Fuel System Vacuum Gauge

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mark1882

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Hi

In response to a previous post of mine about the frequency for changing pre/primary fuel filters a few people in reply said that the had fitted some form of gauge, I think they referred to it as a vacuum gauge to help decide when it was time to replace the pre/primary fuel filter.

Any advice as to what sort of gauge is appropriate eg make/model number etc and how and where in the fuel system it is fitted and how it works would be welcomed. I have a Racor pre/primary fuel filter with water separator.

thanks

Mark
 
Racor also offer a remote-mounted vacuum gauge, which is a standard gauge size so can be fitted neatly to a panel. I fitted one in my last boat after I'd suffered from a bout of diesel bug. The gauge shows increased vacuum as the filter gradually gets clogged with dirt. Once I'd fixed the bug outbreak, I found that I hardly ever needed to change fuel filters. However, the Racor gauge isn't cheap, any alternative vacuum gauge showing up to -50kPa would do the job. The gauge is simply connected to a T-joint in the pipe from the primary filter to the lift pump.

instruments.jpg
 
I used one like this: http://www.ebay.co.uk/itm/Vacuum-Ga...-8-BSPT-BACK-Optional-Hose-Tails/321687215074 . Note the price is actually £6.95, the lower one is just for the hose-tail (a common trick to bump them up an eBay search).

You fit a T anywhere in the fuel line between the engine lift pump and the filter, and connect the gauge via a hose to the third leg of the T. This hose can be quite small, there will be no flow along it. I led mine upwards from the T so that diesel doesn't flow into the gauge, but I don't know if this is actually necessary. The gauge may well in fact be happy with diesel in it.

With a healthy system, the needle should remain on the stop or very slightly above it. As the filter starts to block, so the needle will rise. How far it can rise before your engine stops will depend on the engine - mine can get to at least 0.5bar or halfway round the gauge.

Pete
 
I've often wondered, when changing an apparently spotless diesel filter element, if a yearly change was really necessary. Would it be safe to fit a vacuum gauge and only change the filter when the gauge showed it was clogging?
 
chuzzlewit said:
I've often wondered, when changing an apparently spotless diesel filter element, if a yearly change was really necessary. Would it be safe to fit a vacuum gauge and only change the filter when the gauge showed it was clogging?
Click to expand...

Yes
 
I've just read the article and agree it explains the problem and solutions well. My fuel system is by gravity only so I don't think a vacuum gauge would be any use unfortunately.
 
ghostlymoron said:
But, being gravity fed, I don't have a lift pump. It just goes; tank, primary filter, secondary filter, pressure pump, injector.
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Your engine sucks fuel in - it wasn't designed purely for gravity feed. The amount of suction increases as the fuel filter gets progressively blocked. A vacuum gauge will show that change.
 
ghostlymoron said:
But, being gravity fed, I don't have a lift pump. It just goes; tank, primary filter, secondary filter, pressure pump, injector.
Click to expand...

Let's say a pressure gauge on your fuel line reads 5 psi when the engine is off due to the hydrostatic head of fuel. You start the engine and at 1000 rpm a certain flow rate is being drawn through the line. The velocity of the fuel at this flow rate will cause friction and let's say that is 2 psi. This friction is consumed, it is parasitic. Your gauge will now reads 5-2=3psi. Now the filter starts to block but not enough to stop the engine, the flow rate falls the engine slows and the governor responds by changing the position of the scrolls on the Pistons of the injection pump so that more fuel is injected. The 1000 rpm is maintained on the engine and all is well. Now the velocity has increased in your fuel line, this means extra friction, say 4psi. Now your gauge reads 3-4=-1 psi. This keeps happening until the suction head specification of the pump is exceeded i.e. The minimum amount of pressure that is required to feed the pump. The 5 psi is still there in the tank to force fuel along the line, the velocity is causing the pressure drops due to friction, this will keep going until the rack on the injection pump has moved the scrolls to maximum injection volume, then it can't any more and rpm drops and the engine stops. Your gauge will now read 5 psi again, assuming the tank level has not changed significantly.

I think that's how it works but happy to be corrected.
 
I'm just asking whether a vacuum gauge would work in my very simple system. I am aware that most modern systems have : fuel tank ( above or below the engine), lift pump usually driven off the engine, primary filter, secondary filter, pressure (metering) pump, injector(s). Mine doesn't have a lift pump so would a vacuum gauge after the primary filter show anything useful?
BlowingOldBoots said:
Let's say a pressure gauge on your fuel line reads 5 psi when the engine is off due to the hydrostatic head of fuel. You start the engine and at 1000 rpm a certain flow rate is being drawn through the line. The velocity of the fuel at this flow rate will cause friction and let's say that is 2 psi. This friction is consumed, it is parasitic. Your gauge will now reads 5-2=3psi. Now the filter starts to block but not enough to stop the engine, the flow rate falls the engine slows and the governor responds by changing the position of the scrolls on the Pistons of the injection pump so that more fuel is injected. The 1000 rpm is maintained on the engine and all is well. Now the velocity has increased in your fuel line, this means extra friction, say 4psi. Now your gauge reads 3-4=-1 psi. This keeps happening until the suction head specification of the pump is exceeded i.e. The minimum amount of pressure that is required to feed the pump. The 5 psi is still there in the tank to force fuel along the line, the velocity is causing the pressure drops due to friction, this will keep going until the rack on the injection pump has moved the scrolls to maximum injection volume, then it can't any more and rpm drops and the engine stops. Your gauge will now read 5 psi again, assuming the tank level has not changed significantly.

I think that's how it works but happy to be corrected.
Click to expand...
 
William_H said:
What might also serve the purpose is a differential pressure guage rather than a vaccuum guage. This electronic transducer might be a cheaper option if you cna build up a sensor and light driver circuit. Just a thought olewill http://www.ebay.com/itm/MPXV7002DP-...sure-Sensor-/281687118227?hash=item4195dc5193
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olewill

I have been looking at using one of these to read the fuel flow using a venturi, but my quick calcs show a very small venturi (throat dia of around 1 mm)

Been trying to work out a way to machine the small tapers needed.
 
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ghostlymoron said:
I'm just asking whether a vacuum gauge would work in my very simple system. .....
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I was just demonstrating why it would work in your system without a lift pump. You need to install the vacuum gauge between the tank and primary filter. Mine, for example, is on the inlet to the primary filter via a T piece before the lift pump: fuel from twin tanks goes into the inlet manifold, the inlet manifold has the vacuum gauge, fuel flows out of the outlet manifold, to the lift pump then to the engine filter and onwards. The excess fuel return from the injection pump effects the pressure in the line between the primary filter, engine filter and inlet to the injection pump, so you will not get a true reading of filter performance as it is upstream of the primary filter. The vacuum gauge has to be down stream of the primary filter.

I have checked the manual for my Vetus primary filter and in all installation options they show the primary filter housing with vacuum gauge between the tank and lift pump. If you want to fit a vacuum gauge then fit it between the tank and primary filter.
 
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BlowingOldBoots said:
I was just demonstrating why it would work in your system without a lift pump. You need to install the vacuum gauge between the tank and primary filter. Mine, for example, is on the inlet to the primary filter via a T piece. The excess fuel return from the injection pump effects the pressure in the line between the primary filter, engine filter and inlet to the injection pump, so you will not get a true reading of filter performance as it is upstream of the primary filter. The vacuum gauge has to be down stream of the primary filter.

I have checked the manual for my Vetus primary filter and in all installation options they show the primary filter housing with vacuum gauge between the tank and lift pump. If you want to fit a vacuum gauge then fit it between the tank and primary filter.
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That's incorrect - a vacuum gauge between the tank and the primary filter won't show any relevant reading.
 
I agree with Pete, any gauge between the primary filter and the tank must surely show the pressure due to the fuel level in the tank, assuming the tank is above the filter.
 
BlowingOldBoots said:
You are wrong, it does on mine and it is installed as per the manual, page 28

http://www.vetus.com/media/magentominds/sasdocument/20121107005241_0.pdf
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I'm certainly not wrong, and you seem to have a very short memory as you argued the same ridiculous theory about 3 years ago. Back then, you also incorrectly claimed that the vacuum gauge had to be on the inlet side of the filter, and you also claimed that it was installed on the inlet side of your Vetus filter. Eventually, you realised how wrong you were and posted a correction, although not an apology. I expect you won't have the grace to apologise now either. Just to remind you, here's your post from Jan 2012...

BlowingOldBoots said:
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.
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