2 replies to this topic

[shaun4d]

Hi All,

 

I am currently carrying out a relief assessment for a number of low pressure storage tanks for a client.

 

I am aware that when determining the pressure relief valve inlet and outlet pressure drops, the worst case flow (relief valves actual relieving capacity) should be used and the pressure drops limited to 3% (inlet) and 10% (outlet) of the relief valves set pressure.

 

However, for a storage tank breather valve (Pressure Vacuum Valve) is this still applicable? Should the breather valves actual relieving capacity be used to determine the pressure drop or should the calculated relief rate be used?

 

When using the actual breather valve capacity, a 6" flame arrestor is required whereas if the calculated relief rate is used a 3" flame arrestor is sufficient.

 

Thanks in advance

[latexman]

It depends.  Do the breather valves "pop" wide open or do they modulate with flow?  There are low pressure breather valves on the market with "full lift disc" (i.e. huddling chamber).  I know of one.  Breather valves that modulate are much more common.  So, you have to be careful.

 

With a modulating breather valve, use calculated relief rate.  In fact, for these you don't need to do the 3% and 10% checks at the end like a PSV, that I know of.  You do have to determine the pressure at a point at the breather valve inlet and outlet to establish it has the capacity, so it's similar, but different.

 

With a breather valve that "pops" wide open, use the valve capacity.  Ditto, as above.

Edited by latexman, 21 November 2017 - 09:22 AM.

[shaun4d]

Thank you for your response

 

It depends.  Do the breather valves "pop" wide open or do they modulate with flow?  There are low pressure breather valves on the market with "full lift disc" (i.e. huddling chamber.  I know of one.  So, you must be careful.

 

With a modulating breather valve, use calculated relief rate.  In fact, for these you don't need to do the 3% and 10% checks at the end like a PSV, that I know of.  You do have to determine the pressure at a point at the breather valve inlet and outlet to establish it has the capacity, so it's similar, but different.

 

With a breather valve that "pops" wide open, use the valve capacity.  Ditto, as above.

 

Thank you for your response.