There are particular issues related to safe storage of pellets. This page sets out some of the key issues of which to be aware.
The following guidelines apply to the design and operation of bulk pellet stores. Storage of bagged pellets in limited quantities is generally very safe, and it is sufficient to ensure that they are kep away from moisture and are not damaged within the bags by rough handling.
Good quality wood pellets have good mechanical durability and include low levels of "fines" (dust), and it is important always to buy good quality fuel. However even these can be damaged in transit, delivery and storage if care is not taken, or the store is badly designed. If pellets are damaged then they can crumble into fine, dry dust. In particular, if pellets are abraded during delivery they can generate an extremely fine dust and if this is allowed to build up and form a suspension in the air there is the potential to form an explosive mixture. While this is only dangerous if there is also a source of ignition, it is clearly a situation that must be avoided.
Another hazard can arise as a result of the potential for pellets to outgas carbon monoxide CO in storage. This gas is both flammable and toxic and so great care must be exercised to ensure adequate ventilation before entering an enclosed pellet store. This risk is considerably greater in larger stores. The HSE has issues a Safety Notice on the risk of CO outgasing in pellet storage.
Care in design and operation can keep the pellets in good condition and provide safe storage, however there are a number of guidelines that should always be observed. These will help to minimize damage to pellets during delivery and ensure there are no potential ignition sources.
- There must be no tight bends in delivery pipes. Any corners should be gentle curves with a radius not less than 0.5 m. The number of bends must be kept to an absolute minimum
- There should usually be a suitable soft baffle opposite the end of the delivery pipe. This helps safely absorb the energy of the pellets as they are blown into the silo, preventing damage as they hit the opposite wall. Other designs based on deflection can offer an alternative
- Delivery pipework must be internally smooth to minimize abrasion during delivery
- Delivery pipework must be made of metal or conductive material and securely earthed to avoid the build up of static charge, potentially giving rise to a source of ignition
- Delivery pressure should be kept as low as possible consistent with efficiently delivering the pellets into the store. Minimizing the length of delivery pipe required (by ensuring good access to the store for the tanker) and minimizing the vertical rise will assist this
- If at all possible there should be no electrical appliances within the store. If this is unavoidable, such as a requirement for a light, this must meet the apptopriate ATEX standard, typically for Zone 22
- A second pipe or vent is required to relieve the pressure during delivery. This must either be connected back to the tanker, or have a suitable filter attached during delivery to collect dust
- Large quantities of dust must not be allowed to build up over the years. The store must be periodically checked for build up of dust and from time to time emptied completely and residual dust removed
- Under some circumstances it can be appropriate to consider incorporating an explosion relief panel. Factors such as size and siting of the store may be relevant, but good design and maintenance, as set out above, should reduce the possibility of explosion to a negligible level.
The size of the pellet store should be considered carefully at the outset. It should be sufficiently large to allow at least the minimum delivery quantity of local suppliers (typically 5 tonnes, approximately 7.5 m3) without needing to be completely empty first. To allow a little "headroom", a sensible minimum volume is 10 m3. Larger users will need a larger store, and a sensible minimum size is 3 weeks storage. A larger store will allow larger deliveries and this can often attract a lower price.
It is vital that the store is completely waterproof as pellets can absorb moisture and will crumble to sawdust. They will also expand during this process, potentially putting considerable pressure on the store structure. It also needs ventillation (filtered) to avoid the build up of condensation, however the structure should be sufficiently air tight, including appropriate seals on access doors, to prevent the escape of dust into other areas, in particular the boiler room.
If a person needs to enter an enclosed space that has been used to store pellets they must always ensure that it has been adequately ventilated before entry, that the door remains open while they are inside, and that they only do this under the constant supervision of a second person who remains in the vicinity, outside the store.