Comissioning of a blasting pot is shown as an example here.

Commissioning of a blasting pot

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A blasting pot with pressure conditions as shown in this picture does not function because there is lower pressure at the dosing valve (p3) than at dosing valve (p4). By this, the abrasive can flow through the dosing valve out of the blasting pot; only compressed air flows out of the jet nozzle.
The pressure p4 will be reduced only by slight closing of the lateral ball valve. If p4 will reach the value of p3 (or less) the abrasive will start flowing through the dosing valve and the blasting pot is operational.

Too hard closing of the ball valve leads to the fact that too much abrasive flows through the dosing valve at a very high speed (pressure p4 is much lower than pressure p3). This leads to extraordinary high wear and tear at the dosing valve. If the flow volume will be additionally reduced at the dosing valve, i.e. the flow volume cross-section reduces, the dosing valve acts like a nozzle. By this life time of the dosing valve as well of the components getting in contact with the abrasive will reduce many times over.