I’m looking for some experienced tankerman and cargo mates who have time on an IG barge/tank ship to answer some questions. At the risk of asking dumb questions…what is the reality of the amount of pressure build up in a tank with gas and/or ULSD? Expansion and contraction of the product in the tank occurs throughout the day depending on outside ambient temperature. With that said, are you supposed to manually release the pressure through individual tanks or utilize the high hat? Or should there not be enough pressure built up anyway within the tanks?
You asked for an experienced opinion, which isn’t me, but based on what I learned in my tanker class I would expect to see a P/V valve to take care of this? I can dig out my book and quote you some figures, if that’s helpful.
Typically when I go to load I like to start with around .2 pounds of deck pressure. If more than that the I usually have problems with pressure alarms and shutdowns shoreside. The weather has been pretty warm and the deck pressure usually creeps up to where the the PV valves open. The problem I have is the deck pressure alarm goes off on the IG system before the valves release the pressure. I typically adjust the pressure through the high hat before starting the IG or loading under vapor recovery.
The IG system high deck pressure alarm goes off at 1.5 on my barge. The PV valves dump up around 1.8. To make my life simple I bleed some pressure off through the high hat before starting the system. In this warm weather the deck pressure climbs up where the tank PV valves dump pressure. The big issue is when I go to load. The pressure alarms shoreside don’t like it when I open the vapor recovery line with over .2 pounds of deck pressure. I usually bleed some off through the high hat before I start loading.
Swamper, thanks for the replies. That’s the info I’m looking for. Is there something about antifreeze being dumped on deck if you let the p/v do its job…this sounds odd to me. We’ll be underway and all of a sudden we get a blast of gasoline smell coming from the barge.
Be careful of where you are when venting IG or other. Think about all the flammable vapors you are releasing and their relationship to your stack and potential sparks from it or other activities on the shore that could be causing an ignition source. Also purposely venting isn’t always allowed at facilities or in some states waters. You can usually allow your high hat/ pv vents to release, without having to report, as it is saving the ship or barge from structural damage but if you do it manually that may be reportable.
The antifreeze you are talking about is a secondary liquid breaker that will release if your mechanical high hats/ pvs do not. The antifreeze is usually mixed with water to create a specified weight so that it is known at what pressure it will release. If wrong ratio is mixed it will not release at the correct pressure, maybe before any alarm or pv.