ElectroStatic Discharge mitigation from railcar fuel tankers

[jhc_jr]

I've been tasked with performing research on the issue of levels of ESD build up and discharge from railcar tankers before the process of petroleum transfer. Specifically, looking into a specification that our products can be tested to that would cover the connection of an electronic device to a railcar that has not been properly bonded and/or grounded.

API2003 covers a recommended practice for bonding and grounding of a railcar tanker before product transfer, but as can be expected these practices are not universally followed. Although API 2003 discusses charge accumulation _during_ product transfer, it does not discuss charge accumulation as a result of railcar movement.

What I'm looking for is some sort of research that would support waveform and magnitude criteria in order to conduct a reasonable test that would simulate - on some level - the worst-case electrostatic discharge of a railcar. So far all I've managed to come up with is a vague reference to a British study that suggested a typical stored charge on a tanker truck with 30KV of potential is 2250 millijoules. Since road vehicles typically have inherently dissipative paths, and rail vehicles usually have non-conductive bearings and wear pads isolating the tank from the rails, as well as a significantly larger volume of 'dielectric', it’s safe to assume that the stored energies on a rail car will be much larger.

I'm looking into DO-160 EMP waveform 2 stimulus - This seems to be the closest thing I can dig up that might approximate a large ESD event occurring upon the connection to a railcar with significant static accumulation.

Any suggestions of regulatory agency recommendations would be helpful.

James

 

[LittleInch]

I'm not really sure what information you're after. Regulatory and safety bodies normally stipulate the procedure you need to go through to earth the tanker and prevent any static discharge which could cause a fire or explosion. They're not interested in how much charge there is, it's the voltage and spark energy they worry about.

I've seen figures of up to 20kV mentioned. You know the ability of your product to hold charge so what's the question?

Rail cars are normally filled to within 6inches of the roof so there's very little product movement / slopping around - far less than a road tanker.

So if you're looking at preventing some sort of electronic equipment being fried when it connects a charged mass to earth then you will need to use some basic numbers - 20-30kV seems about right.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jhc_jr]

LittleInch,
Thanks for the reply. You're correct that there are recommended practices that should be followed. Unfortunately - as I noted in my OP - There are many cases in which these practices _aren't_ followed. In our case, there have been many instances where the bonding process was not followed, and our equipment was connected resulting in a very large ESD event which blew out our equipment.

Since we can't rely on the field personnel to follow process 100% of the time, it's in our best interest to ensure our product is robust enough to handle such events.

It would be worthwhile to reconsider the amount of electrostatic energy that is created by fluid dynamics in a moving railcar. Unless there are baffles that restrict the movement of the product in the car, there will be constant large fluid currents that generate friction and the resultant static energy. Consider the difference in spinning an uncooked egg versus a hard-boiled egg on your kitchen counter. The uncooked egg slows down rather dramatically due to the friction of the unccoked contents - and there isn't a dynamic 'bubble' in an egg. Fluid dynamics dictate this behaviour. Since there isn't any real research on the issue of electrostatic accumulation as a result of transportation, I would suggest that the energy generated in a rail car - with a typically isolated body - is _much_ larger that a road tanker. Especially since the road tankers are typically separated into compartments of less than 3000 gallons, and most are significantly smaller. These compartments constitute baffles that of course dramatically limit the movement of the product. That, with the fact that road tankers by design have conductive paths that limit the energy buildup, results in significantly larger stored energy in railcars.

Additionally, we have a rather large and long-standing installed product base with the vast majority at road tanker transfer facilities (tank farms). While we have had several instances of large ESD damaging our products in rail facilities, there has not been a single instance of such damage occurring at a road tanker facility - Q.E.D.

The question I'm asking is from an engineering design Validation and Verification perspective (vis a vis en61508 and en61511). It's imperative in SIL approved products that the design criteria are verified using repeatable and justifiable test plans. While the test we have come up with is certainly repeatable, we would like externally traceable justification. If there was an industry study or recommendation from a design (not process) perspective we could refer to, it would make things much easier.

 

[BigInch]

In order to upper-bracket the possiblities, you might want to up the voltage to what aircraft can experience. Potentials there can reach 100,000V

 

[jhc_jr]

We did actually consult a couple of consultants in triboelectric and precipitation static effects. One was an antenna designer and another was an aviation specialist. Neither one wanted to accept the job. One went so far as to say "The lack of definition is such that I can see so many caveats being needed that I am unlikely to get very far and can make no promises – I doubt anyone else could make you any guarantees either."

 

[georgeverghese]

In standard design practice, if the grounding cable is not connected to the stipulated earth device, a loss of grounding safety monitor would inhibit loading operations.

 

[LittleInch]

jhc - first congrats on an well worded OP and response.

I still think though that you're working on incorrect assumptions - in general people fill railcars to close to max fill - 1% ullage seems to be common therefore I just can't envisage "constant large fluid currents that generate friction". Part filled then maybe, but from what I could see, if it is real, then not many people have given it any thought as it's not of benefit to anyone. Rial cars do start and stop, but generally just sit there slowly trundling in straight lines or very gentle curves for long periods.

Not sure what you searched on but I tried "railcar tanker slopping" and some interesting papers came up with other references which might help you, but most were concerned with the physical forces rather than static generated. I guess most people think that the tanker will be earthed like it should be before offloading.

Road tankers are inherently insulating on their rubber tyres, but many have anti-static connections dragging along the road to avoid this. Maybe road tanker off loading is more careful to avoid static than rail cars?

I think to get to the level of justification you are looking for you probably need to go and do a set of tests yourself to measure the potential found over time.

If you can tell us what is this device? Can't you just protect it via a surge divertor or similar? Solid state devices exist for pipeline isolation joints which protect the joint against lightning strikes which might be similar levels of voltage.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jhc_jr]

georgeverghese,
You've hit the nail on the head - that is exactly what happens. The large ESD event damages the ground monitoring device > The ground monitoring device will not allow flow control systems to operate (fail safe) > the individual gantry becomes inoperable. Bear in mind this is a ground _monitoring_ device, not a grounding (bonding) device. In truck transfer facilities there is usually a series of gantries, and it's not a huge problem for the truck to be moved in order to fill. You can understand therefore why this is highly problematic in railcars.

 

[jhc_jr]

LittleInch,
With regards to large fluid currents, it's helpful to remember that even when the mass of liquid has reach some sort of inertial stability, there is the issue of constant and significant vibration and shock being transmitted to the payload. The laboratory test parameters as stipulated by The American Railway Engineering and Maintenance-Of-Way Association (Communications & Signals Manual) for shock and vibration at the truck axle are:

5-20 Hz - 0.9” p-p
20-1000 Hz - 20 g p
operating shock - 11 mS sawtooth 100 G pk

Yes, that's 100 G. The shock aside, 5-20 Hz - 0.9” p-p and 20 - 1000 Hz at 20g is incredibly harsh. That specific requirement calls for a series of 20 sine-sweeps at 17 minutes each for a total of 5 hrs 40 minutes on each axis of the product under test.

That spec is at the axle. However, I digress - vibration isolation isn't the issue I'm exploring, I only presented it to make the point that there is a tremendous amount of energy constantly being pumped into the fuel load in a railcar. Contrast this with vibration and shock specifications for equipment mounted to road tankers, and again you can see the factors contributing to stored energy are significantly smaller.

As a side note, the railroad spec mentioned above only address ESD immunity in terms of the EN61000-4-2 human body model (hbm), and only addresses electrical transients at the signal ports in terms of EN61000-4-4 EFT, which is based on typical power line transients - no where near the energies we are seeing in this case. Our testing using an ESD hbm generator up to 30KV showed no ill-effect on the product _before_ the redesign. It turns out the hbm waveform is far too fast, and therefore does not contain the overall energy we are seeing with the railcar discharges.

Addressing the issue of electrostatic dissipation on road tankers, We've done a lot of research in this area to determine why it isn't as much of a problem as we had first thought:

- You are correct in that some road tankers have static discharging devices that perform a near-constant discharge while traveling​

- The road tanks are not isolated from the chassis - as is often the case with railcar tanks​

- Most tires these days have a high enough carbon content that allows for some additional dissipation, and many tire manufacturers are incorporating "antenna tread" - a highly conductive strip of high-carbon rubber in the tread to help dissipate static accumulation further.​

We redesigned the ground-sense input of our products some time ago to be able to handle high energy EMP (per DO-160) as I mentioned earlier, and haven't had a problem since. The issue now, as I also mentioned earlier, is the traceability and justification to a design spec for our new products. We're using DO-160 for now, but it isn't really appropriate to address the design requirement.

 

[georgeverghese]

Would it then be possible to find some way the plant operator would connect the grounding cable on the railcar / roadtanker first to allow the large ESD to dissipate, and then connect the grounding loss safety monitor?

 

[jhc_jr]

georgeverghese,
Connection of a bonding/earthing device before a monitoring device is generally stipulated in most facilities as a combination of corporate policy and insurance requirements. It's also part of the recommended practice as defined in API 2003. However, as previously noted, it's not uncommon that the process of discharging the tanker _before_ connection of the ground monitoring device is not followed. Many of the operators are under the impression that the monitor is also a bonding device, but the resistance characteristics of the ground path through the monitor aren't designed to handle large esd currents. In the vast majority of instances, it works as such, but it isn't the recommended (and often required) method.

 

[georgeverghese]

These vendors who provide these safety devices may be able to come up with some tamper proof mechanical contraption that will force the operator to ground the railcar / roadtanker first before hooking up the loss of grounding safety monitor?