I suspect we may be overlooking some important points of information...
But realize,this is not a FLUID POWER application, and it is not a 'hydraulic pump'. It's a boiler feed pump.
There cannot be a pressure relief valve isn't in the water delivery line... that would risk having the boiler discharge itself and overheat.
What defines the pressure to which the feedwater pump is working, is the temperature of the boiler, and the rate at which it is developing steam. As the steam is drawn off, boiler water level falls, water boiling rate rises, and pressure in the boiler STARTS to fall.
As that pressure falls, the multistage centrifugal pump progresses from a minimal-flow circumstance, to a higher flow circumstance, and water then moves into the boiler. This CHILLS the boiler slightly (incoming feed water is absorbing heat, hence, boiler pressure slumps for a little bit... but the water expands, recovering the hump.
All this time, the feedwater pump is running, and must remain so in order to maintain the differential pressure from feedwater input, to boiler output.
To further complicate matters, we do not know if this pump is in the path immediately PRIOR, or POST a feedwater preheater, if it is BEFORE, there'll be a return passage from the after port, back to a point BEFORE the feedwater pump, as 'stalling' water in the preheater loop will turn the preheater into a BOILER.
The feedwater pump is centrifugal, and it is designed to be stalled against an intended PRESSURE. It is not positive displacement... it is variable. Remember, it is first and foremost, centrifugal FORCE which allows it to work, and the motor's RPM determines how much centrifugal force can appear at the edges of the impellers (it's multi-stage). The only thing that will PREVENT it from generating a static pressure, is CAVITATION, which won't occur if the inlet pressure (water supply source)is at an excessively LOW pressure to start with.
A PID control signal from the boiler's master controller probably commands the pump's drive in response to several factors, as most complex boilers nowdays run nifty full-authority controls that can do instantly, what a well-seasoned boiler operator needed several minutes to sort out.