Centrifugal pump in series with a rotating positive displacement pump
I am especially interested in the sequence for starting and stopping.
Reading the email thread, there appears to be some confusion as to whether the centrif is used to feed the pd or vice versa.
My comment if you have centrifs feeding PD.
Centrifs have varying flow charcteristics and displace fluid via centrifugal force. The flow you get depends on the system resistance it sees. Ie basically the higher the resistance the less the flow ; so you need to look at the selected centrif curve. For PDs they are basically fixed flow machines . In principle, will deliver the flow rate at whatever the resistance is. A PSV is required to avoid exceeding your line ratings. The PD flow charcteristics is pulsating so one needs have pulsation damperners to take out the pressure spikes.
Centrif feeding PD is not an ideal combination but not impossible if you have to. You need to ensure you match the flow. Flow matching in series pumping is always the key. Primarily you need to ensure your centrif duty point (flow wise ) marginally exceed that of the PD flow. This way you flood the suction to the PD . The centrif flow is only dictated by the system resistance so if resistance resistnace is actually higher than what you have calculated ; your centrif will end up give you lower flow than you calculated. This spells disaster for the PD . You will have flow detachment problems and cavitation issues on the pD pump. If the system resistance on the centrif is lower than calculated, this is not an issue the PD pump will act as a choke and throttle back the flow ie basically takes what it needs.
So ensure your centrif can deliver flow higher than the PD; not lower; ie not starve your PD pump.
Hope above helps without going into specifc numbers etc.