Fitting vacuum gauge to CAV filter

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RobbieH

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I've just removed the diesel tank from my boat to clean it out- when the douanes came to dip the tank (looking for red diesel) I noticed that the sample bottle they took away had water and some ugly globules in it :eek:

Luckily no water/gunk seems to have got to the separator. The filter and glass bowl looked clean.

I'm changing the filters etc. at the same time and while I've got the fuek system apart I am toying with the idea of mounting a vacuum gauge between the primary filter and the lift pump - to give me an early signal of problems if I get a similar bad fill. Water trap will tell me if I'm drawing water, vacuum will tell me if the filter is clogging (time to change), hopefully before the engine stops (always at a bad moment).

Engine is a Betamarine 25hp so doesn't draw too much diesel.

The filter housing is a CAV 296 with 4 ports, two inlet, two outlet. As I have a spare outlet port is it possible to plumb the gauge into that or do I have to tee into the line going from the primary filter to the engine?

Probably going to remote mount the gauge with tachometer etc. where I can see it easily - a fuel line run of about 1.5 meters.

And does anyone have any recommendations for which gauge to go for and the names of any helpful suppliers?
 
I have no advice to offer regarding vacuum gauges, but would strongly suggest that you consider fitting a drain valve in the lowest point of the tank, since it is already out of the boat.
Routinely tapping the tank for water is the best way to avoid problems with diesel bug, I believe.
Obviously you have to have access to operate the valve, depends on how your tank is installed. A dedicated drain pipe from the top of the tank to the lowest point might be an alternative.
 
Mine is mounted on the inlet to the filter housing. I have two filters with a common supply to an inlet manifold. The gauge is stuck on top of the inlet manifold. So plumb the gauge into the spare inlet port.

An excellent idea I picked up here is to extend the gauge to a location where it can be easily read by using an appropriate length of plastic pipe. You can buy the various fittings quite easily. For example run the pipe such that you can mount the gauge near the instrument panel.
 
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just out of interest, from a catalogue I see these fuel vacuum gauges go from 0 to 30inHg: what is the approximate reading with a new filter ? and what reading should be considered excessive ?



also, if one fits an extension hose, how is it primed of air when the filter is replaced ?


thanks
 
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BlowingOldBoots said:
Mine is mounted on the inlet to the filter housing. I have two filters with a common supply to an inlet manifold. The gauge is stuck on top of the inlet manifold. So plumb the gauge into the spare inlet port.

An excellent idea I picked up here is to extend the gauge to a location where it can be easily read by using an appropriate length of plastic pipe. You can buy the various fittings quite easily. For example run the pipe such that you can mount the gauge near the instrument panel.
Click to expand...

I don't have an inlet manifold because I don't need one (having only one filter). I have two outlets on the CAV filter - one has a pipe feeding the diesel lift pump and the other has a blanking plug. My question is - is it possible to plumb the vacuum gauge into the (currently blanked) spare OUTLET port on the primary filter (for a neater installation). If not I'll just tee into the exisiting pipe between the primary filter and the lift pump.

Good idea about the remote gauge near the instrument panel.

Do you know who made your vacuum gauge and where you got hold of it?

The other possibility I'm currrently toying with is swapping out the CAV for the smallest Racor turbine (500FG) with gauge. I'm told they are much easier to change the filters on and having just changed the filter on my CAV -upside down, head jammed against (luckily cold) exhaust, dropping small bolts in bilge etc. etc.- I think it would want to be easier if i had to do this just off a lee shore in a heavy swell with a hot engine.

But $$$$$$ for the Racor solution. :(
 
As far as I can see the pressure drop across a CAV 296 10 micron element, even at 15gph is less than 0.1 PSI ( *2.036 for in Hg or .2036 in ) so a 30 in Hg gauge will only start reading something different from normal once something is seriously wrong!
 
BabaYaga said:
I have no advice to offer regarding vacuum gauges, but would strongly suggest that you consider fitting a drain valve in the lowest point of the tank, since it is already out of the boat.
Routinely tapping the tank for water is the best way to avoid problems with diesel bug, I believe.
Obviously you have to have access to operate the valve, depends on how your tank is installed. A dedicated drain pipe from the top of the tank to the lowest point might be an alternative.
Click to expand...

Probably going to cut a small sump into the bottom of the tank (weld on a short piece of 80mm diam s/s tube, cut through from the tank inside the tube, blank the end of it with some thick plate and tap for a drain tap to which will be connected a drain hose going towards the bilge. Access to the bottom of the tank is pretty good (I have room to fit this small sump) close to a bulkhead where I can cut a small access hatch to operate the tap. Obviously I will catch the drainings in something and not just flush to the bilge.

I think without the sump unless the tank bottom is very narrow and the "downhill end" of the tank is clear fitting a drain is a waste of time. With a sump as the boat bounces the water/gunk should accumulate there to await draining. Not guaranteed with a flat tank bottom that the water will be where you want it at draining time.
 
nimbusgb said:
As far as I can see the pressure drop across a CAV 296 10 micron element, even at 15gph is less than 0.1 PSI ( *2.036 for in Hg or .2036 in ) so a 30 in Hg gauge will only start reading something different from normal once something is seriously wrong!
Click to expand...

Extensive research (a bit of Googling) reveals that RichH on Sailnet reckons you should change filters at 75% of rated lift pump capacity. He reckons that is typically around 4-5"Hg. Getting hold of manufacturers specs for the lift pump might be a challenge. But maybe this is in industrial applications where they change filters real early.

Other less authoratitive sources reckon 8-10"Hg. I notice that the Racor gauges go into the yellow at 7"Hg and red at 10"Hg (Google "Racor vacuum gauge" - images).
 
Vacuum Gauge By Racor - Example Set Up

RobbieH said:
...... Do you know who made your vacuum gauge and where you got hold of it?
Click to expand...

My own vacuum gauge came with the Vetus twin filter unit, Model 75704VTEB. I have not yet moved it as it will be done when I get round to renovating the engine space.

The remote vacuum gauge idea was from pvb Post No 6. Click on the link and it will take you to the thread where pvb has a picture of his set up. The vacuum gauge is by Racor and you can probably buy the fittings from ASAP Supplies. I haven't checked their web page but they have all this sort of stuff.

You could fit the vacuum gauge to the spare outlet as this outlet will see the same pressure as as the engines fuel pump is trying pull on your actual fuel line from the filter.
 
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nimbusgb said:
As far as I can see the pressure drop across a CAV 296 10 micron element, even at 15gph is less than 0.1 PSI ( *2.036 for in Hg or .2036 in ) so a 30 in Hg gauge will only start reading something different from normal once something is seriously wrong!
Click to expand...



thanks !

so, as I have a 4108 Perkins which at the average rpm I use it might have a fuel flow rate of say 10ish *litres* per hours, 2-3 gallons per hour...
the needle wouldn't probably ever move except once the engine had already stopped :D

I was looking at Vacuum gauges on asap supplies site and both Racor and other brands are all 0-30inHg, the Racor one has a yellow coloured area at around -10inHg, so that may mean something ?



just noticed other previous contributions
 
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Think your flow rates is wrong

Roberto said:
thanks !

so, as I have a 4108 Perkins which at the average rpm I use it might have a fuel flow rate of say 10ish *litres* per hours, 2-3 gallons per hour...
the needle wouldn't probably ever move except once the engine had already stopped :D

I was looking at Vacuum gauges on asap supplies site and both Racor and other brands are all 0-30inHg, the Racor one has a yellow coloured area at around -10inHg, so that may mean something ?



just noticed other previous contributions
Click to expand...

Got to point out a bit about diesel engine operation here. Although the fuel consumption rate you quote of 10 litres per hour is correct, the actual flow rate through the filter is very much higher as the engine returns a very high proportion of fuel to the tank via the fuel pump and the injector spill off pipes.

I once had to test my Ford engine by feeding it from a 5 litre tank on top of my wheel house. The return line was of course still connected to the main tank. At a fast tickover the 5 litre tank was empty within 5 mins.

At a guess, the flow rate through your filters will be in the order of 60 to 80 litre per hour. Best to talk to your local diesel repair shop and get a figure from them.
 
FILTERS

Rather than a vacuum gauge which I think might be intended for use as a manifold pressure gauge for petrol engines I would suggest a pressure gauge plumbed from inlet to outlet of the filter. A rise in pressure indicates a filter problem. This is used in many industries. However better than a gauge would be a switch to operate a warning buzzer. You might consider the switches used for washing machine pressure level. Although this might be too sensitive. some experimentation required. However IMHO good inspection and cleaning regime for tank would be far more useful. good luck olewill
 
Jcorstorphine said:
Got to point out a bit about diesel engine operation here. Although the fuel consumption rate you quote of 10 litres per hour is correct, the actual flow rate through the filter is very much higher as the engine returns a very high proportion of fuel to the tank via the fuel pump and the injector spill off pipes.

.
Click to expand...


I was indicating 10 litres per hour as *fuel flow rate*, the engine *fuel consumption* is about 2-2.5 litres per hour at the rpm I usually run it.

I had found (sorry can't remember where) that diesel flow rate could be approximate by

a. 3.5-4 times hourly consumption, or
b. 0.7 times Horsepower

calculated in either way, the results for my engine are more or less the same fuel flow 10 lph.


However, I would welcome a more proper, official reference to a method of approximating fuel flow rate than my scribbled notes :o
 
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