Situation:

We have a remote steam line 300# and the only place remove the condensate from the live line is into a nearby oil line which has a lower rating of 150#.

The oil system has other ASME vessels in it that need to be safeguarded from the higher pressure steam line in the event that:
1. 1-inch stream traps breaks
2. accidental opening of 1-inch by-pass valves

Other issues:
1. Simulation results show that when steam flows into the oil pipeline, it would flash some of the lighter ends in the oil pipeline and result in an over all vapour fraction of 20%

2. This flashed fluid has a very high 2-phase volumetric flow, which would push the oil out the other end, but because the other end of the line has not heated up in the instant of the upset event, this volumetric 2-phase flow will push an equivalent flow of single phase liquid oil.

3. When we size the PSV for the single phase mass flow rate of oil to be relieved using a PSV, the PSV orifice size is very large than one would expect from failure of 1-inch trap or valve(Check Valves).

Concern:
1. Is there a normal set of assumptions used when sizing PSV for upstream steam trap failures?
2. How does one calculate the flow of steam though a steam trap whose vendor data is only for flow of liquid condensate?

The steam trap has an orifice size.  Calculate the steam flow using an orifice equation from Crane.  Is the steam trap failure coincident with the hot oil system being blocked in?  Can the expansion tank absorb any of the vapor?
There are no normal assumptions for this type of failure other than if a small steam flow goes into a very large open hot oil system then you might convince yourself that overpressure might not occur.

I do not understand why you are bound to have the release into an oil-pipe.

To arrange the release and/or possible return of condensate separately seems thechnically better and perhaps both safer and cheaper.

As you point out, in addition to the amount directly from steam, you will get a heating of oil fractions, and expanding volume/gases needed to be depressurized (released).

a) Sizing is one thing, of course you would need a fairly large orifice. You are after all overpressurizing and overheating a 150# line (of some size and normal operating pressure/temperatur) from a fully operating 300# line (of some size, operating pressure and temperature), even if it is through a limitid steamtrap orifice.

b) If you do it your way: How fast and how would you be able to stop the release from the safety device? How much and what do you put into the surrounding atmosphere by this release? Is this within rules, safety standards and concessions? Harmful? Toxic? Explosive? Backlash into 300# line?

If this is a new construction I would have put my foot down and said 'no' until someone really could have convinced me this was sensible.

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