VRS
IBC code requires that
toxic and carcinogenic vapors must not be vented to atmosphere while loading,
but must be returned to shore.
IMO recommends that
maximum tank pressure should be 80% ( 1600mm )of the set pressure.
If VRS has to be used ,
then the pressure drop calculations must be completed before arrival. The class
approved vapour control system manuals should be on board, along with sample
calculations.
The ship should choose
the maximum loading rate for the cargo and then calculate the drop to ensure
that the system is capable of effectively delivering all vapour back to the
shore tanks. There will be a meeting between mate and the terminal to properly
set the HP alarm at the shore facility vapor connection.
Before using VRS in US
ports it is a good practise to make the officer/ crew familiar , explain the
VRS class approved manual ,safety precautions, USCG regulations wrt vapour
recovery systems and then issue a
certificate in accordance with 33 CFR 154.94—signed by master.
Vapor
return calculations:
Find out the VD of the
chemical. Say for EDC it is 3.42. from your VCS manual use the graph drawn for
pressure drop in mm Aq on Y axis and loading rate in cum/ hr on the X axis.
Use the relevant curve
depending on the number of tanks loaded simultaneously—to read off the pressure
drop . Multiply this with VD to get A.
Total pressure should
be PV valve setting 2000 mm Aq X 0.8 ---call this figure B.
Anytime the manifold
vapor pressure exceeds B-A---then reduce the loading rate.
DOP has a VD of 13.5 .
it has a very high pressure drop.
Vapor
growth rate:--
As the vapors are
removed from cargo tank while loading, the evaporation of chemical in tank
continues. This means that the vapor
recovered is loaded liquid volume plus evaporated volume.
The VP of a chemical is
closely connected to the BP. Chemicals having high VP evaporate more quickly
that substances with low VP.
Any information on VP
must be tagged to a temperature. At BP, VP = AP.
Concentration of gas
vapors which can be produced in an enclosed space is –
PPM=VP (mm Hg ) X 1300
Using gasoline as a
baseline cargo with an assumed vapour growth rate of 25% the foll formula can
be used to determine the VGR of other cargoes
VGR = 1.25 X saturated
VP at 155 deg F in psi/ 12.5
On the VCS high
pressure alarm is set at 90% of 2000 mm Aq or 1800 mm Aq.
Low pressure alarm is
set at -350mm Aq.
The last 1 metre of
vapor piping before the ship's vapor connection must be painted R/Y/R with the
red bands 10 cms wide. This means the middle yellow band is 80 cms wide. It
must be stencilled VAPOR in black letters.
Each vapor connection
flange of ship must have a permanently
attached 0.5 inch dia stud at least one inch long projecting outwards from the
flange face. The stud fits into a hole in the hose flange. This prevents liquid
hose from being connected to vapor system.
VCS must be capable of
dischg vapor at 1.25 times the max transfer rate.
Vapors are processed,
incinerated or dispersed far away from ship.
Vapors are recovered by
refrigeration, aBsorbsion in lean oil or aDsorbsion.
If your ship has to use
VCS ensure the metal spool pieces are connected in good time—if you do it at
the eleventh hour cargo operations may get delayed as often a couple of spools
do not fit well. Instead of fixed spool pieces upto 3 metre long flexibles are
allowed.
Always ensure the vapor
lines are drained at the lowest point aft. If liquid builds up in vapor linethe
flow of vapor could be restricted causing sudden increase of tank pressure.
USCG wants a VCS drill
every 6 months .Training of personnel and testing of systems must be done prior arrival port.
As a minimum, the
following items of equipment should be located/demonstrated/explained.
Pipelines, drains,
Pipeline bonding strips, labeling, Vapour Connection/Manifolds incl manual
valve, protection stud, Vapour Connection
colouring /paint, Individual tank
isolation valves, Pressure monitoring points/devices/gauges and recorders, Closed gauging system including high level
alarms, Independent High-High/Tank
Overfill alarms, audible/visual alarms,
P.V. protection devices associated with system, Ship’s Oil Transfer Procedures
notice, Maximum allowable
cargo transfer rate and maximum allowable venting rates for individual cargo
tanks.
Read the VCS class
approved manual. It will be a taken amiss if the mate is not familiar with
pressure drop calculations involving pipe friction factors and pipeline ID
for resistance coefficients as explained
in the ships VCS manual. The groups of pipelines being from tank to spool,
spool to manifold and at the vapor manifold.
NOTE: A good VRS manual
will NOT require you to do below
calculations-- as all info can be lifted off the graphs- pressure drop
versus loading rate , on curve or the target vapor specific gravity.
VRS PD units—
PD/ mm Aq
Friction coeff factor/
0.02 ( 0.017 to 0.022approx for a standard 20000 DWT chem. Tanker )
L and D in metres
Flow velocity in pipe/
Mtrs per sec
G=9.8 mtrs/ sec2
S= specific gravity of
vapour in kg/ cum –VD multiplied 1.2818
Pressure drop= (F X L/D +
R) X V2/ 2G
X S
Where –
F/ friction factor
L/ length of pipe
D/ inner dia
R/ partial resistance
factor OF ELBOWS TEE AND VVS
V/ velocity in pipe
G/ 9.8 m/s2
acceleration due to gravity
S/ vapor SG IN KG/ CUM
( VD X 1.2818 )
PD= total branch res
coeff X branch Vel head X SG of vapour
TOTAL PD in MM AQ= PD+
vent pipe head (VPH =height of vent in metres
3.2 Mtrs X SG of vapour in kg/ cum)
Branches are:--
A/ tank to spectacle of
VRS pipe ( PV vent pipe head has to be added only here also –total resistance
coeff and vel head is max for this branch so pressure drop is highest for
branch A , B and C branches have very little pressure drop )
B/ spectacle to VR
manifold
C/ vapor manifold
Res coeff of ONE number
( typical ):
tee in small/ 1.4
tee in same/ 0.38
tee in big/ 0.33
butterfly VV/ 0.9
entrance/ 0.5
add up to get partial
res factors of branch A B and C
V= VAPOR FLOW RATE/
PIPE SECTIONAL AREA
VFR IN CUM/ SEC=1.25 x
CARGO LOADING RATE IN CUM PER HR / 3600
MAX LIQUID VEL IN CARGO
LINE TAKEN AS 7 MTRS / SEC ( 476 CUM/ HR FOR 150 MM DIA AND 340 FOR 100MM DIA )
MAX LIQUID PRESSURE AT
MANIFOLD/ 10 KG
Before the pipework
losses can be established, the friction factor must be calculated. The friction
factor will be dependant on the pipe size, inner roughness of the pipe, flow
velocity and fluid viscosity.
The Darcy friction
factor is a dimensionless number used in internal flow calculations. It
expresses the linear relationship between mean flow velocity and pressure
gradient. The pipe diameter times the drop in pressure in the fluid due to
friction as it passes through the pipe, is divided by the product of the pipe
length and the kinetic energy of the fluid per unit volume.
French engineer Henry
Darcy’s equation is used to measure the
PD through the PV in some VRS manuals .
Be aware that Darcy’s
FF is 4 times larger than British engineer Fannings FF which is used in some
other manuals.
Test vacuum of the PV
by using a graco pump to create a vacuum of 0.035 bars in the pipeline , till
the LP alarm rings. To test the pressure side inject compressed air to 0.21
bars . let is hold for a time test of 30 minutes.
Definition of USCG max
loading rate is the lower of ------
1) 80% of total venting
capacity for pressure relief valve.
2)The rate based on
pressure drop calculations at which, for a given pressure at the facility
vapour connection the pressure in any cargo tank connected to the vapour
collecting system exceeds 80% of the setting of any pressure relief valve (2000
mmAq) in the cargo venting system.
Condensed vapors cause
blockage which increases pressure drop. Also certain cargoes can cause polymer
build up. High VP cargoes containing Hydrogen Sulphide deposit pyrophoric iron
deposits in the vapor line.
There are 2 systems—
One uses vessels
pressure to push vapors.
Other requires a
booster or blower or evacuator because pressures become too high on
vessel. If this device
runs without liquid being loaded there could be vacuum rupture of tank.
Pressure drop in vapor
collection header is equivalent to build up of pressure in tank.
Some ships are fitted
with rupture disc to prevent tank collapse. Remember that the graphite discs
are directional and cannot be put upside down. The range is +2000 to -500mm Aq.
All vessels must have a
vacuum valve at the header ( with a means to test ) as per 46 CFR 39.2 -11 (b)
2. this vacuum relief setting must not be less than the lowest vacuum relief
setting of cargo tank PV vent.
There should be an
insulating flange between vapor hose and shore facility connection.
During STS vapor
balancing is not allowed between inerted and non inerted tanks.
Vapour Balancing means
the transfer of vapour displaced by incoming cargo from the tank of a vessel
receiving cargo into a tank of the vessel or facility delivering cargo via a
vapour collection system
Irrespective of whether
the vessels are inerted or not, an electrical insulating flange, or one length
of non-conductive hose, must be provided between the vapour collection system
connections of the service vessel and the vessel being serviced.
VECS can serve tankers
fitted with inert gas as well as non-inerted tankers.
The vapor connection at
the ships manifold must have a 1 inch stud, half inch in diameter at the 12 o
clock position of the presentation flange, in order to prevent accidental
misconnection of liquid cargo line. The vapour return line should be provided
with a drain to collect any liquid condensate , at the aftermost lowest point.
USCG requirements:--
There must be a
pressure indicator located in the CCR, 46 CFR 20.13(a)
There must be an
audible and visible HP and LP alarm in CCR,46 CFR 20.13(b)(1).
The pressure drop
through the vapor collection system for the most remote cargo tank to the
vessel vapor connection must be determined for each cargo handled, 46 CFR
39.30-1(b)(1). It must be included in
the oil transfer procedures as a graph or table, 46 CFR 39.30-1(3).
Information on the
vessels VCS must be contained in the oil transfer procedures. They must include the information required by
33 CFR 156.120(aa), 33 CFR 156.170(g) and 46 CFR 39.30(b)(3)
If the vessel collects
vapors from incompatible cargoes simultaneously, cargo vapors must be kept
separated throughout the entire system, 46 CFR 39.20-1(a)(2).
Piping must be
electrically bonded to the hull, and must be electrically continuous, 46 CFR
39.20-1(a)(5).
An inerted tank ship
must have a means to isolate the inert gas supply from the vapor collection
system, 46 CFR 39.20-1(a)(6). (Isolation valve as per SOLAS II-2 Reg. 62.10.2
may be used).
Vapor collection must
not interfere with cargo tank venting, 46 CFR 39.20-1(6)(b).
A manual isolation
valve must be provided at the vessel Vapor connection. Clearly indicating whether the valve is
opened or closed, 46 CFR 39.20-1(6)(c).
Unless, the valve position can readily determined from the valve handle
or valve stem.
Each hose used for
transferring Vapors must have a 25-psig design burst pressure and must be
tested in accordance with, 33 CFR 156.170(g), 46 CFR 39.20-1(f)(1).
Hose must have 5-psig
Maximum Allowable Working Pressure (MAWP), 46 CFR 39.20-1(f)(2).
Hose must withstand
2-psig vacuum without collapsing or constricting, 46 CFR 39.20-1(f)(3). Electrically continuous with maximum
resistance of 10,000 OHM RESISTANCE, 46 CFR 39.20-1(f)(4).
Vapor hose must be
abrasion resistant and resist kinking, 46 CFR 39.20-1(f)(6).
RED / YELLOW / RED band
at last 1 meter at each end of the hose, 46 CFR 39.20-1(f)(7).
Vapor hose handling
equipment must provide adequate support, 46 CFR 39.20-1(7)(g).
Provide a closed
gauging as per, 46 CFR 151.15.10. it must allows the operator to determine full
range of liquid levels in tank, 46 CFR 39.20-3
(a)(2). Indicates liquid level in CCR, 46 CFR
39.20-3
(a)(3) If portable, is
installed on the tank during entire operation, 46 CFR 39-20-3 (a)(4).
Provides visual
indication of the liquid level when level is within 1 meter from the tank top,
46 CFR 39-20-3(4)(b)(1)
Fluid flow in pipes is
affected by many different factors:
The viscosity, density,
and velocity of the fluid.
Changes in the fluid
temperature will change the viscosity & density of the fluid.
The length, inner
diameter, and in the case of turbulent flow, the internal roughness of the
pipe.
The position of the
supply and discharge containers relative to the pump position.
The addition of rises
& falls within the pipe layout.
The number &
types of bends in the pipe layout.
The number &
types of valves, & other fittings, in the pipe layout.
Entrance & exit
conditions of the pipe work.
Max loading rates—
Cargo loading rate can
be restricted by the least of:--
Pressure drop
Cargo velocity
Data:--
PV valve setting
pressure ( vapour volume / 1.25 for
pressure side=liquid rate )
SUS 316L/ Sch 20 / 125A
dia is 0.1298—285 cum/ hr
150A dia is 0.1552 --408 cum/ hr
250A is 0.2544—1097
cum/ hr
Pi R 2 x 6 x 3600 (
velocity of 6 mtrs per sec )
DOP/ VD 13.5/ 300 cum
hr ( 10.4 to 13.5 )
Butyl benzyl phthalate/
VD 10.3/ 350 cum per hr ( 7.7 to 10.3 )
Di ethyl phthalate/ VD
7.66/ 400 cum hr ( 4 TO 7.66 )
Gasoline/ VD 4/ 550 cum
hr ( <4)
Vapor Density. The
weight of vapor or gas compared with an equal volume of air.
Air has been assigned a
value of one. Vapors that are heavier than air, such as gasoline or hydrogen sulfide, have a vapor density greater than one and accumulate in low places.
The vapor tightness
certificate signed by ch officer and master has to be sent to the terminal 24 hrs in advance in standard
ports of USA. ( remember they want it
in their corny units-- their CFR style -- so get used to it!)
Method is as per USCG
CFR 63.563 (a)-2-1
Pressurize tha tank
using dry air to say +1000 mm aq ( Pi ).
Then cut off the air. Wait for 30 minutes read off the pressure ( Pf).
Reduction of pressure
P= Pi-Pf ( in inches of
water ).
1000mm Aq is 100mb or
40.146 inches Aq.
1 Psi= 7.379 Psia.
Reduction of pressure
shall be now compared to the pressure drop using the formula –
Pm= (0.861 X Pia X L) /
V
Where Pm = max
allowable pressure change in inches of Aq
Pia= Pressure in tank
when air source is cut off
L= Max permissible
loading rate in barrels.
V= volume of tank in
barrels. ( Cum X 6.2898 ) . Ship has big
and small tanks.
If P is < than
Pm tanks are vapour TIGHT. Reverse means tanks are NOT tight.
Loading rate Cum/ hr=
3.14 X R X R X 6X 3600 ( for 7 metres/ sec flow, where R is inner radius )
PRACTICAL ASPECTS :
Loading rate Cum/ hr=
3.14 X R X R X 6X 3600 ( for 7 metres/ sec flow, where R is inner radius )
On FCOL small tank is
thus 333 Cum/ hr -- and big tank 477 Cum/ hr max loading rate
Tank PV riser HP
setting is +2000mm Aq ( 0.2 Kg or 17.12 Psia)
Vapor growth rate is
tabled readymade by DNV (or in some ship’s VRS manual).
Liquid flow rate= gas
transfer rate / vapor growth rate.
VGR for Ethanol = 1.04
Lamda or Vapor air
density in DNV tables= 1.498 kg/ cum ( for the graph )
USCG regulations:
For shore= 33 CFR 154
For ship= 46 CFR 39.
All systems must be
tested 24 hrs prior arrival.
VD X 1.2818 is air
specific weight
GRAIN:
A + B = C
600 + 900 = 1600 MM Aq
A= Max allowable shore
header Pressure Drop ( manifold back pressure )
B= Ships pressure drop
lifted off from vertical axis graph in VRS manual
( against loading rate
on horizontal axis and curve for Y Lamda = 1.498 for
Ethanol . This includes
vent head of 3.2 mtrs of that vapour of SG in kg/cum.
C ( VRL indicator in
CCR ) = 1600 mm Aq ( 80% of +2000 mm Aq – CCR
alarm to call for
imminent loading rate reduction ).
Pressure drop depends
on:--
VD of Ethanol
Friction factor of
pipelines , bends, spools, valves
Length of vapour
pipelines from tank till manifold
Velocity of flow
ID of pipelines
Condensed vapours
increase PD
Acceleration 9.81
Max VCS high pressure
alarm setting is 90% or + 1800mmAQ/ -350
VRS has insulating
flange
VRS must be capable of
dischg vapour > 1.25 times max loading rate.
PV valve +2000 mm Aq
must never lift . Shore must be able to process the returned
vapour. Meeting between
ship and shore in CCR is to calculate the alarm pressure setting
ashore. Ensure VRL cock is well drained aft.
VCS drill every 6
months. Test everything 24 hrs before .
Vapour recovered
ashore= loaded liquid volume + evaporated vapor growth volume.
Shore VR hose designed
to burst at 2 bars. At 2 psi vacuum , hose should not collapse.
For chemical tankers (
vapour +_air ) mixture density of 3.6 kg/ cum and VGR 1.2
will cover all chemicals except those with a
RVP > AP ( IBC code 15.14 )
Ch Officer to fill up
VRS Pre arr and Pre-operational checklist. At berth in pre-load meeting
find out if ship’s tank pressure pushes gases
out OR if shore is using a vacuum—in
which case if cargo stops suddenly there will be sever vacuum in ship’s tanks.
Shore Vapor return hose
must have support.
Include Vapor return
system in USCG oil transfer procedures. Test H/L VRS pressure sensor
alarms CFR 20.13 (b)(1)
Always watch the CCR
VRL pressure indicator to ensure it never reaches 1600mm , if so the cargo rate
has to be reduced.
There has to be an
insulating flange between ship’s VRL manifold and shore VR hose.—or a length of
non-conducting hose.
Use the correct curve
for Lamda Y to read off pressure drop on vertical axis—against loading rate on
horizontal axis.
Choose for ONE vapor
line or TWO vapor lines.
Choose for 4 tanks loading at common rail or
whatever.
Pressure drop depends
on Friction factor of ships vapor pipeline lenghts, diameters, spools , valves
and bends. And also VD of Ethanol and velocity of gas flow..
VCS must remove gas at
125% of the loading rate.
ANY TIME MANIFOLD
PRESSURE (A) EXCEEDS 1600 MM AQ (C) MINUS PRESSURE DROP (B) —REDUCE LOADING
RATE.
OR IN OTHER WORDS
MANIFOLD PRESSURE (A) +
PRESSURE DROP OF CURVE (B) MUST NEVER EXCEED 1600MM AQ (C)
CAPT AJIT VADAKAYIL (
28 years in command )
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