Safety relief valve Rated flow

[Omid1977]

I am designing PSV for a fuel gas scrubber,

Governing case is gas blow-by through two pressure regulators (PCV)upstream fuel gas scrubber. Flow through these two valves is two-phase and gas and liquid are separated in fuel gas scrubber therefore I only considered gas portion of flow through regulators as my design relief load of PSV(is it true?).

Upstream pressure of PCV's considered 68barg and down stream pressure considered 17barg(set pressure of PSV on fuel gas scrubber) + 10%

I know that if both regulators fail(my design case for considering gas blow-by) generated gas flow rate will be less than rated flow rate of fuel gas PSV. But do I need to size line upstream this PSV according to Rated flow of PSV or Relief load (max possible flow during gas blow by) is sufficient ?

 

[Chance17]

Are the regulators in series or in parallel ??

For the last question: The line upstream of the PSV should be sized for actual valve capacity (not scenario rate).

 

[Omid1977]

Chance17,

pressure regulators(Mechanical devices) are parallel and according to company design criteria, we must design this PSV for failing of 2 regulators.

But why we should design upstream valves for rated flow when there is not any possibility for providing this flow rate by any source?

 

[Chance17]

Safety Valves in gas service are designed with "pop" action.
They go to 100% capacity when activated.
It requires kinetic energy to maintain the valve opening.
If the inlet pipe has too great friction loss, the kinetic energy is lost and the valve will shut.
If the overpressure scenario is not corrected, the valve will re-open.
Successive open and reopen is called "chatter".
This will damage the safety valve.
The ASME code requires the safety valve be designed to not "chatter".
This typically means the inlet pipe have friction loss less than 3% of set pressure at actual valve capacity.
If the valve has a conventional spring, the outlet pipe must be designed to have less than 10% friction loss at actual valve capacity.