Unpumpable is defined as a substance which can not be "induced" to flow through a pipe.
You can pump solids - such as powders as they act just like a fluid when in motion. The key is how do you get the supply into the pump. What happens to the solid after it is pumped...
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What flow rate and size of pump?
What are the inlet conditions? Low NPSHA could cause solids to flash out of solution in the eye of the first impeller.
The duty cycle is key - This fluid needs to be flushed prior to any shut down. I suspect that this is not done and the...