Pump Ops - Maximum Theoretical and Practical Lifts

[Sam77]

hi

been questioning this for years and never been given an answer thats satisfactory, and its just reared its head again this week, so im trying on here for a definitive answer 🙂

ok, the main pump, max theoretical and practical lifts are given as 10m and 8m.  i have no problem with this and understand the theory behind atmospheric pressure at 1 bar etc.

LPP we teach max theoretical and practical lifts as 8m and 6m.  this is where im flummoxed.  i accept the 6m practical lift difference, but how can the theoretical lift change?  its still the same science isnt it, the same atmospheric pressure at 1 bar, so why is the 'theoretical' lift not the same? 

thank you, and heres hoping i can get an answer to put this to bed  

[Keith]

I wonder is it anything to do with the type of primer? Majority of LPP's use exhaust gas ejector primers, could it be that it doesn't get the pressure fully down to zero, so you can never get the maximum theoretical.

Other thing with a lot of LPP's is there's a trade off in size against performance. Most people think they have similar capacities as the main pump, but if you check the spec sheets they are often well down. I can't remember the model, but one we used for a while had a maximum practical lift of 3.5m.

[Sam77]

Well thats the best answer ive heard so far so thank you for that, im hoping it isnt true tho as i really want to be right and blow their theory out of the water 

but as you say, that would be the 'practical' lift, the theoretical should still be the same?

thanks

cant see how to edit, so apologies for the extra post but ......

"so you can never get the maximum theoretical."

still suggests that 10m is the maximum theoretical 

[OscarTango]

Where is the original source of this "8 and 6" rule? My guess is someone at some point has written down the general practical lift figures of a main pump and LPP, and for whatever reason that's been been mixed up as two different theoretical lifts and practical for two different pumps. Chinese powerpoint whispers.

Like you say, the theoretical lift should be the same.

[Sam77]

thats my guess too, im chuffed that someone agrees with me and we can maybe get it changed (if correct of course).

out of interest, what do your brigades teach for this on your pump op courses?

thanks

Edited July 28, 2023 by Carl
Quote removed as its a direct quote of the post above. Please see FAQ

[Br9mp81]

Years back,Godiva from Warwick had the service contract for both our main and LPP, the engineer hated with a passion the FS method of raving the balls off LPPs, he always maintained that you just needed to increase the revs till you got a lift, and reving the balls off a cold engine wrecked them.