Estimating the real weight of product above a valve and its effect on valve torque is very complex with factors such as the apparent density of the product, the shape and size of the particles, the mixture of different material, the shape of the container, moisture content, and many more affecting the outcome. Often the most cost-effective method is trial and error. In the absent of empirical data a simple lab test can yield good information. Companies like Jenike & Johanson have a lot of experience with bulk solids and could help.
Here is an expert from http://gemcovalve.com/choosing.htm “When handling solids a higher factor of safety is used to calculate seat torque requirements. The factor is typically 1.5 instead of the 1.25 used for liquid and gas valve calculations. For solids that “set-up” or harden oversized actuator and specially designed discs that can break through the hardened cake are used.”
Also a ball disc which opens and close within its own spherical radius and does not have to “lift” product tends to work better than flap or butterfly valves.
Sanitary rotary valves used to convey a steady flow of material work well in a lot of gravity feed applications. They can be limited when trying to hold vacuum and sanitary airlocks are often a good alternative.
When handling solids or slurries a higher factor of safety is used to calculate seat torque requirements. The factor is typically 1.5 instead of the 1.25 used for liquid and gas valve calculations. So use your torque wrench to see what is required to seat the valve then multiply it by 1.5 and pick the next size up actuator.
For solids that “set-up” or harden, an oversized actuator and specially designed discs that can break through the hardened cake are used. Actuators are typically sized for 80-PSI (5.3 bar) pressure. If the available supply air pressure is dependably higher (100 to 120 PSI) or lower (40 to 60 PSI), this will factor in the sizing of the actuator.