The flow rate of my decreases by plateau

[ncdem1983]

Hello,
I use a magnetically driven gear pump to transfer a solution in a process that operates under vaccum (suction = 14 mbars; discharge = 17 mbars).
The problem is that, I have often a decrease in the flow rate by plateau without a significant modification of the pressures. The speed of the pump is controlled by a 0-5V button command. If I increase the speed when the flow rate is decreasing, I do not get a response. But if a stop the pump (or put the speed command on 0) and start again (or increase the command voltage), I get a higher flow rate. Unfortunately, It would begin to decrease again after 5 minutes and slowly toward to 0.
What is surprising is that some days, the pump run very well without any problem during hours. Unfortunately, this is not usual !
Do somebody have an idea to help me understand what is happening?
Thanks.
Nota: The pump is under both vessels.

 

[MikeHalloran]

It sounds like the suction is being starved.
On the days when it runs well, is the static height of solution in the supply vessel higher than usual?



Mike Halloran
Pembroke Pines, FL, USA

 

[ncdem1983]

Thank you for your answer. I must admit that the static height is slightly higher (about 10 cm; the total height is 100 cm). But I have excluded that hypothesis since I have another process (identic component) with lower height and there is no problem.
But may be...
Thanks

 

[Compositepro]

You are operating at low absolute pressure. Your solution is probably vaporizing in the pump when it gets warm.

 

[ncdem1983]

Thank you for the answer. I will try to cool the solution before pumping today.