Need your help in selection of a Steam Trap..
I got a exchanger which is operated on a temperature control, the condendate load is about 2600 lb/hr The operating pressure is 60 psig and the condensate is drained to atmosphere.

The vendor quoted me a 1.5" flat type trap with a capacity of 4500 lb/hr. He insists that is the size I should use becaue the manufacturer recommends 1.5 excess capacity over the normal capacity.

There is a 1" trap available which has a capacity of 3200 lb/hr. I think, 1" is more suitable.

Think about what your trap has to do.  First it has to "trap" the condensate (and in the case of some types, fill), then vent it.  Most types of traps can't do both at the same time.  Hence the need to size your trap so that when it is venting it can vent at a rate sufficient to allow it time to fill (or let condensate collect.

If your trap required 50% of the time to fill, then it would need 2X the flow rate to be able to vent what it had collected to keep up with your Hx.

A trap is not a level control valve, it is a trap.

And another tip; select a trap that has the capability to pass air (non-condensables.)  The steam that will enter your Hx will have some air in it, typically.  If the trap can't release it, it will build up in the Hx and eventually air bind it.

It is a common practice to apply safety factors to the max condensate load when sizing steam traps. These safety factors account for the type of traps, type of service ( intermittent or continuous), presence of non-condensable gases (as pointed out above by rmw), peak loads at start-up, changes (drops) in pressure. It is also very important to take particular care to the design of the unit to be drained. I wouldn't trust a traps' manufacturer who makes statement on safety factors just knowing the condensate rates and pressures, but without any information on the configuration of the heated unit. Please consider that oversizing steam traps results not only in higher initial investment, but also in a waste of energy through the traps (operational cost).

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