Cascade loops?

I need to program a PID loop here are the componants I have:
Flowmeter (4to20ma signal)
Control valve (4to20ma signal)
Pressure transmitter (4to20ma signal)

------WATER----PRESSURE TRANS---FLOWMETER---CONTROL VALVE--MANUAL VALVE

A flow setpoint will be given by an operator, the controle valve will be controlled by the PID to give the demanded flow...BUT I must always make sure that the pressure does not drop below a certain point...

How do I incorporate this to my PID loop? Do I need a cascade loop if so how do I program it?

If the pressure drops I must close the valve until I reach the minimum allowed pressure...

Rather than cascade a PID loop I would clamp the output of the flow control loop when pressure falls below a low threshold set pressure value. Then I would enable a deadband controller to increment the valve closed until the pressure rises above the deadband. Then you can re-enable your PID.

I think Tom's suggestion is a good start but..

Clamping the output may not increase the pressure. The valve may need to close to increases pressure if the pressure goes below the low limit.

One can use two PIDs and use the minimum of the two outputs. The pressure PID would be reverse acting and try to control to the lower limit. It would try open the control valve 100% trying to get the pressure to the lower limit but the flow PID would probably have a lower output so it would be used. However, as the pressure dropped the pressure PID's output would get lower and approach zero as the pressure approached the lower limit. Then the pressure PID's would be the minimum and restrict flow to keep the pressure at the low limit.

Work it out in your head using ONLY P control until you understand the concept.
Then add the integrator.