Governing scenario for PSV?

[Orsiz]

Dear all,

I need your help in determining the reasoning behind a PSV in on of our water heating systems. The system is as follows: a shell-and-tube heat exchanger in which water (tube side, 6 barg, 20°C) is heated by low pressure saturated steam (shell side, 1,7 barg; 131°C) to approximately 65°C. The water side is protected by a thermal relief valve located between two manual valves. The shell side is also protected by a 3/4"x1" relief valve. I found an old datasheet of the PSV and it said 'condensate' as relieving medium. Does anyone know why there is a PSV on this side?

I did a P&ID check, a field check and looked into the ODDR but nothing indicated the need for this PSV?

Orsiz

 

[georgeverghese]

Presume you are asking about the need for the PSV on the shellside and not for the tubeside? A shellside PSV is required for all fixed tubesheet HXs' with ASME certification to be operated as as pressure vessels.

 

[LittleInch]

Also you need to consider a leak / rupture of the tube when the system is in a shut in condition.

Your shell will be designed for min 1.7 bar, but you could end up with 6 bar inside if it fills with water in a shut in condition, which is possibly why it says condensate ?( unless it is mounted on the bottom of the vessel?)

Is there any other form of relief such as a bursting disc?

BTW, what's an ODDR? Always better to spell out abbreviations at least the first time you see it in a post (and report for that matter).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mk3223]

IMO, the 3/4x1 PSV of the shell side sounds like a "condensate" thermal relief to me. Does the data sheet specify the PSV process conditions, such as set pressure, capacity, etc., to give you some clues for the valve design? So, it may be compared with the process of the water heating system.

Or, you may see if any HAZPO or PHA doc available to confirm the design purpose for the PSV.

 

[georgeverghese]

The shellside PSV governing relief scenario may most likely be tube rupture for the case when the shellside design pressure is less than that for the tubeside, while firecase would be the only valid relief case for the shellside if both shellside and tubeside design pressures are the same.

 

[Orsiz]

@LittleInch: The shell is designed for 6 barg, which is the set pressure of the mentioned PSV (that's basically the only process related thing I know about this relief valve). The PSV is the only form of relief, no other form present. ODDR is the orignal design data report, the relief valve was installed when our plant was build.

@mk3223: I don't have any information available except for the relief medium (condensate) en the set pressure (6 barg). No documentation regarding safety reviews of the original design are available.

@georgeverghese: the shell side design pressure (6 barg) is indeed < tube side design pressure (10,6 barg). Based on this, I'll go for the tube rupture case but I have to check if a D orifice is sufficient for this scenario.

Thank you for your replies

 

[vladrath]

The PSV could be sized for the relief of the tube rupture scenario or a fire case scenario.

It is really difficult to make these types of decisions on a forum. You should consider having this system evaluated by a experienced relief design person at your plant, if there is real cause for concern.