Hi,

Are there any specific requirements regarding how a vehicle unloading biomass pellets should be earthed?

In the past I have seen installations which have a designated earth rod connected to a unit which checks resistance once connected to a tanker (Newson Gale are the most common provider it seems).

Alternatively, I have seen earthing connections going back to a known earth in an electrical panel, with a crocodile clip on the end.

Providing these are periodically tested, is there an right or wrong between these two systems?

Thanks in advance.

My experience is with bulk delivery of powdered starch.

Sites accepting deliveries would have a cable fixed to the metal silo filling pipe (naturally connected to earth) whose other end would have a crocodile clip which would be attached to the designated earthing point on the tanker (an exposed metal stump).

The "static" from a delivery derives from the product rushing through typically rubber hosing - the cable between pipe and tanker shorts the insulated filling pipe preventing static build-up.

Muts admit never coming across a delivery earth that was connected within an electrical cabinet.

Hi Jdpnsp

First let me mention that I don’t know what material your biomass pellets are made up from and if you have had a DSEAR Assessment completed to identify if there is an actual explosion or increased fire risk identified during the delivery process or at any other time that during the consumption of the biomass pellets.

It really depends on particulate size and amount of dust being generated. The pellets themselves are not an issue, it is the fine dust that forms part of the delivery and is also generated when they are delivered the pellets to the silo form the delivery vehicle under pressure. You don’t mention what the pellets are made from so on this occasion I am assuming they are compressed wood powder, sawdust and chippings.

The dust generated from the aforementioned substrates is the same particulate size if not smaller than wood flour.  Normally the vehicle delivering the pellets connects an extraction fan to the silo to extract the dust during the delivery process through a filter to a safe location outside the building.

This has a two-fold function. First it reduces the pressure within the silo during delivery as the pellets are delivered. This prevents the dust that is generated being expelled into the room were the silo is located through gaps in the structure as it is not a certified pressure vessel.

Secondly the dust is potentially explosive and preventing it leaving the silo into an enclosed room with potential ignition sources (as these biomass silos are normally located in the same room as the boilers) which can also be sometimes gas fed as an alternative fuel source.

Generally hard wood flour has a Minimum Ignition Energy of 20mJ but needs 50g/m³ to become ignitable, whereas Pine wood flour has a Minimum Ignition Energy of 40mJ but surprisingly only needs 35g/m³ to become ignitable.

Passing particulate either pellets or dust along a duct from the delivery vehicle into the silo has a charging affect and to reduce this charging action it is important to ensure that the silo and delivery vehicle are at the same potential if the ducting is not static dissipative. To aid this a grounding wire is attached form the delivery vehicle to a common earth point where the silo is also grounded.

 The guidance you require on this subject matter is held within PD CLC/TR 60079-32-1 Explosive Atmospheres Electrostatic Hazards, Guidance.

Section 7.2.4 Promoting charge dissipation

“In situations where it is not possible to avoid a flammable atmosphere, the risk of ignition can be controlled by limiting charge accumulation. For the components of the handling system, this involves bonding solid conductors and people to ground and, if necessary, replacing insulating system components with earthed dissipative or conductive components “.

This applies to both dusts and liquid delivery where there is a risk of an explosive atmosphere being generated within a silo or tank which is why they are grounded during delivery.

As you are only looking for bonding protection against static you can connect all the conducting parts together to a common earth point or common earth connection on the electrical system in the hazardous area. 

You will probably find that the pellet silo itself will have electrical systems installed upon its structure and the metalwork  is connected to that common earth point on the electrical system.

You do not need a separate earth on this occasion and can use a common system earth for static protection.

A separate earth is only needed (Excluding Lightning Protection and Electrically Conductive Flooring) in hazardous areas when you require a clean earth connection for the explosion protection concept intrinsic safety, which in that case is connected back directly to the Main Earth Terminal (MET) of the building or plant which is normally at the metering position in a Non-Hazardous Area.

The reason for this is to ensure that any potential electrical earth fault flows back to the main earth terminal in the Non-Hazardous area as opposed to potential earth faults in Non-Hazardous areas using what are called parallel paths to earth which might allow electrical earth faults to flow to earth within the hazardous area presenting a potential ignition hazard.