I think you have a good explation from the 3 replies so far, but let me put this in a slightly different way.
Let us say that the discharge valve is closed, and the pump is started. As you have pointed out, the fluid exiting the impeller has a high velocity, which is converted to pressure energy by reducing its velocity in the volute. When the valve is closed, the chamber quickly fills up, so there is no place for more fluid to enter the volute. In effect, the back pressure in the volute keeps rising, and eventually the velocity energy of the fluid in the impeller cannot overcome the back oressure any longer, so there is no flow. Now for the pump to do work, you need to raise the pressure, but you also need to have a mass flow. If you raise a stone 10 feet up and leave it there, it does no work. If you drop it, its mass and the height it falls determines how much destruction(i.e. work) it does.
As you open the discharge valve, the flow invreases. This volume of fluid with the pressure the pump has raised contributes to th work done or BHP. You need both flow and pressure, not just one.
As to why the velocity is not 'equal' to the pump circumferential speed, consider;
- at no flow, i.e valve closed, the back pressure equals the supply pressure, so vel. is zero.
- at full flow, the vel is max.
It is like opening a garden hose, and closing off the hose at the far end, initially there is a flow, the hose is under pressure, but by shutting off the hose-end valve, you stop the flow.
I hope this explanation is less technical and easier to understand. You can get the maths involved in a vook such as one by Hicks.