Choice of vacuum gauge
- Thread starter srah1953
- Start date
mikemonty
New member
So what's the lift pump for, then?
Pete
Err... Gents, as far as I can see this thread originates from the premise that we are talking about fuel filters - the link to the vacuum gauge refers to a fitting for the air filter and bolts onto the top cover of the filter - thus measuring the air pressure AFTER the air filter.
A vacuum gauge makes more sense in this context where is IS measuring differential pressure across the air filter, but none as a fuel filter gauge.
vyv_cox
Well-known member
Perhaps you should read the whole thread? Especially post #19, then maybe look at the Vetus catalogue pages on fuel filters.
mikemonty
New member
I have.
And perhaps you should read post #4, from where the thread originates where a link is given to "Instalation instructions for a ParkerRacor vacuum gauge" which refers to an air filter gauge.
If you view this resulting thread hanging off this post it appears to be based largely on the argument that a vacuum gauge at the inlet of a fuel filter will tell you something about the filter condition whereas the equipment cited was a vacuum gauge for an air filter.
In any case:
I'm not entirely convinced that the vetus gauge is hooked to the inlet of the filter shown - it's not clear and seems a bit contorted to make either a really short, small radius elbow to mount the gauge on or some kind of "box" at the back of the inlet pipe, which looks like the only way to interpret the photo.
None of the catalog descriptions I've found suggest a vacuum gauge fitting on the inlet.
In fact I've found plenty that either show or decsribe the gauge going to the outlet.
I can't believe I'm spending my Christmas Eve on this ****!
Merry Christmas one and all!
pvb
Well-known member
I reckon you've had too much Christmas cheer already! The Parker Racor link is to a vacuum gauge for a fuel filter, not an air filter. Look at it again!
Merry Christmas.
Avocet
Well-known member
If it helps any, the latest crop of Euro5 diesel cars, fitted with particulate filters, have a pressure tapping before and after the particulate filter. The pressure immediately before and immediately after the filter is monitored and the two are compared so that the engine management system knows when the filter is getting blocked and needs a regeneration. Obviously, this is all positive pressure stuff, but clearly, the only way the car manufacturers feel this can work is by monitoring the pressure drop across the filter, with two transducers.
aluijten
New member
Correct, I found out the expensive way :-(
Not only on the exhaust part but also in the airfilter/inlet manifold by the way.
The point is, if you want to spot ANY blocking in circuit between the tank and the lift-pump a (dampened) vacuum gauge can be used, if you want to really monitor the filter itself , a differential gauge is required. It will be more precise as well.
Latestarter1
New member
Seems like time for some 'back to basics'.
Looks like after an inordinate amount of uphill thinking we have nailed the concept of measuring pressure drop across fuel filters. However there seems to be a lot of misunderstanding as to the correct fuel line sizes to use for various engines when repowering a boat and how fuel line sizes contribute to pressure drop as well as the filters themselves. It is really a very simple concept to understand once you understand how fuel flow vs. fuel line sizes interact and what the engines need to make them perform to specification.
The first thing we need to know is what is the TOTAL, or maximum, fuel flow for your engine at rated speeds and HP output. In this case, and most any 250-450 HP 6 cylinder diesel that uses an ND EP-9 type or Bosch P7100 fuel injection pump (or a system of similar design), just assume the fuel flow is about 1 GPM. It is usually a touch less, but using 1 GPM or 60 GPH is a good number to use when calculating fuel system design.
The max HP of the particular engine does not matter much - It is more the design of the fuel system that dictates the total fuel flow. In comparing fuel injection systems, these are "inline" type injection pumps, and the typical flow is about twice to three times of the fuel flow in a rotary or distributor type pump diesel engine (Bosch VE, CAV / Lucas or Stanadyne type).
Next, we need to know you engine's maximum values, numbers, or pressure drop that the engine can tolerate to work properly for both the supply side (suction) and the return side (restriction-pressure of the return line). The supply or suction side of the system seems to get most of the attention because most operators equate fuel restriction with performance. Yes, you can equate the two but not as most think.
Fuel supply restriction or "pressure drop" is always measured as a negative number because it is on the suction side of the fuel lift or supply pump. Units of measurement can vary, but inches of Mercury (Hg) seems to be most accepted because most decent gauges seem to originate from US. Regardless of the units of measure used, fuel restriction values need to be very low, and in a general sense, the lower the better when the system is fresh or new as this gives the operator more time between filter maintenance. Typical maximum values are 5" Hg with CLEAN filters.
There is no such thing as too low of a number or restriction. Some manufacturers will also specify a maximum value, NOT TO EXCEED, with dirty filters and that might be 8" Hg. Depending upon the engine, my experience has shown that most engines DO NOT lose any performance when running up to 10-12" Hg restriction. In my opinion, the "dirty" filter number or maximum restriction you can have, or can get by with, is somewhat installation engine unique, and the owner can make the call as to when fuel restriction or filter changes are required. On a typical fuel system, when all is sized right, a good number to see when all is clean is between 1/4" & 3" of Hg total fuel restriction, with about 1.0 to 2" Hg most common in a well designed fuel system. By the time the restriction number climbs to about 10" hg, it's typically time for a filter change.
Just about everybody seems to equate fuel restriction with the fuel filtration system and it's design, and never thinks about the fuel line as being a major factor in this equation, even when all is new and clean. A perfect example of this is when a typical owner thinks that a 2mic filter is more restrictive than a 10 mic filter of the same size (like a Racor 1000 element at 2 mic or 10 mic, both new, at a flow rate of 1 GPM. In actuality, both elements would be under .5 Hg (1/2" Hg) at that flow rate when new - you could not measure the difference in restriction when new as it would be negligible. Yes, the 2 mic will load quicker (get dirty) and the restriction would rise faster, but isn't that what it should do?.
But, here is what all seem to not realize. Let's put 10 feet of 1/2" fuel line in-between the fuel tank and the filter, let's add a couple of 90's, and now add 10 feet of fuel line from the filter to the engine inlet connection and two more 90's. Pretty typical in a 35-40 ft boat as to fuel line routing... Let's take our 1 GPM and do a "fuel line restriction calculation" based on typical values of standard diesel at a temp of 38C use a specific gravity of .85 and a viscosity of 3.0 centipoises (these would be reasonable numbers). We just ended up with a .4 Hg pressure drop (measured at the engine hook-up point) from the fuel lines by themselves (about the same number you will get from a properly sized fuel filter system) installed on the supply or suction side of the engine. This would be about normal. Now substitute the fuel line for 3/8" ID on the more critical supply or suction side (trying to save a a bob or two) and you have now upped the fuel restriction to 3" Hg pressure drop without even adding the fuel filtration system. Inside the ½" fuel line the fuel would be traveling at 2 ft/second and inside the 3/8" fuel line just over 3 ft per second.
This is why fuel supply lines must be large, and the longer the lines get, requiring more fittings, valves, manifolds, etc., the larger they need to be. I typically use 5/8" ID line for supply on any runs greater than 15 feet from the tank to the filters on engines that flow about 1 GPM. Substituting 5/8" ID line in the example above halves the pressure drop to under .2 Hg.
This guy does the very best fuel restriction measurement kit at very reasonable cost:
http://www.designatedengineer.com/index.html
US stuff but has Europe and UK agents.
Looks like after an inordinate amount of uphill thinking we have nailed the concept of measuring pressure drop across fuel filters. However there seems to be a lot of misunderstanding as to the correct fuel line sizes to use for various engines when repowering a boat and how fuel line sizes contribute to pressure drop as well as the filters themselves. It is really a very simple concept to understand once you understand how fuel flow vs. fuel line sizes interact and what the engines need to make them perform to specification.
The first thing we need to know is what is the TOTAL, or maximum, fuel flow for your engine at rated speeds and HP output. In this case, and most any 250-450 HP 6 cylinder diesel that uses an ND EP-9 type or Bosch P7100 fuel injection pump (or a system of similar design), just assume the fuel flow is about 1 GPM. It is usually a touch less, but using 1 GPM or 60 GPH is a good number to use when calculating fuel system design.
The max HP of the particular engine does not matter much - It is more the design of the fuel system that dictates the total fuel flow. In comparing fuel injection systems, these are "inline" type injection pumps, and the typical flow is about twice to three times of the fuel flow in a rotary or distributor type pump diesel engine (Bosch VE, CAV / Lucas or Stanadyne type).
Next, we need to know you engine's maximum values, numbers, or pressure drop that the engine can tolerate to work properly for both the supply side (suction) and the return side (restriction-pressure of the return line). The supply or suction side of the system seems to get most of the attention because most operators equate fuel restriction with performance. Yes, you can equate the two but not as most think.
Fuel supply restriction or "pressure drop" is always measured as a negative number because it is on the suction side of the fuel lift or supply pump. Units of measurement can vary, but inches of Mercury (Hg) seems to be most accepted because most decent gauges seem to originate from US. Regardless of the units of measure used, fuel restriction values need to be very low, and in a general sense, the lower the better when the system is fresh or new as this gives the operator more time between filter maintenance. Typical maximum values are 5" Hg with CLEAN filters.
There is no such thing as too low of a number or restriction. Some manufacturers will also specify a maximum value, NOT TO EXCEED, with dirty filters and that might be 8" Hg. Depending upon the engine, my experience has shown that most engines DO NOT lose any performance when running up to 10-12" Hg restriction. In my opinion, the "dirty" filter number or maximum restriction you can have, or can get by with, is somewhat installation engine unique, and the owner can make the call as to when fuel restriction or filter changes are required. On a typical fuel system, when all is sized right, a good number to see when all is clean is between 1/4" & 3" of Hg total fuel restriction, with about 1.0 to 2" Hg most common in a well designed fuel system. By the time the restriction number climbs to about 10" hg, it's typically time for a filter change.
Just about everybody seems to equate fuel restriction with the fuel filtration system and it's design, and never thinks about the fuel line as being a major factor in this equation, even when all is new and clean. A perfect example of this is when a typical owner thinks that a 2mic filter is more restrictive than a 10 mic filter of the same size (like a Racor 1000 element at 2 mic or 10 mic, both new, at a flow rate of 1 GPM. In actuality, both elements would be under .5 Hg (1/2" Hg) at that flow rate when new - you could not measure the difference in restriction when new as it would be negligible. Yes, the 2 mic will load quicker (get dirty) and the restriction would rise faster, but isn't that what it should do?.
But, here is what all seem to not realize. Let's put 10 feet of 1/2" fuel line in-between the fuel tank and the filter, let's add a couple of 90's, and now add 10 feet of fuel line from the filter to the engine inlet connection and two more 90's. Pretty typical in a 35-40 ft boat as to fuel line routing... Let's take our 1 GPM and do a "fuel line restriction calculation" based on typical values of standard diesel at a temp of 38C use a specific gravity of .85 and a viscosity of 3.0 centipoises (these would be reasonable numbers). We just ended up with a .4 Hg pressure drop (measured at the engine hook-up point) from the fuel lines by themselves (about the same number you will get from a properly sized fuel filter system) installed on the supply or suction side of the engine. This would be about normal. Now substitute the fuel line for 3/8" ID on the more critical supply or suction side (trying to save a a bob or two) and you have now upped the fuel restriction to 3" Hg pressure drop without even adding the fuel filtration system. Inside the ½" fuel line the fuel would be traveling at 2 ft/second and inside the 3/8" fuel line just over 3 ft per second.
This is why fuel supply lines must be large, and the longer the lines get, requiring more fittings, valves, manifolds, etc., the larger they need to be. I typically use 5/8" ID line for supply on any runs greater than 15 feet from the tank to the filters on engines that flow about 1 GPM. Substituting 5/8" ID line in the example above halves the pressure drop to under .2 Hg.
This guy does the very best fuel restriction measurement kit at very reasonable cost:
http://www.designatedengineer.com/index.html
US stuff but has Europe and UK agents.
vyv_cox
Well-known member
All good stuff I'm sure, but my engine consumes about 1.5 litres per hour on average. No doubt the flow rate at maximum revs is a good bit more but the fuel supply pipework on my Yanmar 3GM and supplied by them is 6 mm, from memory, and it seems to manage OK.
nigelmercier
RIP
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- 16,234
- Location
- Live in Kent, boat in Canary Islands
The fuel consumption is not relevant, it is the flow rate through the filters that matters. Remember that most of the fuel gets pumped back into the tank.
vyv_cox
Well-known member
Most? The injector return nozzles and return lines are 2 mm bore, maybe 3 mm, from memory. My filter is rated at 50 litres/hour, which I'm sure is vastly over-sized.
penfold
Well-known member
This thread's still going? Are people that bored at christmas they need to argue about vacuum gauges on teh internetz? 
nigelmercier
RIP
- Joined
- 20 Jun 2007
- Messages
- 16,234
- Location
- Live in Kent, boat in Canary Islands
Yes, most. When fault-finding I bypassed my pre-filter with an inverted bottle of diesel. It lost about 1 litre in a minute.
Tranona
Well-known member
This thread's still going? Are people that bored at christmas they need to argue about vacuum gauges on teh internetz?
'course its important. If you don't agree about this process, how will you ever cope with working out how quickly the snow blocks Rudoph's hooves - and therefore slows down delivery of presents!
ghostlymoron
Well-known member
Vyv and Latestarter are talking about completely different engines the former a sailboat auxillary and the latter a RN gunboat. Both are correct!
Vyvs stats are:
pipe size 6.0000 mm
flow rate 1.5000 l/hr
flow rate 0.0001 gal/min
flow velocity 0.0147 m/s
flow velocity 0.0483 ft/sec
So there are very low head losses in the fuel feed pipes as head loss is proportional to velocity squared
The original question which has exercised so many minds over Christmas was how to tell whether a fuel filter is blocked enought to need replacement and what sort of gauge is required (not guage - which is something different). Whilst a pressure/vacuum gauge anywhere in the fuel line could be of use but difficult to interpret, the best solution is a differential gauge across the filter. This eliminates variation due to different levels in the fuel tank and, since the lift pump is more or less a positive displacement type the flow rate will always be the same (ish).
I think this was suggested early on in this thread.
Vyvs stats are:
pipe size 6.0000 mm
flow rate 1.5000 l/hr
flow rate 0.0001 gal/min
flow velocity 0.0147 m/s
flow velocity 0.0483 ft/sec
So there are very low head losses in the fuel feed pipes as head loss is proportional to velocity squared
The original question which has exercised so many minds over Christmas was how to tell whether a fuel filter is blocked enought to need replacement and what sort of gauge is required (not guage - which is something different). Whilst a pressure/vacuum gauge anywhere in the fuel line could be of use but difficult to interpret, the best solution is a differential gauge across the filter. This eliminates variation due to different levels in the fuel tank and, since the lift pump is more or less a positive displacement type the flow rate will always be the same (ish).
I think this was suggested early on in this thread.
vyv_cox
Well-known member
I'm quite familiar with DP gauges used on pressurised systems such as lub oil filters, where a differential of around 1 bar might be expected. Are DP gauges available for the very low pressure ranges that might be encountered in fuel filters? With a keel tank the pressure at the filter inlet might be well under 1 bara, with a fouled filter pressure not much more than 0.2 bara.
mikemonty
New member
Actually - no Christmas cheer involved - a stone cold sober mistake on my part based on my presumptions on what makes for good engineering practice.
I suppose there is pressure on fuel system manufacturers and gauge makers to sell you some kit to keep their turnover up - but why they wouldn't offer a differential gauge for this purpose is beyond me. Unless, of course, they were selling an ordinary gauge for the price of a differential one, or they didn't think they could shift differential gauges at an acceptable markup.
mikemonty
New member
You wouldn't be interested in low pressure ranges in a DP gauge - you'd be interested when the DP started getting to that half bar plus range.
It looks like the photo of the single-leg gauge in PVBs post was scaled to -100kPa(g), -30 ftH2O so - extending the principle that it designed for these pressures I can't see why a DP gauge couldnt go down to the same values .
The Colepalmer DP gauge reads ok up to 5 psid and "change" at 7.5 psid, but that would be independant of what pressure losses were up or downstream of the gauge and well within your acceptable delta P for a lube filter.
I don't see why working at subatmospheric should be a problem for a DP Guage. (excessive pressures - yes)
(Though the alarm on any system of mine would be going and it would be on the point of tripping at one bar).
Maine Sail
New member
Contrary to some nonsensical stuff posted in this thread the vacuum gauge would be plumbed on the outlet side of the filter to show the progress of a plugging filter. This is provided the lift pump is sucking and not pushing through the primary filter. If it is pushing through the filter you would not use a vacuum gauge to monitor filter restrictions.
I have had to re-plumb two vacuum gauges incorrectly installed by DIY's that were installed on the inlet side of the filter. In one case the engine died due to a plugged filter and the vacuum gauge never moved off the pin. Flipped it to the outlet side, before changing the filter, and it was showing a massively plugged filter and then the engine would sputter and die.. On the second one the same result. No reading at all on the inlet and a reading of plugged on the outlet side. In both cases these were Racor turbine filters where the owners were too cheap to buy the t-handle vacuum gauge adapter fitting so they used a brass tee..
I use a fluid damped drag needle gauge from Sailor's Solutions in the US along with the t-handle vacuum gauge adapter. The nice thing is you don't have to monitor it while under load to get a reading. I can watch the progress of our filter beginning to clog and change it before it becomes a problem...
VicS
Well-known member
And you were able to do this with a simple vacuum gauge rather than with a much more expensive differential gauge.
Well, well, well. Fancy that !
