Choice of vacuum gauge

[david_bagshaw]

I think pvb and Vics have got it right.
A vacuum guage on the filter inlet is as much use as a chocolate teapot and misses the point of fitting one in the first place i.e. to check for a blocked filter.

+1 for pvb, as I have the racor gauge mounted on the t handle, with a hole drilled down through the middle, into the middle of the filter, which is the outlet side. (otherwise the muck would collect in the smaller area which is nonesense.)


As far as I am aware the injection pump has almost no sucking action to fill it self with fuel, but relies on the lift pump pressure to fill. Afterall in an inline pump the plungers are operated by a cam, so the suction pressure would be that of its return spring, if fitted.

DPA pumps have a lift pump element in its design.

 

[mikemonty]

The vacuum gauge does not give a hoot where the blockage is. It measures a pressure draw-down on the fuel pipes from the tank to the positive displacement injection pump.

As soon as the injection pump has to work harder to pull fuel through because of a blockage in the filters, a vacuum gauge anywhere on the system between the tank and injection pump will show an increase in vacuum, even if mounted between the lift pump and tank. This happens because the injection pump delivers a fixed volume flow rate (constant rpm). So long as the injection pump seals every time the pump piston strokes it pulls fuel up the line and it will continue to increase the suction pressure until something stops i.e. fuel flow to the engine, worn injection pump, collapsed lines.

The reason is because the net positive suction head (NPSH) is dominated by the injection pump suction side and not the lift pump charge pressure. If the lift pump charge pressure was greater than the injection pump suction pressure, then the vacuum gauge would have to be on the filter outlet.

So, what I am proposing, and what is also found on installations, is that an indication of a blocking filter can be had by mounting the vacuum gauge anywhere on the line. It will limit any analysis to no more than noting that something is causing an increase in suction i.e. a restriction in the tank outlet is a possible cause.

To make further analysis of increasing vacuum pressure I agree that you need the gauge mounted on the outlet of the filter unit. You must also know the value of the lift pump pressure at the rpm concerned to differentiate between blocked filters and other issues which affect fuel flow. This is why differential pressure gauges are better for trouble shouting over any single gauge installation.

Thats the best explanation I can make by dragging the stuff from memory from courses over 25 years ago.

I agree with the above but would add that I would never have designed a system where the condition of the filter was indicated by the differential between atmospheric pressure and pressure downstream of the filter. Too many variables - generally other potential blockages or loss of performance of the pump itself.

If you want to instrument the filter, use - as was said early in the thread - a differential pressure gauge across its inlet and outlet. Otherwise, all you can tell from a "vacuum" guage is that there is a blockage (or perhaps a ruptured filter element if your pressure suddenly improves, or maybe an air leak) somewhere upstream of the GUAGE.

 

[Pagetslady]

On My VP 2003 there are two filters one with a glass bowl and one which is the fine filter bolted to the engine, were would you put the vacume gauge to warn of a potential blockage in the final fine filter before the injector pump.
Mike

 

[pvb]

On My VP 2003 there are two filters one with a glass bowl and one which is the fine filter bolted to the engine, were would you put the vacume gauge to warn of a potential blockage in the final fine filter before the injector pump.
Mike

You can't use a vacuum gauge for this, and indeed the engine-mounted filter is highly unlikely to get blocked if it's replaced according to schedule.

 

[2Tizwoz]

A differential pressure gauge indicates the pressure drop across the filter so it can be monitored.

From the Cole=Parmer http://www.coleparmer.co.uk/catalog/product_view.asp?sku=6840504&pfx=website:-

'Commonly used to show when to change or clean a filter, these pressure indicators are divided into three transition points, each clearly marked for ease of understanding. For example, model 68405-02 changes from green to yellow at 5 psid and from yellow to red at 7.5 psid. Mounting block has a 1⁄4" NPT(F) in-line process connection.'

 

[Deleted member 36384]

..... I'd suggest that if anyone's still reading this thread and is considering fitting a remote vacuum gauge, they should fit it on the outlet side of the filter ......

It is the better position if one has a choice as it can be used for more detailed analysis if other data is known. At best the vacuum gauge on the inlet side can only be an indirect measure of filter blocking, if all other system losses remain unchanged. However, what I have said before is still applicable and I am sure that you must know this having "an Honours degree in Engineering, gained in an era when a degree meant something".

 

[pvb]

A differential pressure gauge indicates the pressure drop across the filter so it can be monitored.

From the Cole=Parmer http://www.coleparmer.co.uk/catalog/product_view.asp?sku=6840504&pfx=website:-

'Commonly used to show when to change or clean a filter, these pressure indicators are divided into three transition points, each clearly marked for ease of understanding. For example, model 68405-02 changes from green to yellow at 5 psid and from yellow to red at 7.5 psid. Mounting block has a 1⁄4" NPT(F) in-line process connection.'

Unfortunately, that wouldn't work on most boat primary filters, as it requires positive pressure. Most primary filters work under vacuum.

 

[pvb]

It is the better position if one has a choice as it can be used for more detailed analysis if other data is known. At best the vacuum gauge on the inlet side can only be an indirect measure of filter blocking, if all other system losses remain unchanged. However, what I have said before is still applicable and I am sure that you must know this having "an Honours degree in Engineering, gained in an era when a degree meant something".

No, frankly I'm mystified as to how a vacuum gauge fitted on the inlet side of a primary filter can possibly indicate if the filter is becoming blocked.

 

[Deleted member 36384]

No, frankly I'm mystified as to how a vacuum gauge fitted on the inlet side of a primary filter can possibly indicate if the filter is becoming blocked.

Go back to your books then.

 

[pvb]

Go back to your books then.

It won't be there, because the claim is illogical.

 

[Deleted member 36384]

It won't be there, because the claim is illogical.

It is you who is illogical. On one hand you state it is a vacuum system, and other the other you insist that the gauge must be mounted between the lift pump and the filters. The lift pump is a positive pressure device, so one only needs a pressure gauge to determine drop in pressure on the filter outlet caused by blockage if relying on the lift pump as the source of energy. So why is vacuum gauge used pvb?

 

[pvb]

It is you who is illogical. On one hand you state it is a vacuum system, and other the other you insist that the gauge must be mounted between the lift pump and the filters. The lift pump is a positive pressure device, so one only needs a pressure gauge to determine drop in pressure on the filter outlet caused by blockage if relying on the lift pump as the source of energy. So why is vacuum gauge used pvb?

Maybe you're confused by terminology. In simple terms, in most small boat engines, the engine's fuel pump sucks fuel from the fuel tank, usually via a primary fuel filter. It follows that the primary fuel filter is operating usually under some vacuum. If the fuel level in the tank is higher than the fuel pump on the engine, there may be little or no vacuum. If the filter starts to become blocked, it is harder for fuel to get through the filter, and the vacuum will increase in the pipe between the primary filter outlet and the engine fuel pump. That's why remote vacuum gauges are plumbed in to this pipe.

 

[vyv_cox]

Assuming the outlet from the tank to be clear, and using VicS' drawing as a basis, the pressure at the inlet side of the filter is determined by the head of fuel, and therefore constant over some length of time. With a new filter there is a small pressure drop through it, increasing as the filter fouls. The suction pressure at the lift pump therefore reduces as the fouling takes place, ultimately becoming negative if the filter becomes completely blocked. At this point a vacuum gauge would register the negative pressure in the line between the filter and lift pump.

 

[Bilgediver]

The vacuum gauge does not give a hoot where the blockage is. It measures a pressure draw-down on the fuel pipes from the tank to the positive displacement injection pump.

As soon as the injection pump has to work harder to pull fuel through because of a blockage in the filters, a vacuum gauge anywhere on the system between the tank and injection pump will show an increase in vacuum, even if mounted between the lift pump and tank. This happens because the injection pump delivers a fixed volume flow rate (constant rpm). So long as the injection pump seals every time the pump piston strokes it pulls fuel up the line and it will continue to increase the suction pressure until something stops i.e. fuel flow to the engine, worn injection pump, collapsed lines.

The reason is because the net positive suction head (NPSH) is dominated by the injection pump suction side and not the lift pump charge pressure. If the lift pump charge pressure was greater than the injection pump suction pressure, then the vacuum gauge would have to be on the filter outlet.

So, what I am proposing, and what is also found on installations, is that an indication of a blocking filter can be had by mounting the vacuum gauge anywhere on the line. It will limit any analysis to no more than noting that something is causing an increase in suction i.e. a restriction in the tank outlet is a possible cause.

To make further analysis of increasing vacuum pressure I agree that you need the gauge mounted on the outlet of the filter unit. You must also know the value of the lift pump pressure at the rpm concerned to differentiate between blocked filters and other issues which affect fuel flow. This is why differential pressure gauges are better for trouble shouting over any single gauge installation.

Thats the best explanation I can make by dragging the stuff from memory from courses over 25 years ago.

Nop

This is not right.......The HP fuel pump relies on positive head to operate hence the need for a lift pump or day tank. The HP fuel pump can NOT draw fuel as it has no effective suction strioke due to open ports and relies on the lift pump to supply it under pressure. The vacuum gauge has to be after the primary filter and before the lift pump so that the change in conditions at the filter can be registered. A differential gauge is batter but more expensive.

Have a good shift in the desert as suspect you left EDI recently???

 

[BruceDanforth]

Recommendation for type of vacuum gauge :

there's nothing in it.

 

[theoldsalt]

Reading this thread just goes to emphasise that it is just (if not more) important to know what you don't know than what you do know.

At least common sense prevailed......eventually. Thank you Viv.

 

[mikemonty]

Unfortunately, that wouldn't work on most boat primary filters, as it requires positive pressure. Most primary filters work under vacuum.

As far as I can see (and I admit to not looking too hard - life's too short) these gauges are rated for a maximum PSI d (d for differential).
The gauge doesn't care if it is less than atmospheric - theoretically the low pressure leg could be at absolute vacuum (0 psia) as long as the other doesn't exceed 43psia (absolute), in the widest ranged unit, before damage to the gauge is likely.

 

[mikemonty]

On My VP 2003 there are two filters one with a glass bowl and one which is the fine filter bolted to the engine, were would you put the vacume gauge to warn of a potential blockage in the final fine filter before the injector pump.
Mike

The gauge would have to be taken off somewhere between the fine filter and the injector pump.
But, as I've said before - its a very bad indicator of filter condition - changes in pressure - up or down can be caused by so many things (fuel tank level for Gawds sake!) that you might as well be using tarot cards.

Use the correct instrument for the job - a differential pressure indicator across the filter.

 

[VicS]

The gauge would have to be taken off somewhere between the fine filter and the injector pump.
But, as I've said before - its a very bad indicator of filter condition - changes in pressure - up or down can be caused by so many things (fuel tank level for Gawds sake!) that you might as well be using tarot cards.

Use the correct instrument for the job - a differential pressure indicator across the filter.

I think to monitor the fine filter between the lift pump and the injector pump there is no sensible alternative but to use a differential pressure gauge.

When monitoring a filter between tank and a lift pump you will get away with a simple gauge on the filter outlet provided you are aware that the reading will be affected by the fuel level in the tank. It would be sensible to know what the normal readings are with a full tank and with a ( nearly) empty tank.

 

[mikemonty]

I think to monitor the fine filter between the lift pump and the injector pump there is no sensible alternative but to use a differential pressure gauge.

When monitoring a filter between tank and a lift pump you will get away with a simple gauge on the filter outlet provided you are aware that the reading will be affected by the fuel level in the tank. It would be sensible to know what the normal readings are with a full tank and with a ( nearly) empty tank.

Rho x g x h

However the whole thing is probably pointless - the gauge will only show a significant pressure when there is flow. So the reading will fluctuate with both the action of the pump and the throttle setting.
The best he can hope for is that the gauge is glycerine damped and will give something approaching an averaged reading as the needle jerks around.

I can only assume that the differentiial gauge shown in the link previously has damping and inertia sufficient to make it reasonably steady. Its probably also the main reason it is scaled as good-intermediate-bad and makes no claim to represent actual pressure.