You can operate the system with two individual pressure control loops, or you can have a cascade loop with a master and slave controller arrangement. The main issue is to define the actual time delay of pump responses and ramp-up speeds (to avoid the surges and cavitation). Obviously the final arrangement will depend on the detailed configuration of the system.
The basic control scheme is presented below.
It must be noted that for clarity's sake all the pressure gauges, switches, flow elements, etc., are not shown—just the basic concept of an option to control the pump arrangement.
This system allows the ramp-up of speed and control of the recirculation, minimizing cavitation, surges or overpressuring the system.
If there are several booster(s) that feed several main line pumps, the basic control would need to include flow measurement for better distribution and probably ratio control for the dosification of the pressures and flows to minimize upsets.
To best develop the required control scheme, more detailed information is required. This is only a basic scheme.
A: Unlike centrifugal pumps, the torque requirements of PD pumps are directly related to the DP across the pumps and independent of speed. Hence, a pressure-based speed control will accurately maintain stable flow across the pumps with good turn down as long as you maintain net positive inlet pressure (NPIP) requirements of both the pumps (minimum pressure limit based on current process fluid density in your control loops).
Since PD pumps cannot run on shut head, you need to shut off booster pumps any time the main pump trips along with a PSV in the booster bypass loop. Similarly the main pump will trip on NPIP protection when the booster trips.
A bypass control valve on booster pump is used when speed control is not implemented due to cost or pump design or electrical supply system harmonics limitations.
A: Rules of thumb for series pump start-up/control are mentioned below.
NPSH is maintained prior to start of the second pumps.
Second pump will not run dry.
Capacity of both pumps will be adjusted and similar, unless second pump operates on the pressure controlled manifold.
Pressure control valve on the discharge line.
A: Assuming that crude oil is incompressible, a buffer tank will be required between the pumps to absorb the variations as the pistons reciprocate. The first pump should be controlled by the level in the tank. The second pump should be controlled by whatever it pumps into. But I am a simple mechanical engineer with no experience in crude things, and I can't afford a copy of the IEH.
A: I have never set up such a system, but have a couple comments. First, it might be interesting to measure the constant-speed case total recirculation flow to enable some efficiency optimization, similar to the variable-speed case. Second, I like the variable-speed method too, but wonder if the cost (vs fixed speed) would be justifiable at the large HP mentioned.