Hi all, Thought I would ask for opinions in this newsgroup regarding a new project I'm involved in. I'm a PLC programmer who has been tasked with programming a new clean in place (CIP) project for a large spray dryer and it's associated duct work. There are 135 nozzles split unevenly between 11 defined "zones". Each zone is to wash for a predefined period of time by sequentially actuating ball valves off the main CIP header. Differences in elevation between the zones differ by as much as 110 feet, and there are several pressure transmitters to be installed to detect pressure at each zone. I'll move the PT signals into the PID instruction when the appropriate zone is washing. Each nozzle is designed to operate at 100 p.s.i.. At the designed pressure the nozzles will use 12 g.p.m.
The pump is a centrifugal pump and a frequency drive has already been specified to control pressure in each zone. Having been through the extremely unpleasant experience of having to control pressure with a drive and a centrifugal pump I raised the possibility of using a modulating valve to either control pressure, or deliver the designed flow rate, for ex: 12 nozzle at 8 g.p.m. for 96 g.p.m.
I was told by the chemical engineer who's running the job (he *seemed* like a knowledgeable fellow) that a modulating valve wouldn't work and the frequency drive was the only way to go. Because of the head. After much discussion (most of which I didn't understand :) ), he convinced me that as much as I hated to hear it the drive was the way to go.
The project was approved and materials were already being delivered when I became involved. Was he yanking my chain about the drive vs. valve issue? Or does the head on the pump demand a frequency drive in this application? I'll also add that there's 160 P.S.I. direct steam injection controlled by another PID loop to the mixing chamber of a Pick heater to heat the water to
160 Deg. F. Anyone who has had to control pressure with a centrifugal pump will understand my trepidation.