What Do Ships Use THESE For?
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- Опубликовано: 10 мар 2022
- In this video, we explore what happened on the Fairchem Filly when its PV Valves did not have sufficient capacity to vent the gas that was pumped on board.
We see how PV Valves and PV Breakers are used to maintain a safe pressure on board cargo ships.
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I think discussing non-dramatic (i.e. no fire or explosion or loss of life) accidents like this one is immensely valuable.
It really is, these sorts can be just as dangerous and just as expensive. As say a fuel leak followed by a fire. Not to mention most accidents are of these sort of non dramatic nature. It’s good to have knowledge like this for sure.
@@87slashers34 This story also illustrates the value of safety measures. The combination of safety cutout and double-hulled tanker surely prevented a much worse incident.
Basically "This accident cost $100,000. Next time it could cost lives. Let's make sure there isn't a next time."
I like to cover these too because they provide equally valuable learning opportunities, but get far less publicity.
@@toddkes5890 It cost 100K in lost cargo. Do you think tank repair and time lost in dock were free?
1:48 Yay, a Finnish emergency shutdown button. :D
(Nödstopp is obviously Swedish but the other side says "Hätäseis" and this combo would really only happen here.)
Torilla tavataan
Meh, jag som var glad den var på svenska!
I figured it was Swedish, (had no idea about the other word) which is funny since this accident happened in Texas.
@@Phootaba the other side is in Finnish, which means it’s a Finnish button.
@@chri-k It could be Swedish. You'll find plenty of stuff in Sweden that uses both Swedish and Finnish/Danish/Norwegian, as that allows an identical product to be sold in several countries. If the word is similar in more than one language, Danish and/or Norwegian may be omitted.
As someone versed in railway boilers, hearing about pressure relief valves with variable pressure release points scares me. Even though it wasn't the problem here, it still frightens me that you can overpress your tanks by simply overweighting/overspringing the PV Valves.
Well that's a matter of negligence/incompetence if they ever mess with the weights/springs, for something like the ship's tanks it would make sense to just set it bellow rupture and never increase, but I can imagine some specific scenarios where you would want to decrease the maximum pressure (say if the chemicals being loaded can end up diffused with the gas loaded if the pressure is above a certain point... that's just a scenario I imagined tho nothing I can name).
In other hand I think it's something you just set on install instead of making several different valves. More of a matter of having common parts instead of several, not needing adjustment itself.
@@Kalvinjj you are right, of course, but in the early days of the railways, engine drivers would overpress their engine's safety valves to get more power out of the engine (often leading to an explosion.) I suspect it isn't unheard of that shippers unofficially raise tank pressures to fit in more product - there just hasn't been a disaster yet. Intelligence and safety go out the window when 'efficiency' could be promoted.
@@ajaxengineco Oh you do rise an interesting question... Haven't thought of that frankly, would be important in vessels that carry gasses. Wrong (by a huge margin) weight listings on containers sure is a thing after all (even tho that's more on the one paying the shipping).
@@ajaxengineco Interesting point.
I'd gues that in case of engine even small increase in pressure will give you significantly more power but in case of cargo hold every additional rise in pressure will give you significantly less extra product. And the risks are basicaly same.
I can't see any Incentive to do so though. Messing with the Safety Valves of a Boiler increases Power and Efficiency at the Cost of Safety. But since the Cargo itself isn't gaseous, increasing the Tank Pressure doesn't increase the Amount of Cargo you're transporting. So why should you intentionally mess with the Safety Valves? The only Danger I can see is forgetting to adjust them.
Excellent presentation of a safety mechanism.
It'd be interesting to see why did the low pressure alarm go in the first place. Or was the pressure too low indeed but the nitrogen valve then was opened way more than needed?
My guess would be a sensor failure, but I could be wrong. Since the cargo is forced out by the nitrogen line, I can't see how an actual underpressure could happen there
The cargo was being pumped out faster than the nitrogen came in initially. That's why they opened the valve more.
@@CasualNavigation I still find it interesting how this issue came to be. I assume that before unloading, you'd make some sort of calculation on the release rate of the load, and thus how much nitrogen must flow in, and thus how far a valve must open. Was this all manual guesswork then?
@@CasualNavigation Couldn't the chemical just be pumped out by the nitrogen pressure? or it would be too risky and slow?
@@DEEJAY88 Or nitrogen sucked in by underpressure from hold being emptied?
A few things:
The PV valves are seperated into three segments, not two: Main body (the pipe) Upper house (pressure side) and vac side (vacuum side)
When releasing pressure the high speed is also needed in case there is a fire on the deck. As the air travels faster than the burnspeed, fire can't make its way down through the release.
At the vacuum side there are two very fine metal grids, these prevents fire from comming in when it sucks in air.
Source: Me, i used to make these things at my old work (Pres-vac engeneering)
Very cool! Any interesting stories to share from your time in that industry? New advancements, strange problems you weren't expecting, etc?
@@itsmeepo4203 I wasn't realy involved that much in RnD, so can't say much about that kind of thing.
And most of the problems i faced was generic factory problems of the RnD people mocking around with something, and forgetting to tell people making a whole bunch of confusion.
Think the wildest we had was a russian tanker back in 2017 or so which needed new valves becouse their valves all just kinda died, it was a super mega rush order that was picked up by helicopter
I don't work on a ship and I'm rarely on the water. I still find these videos fascinating.
Cheers Conrad!
I love the channel and your videos! As a licensed engineer under STCW and port state officer, I use them to train others and they have increased my own knowledge as well.
For the first time I do actually have a small bit of feedback. SOLAS II-2/4.5.3 and II-2/11.6.3 and IBC Code chapter 8 are some of the most convluted, annoyingly written regulations in the industry. But they do require tankers to have two means of protection for over/underpressurization of cargo tanks. For most oil tankers and low-grade chem tankers this will be the P/V valves on each tank, and a P/V breaker on the main vent header.
But most Type 1 chem tankers which carry the grades of cargo in this scenario do not use P/V breakers because of cargo compatability concerns. But because they have small cargo pumps installed within each tank, they can use high and low pressure alarms on the tanks to replace the P/V breaker.
I'm not intimately familiar with this vessel, but would be extremely surprised to see a type 1 chem tanker with a P/V breaker. Because a P/V breaker on such a vessel would conflict with regulations on cargo segregation found in IBC Code 3.1.3 and 46 CFR 150
Now you mention it, I am not sure if this tanker did have a breaker, but I wanted to talk about them as they are one type of pressure relief system. Maybe I should have demonstrated that part on a different vessel though.
I can only imagine how many fingers are pointed at how many people in the first 48 hours of that accident. …. He did it, she did it , must be a defective valve from some company, someone put the wrong pressure in the system, Tanks were improperly manufactured….
Blamestorming?
Nope the Accountant was just too cheap with Tank Materials …
I'm going with Jethro manning the nitrogen valve...
Makes me wonder why there wasn't a flow control valve on the ships side just integrated into the piping. This would enable you to hook up larger than designed for gas systems and still not exceed the 17.000 cfh limit.
Same, I mean you can't go full "only accept the right type of line" that would ideal, but some har ors might not be equipped to handle it, so having a flow restrictor on the ships side would be second best.
After watching your videos as soon as I can and rewatching many others I really believe that you would make a great teacher. I know I've learned a lot about things I would have had no interest in otherwise so thanks for what you do. 👍
Thanks. I've always enjoyed teaching, so I count myself lucky that I can help so many people learn through this channel.
I would watch a full hour documentary about almost every topic this channel does as long as they produce it
Currently working on a chemical tanker, I love your videos. Thank you and keep 'em coming.
The real question here is actually, why wasn't the nitrogen supply stopped quickly enough after the overpressure alarm went off? Too small margins? Wrong alarm level set on the sensors? Failure of communciation? Too slow valve actuation on the supply line?
Casual navigation videos are my absolute favorite on RUclips. I get really excited every time there's a new video, it's that same feeling I got in high school when I was about to go to "the fun class" with the fun teacher after surviving the other boring classes
as a chemical engineer i'm kind of horrified that there wasn't any secondary pressure relief device in form of a burst disc. is it common on cargo vessels to not have anything like that? it would still have ruined the cargo, but at least it should have prevented a rupture of the cargo tank
Wouldn't a burst disc endanger the crew again?
As said in this video the pressure relief is build in such a way to safely discard the gas.
Whereas with a burst disc the gas would stay on the deck.
@@Jehty_ fair enough, though you can also install it high and away from the deck, to minimize that. and i'd rather have just the gas collcting on deck and be warned with an alarm, then have the tank under my feet rupture and then most likely still have the gas collecting on deck or worse in the hull
@@saladiniv7968 or have it be connected to a pipe on the side of the tanker, so it spews over the side, away from the deck, cause it’s going to end up in the ocean either way
@@Jehty_ A burst disc is a last resort and it endangers the crew less than the cargo tank bursting!
All of my rocket test stands and rocket vehicles had relief valves AND burst discs, with pipeaways to control flammable vapors if needed. For the Lynx vehicle with integral low pressure fuel tanks in the wing strakes, I invented a resettable 1.25" burst disc that used magnets to hold a plug in place. It could be adjusted to open at pressure differences from 1.2 to 11 psi, with repeatability within 0.15 psid.
Okay, maybe it *is* rocket science, but it's really damn *basic* rocket science.
I love your channel bro, supreme part of RUclips, creators like you make this platform worth watching, keep it up
Classic example of workers doing things because they were told, and not understanding why. There should be someone managing this who is well trained. But likely that person would cost more, so they weren't hired.
If you’ve ever worked on these Chem ships especially in Houston area 99.99% of the time it’s foreigners working the ship that really don’t know what they’re doing except for the pump master other then that they couldn’t be bothered to know shit like this.
Yeah. One accident every few years (that might be covered by insurance) is cheaper than paying skilled workers in the long run.
I had no idea I needed to know this - until I knew it. (Pretty sure I've said that after watching other of your presentations!)
Great stuff - thanks for the work it takes to put these together.
damn, I love that you're getting more videos out! Quality stuff. I genuinely enjoy content like yours so I always love to see it when it comes out
Thanks Model Gaming. I am enjoying making them more frequently now, as it lets me experiment a little more.
Sir, thank You for the another one of these... You are one of the kind and I love Your videos, every second of them!
Glad you like them!
"Wow, there's a low pressure alarm on the tank, reminding us all that since nothing is automated we have to watch it carefully and regulate it by hand ! So let's just open the Nitrogen full blast and go have a beer without paying attention to it anymore"
A very broad and “offensive” message. A pure blaim culture idea.
Maybe the terminal increased the flow of nitogen themself. Opening up a valve a shore and the high inflow if N2 out of a 4” hose cant be handled by any normal pv valve.
Yes, an incident occured, but why the blame directly towards the vessel?
@@bas6983 No. If the low pressure had gone worse and caused damage it would have been an accident, the second time should not have gone unnoticed. I work in automation so basically i replace human control with machine control, i know it's not possible to have someone check everuthing all the time, but once a red flag is raised you can't ignore it anymore.
Once you're warned if you fail to pay attention to it it's someone's fault. Either the guy in charge or a more global reason which caused the person to be otherwise occupied.
I blame no one because i don't even know if it's the vessel crew or the terminal that should check this and I don't care. I just pointed out the fact that basically "fool me once shame on you, fool me twice shame on me" happened here, and worded it this way for humorous reasons
@@crezychameau the ship has the pressure readings of the cargo tanks. The low pressure was noter and acted upon. At that moment the deck watch was most likely asked to open the valve a bit more. Check opened valve chief. At that moment you’re waiting what the situation and pressures are doing. If the valve was fully opened and a large amount of high pressure n2 entered the tank. This will lead to a high pressure alarm and potentially before the situation was fully understood the incident occured. You work in automation. But you do not work at a ship or in the tanker world. Not a single ship is the same as the next one. Not even full sisterships. All terminals over the world ar different and. Crews are limited. Mulitple layers failed in this instance and defenitly not only at ship side. The terminal should have reduced the max flow available to the ship to a more or less acceptable rate. The crew partly failed as they accepted the n2 hose etc etc. But pointing directly at persons is exactly what not should happen.
You mentioned. Low pressure alarm. Open the valve and forget. Get a beer/tea/softdrink and forget. Thats blaiming a crew in western european non native english speaking terms. We’re looking at a 10m clip and making conclusions. Thats just dangerous.
I dont work in maritime, and have very little intrest in ships, but you just draw me into every video. Its the quality and your voice that keeps me coming back for more.
Thanks. I'm glad you are enjoying them!
As always, beautiful video! Relaxing and informative!
Thank you! Cheers!
Thanks for sharing. As a Process Engineer, i think that Electronic Pressure Vacuum valves are more effective than ordinary one but the operation may go expensive.
I like learning random knowledge that I will probably never use in my entire life. Great video!
Glad you liked it!
Great work, thank you very much
These are amazing please keep going - very much enjoyed!!!! :))
Found this channel on accident. Not disappointed, good work.
Hasn't anyone in the shipbuilding industry ever hear of an orifice? If the ship's manual recommended a 1" hose, install a 1.2" orifice and the hose size is immaterial. It shouldn't take a rocket scientist...
Variable speed pumps may be more expensive, but they'll pay for themselves in just a few voyages by allowing better control.
The industrie has. But the system is not designed tubbe used with an orfice. Extra flanges in the system are not allowed by many terminals. The cargo pumps of a vessel are mostly variable speed. The rate is depending again on a lot of variables depending shore line diameter. Max flow rate approved by vessel and terminal.
I was thinking of a pressure regulator like high pressure gasses. I guess the flow rates are too huge
@@MrNoobed and these are available but not fail safe. There is reason why the crew appearantly asked for a 1” n2 hose and as I mentioned before in most cases the vessel gets displaced shore tank atmosphere back on board. Supplying high pressure n2 is in NW Europe very uncommon when using it as padding system.
Actually, the respective company and it's qhse manual does recommend an orifice, it's supplied almost on every ship. They just didnt install it, probably because they downplayed the risk and ofc their overconfidence to manage it.
@@bas6983 That's much like the vapor return systems used in gas (petrol) stations in California- as the fuel fills the car's tanks, the vapor is transferred back to the station's storage tank, keeping the noxious fuel vapors from being released into the air. Using high-pressure N2 to fill the ship's ullage leaves the hexane vapor in the shore tank nowhere to go but out into the air, which is both wasteful and polluting.
This is another case where doing good (not polluting) also allows doing well (not breaking a ship).
I think discussing non dramatic ones but it's a worth of shot
These are always amazing
A clear case of overconfidence by part of the port workers...
Thank you for the clear explanation of the technical details of the incident, which are very useful for nonspecialists of this branch of applied engineering...
CashNav your video graphics are getting better and better! Nice!
Thanks. Its always fun to continuously try to improve.
Great video
Fascinating!
The more I see of videos like yours......the more I realize how little I really know. It's quite humbling.
Excellent vidéo!
Anyone else like me who didn't understand pretty much anything but enjoyed the video nevertheless?
What exactly didn't you understand? Maybe we can help you?
Glad you enjoyed it Stanimir!
Great video sir!! 😀👍👍
Thanks!
Based on the picture that advertises the video those are vent covers to prevent the oil storage tanks for being over pressured and it has a release valve to that does not allow fire to go back into the tank
Very interesting video!
Glad you enjoyed it
My guy I know these are meant to be educational but I have no knowledge of sailing of any type and I find these all fascinating
Funny timing, I just had an exam 1h ago on this exact topic! :P
Hope it went well.
As a Nautical Student who is about to become a Officer soon i keep coming back to your videos. not that i learn anxthing whatsoever as it should bne obvious to you but i really really enjoy the style of presentation. Its always nice to see the effortt put into making easily understandable shorter vids to explain the really really different world at sea. Whenever someone asks me about the stuff i tell or experience and they come up wiuth question i just direct them to this channel and know that it will provide a excellent job explaining it - far better than i wouldve done even though i know what im talking about :D
I also watch them sometimes to just rebrief me one sth short.
As you are a group and i know how small the seafearing world is ill give a hint to the whereabout of my location and maybe some seaferer will recognize it: Close to me is one of the Main fabricants of the Lifeboats (free fall and David launch). It starts with "Fa" and while i grew up next to this shipyard i always get happy when im on board somewhere on the other side of the globe and see the branding inside the device that is supposed to save me - knowing it was manuifactured in a country which is known to work percisely and efficient
Cheers Aaron. I am glad you enjoy them and they are useful for helping others to understand the maritime world.
Sorry but your comment was complete nonsense and you should humble yourself. I might also add that proof reading might be part of an officers tasks.
@@ag4103 i apologize for that And you are right.
netflix: "are you still watching?"
someone's daughter: 0:14
Tldr ships use these to Dock into wet Dock connectors on other ships, commonly referred to as a Bussy
😳
Fuck yea new CasNav!
I like that thx that also work at tiny sailors world
We also had these on our barges.
After watching so much Plainly Difficult, I was worried this was going to lead to a massive explosion that wiped out half the city lol
Interesting
Judging from the shape, I'd say it's used on naughty cabin boys
So basically like when you inflate an air mattress until it bursts. Works with air mattresses, works with balloons, works with dinghies, works with tankers!
Exactly!
So the initial alarm of low pressure was a true reading and the crew simply overcompensated due to the large hose diameter?
If that's the case, then couldn't they have just slowed down the rate you pump out the chemical instead of increasing the rate you fill with gas?
Obviously, hindsight is 20/20... but was that an option?
@@loayzag91 It would depend on the motor that the pump used. If the motor was powered from a VFD/VSD (variable speed drive) then it would be technically possible to adjust the flow rate. Many motors just run straight off the mains and as such are either on or off.
Technical reasons aside, this still isn't a great option because time is money! Slowing down the offloading would not be the best option from a financial perspective (especially as there was no reason to do so, if the inert gas system was operated correctly).
@@charmio Thanks for the response! I figured you wouldn’t want to slow down the rate of pumping chemical out because of time limitations… but I was just wondering if slow, gradual changes in the case of any doubt among the crew could have led them to identifying the problem before parts started failing. But like I said, hindsight is 20/20.
@@charmio Most cargo pumps on chemical tankers are operated by a hydraulic motor. The motor's speed can be controlled by a speed control valve, which determines hydraulic flow rate to motor. Framo makes the most commonly installed system, to the point where hydraulic driven deepwell centrifugal cargo pumps are usually just called Framo pumps. So they can control pump speed, but I'm not at all surprised that they instead tried to regulate the N2 flowrate instead.
And most chemical tankers operating electric pumps are using Marflex systems. These are var rpm systems. Only the smallest variants have a direct drive. In that case you can close the delivery valve to reduce the flow.
In a lot of cases its about “production” and 300m3/hour (or a value set in the pre cargo meeting) has to be kept to keep charteres and owners happy and prevent a letter of protest for slow discharge.
Opening up a nitrogen supply a bit further is very common. You start up and do not have a balanced system directly. The pumprate can be calculated easily but the rate of the n2 will be trial and error. In many cases the supplied n2 is coming directly out of the shoretank as displaced volume.
You blew it.
The thing is, it does not only "absorb oxygen". It reactes with it (oxydisation). That releases heat which in turn can accelerate the reaction. The whole thing can "run away". They were kind of lucky for it not to block up. Second reason für inert gas is the so it can't form a combustible athmossphere (no oxygen no explosion).
Should the shore supply of nitrogen go through the ship's nitrogen system where it can be properly controlled? or was this broken and bypassed?
I think there‘s a typo, that is through the whole Video
And I watch this the second time atm, watched it on release
Isn‘t Nitrogen usually in N2?
At least she didn't explode... And I didn't hear about injuries here, don't remember the original accident, so that's a good sign.
Obviously the problem is at 1:48; they're using a foreign part for the emergency stop. "What does this *Nödstopp* mean?" "Stop _something_ I guess." "What does the other side say?" *"Hätäseis;* Hot as it is, maybe?" "Yep, let's push it that way." ;)
(Special thanks to Petteri for pointing out the Hätäseis.)
In Dutch, "nood stop" would be "emergency stop"
So I have a hunch xD
Nödstopp is emergency stop in Swedish
Hätäseis means emergency stop in Finnish. And you don't push an emergency stop "that way". You just push it in and it stops whatever system it is meant to stop. They look basically the same worldwide, so it doesn't matter whether its a foreign part or not. Total nonsense.
@@TigruArdavi
Joke ----------------->
Your head -------->
5:00. So wouldn’t the smart thing to do be to make the fixed pipe on the ship one inch in diameter so the pressure restriction can’t be overridden by someone selecting the wrong hose?
I'm curios about this as well - good question.
Any naval / engineering types got the answer?
That poor chief mate, lol
I hate Vopak and ITC
basically it's fixed with automatic pressure regulating valve from IG system, I think there have some sensor issue
The valves don’t need sensors or other electrics to work. They just work manually by themselves, the problem was that the valves couldn’t release enough pressure because the supply was way to big.
They look... interesting
I recon they were rushing. Hence the 4 gauge hose.
Interesting that the didn't use burst-plates or designated weakpoints to at least prevent structural damage if the PV-valves fail.
TIL: if a ships says you use the 1 inch hose. you use that. no substitues.
I am surprised this was not enforced by a restrictive plate limiting the diameter.
If there was no 1” hose available?? I
Great video but youtube comments are glitching today lol.
Glad you liked it!
If they were pumping out the Hexene too fast and it caused the vacuum protection to open could the air that comes in ruin the Hexene as well? If it could is there some kind of countermeasure used to stop the air from damaging the Hexene?
Air won't ruin hexane. I think it's a mistake explanation. The purpose of nitrogen is to prevent hexane vapor to build up and possibly explode.
A (elektro)mechanical over pressure system would probably have prevented this situation. When the pressure in the tanks rises or drops to much it would automaticaly stop all pumps and closed al ingoing and outgoing valves of the tank until manual reset.
One day I'll be mature enough to not laugh at the thumbnail. Today is not that day 😆
What did they do with the contaminated hexane?
Not good but it seems safer than lifting safeties on a 1200psi boiler. That will seriously increase the watch-stander's pucker factor.
A good night out.
You would think that a large refinery operation would have different diameters of chemical hoses and adaptors to insure the wrong hose is never used. Using a 4" high pressure N2 filler line into a ship receptacle only designed to accept a 1" high pressure receptacle. It could have been much worse involving far deadlier chemical you never ever want mixed together.
Shouldn't they use pressure regulator on nitrogen line? That would ensure supplying the gas with proper pressure that does not exceeds the structural limits of the tank.
Weird that the input of nitrogen should be controlled by hose diameter, not by a flow regulator inside the ship...
Surprised there no automatic pressure control system on there
Yea, but how do you determine which Santa Claus is the _real one_ in a shopping mall?
one of the properties of Nitrogen is that it wont compress. So as the tank fills and the vapor volume is replaced by liquid, the pressure goes up rapidly. This problem can be compounded by the shape of the container (one that gets narrower at the top, or a tall thin compartment) Also only a small amount of Nitrogen is needed to make an inert atmosphere so... some planning was not done correctly. also the compartments that are pressurized need to have a separate pressure relief valve designed to lift at a certain pressure different than the ons in this video.
What? No.
They dont use liquid nitrogen, they use gas nitrogen, that is compressible and that need to replace all other gases to make an inert atmosphere, not only a small amount
nitrogen absolutely is compressible, it’s a gas.
Nitrogen gas compresses quite easily. Just as easily as air, actually.
You may be thinking of liquid nitrogen, which compresses just as easily as water, but is also far colder, and likely wasn't involved in this scenario.
BS, it's replased by gaseous N2. Every gas is compressable.
kinda interesting
wondering if any crew were injured?
Thank you for your videos! One thing I'd like to argue about this video, is your explanation of pressurizing hexane tanks with inert gas. I really don't think it's done to prevent oxygen or moisture absorption by hexane. Hexane is non-polar solvent and it doesn't mix with water or gases. But it's flammable. So I would suggest that pumping non flammable nitrogen is done to prevent build up of hexane vapor and eliminate risk of fire and explosion.
A word on how the situation was handled and perceived could have been added - the story now stops a bit short of being a story. Did the crew get away safely, were they in any danger to begin with, was this accident reason for any changes to design / setup of such ships, their tanks and safety systems?
Are you swedish or did you just find a picture of an emergency stop button in Swedish?
In my opinion a rupture disc should be installed on every tank that could ever break from over pressure.
$100k seems like a small number here
It is, but it sounds good!
I will be hunted in my dreams by how you draw a human. Scarry stuff, nit gonna lie.
11:49 Nödstopp 🇸🇪
It's concerning still that hazardous chemicals are just released into the air instead of a tank of some kind. I would say filters, but I would imagine that would slow the escape rate too much.
Silly, how do you think Baby Tankers are made?
Wait how was a 4 inch line hooked to a smaller line?
This accident is simply due to poor craftsmanship/imcompetency on the ship's crew and especially on the duty officer.
When you transport Hexene-1, there is normally a burst disc inserted between the PV and the PV line pipework. Standard orders dictate that the pressure in the cargo tanks need to be maintained between certain parameters, the setting for a non overpressure or underpressure cargo tank is normally +250 mbar and -50 mbar. The rupture discs are below that setting and usually you would keep the range into something like 0-150 mbar. This is actually the most difficult when starting up a cargo discharge since the tank is most full. So you start up with the nitrogen a crack open and then start discharging & monitor pressure. Pressure falls, add more nitrogen supply. Pressure increases, increase pump rate. The AB's that have manifold watch are normally controlling the nitrogen supply since you do that with the valves on the vapor return line. The vapor return line is normally connected up to your PV lines with elbows or another connecting piece. If you are discharging multiple tanks at the same time, you would have all of the PV lines to vaporline open to maximize the volume you pumping nitrogen into as this acts as a cushion. You just need to adjust total nitrogen rate to match the discharging rate. Where this can become an issue is when one of the cargo tanks is empty and your discharging rate is so high that you cannot replace the lost rate by pumping faster on the other tanks. In this case you would need to turn down the nitrogen supply or pressure rises. In a calamity where pressure in the cargo tank would rise too sharp you would simply have the AB's shut the nitrogen supply off.
The main issue is that pressure in the cargo tank should not have been near to the PV opening pressure in case of discharging operations with Hexene-1. You insert burst discs so you prevent any opening to outside air (because in static condition, these PV valves usually leak) and it is the duty officer's job to maintain that pressure. So you really already have 2 failures going on here, duty officer not doing his job properly and PV output capacity being too small. This also can be exaggerated if the duty officer started operations on a single tank with nitrogen lined up to a single tank instead of opening them all up in which case pressure is much more controllable.
These accidents with ruptured cargo tanks happen more often, ie when loading phenols and cresols with a high melting point. Often the PV freezes stuck. But in a operation like Hexene-1 discharge there would normally be a lot of focus on maintaining the correct pressure in the tank. Also note that before a tank bursts, you have already well exceeded the operational parameters, then the maximum setting of the PV and then the actual pressure at which the tank bursts.
Also another famous cause for ruptured cargo tanks is pigging of the lines after completion of loading. Ie most designs require you to bleed pressure from the lines and then take the pig out. But in some cases they use a design where the pig goes into the catcher and full line pressure is released into the cargo tank. Durban used to be famous for that.
PV valve. On tankers.
What's break wind
Small correction: its Hexane no hexene
The chemical that is used in fuels, flavours, perfumes, dyes and such things would indeed be hexane. However hexane has properties similar to octane and will not be contaminated by simple contact with oxygen.
Therefore, it was probably 1-hexene, which can be used as a modifier in the production of polyethylene. The NTSB report mentions hexene, but does not specify which of the 13 isomers of hexene this was.
That’s dangerous
Pipe bore is a pretty terrible way to regulate gas flow.
Why do Tankers need these?
Well, Krakens and Godzillas sometime like to, you know, have fun.
I neve thought I would see the more arcane parts of my professuon presented for genwral intetrst. Jahaha.