would anyone suggest using an isolation valve at a pump discharge as a means of controlling operating pressure in a closed system.

I think there are more energy efficient alternatives. And you have the pressure only between pump's discharge and valve and not in the system after the valve.

You should use a control valve as noticed above, in order to adjust the flow to your process requirements.

There are other strategies available.
1)    Use a VSD.
2)    Use "by-pass approach". The pump runs continuously at the maximum process requirement and the excess fluid flow rate is continuously diverted and sent back to the supply.
3)    Trim the impeller  (when the pump has been sized wrongly, that is oversized, machining the impeller is an option to reduce energy added to the fluid).

Throttle with a control valve.  Do not throttle with the isolation valve.

Isolation valves are generally not designed with flow coefficients that make them particularly good at controlling flow.  Consequently, you don't have very smooth or accurate control and you could easily end up with a situation where the control system is not able to find a stable operating point and bounces back and forth opening the valve too much, closing the valve too much, etc.

Other than that, you will likely cause damage to the valve thus negating it's effectiveness for isolation as mentioned by TenPenny.

Generally, just not a good idea but it would really help to know specifically what type of valve you are talking about (gate, ball, butterfly, etc.).

Another way of saying what rneil and BigInch said is that the linearity of a block valve is not very good.  So going from 5% open to 10% open will increase the flow 50% or so.  Going from 75% to 85% will not change flow at all.  Programing that kind of a valve performance curve into a PLC is pretty difficult (I've never seen anyone get it right).  On the other hand, a pressure regulating valve has pretty good linearity and is self correcting.

The most energy efficient way to do pressure control is to never have increased the pressure beyond what you need.  That means that a VFD or a constant pressure valve (i.e., a valve that has a pressure regulator that dumps excess flow) are both far better than throttling the full stream.  Anything is better than throttling the full stream with a butterfly, gate, or ball valve.

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