VAPOR PRESSURE , BLEVE ON CHEMICAL TANKERS – CAPT AJIT VADAKAYIL
When we have a fire on board a ship we are careful to make sure that Acetylene bottle bank is NOT on the top face of the compartment on fire.
If there is danger of fire spreading we evacuate these bottles.
Once an Acetylene bottle does a BLEVE, all on board can kiss their asses goodbye ( yes- this sentence is deliberately put this way --I have kept my entire Chemical section clean and dour.).
BLEVE is a short form for Boiling Liquid Expanding Vapor Explosion.
A boiling liquid expanding vapor explosion (BLEVE) is an explosion caused by the rupture of a vessel containing a pressurized liquid above its boiling point.
BLEVEs can be caused by an external fire near the storage vessel causing heating of the contents and pressure build-up. While tanks are often designed to withstand great pressure, constant heating can cause the metal to weaken and eventually fail.
BLEVE accounts for the following key coupled processes:
from flame to tank
from tank walls to the liquid and vapor phases
between liquid and vapor phases
thermodynamic transformations within the tank and pressure buildup
activation of pressure relief valve (or PRV) and venting
wall stresses and material property degradation
depressurization of liquid upon tank failure with concomitant thermodynamic transformations
At sea ship Captains and senior officers do an advanced fire fighting course and we witness a stimulated BLEVE from a safe place, to pound into our heads what a deadly thing it can be.
Again, the fire pressurizes the vessel, causing the relief valve to open, which allows the pressurized vapor to escape. As the liquid level in the vessel decreases, the flames impinge on the vessel wall above the liquid level. The vessel wall rapidly heats up due to the poor heat transfer provided by the vapor on the inner side of the vessel wall. The wall weakens and then tears, resulting in a sudden catastrophic failure of the vessel.
Vapour pressure is defined as the pressure exerted when a solid or a liquid is in equilibrium with its own vapour. At thermodynamic equilibrium, the vapour pressure is a function of temperature only.
Cargoes with high VP, in addition to causing excessive ocean loss at sea, causing global warming , can also cause dischg difficulties due to gassing up of centrifugal pumps ( which are not deepwell ).
Avoid loading in still air conditions.
Products with a high vapour pressure (higher than some 50 mbar at 200 C) can be removed from the tank by evaporation.
Products with low vapor pressure, or products which leave residues or contain inhibitors should never be cleaned by evaporation (ventilation). The non-volatile matter (NVM) content in the specification of the product is a good indication to determine, whether a product will leave residues.
Substances containing inhibitors, even if the quantities are low, must be washed thoroughly. Any inhibitor left in the tank is difficult to remove and nearly always has a negative influence on the wall wash and on the next product to be loaded.
Product mixtures containing low vapor pressure components or crude products should never be cleaned by evaporation (ventilation). The evaporation of the light substances from the mixture may result in residues being left in the tank, which are very difficult to remove.
To understand the dynamics of vapor control system and evaporation growth it is important to understand the theory. Vapour pressure is higher at higher temperatures.
Vapor pressure and boiling point have an intimate relationship. The boiling point is the temperature at which the vapor pressure of the liquid equals the external atmospheric pressure.
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.
VCS must be capable of dischg vapor at 1.25 times the max transfer rate due to this factor.
The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium.
1 Atmos= 760mm HG= 29.92 inch HG= 33.899 feet FW= 1.033 kg/cm2= 14.69 psi= 1.0133 bars= 101.3 kPa (KN m2)= 1013.3 HPa (mb)
Pressure guage pressure + Atmospheric Pressure (10348mm AQ) = Absolute Pressure
Vapour pressure is measured in Pa (Pascal).
1 cm Hg = 1.333 × 103 Pa 1 bar = 105 Pa
1 atm = 1.01325 × 105 Pa 1 mbar = 1 kPa
No evaporation: < 0.3 kPa
Partial evaporation: 0.3–3 kPa
Rapid evaporation: ≥ 3 kPa
THE P OF BOYLES LAW OF P1V1=P2V2 IS ABSOLUTE PRESSURE.
It is important to specify the temperature when stating a vapor pressure because vapor pressures increase with temperature. Also, be aware that there are several different units for pressure. Note that the vapor pressure scale is logarithmic.
FW Temp Vapor Pressure Density
(oC) (kPa) (kg/m3)
0.01 0.61173 0.99978
1 0.65716 0.99985
4 0.81359 0.99995
5 0.87260 0.99994
10 1.2281 0.99969
15 1.7056 0.99909
20 2.3388 0.99819
25 3.1691 0.99702
30 4.2455 0.99561
35 5.6267 0.99399
40 7.3814 0.99217
45 9.5898 0.99017
50 12.344 0.98799
55 15.752 0.98565
60 19.932 0.98316
65 25.022 0.98053
70 31.176 0.97775
75 38.563 0.97484
80 47.373 0.97179
85 57.815 0.96991
90 70.117 0.96533
95 84.529 0.96192
100 101.32 0.95839
105 120.79 0.95475
If we were to place a substance in an evacuated, closed container, some of it would vaporize. The pressure in the space above the liquid would increase from zero and eventually stabilize at a constant value, the vapor pressure.
Finally, recognize that liquids that aren't in a closed container still have a vapor pressure. However, the material will eventually evaporate or vaporize (turn into a gas) completely.
Even though the pressure in our closed container is constant, molecules of the vapor are still condensing into the liquid phase and molecules of the liquid are still evaporating into the vapor phase.
However, the rate of these two processes is equal, so there is no net change in the amount of vapor or liquid. This process is called dynamic equilibrium. For this reason, the term equilibrium vapor pressure is sometimes used.
Most materials have very low vapor pressures. For example, water has a vapor pressure of approximately 15 torr at room temperature. But remember that vapor pressures increase with temperature; water will have a vapor pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).
In general, the higher the vapor pressure of a material at a given temperature, the lower the boiling point. In other words, compounds with high vapor pressures are volatile, forming a high concentration of vapor above the liquid; this can sometimes pose a fire hazard.
The process of evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping. At this point the vapor is said to be saturated, and the pressure of that vapor (usually expressed in mmHg) is called the saturated vapor pressure.
Ordinary evaporation is a surface phenomenon - some molecules have enough kinetic energy to escape. If the container is closed, an equilibrium is reached where an equal number of molecules return to the surface. The pressure of this equilibrium is called the saturation vapor pressure.
Ordinary evaporation is a surface phenomenon - since the vapor pressure is low and since the pressure inside the liquid is equal to atmospheric pressure plus the liquid pressure, bubbles of water vapor cannot form. But at the boiling point, the saturated vapor pressure is equal to atmospheric pressure, bubbles form, and the vaporization becomes a volume phenomenon.
The standard boiling point for water at 100°C is for standard atmospheric pressure, 760 mmHg. It is the experience of high altitude hikers that it takes longer to cook food at altitude because the boiling point of water is lower. On the other hand, food cooks more quickly in a pressure cooker because the boiling point is elevated. Raising or lowering the pressure by about 28 mmHg will change the boiling point by 1°C.
Reid Vapor Pressure is the equilibrium pressure exerted by vapor over the liquid at 100oF., expressed as pounds per square inch absolute, as defined in 46 CFR 30.10-59.
For measuring RVP a container is filled 1/5 and shaken up at in a water bath at 100F and pressure measured. The measurement of RVP said to be “absolute VP” is contrary to TVP as RVP evaporates only the light fractions at 37.8C or 100F using reids apparatus, with a ratio of gas to liquid volume .
HVP cargoes are the ones whose RVP is 14 psi or more—IVP means 8.5 to 14 psi--LVP means less than 8.5 psi
TVP pressure is a useful indication of the extent of vapour pressure which can be expected from a particular cargo especially when it is near atmospheric pressure. Unfortunately TVP is not easily measurable and its value when required is usually estimated form RVP of the cargo and its ambient temp.
Cool deck using sprinklers in case of high VP cargoes lifting vents. TVP is the absolute pressure exerted by the gas produced by evaporation, when gas and liquid are in equilibrium at the prevailing temp.
TVP for gasoline 0.75 bars for crude oils 1 bar.
Cargoes of VP > than 50 mbar at 200C may be removed from the tank by ventilation/evaporation with precautions taken regarding wind direction for poisonous vapors. The NVM content can then be removed from tank by washing.
Vapour lock means boiling in cargo lines due to excessive VP.—or vapour bubble formation is liquid contained in vaccum pipelines .When discharging high VP cargoes the pump RPM has to be reduced and discharge valve throttled at the stripping stage—for Framo pumps with suction pits it is not so important.
High RVP cargoes reduce pumping performance—the Master must issue a letter of prorest.
On VLCC's which load high TVP cargoes up to one bar-- magnetic pressure cone will evaporate cargo excessively , and such huge quantities have added to global warming.
Water boils at 100 deg C , but if we increase the tank pressure theoretically to 217 Bars, water will boil only at 374 deg C. That is why on Propylene oxide carriers, they load tanks with PV lift at +6000 mm AQ, if they have no cooling systems like on refrigerated gas carriers..
VP and BP have an intimate relationship. -- for BP is the temp at which VP of cargo equals AP.
If the siphoning height is greater than 10 mtrs the FW becomes vapour at the top bend.
TVP is the absolute pressure ( pressure guage presure plus atmospheric pressure of 10348mm aq ) exerted by evaporated gas , when gas and liquid are in equilibrium at the prevailing temp.
TVP cannot be measured and its value when reqd is estimated from the RVP of cargo at its ambient temp.
Boiling commences when the TVP exceeds 1 bar. Crude oils may be stabilised so that their TVPs are near, or somewhat above, 1 bar as they enter the ship. In boiling, gas bubbles form below the surface of the liquid, but only down to a depth at which the total pressure (atmospheric plus hydrostatic) is equal to the TVP.
The consequent loss of gas in this region may lead to a local fall in TVP; moreover the latent heat required to evaporate the gas results in cooling which also reduces the TVP. The reduction in TVP in the liquid near the surface from both these causes tends to delay boiling, despite the fact that the TVP of the bulk of the liquid is above 1 bar. That is why crude oils can be handled with their TVPs somewhat above 1 bar.
It does not apply to the same extent to the natural gasoline type of product because the gaseous constituents in a crude oil are only a small proportion of the total, whereas a natural gasoline usually consists almost entirely of potentially gaseous compounds. this means that the availability of gas, where boiling is taking place, is far greater with the natural gasolines than with crude oils.
Natural gasolines suffer hardly any decrease of TVP due to gas depletion when they begin to boil, and boiling is much more likely to continue in their case than in the case of crude oils.
The TVP at the loading temperature of the cargo should be used as the criterion for determining when special precautions are necessary. The Reid vapour pressure of a cargo gives very little guidance unless the temperature of the cargo when loaded is also specified.
TVP is expected to exceed the following values:
--- for natural gasoline type cargoes, for example pentanes plus, 0.75 bar.
--- for crude oils, with or without added gas, 1.0 bar.
To prevent gassing up of cargo pumps ( which is not deepwell ) , the expected TVP of the cargo at the dischg port should, under normal circumstance not exceed 0.7 bars for CPP.
Order for jettison can be given by Captain alone. A risk assessment must be done as per company SMS.
Jettison of cargo is an extreme measure, when all all solutions are impossible. Jettisoning cargo is to be taken when it is the ONLY means to save life, prevent personal injury or to prevent an even greater loss of cargo due to the ship breaking up.
If it is necessary to jettison cargo in the foregoing circumstances, whenever possible, the Master must notify local authorities of the intention to jettison and explain the likely consequences of failure to jettison. Such notification is not a request for permission to jettison.
If an emergency dictates the jettisoning of cargo it is essential that full information is entered in the Master's Official Log Book and the Deck Oil Record Book. Particulars to be recorded include the following:
-Grades of cargo quantities
-ullages and temperatures in tanks before and after jettisoning
-leaks from tanks before and after jettisoning
-cargo valve settings
-time required to jettison
-draft before and after jettisoning
-position of ship
-all other pertinent information.
Jettisoning of cargo must be reported to the nearest coastal state authority.
There are cargoes like Acrylic acid or Propelene Oxide ( violent polymerization ) which may have to be jettisoned for safety of LIFE at sea.
Jettison must be done in concurrence with the company chemical operator , unless an explosion is imminent and time does not permit.
Engine room personnel should be alerted. Consideration should be given to changing over engine room intakes from high to low level and to opposite side of ship.
All non-essential intakes should be closed. Set up water spray curtains with fire hoses , if relevant.
Not to be done underwater—but discharge from manifold with cargo hose set out as much as possible and as close to water level.
Engine sea suction must be from opposite side.
Make sure wind is from aft and go astern
All safety precautions , which involve the presence of flammable/toxic gas in the vicinity of the deck, must be observed. Keep SCBA bottles, EEBD, Oxygen resuscitators in readiness.
A GMDSS warning should be broadcast.
Crew involved must wear PPE.
Potential sources for ignition must be verified.
Every year one “jettison” emergency drill shall be reported to OOPS as a ‘QI Notification’, on Chemical tankers.
A SAMPLE PROCEDURE:--
ACRYLIC ACID JETTISON PROCEDURE
A: Should the Cargo Temperature rise to 34C, inform the chemical operator.
Begin checking the temperature every 30 minutes. Plot on graph paper. Notify company.
If adjacent tanks or any tanks in vicinity are being heated stop heating these tanks. If adjacent tanks are empty fill those with ballast if possible, provided sea water temperature is between 18-30C.
Line up for circulation either through existing drop line or rig portable hoses.
B. Should the cargo temperature rise to 41C.
Begin circulation of the cargo.
Continue temperature monitoring.
C. Should the cargo temperature rise to 45C.
Prepare to pump cargo overboard.
Add cold fresh water to tank as void space permits. Continue circulation. Continue adding fresh water until tank is full, as fresh water supply permits. Closely monitor temperature.
NOTE: SALT WATER WILL NOT DISSOLVE POLYMER MAKING PUMPING MORE DIFFICULT.
D. Finally at 49C do not wait-- Jettison.
Begin pumping cargo overboard.
Continue adding fresh water while cargo is being discharged overboard, as FW supply permits.
When tank is empty start cleaning with cold water and continue cleaning until tank is clean.
One the ship we are aware of DEW POINT before we paint. We know when to ventilate the stainless steel chemical tank , before we load water sensitive cargoes.
Warm moist air always condense on a cold surface. When you drive at night with car AC on, you must know this.
EVER HEARD A JAGUAR ROAR ?
CAPT AJIT VADAKAYIL
( 30 years in command )