Whats the risk? is it a 30,000 gpm pump or a 3 gpm pump? For small intermitted pumps, I used a fixed RO around the pump, no control valves or instruments, just a 1" to 2" union with an RO in it. Its cheap, it won't fail, and yeah, it's not optimized on energy.
it much depend on the design of the pump and the time the pump is going to run on low flow.if the pump is variable speed, there is no need for min flow protetion at all.
Nik456, in general a min flow return line is not provided for intermittent service. You simply start/stop the pump based on the level in the vessel and make the rated flow above the pump minimum. That being said, if you have a high head multistage pump I would still specify a min flow line (in case the discharge is blocked in accidentally).
Lots of ways to control minimum flows as already stated, however, is this a safety device ? in which case you can look at other cost effective control methods.
An understanding of what you are looking to do - flows, pressures, pump size and type etc will be a great start to advise you further.
Generally, on a batch operation basis I wouldn’t think there is a need to have a minimum flow or recirculation line. That is, for 1 or 2 batches a day. Even though modern plant MCC manufacturers have claimed stunning improvement in operating parameters of their kit, a number of old plant electrical engineers I’ve worked with shy away from starting a drive or prime mover too often. A rule of thumb I often come across is no more than 6 times a day. This figure is of course up for debate.
The recirculation line is primarily to combat intermittent or variable load from the hydraulic system more often than not from a branched / networked reticulation system: since a reticulation system may feed multiple consumers at different parts of the plant, it is difficult to reconcile the myriad system curves possible with combinations of open and closed consumers. Thus the recirculation line may be nominated to attenuate flow through the pump to derive an optimum head and energy profile. I agree with above that a VSD may be used but this is nevertheless limited to the minimum running speed (and hence draft cooling) of the motor.
Another possible way to reconcile your supply and demand, depending on what it fact your process involves, is the use of header tank which is slow filled and quickly discharged.
