A 'solid' ball valve is a ball valve where the ball is machined from polypropylene or PVC. The opposite is a 'hollow ball' where the ball is constructed with an inside hollow, usually by two polypropylene or PVC spheric have joined together, with or without inside walls or ribs as reinforcement.
The hollow-ball concept is used to save weight and/or money in materials and machining.
If there is a choice between solid and hollow ball for the same application, the solid ball is normally considered as the 'best cost/lifetime' solution. Reasons given are less risk for construction, wear, material and fabrication failure over time. The difference between the two could however be from 'practically insignificant' to 'very high', all depending on quality, QA for product and application circumstances.
Best advice to you could probably be given to you if you present your application to a factory that produces both solutions.
Solid ball means the part is made from a solid sphere and a hole is drilled through to make a the flow path. So between the flow port and the outer surface of the sphere, the ball is solid.
Hollow ball is when the inside of the sphere is "hollow" There is a certain thickness to the shell to give it strength, but unless you weld in a piece of pipe, the flow port through the ball will not be circular.
Hollow balls are used to save weight and cost, and are only useful in large valves where the weight of he ball becomes significant. The concern with hollow balls is the strength under pressure when valve is shut, and more importantly, the strength of the ball / stem connection for opening and closing the valve.
Both solid and hollow balls can be made from one material, or a material that is coated with hard facings such as tungsten carbides.
