I have inherited an unstable pumping arrangement. The flow path:
- supply tank for a medium viscosity liquid
- lobe pump
- shell and tube exchanger
- slow speed, high pressure, multi-cylinder piston pump
- the rest of the process
Both pumps are on variable speed drives but the piston pump is usually set to one speed only.
Current set-up has PID control on the pressure between the two pumps by manipulating the speed of the lobe pump. The problem is that almost any disturbance sends the PID crazy - rapid, full scale changes in output. The PID tuning is now very slow but it still happens.
Solutions under consideration all involve the use of a throwback line around the lobe pump (discharge to suction) on the theory that this will provide some degree of self-regulation: discharge pressure rises => throwback flow rises => discharge pressure drops.
Thoughts? Suggestions?