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Post by Fogg on Jan 7, 2023 12:57:34 GMT 12
So Fogg has a slightly annoying setup with primary diesel tank (200L) which feeds engine & genset. Plus a secondary tank (also 200L) which tops up the primary via a cross feed with fuel transfer pump.
Earlier today I calculated that my primary tank was under half full (probably about 80L therefore could take approx 120L to fill up). The notoriously vague fuel gauge aligned with this (showing about 40% full).
Last data point is the fuel transfer pump which has a label on it that says it pumps at 6 LPM.
So I turned it on and expected to see dramatic increases in the primary tank level with a few minutes and to be close to full after about 20 mins (20min x 6 LPM = 120L).
No such luck. After an hour of running the primary tank was showing a small increase from about 40% to about 60%. Suggesting that only about 40L had been transferred over the space of an hour. Which equates to less than 1 LPM.
So what’s the brains-trust thinking about this one?
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Post by fish on Jan 7, 2023 13:11:31 GMT 12
Most marine pumps advertise their max flow at zero head. Pumps never ever operate at zero head, so the figure is about as useful as tits on a bull. In normal operating conditions, the actual flow will be lower than the advertised flow. I would expect at least half, and maybe more.
You need the head flow curve for the pump. If you have the make and model AND it is a reputable pump, we may be able to find it on the interweb.
Then all you need to do is calculate all of the head losses in the pipping system. For your level of accuracy, we just need the length of the pipe, the internal diameter, the elevation of the first tank, the highest point in the line and the height of the discharge. I wouldn't worry about fittings losses, bends and the pick-up stand-pipe. From all of that, I can work it out easy ;-)
Either that, or you have a technical problem. First things to think about are air locks in the discharge line. Most transfer pumps would be positive displacement I would have thought, and would be able to deal with an air lock. I may be a centrifugal pump, in which case it might not.
The other option is the pump impellors have worn or failed, giving feck all flow.
Or you could have a delamination on the inside of the fuel line. That would cause a flow constriction.
Of course, the obvious question, is there a filter on the transfer pump line, and have you cleaned / changed it lately?
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Post by Cantab on Jan 7, 2023 18:08:43 GMT 12
Have seen the transfer pump use the breather and return lines to enter tanks. Have a good look at the line routes, make sure part of it isn't making its way back to the start. The post above covers all the other stuff pretty well too.
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Post by ComfortZone on Jan 8, 2023 13:43:18 GMT 12
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Post by Fogg on Jan 19, 2023 13:48:58 GMT 12
Thanks for the helpful ideas & advice.
In the end I didn’t do any of that clever stuff.
Instead, I simply ran the pump for 2 consecutive long periods of time and then looked at how much fuel was required to refill the 2nd tank. And from that I’ve calculated it transfers at approx 1.5 LPM.
I suppose that might change as the levels in the two tanks change but for now that’s the proxy I’m going to use and then I’ll continue to observe the actual amount vs predicted amount.
And I’ve day I’ll install an accurate fuel gauge in both tanks like I did in my last boat and it was very useful to know within 2-3 litres exactly what you had.
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Post by Fogg on Jan 19, 2023 13:49:08 GMT 12
Thanks for the helpful ideas & advice.
In the end I didn’t do any of that clever stuff.
Instead, I simply ran the pump for 2 consecutive long periods of time and then looked at how much fuel was required to refill the 2nd tank. And from that I’ve calculated it transfers at approx 1.5 LPM.
I suppose that might change as the levels in the two tanks change but for now that’s the proxy I’m going to use and then I’ll continue to observe the actual amount vs predicted amount.
And I’ve day I’ll install an accurate fuel gauge in both tanks like I did in my last boat and it was very useful to know within 2-3 litres exactly what you had.
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Post by GO30 on Jan 19, 2023 20:58:36 GMT 12
Being in the game I can attest that most manufacturers specs are just pile of shit and relate to reality as much as Walt Disney only wrote non fiction.
Every anchor manufacturer we deal with makes the world best anchor, not one of them really does. Every rope manufacture makes the worlds best rope, again B with the follow up of ullshit.
And in rural I have employed a lot of marine gear as we have similar requirements and limitations etc. The 7lpm water pump is more a 4lpm when measured.
So on your first post where it said the pump is 6lpm my very first thought was male bovine excrement and lots of it, shall I bet Fogg it's not? On reading further I see I would have won that bet.
Like so much today basing everything on Manufacturers Spec is quite likely to lead you down a dodgy path so what you have now worked out is probably far more real life. But in saying that 1.5lpm is not large, I would have expected more.
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Post by Fogg on Jan 20, 2023 6:11:08 GMT 12
I agree 1.5 LPM is not big or fast but in the context of Fogg it’s enough. Remember I only burn ~3 LPH at average cruising speeds so I can replace the fuel of 1 hour’s engine time in just 2 mins of transfer. That’s adequate for me, as long as I know roughly what’s happening.
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