Restoring bristol BM 4s 7 compressor cfm pump speed

[kev phillips]

Hi from Scotland,

First post for me here.

Great forum, Lots of info. I wonder if any of you compressor gurus can give me some advice?

I have a new to me Bristol BM4s 7 that is only pumping about 2.8cfm up to about 240bar/3500 psi, the manual indicates 5cfm +or-5% FAD should be achievable. I dont expect to get this due to the compressor age but would like to at least get in the region of 4cfm.

There are no safety valves blowing and the compressor pumps up to 240 bar ok, it just does it a bit slowly for an 8 cylinder/day weekend dive trip.

Does anybody have any suggestions for a plan of attack/investigation? And or what I could replace that would make a big difference i.e 4th stage piston/liner, valve,etc and potential suppliers?

Cheers

Kev

 

[frogman62]

Kev,
Does the compressor have guages showing pressure for each of the 4 stages? This can give the best indication of low output. 1st stage 60psi, 2nd stage 300psi, 3rd stage 8-900psi, 4th stage 3500-5000 psi.
Second check the rpm of the compressor - should be running 1200-1300rpm. Is the motor a 5 HP and what size filters are you using?
The correct method to determine CFM is to fill the system (with the tank turned off) to 3500psi, then open the tank valve and time the "fill" back to the 3500psi. In this manner you don't have to determine how much air it takes to fill the filters and the system.
Jim Shelden

 

[kev phillips]

Kev,
Does the compressor have guages showing pressure for each of the 4 stages? This can give the best indication of low output. 1st stage 60psi, 2nd stage 300psi, 3rd stage 8-900psi, 4th stage 3500-5000 psi.
Second check the rpm of the compressor - should be running 1200-1300rpm. Is the motor a 5 HP and what size filters are you using?
The correct method to determine CFM is to fill the system (with the tank turned off) to 3500psi, then open the tank valve and time the "fill" back to the 3500psi. In this manner you don't have to determine how much air it takes to fill the filters and the system.
Jim Shelden

Unfortunately it doesnt have guages showing the pressure of each of the 4 stages.
The RPM is set at 1100 which is what my 2 manuals say it should be, did later models run a bit faster? Do you think I would do much damage increasing the RPM to 1300? That should give an extra 0.5cfm. This is a 1970 model I think judging by the filter stamp.
The motor is an 8Hp honda gx240. I filled the filter first and started filling after the pressure maintaining valve started to let air through, that was set at about 150 bar. Will use your method next time, it makes more sense. Filter is an MDE LA series, internal volume about 1L.
I have removed all the valves now so I will use your method when Ive replaced the valves.

Fingers crossed.

Thanks for your help


Kev

 

[pescador775]

The basic test is blowby. Measure the blowby at 3500, 900, 300 psi. If the blowby is high at 3500 and drops sharply at 900, the problem is the fourth stage. Rig up a flowmeter to the blowby vent. If the flowmeter fills with oil (not a good sign) it will be necessary to limit that problem before a measurement can be taken. One way is to install a small pressure vessel in line between the compressor and the meter. Typical instrument would be Brooks Rotameter, 1 cfm. Typical blowby for a functional compressor would be 0.5 cfm.

Manuals for military compressors specify blowby as the definitive measurement for compressor competence, particularly piston/cylinder wear. Measuring interstage pressures is more analytical and may also be used if the special fittings and gauges are available. It's purpose is to test valve function but results may be interpreted for other problems as well.