Tube rupture overpressure of heat exchangers

[aikmeam]

Dear all,

Much has been said about the API's 2/3 rule (or the new 10/13 rule) for heat exchangers. And the discussion always resolves around high pressure and low pressure side.

But which sides are we talking about?

What's the likelihood, or credibility to consider tube rupture relief, when the low pressure is in the tube side?

What's the standard practice in determing the "broken" orifice area then? I've seen some company using the 5mm pinhole basis but where does that come from?

 

[EGT01]

Aikmeam,

I think it matters not which side is low side/high side versus shell side/tube side, the likelihood should still be the same. This is a remote but possible failure that API allows you to accept up to the test pressure of the low side for a limited overpressure event.

Regardless of which side is the low side, the flow path is basically the same, through the point of tube failure. I've seen a lot of different equations used to calculate the required relief rate, but generally they follow the two basic methods that API RP-521 suggests. One is a short tube - long tube approach and the second is a single short tube calculation multiplied x2. For a new installation, I tend to use the single short tube calculation x2 (since it is quick and easy) as if the tube broke cleanly at the tube sheet. Flow through the tubesheet hole can estimated as flow through a nozzle.

When the tube side is the low side, flow would pass into different sections of the channel head but keep in mind that there are other tubes remaining in place that can carry the required relief flow from one side to the other (for example inlet channel to outlet outlet).

As for the pinhole basis, I've seen this used only when the tube dimensions were considered equivalent to standard pipe dimensions (for example sch 10s or sch 40). See also API RP-521's discussion regarding double pipe exchangers. I'm not sure where the 5 mm basis came from but this seems to be a common value. I've also seen 1/4 inch and even 1/2 inch. It will be interesting to see if anyone has a reference to the basis for selecting 5 mm versus some other number.