The downthrust increases with differential pressure.
The upthrust increases with velocity.
So we can see on a steady state basis that low flow / high dp favors downthrust and upthrust only can exceed downthrust for high flow low dp.. far to the right of BEP toward runout.
During startup, the curve would need to be adjusted for speed. The consideration favoring upthrust is where the differential pressure does not build up quickly. The most common cause as you mention is air anywhere in the system (not just the pump). But I would venture to say it's not the only cause. For these specific pumps there are three thumbrules given where momentary upthrust may occur during start(at least that's logical reason they would require driver to accomodate momentary upthrust)
Drivers must have 30%* momentary
upthrust capacity if any of the following
conditions are obtained:
A. Where the setting is 75 feet or less
B. Where the head at the surface is 25% or
more of the total head, regardless of setting
(sprinkler irrigation deep will pumps, vertical fire
pumps, industrial pumps)
C. Any submersible
* 30% refers to 30% of the downthrust capacity
Perhaps you can describe the pump and attached system in more detail for more detailed response (not necessarily from me - I've already said most of what I know on the subject - but I'm sure it will help others respond)
Upthrust conditions may occur for a short period during startup and shutdown.
To minimize the upthrust, the pump startup procedure should include starting the pump with a partially closed discharge valve. After a few seconds, the valve can be fully opened because the discharge pressure will be fully developed.
See the attachment, pages 56-57.
