Marine Diesel Gear Pump

[MickMcC]

I have an ultrasonic flowmeter on the supply fuel line measuring volumetric flow rate to a MWM marine diesel generator engine. I also have an equivalent flowmeter on the return fuel line. The capacity at nominal engine speed of 1800 rpm is 8 l/min. The supply line is branched off the fuel supply main, an 80 mm pipe, is reduced to 15 mm pipe, and a valve should reduce the line pressure from 1.3 bar g to 0.4 bar g. This valve is not working and has been bypassed.

The flowmeter on the supply line is showing fairly constant flow for each period of operation. However, over a period of approximately 1 year the supply volumetric flow varies between ~8 L/min and 11.5 L/min for different periods of operation. However, the return line does not react accordingly. This means that when I subtract return from supply to get consumed, and plot against electrical load, I get widely varying fuel consumption per load. The higher the supply rate the higher the fuel consumption.

Since the flow through the engine is controlled by the gear pump, is it possible to “lose volume” in the pump? This would mean the flow to the return line would be constant and only effected by the demand on the generator. The varying supply rate would then account for the apparent variation in fuel consumption per load. Or can this be explained in another way?

 

[LittleInch]

Can you sketch this up to show where all these bits are and how the flow is returned and where it returns to.

Are there any other valves involved somewhere for the return line to work properly?

I suspect a metering error myself as you can't lose mass.

Can any of this start to get bubbles in it?

U/T meters really don't like anything other than 100% liquid.

If this is diesel then pressure shouldn't be an issue for the measurement.

what sort of flows are you getting on the return line?

Is the velocity high enough for a U/T meter? They don't like low velocities.

Please post the specs of the meters - I really think this is a metering issue, not a pump issue.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MickMcC]

Thanks for the reply LittleInch. I'll try to sketch something up later. Originally I thought this was a meter issue but I have flowmeters on seven engines on three ships and they are acting in the same way. If the supply flow rate is around 8 L/min or below I get expected results. When the supply rate starts to increase the fuel consumption seems to increase. I know we cannot lose mass but the calculation uses volumetric flow. Is it possible to lose volumetric flow at the gear pump?

 

[btrueblood]

" Is it possible to lose volumetric flow at the gear pump?"

Depends. Some gear pumps have internal relief valves, to prevent breaking them when the pump is asked to deliver to a dead-headed pipe.

8 Lpm in a 3 in. pipe is pretty low flow velocity, and like LI, I suspect you are in the noise floor of the ultrasonic flowmeter.

 

[MickMcC]

Hi btrueblood, thanks very much for your input. The pipes are 15 mm (3/4 in) ID. I think the flow velocity is around 1.5 m/s.

 

[MickMcC]

I am using Flexim ADM 7407 flowmeters. the specs are here:

https://www.insatech.com/downloads/UMFLUXUS_F7V4-0-2EN.pdf.

It is unlikely there are bubbles in the line and the flowmeter transducers are on a long straight length of pipe. Return line flows are around 5.5 Lpm.

 

[LittleInch]

That's one serious manual.

No way to know from here, but with s many settings to check it would need an hour on site to check and see what is possibly going on.

You can get over readings not being measured, low cut offs, all sorts of things.

U/T meters are great, but those strap on types can often go marginally out of alignment and then it all goes up the spout once the signal quality goes down.

Without understanding the flow path I can't see if there is any slippage possible on the pump but if you're measuring flow in and flow out it really shouldn't matter.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MickMcC]

Hi LittleInch, here are a couple a sketches that may help. In doc.2 FM is the approximate position of the flowmeters a

https://www.eng-tips.com/viewthread.cfm?qid=473109nd

the numbers are pipe diameters in mm.

I only seem to be able to uplaod one file at a time. I'll upload doc.2 in a separate post.

 

[TugboatEng]

This is an application for positive displacement flow meters. Installing the ultrasonic type was an error.

 

[MickMcC]

Here's doc.2

 

[MickMcC]

Hi TugboatEng, cutting the fuel lines was not an option.

 

[TugboatEng]

I understand the issues associated with cutting and welding in fittings but with systems such as Viega Megapress getting class approvals the cost of cutting pipe and inserting instruments is trivial.


Anyways, you have positive pressure (above the design) on the fuel inlet to the engine. The gear pump on the engine does not create a liquid right seal and fuel from the fuel main could be slipping around the fuel pump and out the return during times when the engine is not operating. This might have some affect on your readings.