 M20 12 22 350 292 16 12
300 323.9 445 26 68 370 4 M20 12 22 400 344 16 12
350 355.6 505 26 68 430 4 M20 16 22 460 385 16 12
400 406.4 565 26 72 482 4 M24 16 25 515 440 16 12
450 457.2 615 28 72 532 4 M24 20 26 565 492 16 12
500 508 670 28 75 585 4 M24 20 26 620 542 16 12
600 609.6 780 28 80 685 5 M27 20 30 725 642 18 12
700 711.2 895 30 80 800 5 M27 24 30 840 745 18 12
800 812.8 1015 32 90 905 5 M30 24 33 950 850 18 12
900 914.4 1115 34 95 1005 5 M30 28 33 1050 950 20 12
1000 1016 1230 34 95 1110 5 M33 28 36 1160 1052 20 16
1200 1220 1455 38 115 1330 5 M36 32 39 1380 1255 25 16
1400 1420 1675 42 120 1535 5 M39 36 42 1590 1460 25 16
1600 1620 1915 46 130 1760 5 M45 40 48 1820 1665 25 16
1800 1820 2115 50 140 1960 5 M45 44 48 2020 1868 30 16
2000 2020 2325 54 150 2170 5 M45 48 48 2230 2072 30 16
Interpolation of table 4.20 was done by using D nominal of 280 mm of the inlet pipe
and 67 mm, and 61 mm for the outlet pipes. The following values were obtained for bolt
and flange designs for the separator.
Table 4.21: Values for bolt and flange for the inlet nozzle
Nom. Pipe Flange Raised face Drilling Neck
Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r
d1 ≈
210 230 351 24 66 278 3 M20 9 22 306 246 16 10
Table 4.22: Values for bolt and flange of the vapour outlet nozzle
Nom. Pipe Flange Raised face Drilling Neck
Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r
d1 ≈
67 75.7 194 19 46 130 3 M16 4 14 149 83 9 5
4-36
Table 4.23: Values for bolt and flange for the heavy liquid propanal outlet nozzle
Nom. Pipe Flange Raised face Drilling Neck
Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r
d1 ≈
61 69.3 187 18 45 123 3 M16 4 13 142 77 9 4
4.4.13 Conclusion
Table 4.24: Summary of mechanical design
Item Value
Design pressure 2.189 N/mm2
Design temperature 27.8 oC
Material used Carbon steel
Design stress 135 N/mm2
Tensile stress 369 N/mm2
Wall thickness 34 mm
Ellipsoidal head thickness 34 mm
Weight loads 221.68 kN
Type of support Saddle support
4-37
4.5 Separator Costing
The material cost of the equipment is calculated using the equation below (Turton
et al., Analysis, Synthesis, and Design of Chemical Processes, 3rd Edition, page 906):
log10 Cp° = K1 + K2 log10 (A) + K3 [log10 (A)]
2
Where,
A = capacity or size parameter for the equipment
K1, K2, K3 = constants in Table A.1 (Appendix A)
Process vessel (horizontal):
Material of construction = carbon steel
Diameter, D = 2.1336 m
Length, L = 9.2344 m
Volume, V = 33.016 m3
From Table A.1 (Appendix A);
K1 = 3.5565
K2 = 0.3776
K3 = 0.0905
Therefore,
log10 Cp° = 3.5565+ (0.3776) log10 (33.016) + (0.0905) [log10
(33.016)]2
= 4.3387
Cp° = $ 21 812.23
Pressure factors for process vessels:
tvessel = 0.034 m
P = 2.189 N/mm2
4-38
For pressure vessel, when vessel thickness, ,003.0 mtvessel
𝐹𝑃,𝑣𝑒𝑠𝑠𝑒𝑙 =
𝑃 + 1 𝐷2[850 − 0.6 𝑃 + 1 ]
+ 0.00315
0.003
=
2.189 + 1 2.1336
2[850 − 0.6 2.189 + 1 ] + 0.00315
0.003
= 2.39
The bare module factor for this process vessel (Turton et al., Analysis, Synthesis, and
Design of Chemical Processes, 3rd Edition, page 927) is:
CBM = Cp°FBM = Cp
°(B1 + B2FMFp)
From Table A.4 (Appendix A), B1 = 1.49, B2 = 1.52
From Table A.3 (Appendix A), the identification number for carbon steel horizontal
process vessels is 18.
Hence, from Figure A.18 (Appendix A), material factor, FM = 1.0
And so,
CBM = 21 812.23 [1.49 + (1.52)(1.0)(2.39)]
= $ 111 739.69
Correlation:
CEPCI for year of 2010 is 622.6
CEPCI for year of 2001 is 397
Therefore,
New CBM = $ 111 739.69 x 622.6
397
= $ 175 237.11
= RM 529 917.01
4-39
REFERENCES
Sinnot, R.K., Coulson, J.M., Richardson, J.F., (2005), Chemical Engineering Design,
4th Edition, Vol. 6, UK: Butterworth-Heinemann.
Perry, R.H., Green, D.W., (1997), Chemical Engineer’s Handbook, 7th Edition, McGraw-
Hill Book Company.
API 12J, (1989), Specification for Oil and Gas Separators, 7th Edition, Washington DC:
American Petroleum Institute.
IPS-E-PR-880, (1997), Engineering Standard for Process Design og Gas(Vapour)-
Liquid Separators, Original Edition.
Monarh, D., Separators: Gas/Oil, Monarch Separators Inc.,
<http:www.monarchseparators.com>