Choice of vacuum gauge

[mikemonty]

And you were able to do this with a simple vacuum gauge rather than with a much more expensive differential gauge.

Well, well, well. Fancy that !

Yes fancy that it needed flipping from inlet to outlet side before he could make the definitive diagnosis that the problem was a clogged filter.
He got a reading of "no pressure drop before the filter" THEN he had to move the gauge to get a reading of "high pressure drop after the filter" before he could say the pressure drop is at the filter.
If he han't measured at the two points he wouldn't have been able to say the problem is (probably) the filter.

Admittedly the most likely scenario is a blocked filter, and it would be easy enough to check if that WAS the problem by changing it, (since you are in there changing instrument piping about anyway).
Mind you, if you change the filter and there is STILL a high "vacuum" on your single leg instrument, you still don't know if the new filter is faulty or there is a blockage upstream - so you are really none the wiser.
But why bother, when the right instrument for the job was a DP Gauge which gives a reading of the state of the filter directly, at a glance, without having to trend it over time.

This was said by others at the start, but if you are happy to advocate a cheap bodge - go for it.

Of course, you could go "instrumentation mad" (I've met a few of these, they cost their companies tens of thousands) and have a DP across the filter and a single leg gauge (or even better, several) reading pipeline pressure along the way.

 

[aluijten]

Isn't one of our forum members keen on developing useless projects (YAPP)?
How about designing a circuit with two solid state pressure sensors, to be installed before and after the fuel filters. This way you could monitor the development of pressure before, after and the differential pressure. Via software you could select the dampening, you may even relate it to the engine RPM.
That would be a nice winterproject

 

[nigelmercier]

He got a reading of "no pressure drop before the filter" THEN he had to move the gauge to get a reading of "high pressure drop after the filter" before he could say the pressure drop is at the filter...

No he didn't. The reading before the filter was obviously meaningless, the reading after the filter gave the solution.

 

[mikemonty]

No he didn't. The reading before the filter was obviously meaningless, the reading after the filter gave the solution.

Not so - the reading before the filter was valid - it said "there is negligible difference between atmospheric pressure and line pressure at this point"
the reading after the filter said "there is significant difference between atmospheric pressure and line pressure at this point"
MaineSail unwittingly subtracted one from the other to get the differential pressure across the filter.

If Mainsails customers gauge had been in the correct position in the first place the best he could have concluded is that the restriction was PROBABLY the filter.
He would have to replace the fiter element to find out.

HE performed the function of a differential gauge by moving the sensing point.

 

[VicS]

No he didn't. The reading before the filter was obviously meaningless, the reading after the filter gave the solution.

Absolutely.
The reading before the filter would have almost entirely the level of the fuel in the tank relative to the position of the filter. It would have varied only by a relatively small amount between the max and min tank levels.

As the filter became blocked the pressure reading at the outlet would have decreased significantly.

 

[Deleted member 36384]

I Was Completely Wrong On All Counts As Pointed Out

I took a position on this thread that the vacuum gauge could be mounted on the inlet side. The position was based on the injection pump having suction capability and a proposed reason why fluid dynamics would make this possible. I also posted a picture of a Vetus dual filter system with a vacuum gauge on the inlet side.

Well I am wrong on all accounts as advised by other posters. I took my own advise offered to pvb and went back to my books.

The injection pump inlet pressure is entirely due to the lift pump and the injection pump has no suction capability. Fuel is metered via a helical slot on the injection piston which aligns with a port / slot on the barrel. The distance between the top of injection piston and the point of the helical groove aligning with the cylinder port / slot depends on the rotation of the piston. A rack controlled by the throttle rotates the injection piston so that the helical slot presents a different part, along the helix, to the barrel port / slot. This means that more or less fuel enters the chamber just above injection piston, before the barrel port slot is blocked by the piston skirt. So the only source of fuel that can get through the port slot in the barrel is from a flooded chamber supplied by the lift pump.

The fluid dynamics proposition was just a load of boll'ocks from me as a period of revision quickly demonstrated. All pressure loss due to a restriction in the filter happens across the filter only and has no affect anywhere else. At constant rpm, and therefore lift pump flow rate, which translates into constant fluid velocity, the pressure loss in the pipe between the tank and the filter is constant. The same principle applies between the pump and the final engine filter or injection pump i.e. constant pressure loss. The pressure losses in the line only changes if the fluid velocity changes and the velocity only changes if the engine speed changes and operates the lift pump more or less frequently.

The Vetus dual filter housing which I have fitted and which I claimed had the vacuum gauge on the inlet was an incorrect claim. A closer inspection reveals that the vacuum gauge is mounted on a pillar connected to the outlet manifold on the filter. The pillar also acts as a mounting point for both manifolds. As my own yachts fuel system is conventionally fitted: tanks (port and starboard), filter, lift pump, engine filter, injection pump, it is clear that vacuum gauge only change as the filter blocks. This was confirmed by closing the inlet valves to the filter and watching the vacuum gauge build pressure.

A bummer because I should have known better on the fluid dynamics. At least now clarity has prevailed for me.

Thanks to Bosch Technical Literature on Inline and VE Injection Pumps and to my Mum for not throwing out my collage notes and books.

 

[ghostlymoron]

Well done BoB, not many contributers are so ready to admit they were wrong. I am still not clear whether the gauge required is pressure or vacuum though.

 

[pvb]

A brave post, but appreciated. Glad you got there in the end! It can get a bit tedious trying to convince some posters that black isn't white.

 

[vyv_cox]

Well said. I appreciate the honesty of your post.