Permissible Accumulation under EN 13445

[rjh63]

Hi,
I've been reviewing a design for a deaerator vessel and associated relief stream to be installed at my company. The vessel is designed to EN 13445 which is pretty new to me - our existing vessel population is mostly PD5500 or ASME VIII. I do have a recently purchased copy of EN13445 but I have been looking in vain for any indication within the code of allowable accumulation during a relief event. By contrast, PD 5500 clearly indicates 10% (3.13.2 Capacity of Relief Devices) and ASME VIII div 1 also generally allows (for non-fire cases) 10%, or 16% with multiple valves (UG 125). All I can find in EN 13445 is a reference (in part 3, 5.3.4) that maximum allowable pressure PS shall not be less than the pressure where the relief valve starts to open. It does not reference the pressure when the relief is fully open. Seems an oversight if this is not included somewhere??
Any thought gratefully received - I am for the time being working on the basis that sticking with 10% should be sensible.

Thanks,
Richard

 

[micalbrch]

Are proportional relief valves even allowed under PED? Does EN 13445 really say "starts to open"? I thought that only full stroke pressure relief valves are allowed.

 

[rjh63]

I'll quote EN 13445 directly:

"...the maximum allowable presssure shall not be less than:
a) the differential pressure which will exist at the same specified location in the vessel (or chamber) when the pressure relieving device starts to relieve"

I confess I haven't looked into whether PED permits proportional valves - I don't have experience of them - but that is not what I am talking about here. Even full stroke sprung relief valves have a quoted overpressure that will be seen above set pressure when the valve is at maximum discharge - though this can be small. In any case, what I am considering is that the overall accumulation above set pressure in the vessel during relief (which is made up of upstream pressure drop in the stream, valve overpressure, and tailpipe backpressure) should have a code limit. It can never be zero surely?
Thanks,
Richard

 

[rjh63]

Ok, I've found the answer to my own question. It's actually stated quite clearly in PED itself, rather than being found in the vessel code. I quote here from PED ANNEX I:

2.11.2. Pressure limiting devices
These devices must be so designed that the pressure will not permanently exceed the maximum allowable pressure PS; however a short duration pressure surge in keeping with the specifications laid down in 7.3 is allowable, where appropriate.

7.3. Pressure limiting devices, particularly for pressure vessels
The momentary pressure surge referred to in 2.11.2 must be kept to 10 % of the maximum allowable pressure.

So there you go.
Richard