Hi David!
We have one of the first MultiPreps constructed here in Albany, but
the valve arrangement in our unit is slightly different from that in
yours. However, we have had infrequent problems with the teflon at
the tip of the valve stems not sealing, usually due to some defect or
scratch. Once a problem with sealing occurs, I've never had much
luck other than to replace the valve, which can get expensive if you
have to do it frequently. The only other thing I do is wrap the
metal threads of the valves with teflon tape. I would be very
surprised if the teflon is being affected by cold temperatures. As
you say, they are not close enough to the chiller's probe to cool
down significantly.
Since you just serviced your chiller, are you sure that the leaks are
around your valves and not at the o-ring that separates the water
trap body from the chiller's probe? With our MultiPrep arrangement,
a thick viton o-ring separates a stainless steel flange on the body
of the water trap from a stainless steel flange, or collar, that is
welded to the end of the chiller's probe that is closest to the
flexible pipe coming from the chiller itself. The o-ring seals by
being squeezed between these two flanges by 8 screws and nuts. If
your system is like ours, not only does the o-ring need to be
perfectly centered on the flanges, but it must also be squeezed
uniformly (by tightening opposing screws carefully) but not overly
tight. Sometimes the screws loosen initially as the trap is brought
down to -100蚓 and then warmed up over several cycles and must be
tightened again. If an o-ring has been in the trap for awhile, I
usually replace it with a new one rather than reuse it as it either
gets scratched or its pliability is no longer good enough to seal
well. I'd check to make sure your leak isn't at this o-ring.
Hope this helps! Please let me know the ultimate outcome of your repairs.
Steve
>Hello,
>
>I have a problem with leaking valves on my Micromass multiprep (old
>style valve arrangement). A couple of months ago I replaced all the
>valve stems and cleaned the inside thoroughly. All valves were leak
>tested and were fine. After that the chiller for the cold finger
>failed and it has been serviced. The refrigeration tech measured
>the probes temperature as -88, which is OK. Judging by the amount
>of ice buildup it is working much better than it has for a long
>time. I also replaced the insulation around the trap.
>
>On Tuesday I realised that valve AD had a smal leak - enough to ruin
>the results (AD is the inlet valve). I cleaned the teflon plug and
>the metal seat but that didn't help. So I reseated the teflon into
>the seat by gently heating the valve block with the valve closed.
>After numerous iterations I got the leak to be very small, so I put
>the chiller on. As soon as it got down to temperature the leak
>returned. I also found that AB (downstream from the cold trap) has
>started leaking. Returning the unit to room temperature has not
>solved the problem.
>
>It is starting to look as though the low temperatures are affecting
>the teflon but I find that surprising. Firstly, the blocks did not
>feel very cold and no frost forms on them. Second, I haven't seen
>this behaviour in the past 10 years. Does anyone know the thermal
>properties of teflon? Have I missed something obvious?
>
>thank you,
>David
>
>內躬偕爻內躬內躬偕爻內躬內躬偕爻內躬內躬偕爻內躬內躬偕爻內躬內躬偕爻
>
>David Wheeler
>
>Professional Officer
>Geochemistry Laboratory
>School of Earth and Environmental Sciences
>University of Wollongong,
>Wollongong, 2522, NSW
>Australia
>
>Phone 02 4221 4586 (office)
> 02 4226 5903 (lab)
>Fax 02 4221 4250
>E-mail [log in to unmask]
>
>
>'Oh no, we won't give in,
>we'll go living in the past'
>
>內躬偕爻內躬內躬偕爻內躬內躬偕爻內躬內躬偕爻內躬內躬偕爻內躬內躬偕爻
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