Question - Pressure drop calculations

Discussion in 'Boat Design' started by netjaws, Jun 4, 2008.

  1. netjaws
    Joined: Jan 2004
    Posts: 75
    Likes: 1, Points: 8, Legacy Rep: 22
    Location: None

    netjaws Junior Member

    It's probably a phenomenally stupid question. Hopefully someone here can make me see what obvious thing I'm missing - knowing a question is probably stupid has never stopped me before-

    I just finished designing a vapor recovery system for a tank barge. 46 CFR 39.30-1 (b) says that I now need to plot the pressure drop through the system, from the most remote tank to the gate valve have on the bow, as a function of volumetric flow rate.

    It then says that I need to include this in the vessel's oil transfer procedures as a table or graph.

    So I used Crane's flow of fluids program to model the system, and plotted pressure drop vs flow rate. I understand what I'm looking at, sort of - it's like any standard pipeline resistance curve you'd find in a textbook... but what I'm trying to figure out is - who on earth actually needs to see this? What is it useful for? I can't imagine the tankerman loading the barge getting any use out of it... and I don't see how this could possibly be of any help to coast guard in figuring whether to approve/reject my design....

    ... other than to prove that there is, in fact, fluid flowing through the pipe... and there is a pressure drop.... oooo........

    My question - what on earth is this for?!
  2. Knut Sand
    Joined: Apr 2003
    Posts: 471
    Likes: 30, Points: 28, Legacy Rep: 451
    Location: Kristiansand, Norway

    Knut Sand Senior Member

    Sorry, I do not have a clue what you need it for, but "I" almost screwed up once... Or at least I used some time to see the light....:rolleyes:
    (Good thing was; I saw the light first..).

    A setup with a HPU (hydraulic power unit) installed with on/ off for pressure switch, hydraulic lines to the "consumers". This thingy (HPU) went on off on off on off, really bothering folk around, during testing the:
    control panel was ok, (I had tested it)
    The accumulators, were ok. ( I had tested it)
    the valve actuators were ok, ( I had tested it)
    the HPU were ok... (I had tested that too!)
    So why in God's name did it operate so completely silly?

    It took some time until I finally got an idea; asked the question how long stretches are there of piping, really... Installed, now.. Really?

    When calculating the flow (resistance) of the HPU, in the hydraulic lines, I found there was a pressure drop, resistance in the piping but; then the accumulators only had 180 bar, the pressure switch at the HPU had 210 bar, stopping the HPU, Starting again, stopping again, a zillion times until the accumulators were at the same pressure as the HPU. Until some silly idiot suddenly operated one valve again... The Solution were to relocate the pressure switch... Closer to the accumulators.

    So; a pressure drop diagram can tell you something. And it can be a tool during fault finding.
  3. netjaws
    Joined: Jan 2004
    Posts: 75
    Likes: 1, Points: 8, Legacy Rep: 22
    Location: None

    netjaws Junior Member

    alright, thanks for that, you shed some light on the practical aspect

    with regards to coast guard, the only thing I can come up with is...

    if we assume pressure at the point of recovery to be atmospheric, it's logical that the pressure inside the cargo tank will be 1 atmosphere plus whatever the pressure drop occurred in my piping. I seriously doubt they're questioning the ability of the system to 'keep up' with the rate of cargo loading, given that one's a liquid and the other's a gas - but the smaller I make the pipe, obviously, the more the tank is stressed because the more pressure builds up inside it.

    so I'm going to attach a quick and dirty tank top scantling review - to show that it can handle the pressure - and send it in

    this is just a guess... if someone knows any better, please chime in

  4. Nordic Cat
    Joined: Jan 2008
    Posts: 164
    Likes: 8, Points: 0, Legacy Rep: 72
    Location: South of Copenhagen, Denmark

    Nordic Cat Senior Member

    If you know the pressure drop through the system, the flow you want, gas type, density etc. you can then size the blower/compressor correctly.

    The vapour recovery system is probably connected to the P/V (Pressure/Vacuum) valves that limit the pressures in the tanks. These also have to be designed so that there is no uneccessary venting when the blower is running.

    It can also have safety aspects, to ensure that oxygen is kept out of the tanks even with a massive cool down.

    I hope this makes it clearer, otherwise get back to me.


Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.