I've been asked by our process guys for a steam trap for which the line's design conditions are:
Press: 30 kgs/cm2
Temp: 330 ?C
But (always a but!) they recon the line won't see that much pressure constantly... on the contrary... it is going to be much lower.

Their cuestion was: How little the pressure should be to assure that the (inverted bucket) steam trap will work? (without having a long steam column on the line)

I've done my homework and checked the vendor (Armstrong) and they said that it will be 5 psi (0.35 kg/cm2-filter) but that is just what the datasheet says... so I'm wondering if you guys could bring a bit more light on this!

What do you mean by "to assure that the steam trap will work"?  

If you are talking about sealing when there is no flow then an inverted bucket will seal better at low pressures than at high pressures because the lower steam density makes for more bouyancy.

But if you are talking about getting sufficient condensate through the orifice then I would go with the Armstrong spec sheet.  They have been around a long time and have an excellent reputation.  I don't see any reason to doubt them.

Any steam trap must be capable of containing the steam at supply pressure, 30 kg/cm2 for your application.  The body rating might be PN40 (ANSI CL300).

Operating mechanisms for inverted bucket traps have a limit of the pressure that works against the valve seat area.  The weight of the bucket and mechanism needs to open valve against the force of steam pressure acting on the orifice  seat.  The orifice seat diameter could be larger to allow more flow for the lower pressure applications. For the higher pressure applications the orifice seat diameter will be smaller for the weight of mechanism to operate yet have adequate flow driven by the higher steam pressures.

The optimum sizing of the steam trap is a choice of orifice seat size that provides the capacity at expected pressures, but will operate at full steam supply pressure.  If you study the capacity rating charts for the inverted bucket traps, there are flow ratings for each orifice size that have a limiting peak. Each succesive smaller orifice has curve to show flow rating for next higher range steam pressure.

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