I have a flow from a pump, and soon after the pump discharge want to be able to take off half the flow at times, and regulate the pressure.  Is this possible with an isolation valve and single control valve as shown in the attached diagram?

Which pressure do you want to regulate, and how?

The flow through a valve (roughly speaking) is Q = Cv (Cv of valve or total constriction) multiplied by the root av delta p  (pressure difference before and after the valve) / divided by specific gravity of fluid.

After the control valve you can either regulate the flow, or you can regulate the pressure, not both at the same time.

If you have a direct pilot operated control valve (operated by spring and liquid pressure before and after the valve) it is normal to install and set the valve to keep a constant pressure before or after the valve.

If 'everything' is constant (pump variatons, pipelines, pressures etc) this will also give a constant flow through the valve (Note!: only if variations are damped enough to be considered constant within tolerance limits for he valve)

If you want to limit the flow, you will either have to select components that at max possible pressure difference do not have a througlet (a CV) that gives a flow larger than your limitation, or you have to measure the throughlet and poweroperate or power-close your valve.

The only reason I said Y in to the pipe is that I'm assuming that they want to get the same mixture/concentration down the sampling system as they have in the main flow.  Coming off at 45 does a better job of that than a T.  Well, in some situations it makes a difference.  If it's a homogeneous fluid mixture, it doesn't matter, or fine suspended solids.

I saw a situation once where they were pumping sand plant wash tailings and had a 2" sample line T'd into the top of the pipe.  The pump vendor had calculated the speed for a new pump and it was delivered skid-mounted with drive system installed.  It started up fine pumping water, but very quickly after the material started flowing, they plugged the line.  Their samples had not been representative of the true % solids, in fact were WAY off.

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