One of the most common problems with bulk material flow is a condition known as ratholing. Ratholing occurs when cohesive bulk materials are stored and conveyed in vessels with a funnel flow discharge pattern. When a stable rathole develops, the bulk material remaining in the vessel is stagnant and will not discharge. This stagnant inventory of bulk material can spoil, causing cross batch contamination, spontaneous combustion, and a reduction in live storage capacity.
In many situations, the condition of ratholing occurs because the wall surfaces of the vessel are not steep or smooth enough for materials to flow. This condition is highlighted in the adjacent image, which shows the rathole flow channel in green and the bulk material highlighted in brown.
Below is an actual photograph showing a stable rathole developing in a coal bunker discharging Powder River Basin sub- bituminous coal. This pyramidal shape coal bunker, built in the early 50’s, was constructed of carbon steel and coated with gunite on both the vertical and sloping walls for corrosion protection. This high friction coating worked well, until the coal was switched from high sulfur bituminous to low sulfur sub-bituminous. The finer sub-bituminous coal with higher internal moisture and clay content flowed poorly on the gunite coating, resulting in pockets of stagnant coal, which then lead to bunker fires.
An efficient way of achieving the reliable discharge of bulk materials in bins, bunkers, silos and chutes is to line the sloping wall surfaces of the storage vessel with a low coefficient of friction material. TIVAR® 88-2 is an ideal product for this application. The frictional properties of TIVAR® 88-2 are consistently the lowest in the industry versus a wide variety of bulk materials, with surface lubricity lower than 304 2B stainless steel, baked on coatings, ceramics, chrome plating and sprayed on urethanes. The adjacent chart shows an example of the wall angles required to achieve flow discharge in a silo handling Powder River Basin coal. The lower wall angle provided by TIVAR® 88-2 means bulk materials slide or flow on shallower surfaces as compared to stainless steel. The wall friction angle data supporting this chart was generated using the ASTM test method D-6128. In storage vessels designed for mass flow discharge.
When mass flow discharge develops in a storage vessel the movement of bulk material is uniform and consistent. The flow pattern sequence shown below depicts how mass flow discharge would occur. This flow pattern is described as “first-in, first-out” flow. When any material is withdrawn from the storage vessels all of the product is in movement and full live storage capacity is available. Mass flow discharge is ideal for bulk materials that are classified as non-free flowing and susceptible to spoilage, cross batch contamination or spontaneous combustion.