I would like to learn what type of positive displacement (PD) pumps are used to remove liquids from vacuum applications. I have been searching for an example application, but it seems to be a rare case.

So, imagine you have a liquid trap in a large vacuum system. The flow of water accumulation is so high that it warrants an extraction pump to remove the water. The pressure is about 90% vacuum. The goal is to remove the water to dump so it only need be pumped back up to ambient.

Is there a type of PD pump that is better at this application? From what I have found, very few pump manufacterers publish performance for vacuum operation.

Does anybody have an example of a vacuum application where a PD pump is used to extract liquid?

  • $\begingroup$ What is a "PD pump"? $\endgroup$
    – nibot
    Feb 28, 2017 at 15:34
  • $\begingroup$ @nibot : see the first line of the question. $\endgroup$ Mar 1, 2017 at 11:52

2 Answers 2


Condensors in steam plant are a common application - about 90% vacuum and you have to extract the condensate at the rate the engine is using steam - something like 9 lbs per hp/hour if you'll excuse the non-SI units common in steam days.

So you may be looking for a boiler feedwater pump which not only works against the vacuum, but into a high pressure boiler.

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  • $\begingroup$ This is a great lead! I am looking into BFWPs, but good golly there are a lot of different types. It begs the question, what is the smallest boiler operating from a sub-ambient condenser anybody has heard of? $\endgroup$
    – Murenrb
    Oct 21, 2016 at 3:41
  • $\begingroup$ Smallest I can find offhand, about 400mm long for the complete steam plant ... mainsteam.co.uk/condensers.htm There may well be smaller, but scaling laws limit efficiency... $\endgroup$ Oct 21, 2016 at 9:40
  • $\begingroup$ @Mart : can you double check that figure? I just used tlv.com/global/TI/calculator/steam-table-pressure.html entering 10kPa (abs) and it returned a saturated steam temperature of +45C. (oh right, we agree. I misread ~45 as -45 at first). Yes this is correct, steam plant does work down to 45C or less. A marine environment provides a very convenient way of shedding heat at even lower temperatures; steamships exploit this, many power plants are built on coastlines to exploit it. Inland, you will now understand why cooling towers are so large. $\endgroup$ Mar 1, 2017 at 11:44
  • $\begingroup$ @mart : Apropos of this : it's also why Organic Rankine Cycle plant has a vanishingly small niche : see engineering.stackexchange.com/questions/12108/… for example. You don't want to vent ammonia or ether to atmosphere (!) so you need a condenser. Having a condenser, there is little to be gained from replacing water. Note the linked example of ORC in another answer shows a prototype plant tested about 30 years ago - not widely adopted. $\endgroup$ Mar 1, 2017 at 11:55
  • $\begingroup$ Ok, I stand corrected and remove my comment. $\endgroup$
    – mart
    Mar 1, 2017 at 12:04

One approach is to use a gravity trap so that the liquid flows to the bottom of a sump. The sump can then be isolated from the rest of the system with a valve to allow it to drain under gravity a bit like an airlock.


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