5 replies to this topic

[Yogesh Bhatt]

Dear All,

 

My project is to review a PFD for Diesel Fuel system in FLNG hull. The diesel is supplied from boat to the filter on the topsides. Then it passes thru the metering package before being sent to the storage in the hull.

The pre-FEED drawing shows this metering package.

IMO, metering package is not required because the diesel filling is a manual operation which could be controlled by checking the level in the tank and thereby raising the hooters on the boat.

What are your thoughts on providing metering package here?

 

Thanks in advance!

 

[shan]

Is the tank level gauge visible to the filling operator? 

[JMW]

It depends on the metering package.

It isn't simply about checking how much has been delivered but about protection against frauds. It is all very well tank dipping but a meter package with data logger makes it more difficult to "cheat" the delivery.

Fuel quality and quantity frauds are all too common and in Singapore, for example, suppliers are now required to meter fuel into vessels using coriolis meters. Formerly the fuel would be delivered by tank dipping.

Even a mechanical meter with mechanical register will have a non-resetable cumulative totaliser that  can provide evidence of waht was delivered. But with an electronic register the otential is to record the data and transmit in real time via internet for remote monitoring.

 

So the real answer is to be sure the reason for the metering package is known. Usually by whoever specified it in the first place.

[Yogesh Bhatt]

Shan,

Yes, the gauge is visible and also can be monitored at the control room.

 

JMW,

Thanks for the explanation. But the reason for providing this must be clearly specified in the basis of design which is not. So, I will have to inquire from the client.

 

Thanks

[JMW]

This is not to be confused with any fuel efficiency monitoring system?

It doesn't seem so from your explanation but I thought it best to check.

 

in medium and high speed diesesl the usual oerating profile can include a lot of variable conditions from idle to full power. A simple flow on demand to the engine would result in a very wide flow rate range but usually such installations use a fixed flwo rate supply with a return of unsude fuel to the tank.

In diesel fuel systems, flow to the engine from the tank is usually supplied at twice the maximum consumption rate and the unused fuel returned to the tank thus varies from half the supply flow upto the maximum suply flow so the flow rate ratio of flow to and flow return is thus in the range 2:1.

 

It is usual in such systems to have a flow meter in both the flow and return lines and use a diferential totaliser for consumption monitoring. There are a variety of systems available which correlate the fuel consumption with engine speed, GPS data etc to determine the optimum fuel efficiency conditions.

 

On the other hand, while the same 2:1 flow principle is used for heavy fuel oil engines, the fule is circulated locally in a high pressure loop with fuel from the tank introduced via the mixing tank in the high rpessure circuit. In these systems there is usually only one flowmeter in the make up line. This is adequate because low speed diesels generally spend substantial proprotion of their operation at one speed, or wth only minor variations in speed such as in marine propulsion applications.

 

But as I say, it doesn't seem from your description that this is part of the consumption system and it is only for the medium and high speed engines I would expect to see a flow meter in any flow line to the tank without another in ths supply from the tank to the engine.

 

On the other hand, if the fuel layout is such that tank filling takes place via the return line from the engine to the fuel tank, and the filling point is upstream of the return fuel flow meter it could be a way of making use of the return flow fuel consumption meter to monitor fuel loading.

 

Is the fuel system such that the return from the engine to the tank is seperate from the tank filling operation?

This is a bit of a reach because I would expect the flow rates filling the tank to be far higher than the flow rates in the consumption monitoring meters.

Edited by JMW, 12 November 2014 - 06:40 AM.

[JMW]

OK, been thinking a bit more.

Firstly, the idea of using the return flow meter for monitoring loading may not be quite as off the wall as I at first thought.

In the first place, if we are dealing with diesel at 4cst say, the headloss is OK and the flow meter can be sized with the maximum flow rate way up the flow rate range of the meter. In heavy fuel oil systems with 380 or 500cst fuels, the flow meters, including coriolis (or especially coriolis due to their high pressure drop) the meters are oversized such that the max flow is much lower in the flow rate range to keep headlosses down.

 

But in a diesel fuel application the return flowmeter could be sized suitable to handle the filling flow rates in the top end of its flow range.

The advantage of the flow and return system is that the flow rate when the engine is running though the flow rates may be very much lower down the flow rate range, with modern flow meters and curve fitting for accuracy  (you didn't advise what type.... coriolis? PD? Turbine?) very wide flowranges are possible such that the consumption return flow rate at max fuel consumption could be in range and thus with only a 2:1 flow rate range this is possible.

And/or perhaps the fuel supply flowrate has been increased a bit to bring the return flow rate up within the flow range of the oversized return meter. Say 4 times max cunsumption in which case the minimum return flow is 3/4 of that.

 

But if it is nothing to do with this..... as it probably isn't, just thinking off the top of my head.......

Flow meters are an accurate and verifiable way to monitor the fuel delivered.

Flow meters can also be used to monitor the consumption of the fuel.

But a tank gauge system would still be advised because it will show what fuel actually remains in the tank rather than the fuel calculated to remain in the tank from the difference between fuel delivered and fuel consumed.

Any anomalous diference would be due to either fuel theft after delivery or due to a leak somewhere.

I would imagine that on a vessel it is always advantageous to directly measure the amount of fuel in the tanks at all times for just such a reason .... leaks or pilferage.