Choice of vacuum gauge

[srah1953]

Further to some interesting discussions fuel filters, I would be intersted in finding out how a vacuum gauge is plumbed (if that's the right word) and any recommendations for choice of gauge.
Thanks

 

[aluijten]

I don't think it's a vacuum gauge but a differential gauge, it'll measure the underpressure difference between the feed and the exit line of the filter.
If the filter is clogged the pressure drop over the filter (seen from the fuel pump side) will be bigger compared to a fresh filter.

 

[pvb]

It is a vacuum gauge, and it measures the amount of vacuum in the fuel line from the primary filter to the fuel lift pump. As the filter starts to get blocked, the vacuum gradually increases. It's a good way to see when filters need replacing, and also gives early warning of fuel filter blockage.

Racor sell a nice gauge (think ASAP Supplies sell it). It's what I fitted in my boat, see pic. It's easily fitted - just tee into the pipe and run a flexi plastic tube up to the gauge.

 

[Deleted member 36384]

ASAP Supplies have Vacuum Gauge kits as shown here: -

http://www.asap-supplies.com/search/vacuum+gauges

The vacuum gauge is connected to the inlet side of the filter housing but it doesn't matter if its connected to the outlet either. The pump has to work harder as the filter blocks and this is measured by the vacuum gauge anywhere in the system, upstream of the injection pump.

I would not buy a gauge with any coloured banding just a numbered dial. Note the readings on the dial when a new filter is installed and log the readings with engine hours. You will have to establish the vacuum reading for your system that represents a dirty filter.

Instalation instructions for a ParkerRacor vacuum gauge: -

http://www.parker.com/literature/Racor/14396_Rev_-_RK11-1669_Gauge_Kit_Instructions.pdf

 

[pvb]

The vacuum gauge is connected to the inlet side of the filter housing but it doesn't matter if its connected to the outlet either. The pump has to work harder as the filter blocks and this is measured by the vacuum gauge anywhere in the system, upstream of the injection pump.

Errr, no. The inlet side of the filter housing is certainly not the place to connect it - the gauge won't show anything of value. It has to be plumbed in between the filter outlet and the engine lift pump; in that way it will show the progressively increased vacuum as the filter gets blocked.

 

[Deleted member 36384]

Errr, no. The inlet side of the filter housing is certainly not the place to connect it - the gauge won't show anything of value. It has to be plumbed in between the filter outlet and the engine lift pump; in that way it will show the progressively increased vacuum as the filter gets blocked.

The fuel injection pump draws down a certain pressure to overcome the parasitic losses in the pipe work from the tank to the injection pump inlet. As the filter blocks the parasitic losses increase.

The injection pump is delivering the same quantity of fuel as its a positive displacement pump. The fuel velocity down stream of the blocked filter must increase to keep up with the demands of the pump. As the injection pump is providing the power to pull the fuel through (not the lift pump pushing the fuel) then a pressure drop is realised all along the pipe work from tank to injection pump. Hence, the vacuum gauge does not need to be plumbed in to the outlet on these types of systems.

If the filters were down stream of the injection pumps then what you say would be true. The inlet side is an indirect measure of filter blocking for this type of system.

 

[pampas]

surely you mean down stream and not upstream of the injection pump.

 

[pvb]

If the filters were down stream of the injection pumps then what you say would be true. The inlet side is an indirect measure of filter blocking for this type of system.

Sorry, that's not correct. If you want to have an indication of the extent to which the primary filter is blocked, the vacuum gauge has to be between the filter and the engine lift pump.

 

[VicS]

As PVB says vac gauge is teed in somewhere between filter outlet and lift pump suction

As filter becomes clogged the vacuum reading will increase.

 

[maxi77]

The fuel injection pump draws down a certain pressure to overcome the parasitic losses in the pipe work from the tank to the injection pump inlet. As the filter blocks the parasitic losses increase.

The injection pump is delivering the same quantity of fuel as its a positive displacement pump. The fuel velocity down stream of the blocked filter must increase to keep up with the demands of the pump. As the injection pump is providing the power to pull the fuel through (not the lift pump pushing the fuel) then a pressure drop is realised all along the pipe work from tank to injection pump. Hence, the vacuum gauge does not need to be plumbed in to the outlet on these types of systems.

If the filters were down stream of the injection pumps then what you say would be true. The inlet side is an indirect measure of filter blocking for this type of system.

The guage is trying to see changes caused by the filter, to do this it has to be between the source of the vacumn, and the potential restriction, the filter.

 

[Deleted member 36384]

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.

 

[prv]

As the injection pump is providing the power to pull the fuel through (not the lift pump pushing the fuel) then a pressure drop is realised all along the pipe work from tank to injection pump.

So what's the lift pump for, then?

Pete

 

[pvb]

.....To make further analysis of increasing vacuum pressure I agree that you need the gauge mounted on the outlet of the filter unit.

At last! We got there in the end.

 

[Deleted member 36384]

So what's the lift pump for, then?

Pete

The lift pump is not always required once the engine is up and running. It boosts the inlet pressure at the injection pump (about 3 bar) at very low engine rpm to help it get charged with fuel so it can commence injecting the correct amount, but the non return valve in the lift pump is probably a more important feature. It may be required if the tanks are very low down and it may be required to over come the friction of small fuel lines, maybe for both examples. There are engines which don't have lift pumps relying entirely on the injection pump suction side to draw fuel through the lines.

 

[Deleted member 36384]

At last! We got there in the end.

Yes but if you only know that the vacuum gauge is on the outlet from the fuel filter you are in exactly the same position as if its on the inlet of the fuel filter. That is my point.

 

[pvb]

Yes but if you only know that the vacuum gauge is on the outlet from the fuel filter you are in exactly the same position as if its on the inlet of the fuel filter. That is my point.

There is no point having a vacuum gauge on the inlet of a filter - it won't give any warning of the filter becoming blocked (most likely scenario), although it would warn of the pipe from the tank becoming blocked (highly unlikely scenario).

 

[Deleted member 36384]

There is no point having a vacuum gauge on the inlet of a filter - it won't give any warning of the filter becoming blocked (most likely scenario), although it would warn of the pipe from the tank becoming blocked (highly unlikely scenario).

You are wrong, it will tell you that there is more suction happening on the fuel pipe system which may be due to the filter blocking. The mechanism that it does this has been explained by me as best as I can.

If you mount the vacuum gauge on the outlet of the filter it will still tell you that there is more suction happening on the fuel pipe system which may be due to the filter blocking.

In both mounting positions a restriction at the tank outlet or a blocked filter will show an increase in suction pressure. With no other information all you can do is change filters and see what happened to the vacuum gauge.

If you know the vacuum pressure (outlet mounted) across a clean filter for a range of rpm values and you know the head of pressure at the filter housing for various tank fluid heights or lift pump pressure outlet then you make a conclusion if it is filter blocking or some other blockage (even worn injection pump issues) by comparing the figures. Without the information it doesn't matter diddly squat where the vacuum gauge is. And for most folks who run yachts, they will not know this data, so it doesn't matter where the vacuum gauge is mounted.

I am pretty sure that the above is true. It is to do with hydraulic horse power (available from the engine) and conservation of mass flow rate from the injection pump, where the fuel has to flow faster from the tank to get through the restriction for the pump mass flow rate demand, hence the whole system sees a drawdown because its a suction system. It may even have something to do with the faster velocity increasing the pressure drop across the inlet to the vacuum gauge via Bernoulli's Principle.

Here is a picture of a filter system where the vacuum gauge is on the inlet manifold: -

 

[pvb]

You are wrong, it will tell you that there is more suction happening on the fuel pipe system which may be due to the filter blocking. The mechanism that it does this has been explained by me as best as I can.

No, a vacuum gauge on the inlet side of the filter can't tell you if the filter is blocking, only that the tank pipe might be blocking (highly unlikely).

If you mount the vacuum gauge on the outlet of the filter it will still tell you that there is more suction happening on the fuel pipe system which may be due to the filter blocking.

Mounting the vacuum gauge on the outlet side of the filter is the only way of showing if the filter is becoming blocked.

In both mounting positions a restriction at the tank outlet or a blocked filter will show an increase in suction pressure. With no other information all you can do is change filters and see what happened to the vacuum gauge.

No, only a vacuum gauge mounted on the outlet side will detect both possible blockages.

So, let me summarise:-

• I'm one of the relatively few boatowners who's added a remote vacuum gauge, so I'm qualified to comment
• I fitted it myself, so I know it's plumbed in the outlet pipework from the filter to the engine
• I have an Honours degree in Engineering, gained in an era when a degree meant something, so I've a fair idea of how things work
• Racor's product literature says the vacuum gauge must be on the outlet side of the filter
• VicS and maxi77 agree that the gauge must be on the outlet side of the filter

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 if they want it to be able to show when the filter's starting to become blocked. If they fit it on the inlet side of the filter, it won't give them this very useful warning.

 

[pvb]

Here is a picture of a filter system where the vacuum gauge is on the inlet manifold: -

Yes, it does appear to be, and I know you have one of these on your boat. But the vacuum gauge will not show if the filters are starting to block. Other types of Vetus duplex filters have the vacuum gauge correctly mounted on the outlet piping, so that blocking of the filter can be detected.

 

[SAMYL]

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.