Monday, December 13, 2010

VAPOR RETURN SYSTEM CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL



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 vsls 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 mrts 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 +1000mm 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 pressuredrop 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 = 1600MM 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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1 comment:

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