Important Racor Vacuum Gauge Information!

Few days ago there was some discussion regarding the yellow and red colored portions of the Racor Vacuum gauge for their fuel separators in this thread:

Fuel Filter Reduce WOT?

I wanted to follow up on this and post a little additional information in it's own thread after talking to my Cummins dealer, as I believe many members who have these gauges may not have seen this information before.

The Racor Vacuum Gauge shows vacuum in "Hg (inches of Mercury) from 0 to 30. From 0 to 7 the background is white suggesting the normal operating range. From 7 to 10 the background is yellow suggesting a caution zone. From 10 to 30 the background is red suggesting a warning zone. Through out the life of the filter element it may be normal to see some vacuum. On my trawler when the fuel tank is near empty, it's normal to see about 2"Hg of vacuum on the scale which reflects the effort to lift the the fuel from a lower level up to the level of the fuel pump. Also, it's common to see the vacuum level increase near the end of the filter element's life reflecting the suction required to pull the fuel through the dirty element. Finally, one can also see some vacuum if the engine has a high flow lift pump and the separator is not of an optimal size for that pump.

The point of the thread is to make people aware that the implied warning and danger zones are not reflective of the engine but possibly the separator housing or the element. The issue regarding your engine is the maximum vacuum that can be tolerated by the lift pump. Lift pumps vary by both engine manufacturer and engine model. When excessive vacuum is reached, flow may be reduced, stop, or even damage / break the lift pump. Manufacturers give maximum values for "Hg of vacuum in their engine data sheets. As an example: for the Cummins "C" series engines, the clean filter maximum vacuum is 2.5"Hg (normal operating conditions), and the dirty filter maximum vacuum is 4"Hg (end of filter element life). These are far below the Caution Zone (7 to 10"Hg) of the Racor vacuum gauge. According to my Cummins dealer, operating above the 4"Hg limit can and has caused terminal failure of these pumps.

I'm not suggesting that the particular engine you have in your boat has this same vacuum limit or that your lift pump will even terminally fail if you exceed the limit. What I want people to understand is that warning zones on the Racor gauge likely aren't your engine's limit and that you should consult your manufacture's engine manual to determine the true limits.

As an interesting comparison, the limits for my John Deere 4045 are 9"Hg in clean fuel and 12"Hg in dirty fuel.

Ted

O C Diver said:

As an interesting comparison, the limits for my John Deere 4045 are 9"Hg in clean fuel and 12"Hg in dirty fuel.

Ted

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Extremely useful info, Ted. Given the higher numbers of permissible vacuum allowed by your John Deere, however, one can only deduct that your engine really sucks.

healhustler said:

Extremely useful info, Ted. Given the higher numbers of permissible vacuum allowed by your John Deere, however, one can only deduct that your engine really sucks.

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Guess I should have seen that coming.

Ted

Ted,,

Great post. A quick look in my Yanmar 4JH3E installation manual specifies:

Max. suction head of fuel feed pump: 0.5m, which equates to just under 20"Hg.

In just three engine examples this specification is 4, 12 and 20"Hg.

This really appears to be 'engine-specific' so do follow Ted's recommendation and look up the manufacturer's specifications, don't just guess or rely on the generic colored zones on these gauges.

Hmmm, I guess that means Yanmar's suck more than other brands . . . . damn the strange luck

Thanks Ted....hadn't considered this!

My vacuum meter has an all-white scale.

Cummins B, C with inline inj pump, Yanmar 4LH, 6LY all have steel plunger lift pumps. It will not hurt them to run at higher suction vacuum. If vacuum gets high enough to reduce pump case pressure, then injection is effectively retarded and reduced and that is when you will notice engine running sick. But it won't hurt the lift pump nor the inj pump.

Other kinds of injection pump use either diaphragm pumps, gear pumps or vane pumps. Aside from the diaphragm, I can't see any being damaged by suction vacuum. Even the diaphragm pump should be designed to handle full range of suction pressure, although it will be operating under higher stress.

I don't know where Racor sources their vac gauges, but a high number of them read something other than zero when I'm holding it in my hand!!

Ski, I have also found the Racor gauges to be less than accurate, and they should be calibrated with numbers, not colors.
A "drag pointer" is an absolute necessity for a gauge that isn't on the dashboard, and is normally only visible from inside the ER.
Who wants to stick their head into the ER at full tilt boogie to have a look at the gauge?

Good post Ted.

Further to Ski's comments, there are formulas, tables and college courses on fluid dynamics that show the "lift" issue can be fairly complicated. For starters, in piping and pumping design both vertical lift (static head) and pipe length and fittings (friction head ) enter into the analysis. It is not uncommon that on longer piping runs or those with lots of fittings that the friction head is more than the static head.

All the Racor vacuum gauge is telling you is the amountof vacuum present. It is up to the user to know if it is from too much friction head, tank levels are decreasing or the filters are getting plugged. In vessels with low mounted integral tanks the pumps are sucking from a negative head already so will give a "positive" read out regardless of the filter status.

On most of our low fuel flow vessels the tanks are close to the same elevation or above the Racor gauge. So unless there is dirty fuel or lots of friction head, showing a vacuum is nigh impossible under normal conditions. The only time my Racors show a reading is when I for a moment close the fuel supply valves to see if the gauges are still working OK.

kapnd said:

Who wants to stick their head into the ER at full tilt boogie to have a look at the gauge?

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On many vessels it is routine.

That's a good trick to close racor valve with engine running and see that the needle creeps up, then open valve and it should go back to near zero. Quick and easy op check for the gauge.

Note that it does not work on the dual racors with single valve, I think gauge is valved out when you turn both filters to off. You can still do it, and see the gauge "kick" when you valve in a filter.

One related issue is if your fuel tanks are entirely below the filter and the engines; our Hatteras had the tanks molded into the bottom center line of the hull, a good 18" from top of tanks to the gauge position. So our mechanic noted it would automatically read about 2-3 high.

LarryM said:

Ted,,

Great post. A quick look in my Yanmar 4JH3E installation manual specifies:

Max. suction head of fuel feed pump: 0.5m, which equates to just under 20"Hg.

In just three engine examples this specification is 4, 12 and 20"Hg.

This really appears to be 'engine-specific' so do follow Ted's recommendation and look up the manufacturer's specifications, don't just guess or rely on the generic colored zones on these gauges.

Hmmm, I guess that means Yanmar's suck more than other brands . . . . damn the strange luck

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Larry, I'm lost as to what the value of 0.5m represents. Normally that value would be inches, feet, millimeters, or meters of water . The other is "Hg, mmHg or kpa when expressing a vacuum. What does the units of "M" represent?

Ted

One inch of Hg suction head is about 13" of water suction head, just due to the density difference between Hg and H2O. Diesel is slightly less dense than water, but close enough for discussion. The Yanmar spec is probably 0.5meter of suction head, i.e., tank within half meter of elevation compared to engine. Guessing a bit there. Half meter is 20", in inches Hg that's about 1.5"Hg.

20" Hg is LOTS of suction, atmospheric pressure is only 30"Hg, on a good day!! Literally...

kapnd said:

...
Who wants to stick their head into the ER at full tilt boogie to have a look at the gauge?

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During some future annual maintenance cycle, I'm planning on moving the gauge from the underfloor engine compartment to the pilothouse where all the other gauges are contained.

Ski in NC said:

One inch of Hg suction head is about 13" of water suction head, just due to the density difference between Hg and H2O. Diesel is slightly less dense than water, but close enough for discussion. The Yanmar spec is probably 0.5meter of suction head, i.e., tank within half meter of elevation compared to engine. Guessing a bit there. Half meter is 20", in inches Hg that's about 1.5"Hg.

20" Hg is LOTS of suction, atmospheric pressure is only 30"Hg, on a good day!! Literally...

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I'm with you on that, but I tried to work it out several ways mathematically and frankly lost interest. Then, I found this chart and looked for values near 0.5m or 500mm. It's entirely possible the chart does not relate to this discussion in any way, but it looked like as good a starting point as any.

Larry

That chart is at sea level. Max suction head drops with increasing elevation. Engines in the high Andes or even Lake Powell have many oddities as compared to their brethren at sea level.

Elevation is a big deal when designing and operating large vacuum pumps at elevation.

The best check on a filter is a DP, differential pressure gauge .

This reads the difference between the two sides of the unit.

These can be dash mounted , with an alarm if you choose.2 hoses required to the unit.

See a Murphy switch gauge .

Inshore why bother , passagemaking a good idea.