Vacuum Pressure Question.

[conny]

We do a lot of submersible pump repairs in work and test the mechanical seals to a pressure of 4 bar.
Recently had a 185kW for repair and on reading the seal spec found they should be vacuum tested not pressure tested, (i.e. air drawn out of the seal chamber not pressured in).
It stated that the vacuum should 'have a drop in the mercury of 500mm below absolute and not rise to less than 250mm after 1 hour.'
Can anyone tell me what the equivalent air pressure would be for these figures please.
Just want to satisfy my own curiosity.

 

[TicklyT]

500 mm Hg (mercury), or 500 Torr is about 0.34 Bar (absolute).

A vacuum (-1 Bar) = 760 Torr.

 

[conny]

Thanks TicklyT.
Gives me somethink to think about.

 

[Tipper]

What's a Torr.....never heard that one before!

 

[Corgigrouch]

760 torr equates to 1 atmosphere and is used to measure degrees of vacuum

 

[D_Hailsham]

760 torr equates to 1 atmosphere and is used to measure degrees of vacuum

It's an "old money" unit of measurement; not SI approved. The modern unit is the Pascal - 100,000Pa = 100kPa= 1 bar

 

[conny]

500 mm Hg (mercury), or 500 Torr is about 0.34 Bar (absolute).

A vacuum (-1 Bar) = 760 Torr.

Maths is not my strong point but does this mean we are pressure testing at about 12 or 13 times the required pressure?

 

[matt1e]

no about 6 times conny by my reckoning
this should help

then 4 / 0.6666118421053 = 6.00049346

But I don't think you are pressure testing at the wrong pressure its just a different technique as surely pressure testing will push down on the seal whereas vacuum testing will pull up on it against the spring

Matt

 

[conny]

Thanks Matt.
Thats a brilliant way of explaining it to my boss tomorrow.

I see your reasoning behind the vacuum pulling and the pressure pushing also.

Thanks mate.

 

[matt1e]

Thanks Matt.
Thats a brilliant way of explaining it to my boss tomorrow.

I see your reasoning behind the vacuum pulling and the pressure pushing also.

Thanks mate.

no problem mate, I think its probably the manufacturers prefered way of testing as it also tests the integrity of the loading spring and its a smaller vacuum pressure incomparison because it is pulling the seal off its seat, I wouldn't worry too much as they will know the specs
on another note (same theme) and sorry for hijacking your post just out of curiousity since you obviously do a lot of work on these.....
last week I had to change a flygt 4Kw borehole pump and it failed after a day
I was onsite to fit a low level float to the storage tanks and also noticed the panel PLC was reporting borehole pump 1 O/L had tripped so metered it out and found it had an O/C winding.
they had a spare one onsite (new but unpackaged and lying in a store room for around 5yrs) so went next day to change it (matter of urgency as its a busy hotel/complex and all their water comes out the ground) got the old one out and the new one in and because they have no level sensors in the boreholes and also because the FLYGT doesn't have any internal protection I measured the water table at borehole 1 because I was worried incase the pump had ran dry
the pump was at a depth of 52.2 metres and the water table was at 45.3M pumps off but fell to 49 metres with both pumps running. so really only about 3 metres of head at the pump. anyway because the hotel needs both pumps running to cope with demand I put it in service as I was going back in the morning to measure up to quote for borehole level sensors etc.
got there next day to find the whole plant had tripped out on the main RCD (a bit more work req to sort out discrimination too) and of course found that the culprit was said pump.measured the windings which were ok on continuity but now leaking to earth (0.034 Mohms)
this was friday and haven't been back yet to pull it out or re-measure leakage but what do you reckon on cause
1)moisture ingress due to damage to the case/cable because it was lying unpackaged buried under crap etc for 5 yrs?
2)damage to the machanical seal because of above? or
3)damage because of dry running?(only 3 metres at most of head)

cheers for any input
Matt

 

[conny]

Hi Matt, my first suspicion would be the mech seals have been damaged. Probably the unit has been tossed about a bit over the years and the slightest 'jar' can easily chip the carbon surfaces. As an example one of theses 185kW ones we did last year was being off-loaded at site. Left us in perfect condition but when the crane operator placed it on the ground he came down a fraction to heavy. They installed it and within the hour had tripped out. On getting it back and dismantling it both seals had been chipped and so let water into the oil chamber which thankfully activated the sensor probe and turned it off but water continued to pass the upper seal and contaminate the windings. As it had tripped out the windings just needed drying out rather than a rewind.
Second thing I would suspect but frist I would look for would be the cables for splits/chafes or 'bulging', especially where they enter the terminal head. If the cable looks ok drain the oil chamber and see if there is any water in the oil. Put some in a clear container, (plastic cup/glass jar), and let it settle. If there is water in it it will separate with the oil on top.
So long as the pump is at least half submerged I would discount the running dry aspect.
Sorry to be posting so late but had a busy day and only just come on line.
Hope this helps, let me know how you get on.

 

[matt1e]

cheers for that conny,have not been back up there yet as I've had other jobs on and we are still waiting for the new pump to be delivered but will let you know how it turns out, first thing I'm planning to do when I get back up there is measure the leakage to earth again, any significant change should point to water ingress due to damage to casing/seals/cable which I susspect is the problem, I will let you know how I get on
Matt

 

[conny]

Hi Matt, forgot to mention last night, pay attention to the rubber seals where the cable enters. If they weren't coated with a smear of silicone grease, (not ordinary albida type of grease), then they may well be perished, especially if the storage room is not heated and subject to wide temperature variations over the seasons. Flygt pumps usually have a set of thermistors installed in the windings to protect against overheating, though to be honest, I personally don't think they are totally reliable. If you have a multi-core lead rather than just a 3core +E measure the resistance across the other cores, (usually 5+6 on a 6 core), and see if you get a resistance reading of around 270Ω. this will be your thermistors. But you will also need the correct equipment in your panel to connect them to.
Let me know how you get on.

 

[matt1e]

Hi Matt, forgot to mention last night, pay attention to the rubber seals where the cable enters. If they weren't coated with a smear of silicone grease, (not ordinary albida type of grease), then they may well be perished, especially if the storage room is not heated and subject to wide temperature variations over the seasons.

Thanks for that conny,I think you could be onto something there. the storeroom certainly isn't heated and the building its in is pretty much exposed to the elements.

Matt

 

[conny]

How you getting on with this matt?
Any joy yet?