Pipe connection bonding test

[controlnovice]

We have a powder transfer system: blowers, diverter valves, piping with large radius bends, to transfer powder from one silo to another.

Piping is aluminum (not sure exact spec), about 4" diam.

At the connection where two pipes are joined, the plant has installed a short cable, external to the pipe, from one pipe to the other, connected with a 'tab' welded to each pipe. This is to dissipate any static buildup during the transfer of material.

Every year or so, each of these 'bonds' are tested using a standard Fluke 87 for resistance. The probes of the Fluke 87 are pressed against the side of each pipe.

Is this a good test to verify the bond between the two pipes? I didn't think the 87 would produce enough voltage to give a good reading...and that a milliohmeter would be better.

I've also used a clamp-on ground tester to test grounds. Could this tool be used to test the bond cable?

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[VEBill]

I'm not sure about your sort of installation, but for aircraft bonding the (simplified) limit is 0.5 ohms. Do you have specified pass/fail criteria? Compare those (if they're specified) against the resolution of the meter.

The instrument should be 'better' than the pass/fail limit by roughly a 10-to-1 ratio. For example, if the limit is 1 ohm, then the meter should be capable of reading to 0.1 ohms.

 

[resqcapt19]

NFPA 77, the Recommended Practice on Static Electricity, specifies a maximum resistance of 10 ohms between two points. I installed about 5000 of these bonding jumpers on a pneumatic transfer system for a plastics plant. They are checking about 20% of the connection points per year using a standard DMM. When we did the original install, we had to test the resistance across the couplings before we applied the bonding jumpers and only a few were greater than 10 ohms. Since this was existing piping, there were no tabs to connect the jumper to, so we installed a pipe grounding strap on each side of the coupling and bonding jumper between the couplings. When new transfer piping is installed a tab is welded on the the pipe on each side of the coupling for this purpose.

 

[controlnovice]

resqcapt19:

We work for the same company at the same plant.

My question to all is:

Is a standard DMM (Fluke 87) a good enough tool to verify bonding connection/resistance?

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This is normally the space where people post something insightful.

 

[btrueblood]

The short answer would be yes, a Fluke 87 will measure to 0.1 ohms or so, which is about 100x the limit value of 10 ohms you are checking against. Contact resistance from the probes to the metal pipe is probably on the order of a few tenths of an ohm too, and could be checked by measuring across a pad on one pipe (i.e. touch the pipe with probe tips about 1" apart).

 

[OperaHouse]

If safety is really an issue, I would never trust a test like that. Without putting a signifigant amount of current through it, you do not know what kind of bond it has. ROD-L and HYPATIA make testers that put up to 30A through a connection for a timed period. This stresses a connection and potential problems can be detected.

Even something as simple as a 12V battery with a 1-2 ohm resistor supplying current would be a valid test providing some stress. Just use a small lamp across the resistor to verify current is flowing and measure voltage across the bond. This will tell you if things are about to fail either mechanically or from corosion.

 

[resqcapt19]

OperaHouse,
Given that the issue here is static electricity, do we really need to use a high current test? While NFPA 77 calls for a maximum of 10 ohms, there is information in the IEEE Green Book that suggests that resistances of 1,000,000 ohms or less are sufficient to prevent the build of of static charges.

 

[OperaHouse]

Well, my real bad is this and there should have neen a dozen posts correcting me. If your situation requires making a resistance measurment every year because of potential static, YOU MAY BE IN A HAZARDOUS LOCATION. Using the current I suggested would be a spark hazard and not allowed in that area.

That said, anything over an ohm on a metal to metal connection indicates there is some kind of problem with an oxidation layer forming. It doesn't take much to get rid of a static discharge and often voltage will break down an oxidation layer. My concern is the DVM test gives no indication that a connection will fail in the future. You shouldn't test with a higher current than a DVM unless there is no possibility of ignition from a spark.