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Bart Nabbe
26-06-2008, 10:23 AM
Hello everybody,

I have looked in this forum a lot of times and gave my help in some topics.

This is the first time that i need help so here goes.

We do maintenance on an NH3 system that has Danfoss safetyvalves of the type SFA 15.
We have them in more systems and never had any problems but on this system the problem is as follows.

The safety's are leaking after 4 years (even when the unit is standing still)
We dismounted and checked the safety's at a testcentre. They opened to soon or some didn't even open at all.
We have sent them to Danfoss to inspect them and they couldn't find anything.
The only thing was yellow powder in the safety's.
I have put 2 photo's with this thread.
After examination the main substance of the powder is Zinc oxide.

We have taken an oil sample and after analysing it it showed no Zinc.
We have taken an NH3 sample en after analysing it it showed no Zinc.
In the Danfoss valves there is also no Zinc used with manufacturing.

So i'm out of ideas I think that it's not from within the system but maybe from outside.
Allthough that is also strange because there are 10 safety's on 1 central exaust pipe that goes to the NH3 detection.

Can anyone give me a next clue to look for.

Thank you

PaulZ
26-06-2008, 11:58 AM
Hi Bart
What are the vent lines made of. If they are galvanized pipe the zinc oxide could be coming from the galvanizing breaking down on the inside of the vent lines.
Paul

US Iceman
26-06-2008, 01:53 PM
One of the pictures shows evidence of extreme pitting. The other shows corrosion and something that looks like a broken ring sealing two parts.

As you say, the valves all connect to a common discharge vent line; where is this valve is relation to the others (in the main vent line)? If you could provide an isometric drawing of the relief valve piping it would be very helpful.

My initial guess is that water has accumulated in the vent line and settled on top of the relief valve outlets (perhaps on a regular basis). This might be due to condensation accumulation or simply water getting into the vent piping when it rains.

At some time the water would evaporate, but any solids would remain behind in the pipe since the water would not be agitated (solids sink).

The pitting corrosion appears to be severe. However, I don't know if that is due to localized conditions or perhaps something in the material of that valve that just happened to react at that location.

Bart Nabbe
26-06-2008, 04:21 PM
Hello Paul and Iceman,

Paul, you are correct I was also looking up the specifications of the pipes that where used but i'm looking for a purchaseorder in our archive and you know how archives are.
Normally we don't use galvanised pipes.
I was even thinking of primer that maybe was sprayed in the pipes by accident.

Iceman,
Al the safety's look like the one in the picture (total of 20, 10 x 2 on an exchange valve)
And they are spreaded from very close to the end of the pipe and some in the beginning of the pipe.
Rain is not realy an option because the end of the pipe is put downwards.

So you can see that i'm realy confuesed ??? with this matter.

The next step we will do is investigate the oilfilters in the system and make an analyses of that. maybe there are Zinc parts inthere that you cannot see in the oilanalyses.

And we go and make a dend in the central pipe with an oilfilling in that bend to avoid contact with the outside air.

More suggestions anyone?

US Iceman
26-06-2008, 04:32 PM
Bart,

Even if the vent line cannot allow rain water to enter there is nothing to prevent water from condensing in the vent lines or relief valves.

If the valves are connected to low temperature vessels, the valves can be cold by conduction. If the surface temperature of the valve is below the dew point temperature of air, water will condense.

And water does funny things to metal and materials depending on temperature or the availability of air borne chemicals.

If you put an oil seal on the vent line, where is the discharge of the vent line? Hopefully not some place were a person can be standing... See the problem?

josef
26-06-2008, 07:55 PM
And we go and make a dend in the central pipe with an oilfilling in that bend to avoid contact with the outside air.

Grizzly
26-06-2008, 08:09 PM
Bart.
Just a thought, would it be possible that the fitters where not to careful when they threaded the pipes.
And as a result some galvanised shavings/ contamination has entered the pipework?

I assume the pipes are not welded as on Galvanised Steel.
This produces very fine white Zinc powder inside and outside the welded joint.
So, if I remember correctly Danfoss / Sabroe PRV'S are Welded in situ?
Therefore this would be where the zinc is coming from.

That's only correct if my assumption is right?

Tycho
26-06-2008, 09:05 PM
is it the same locations that fail every time, or is it totally random?

is there tension in the pipes?

The SFV15's I've had fail always had tension in the outlet pipes, so I cut the pipe and put on a straubgrip and after that it has worked.


Either that or they have been full of seawater.