imatasb
Chemical
- Jul 17, 2003
- 32
Pressure relief configuration
Dear colleagues,
I am designing a pressure relief configuration for a vessel. The relief system consists of a bursting disk followed by a relief valve and it is designed basically to prevent the overpressure in the vessel in case of an external fire.
After doing the calculations I obtain the following results:
The vessel operates normally at 8.7 psia (0.6 bara).
The set pressure is 15.5 psig (1.07 barg) so for the calculations I use 15.5 *1.1 +14.7 =31.8 psia.
Reading from API 520 3.5.3 Fire Contingencies I understand that I could use a value of 121% of the MAWP instead of the 10%. Is this correct? The design pressure of the vessel is 17.4 psig (1.2 barg)
The pipe that discharges the vapours is a 2” pipe that vents into an atmospheric dump tank. The pressure drop in this line is 4.6 psig (0.32 barg)
The Critical flow pressure is
PCF=0.6 * (15.5+14.7) = 18.12 psig (1.2 bara)
As the back pressure (4.6+14.7= 19.3 psia) is bigger than PCF (=>critical flow) I use the equations of sizing for subcritical flow.
For the calculations I use a Kd=0.62 for the bursting disk and 0.975 for the relief valve (plus a 0.9 of combination factor) and the results are a diameter of 1.37” and 1.15 “ respectively.
So I take the next size which would be 1.5 in for both. However the pipe is 2 in.
Would the normal configuration be a reduction before the bursting disk and a expansion after the valve or are there any other possibilities (valve of 2” body but with an 1.5” orifice for example).
What about the kind of relief valve?
According to API 520, “in a conventional pressure relief valve application, built-up back pressure should not exceed 10% of the S.Pressure at 10% allowable pressure”, in my case 0.1 * (15.5*1.1) = 1.7 psig.
My pressure drop is 4.6 psig so I should use a balanced one.
A balanced valve can be used when
(Built-up pres. + superimposed) < 50 % Set pressure.
This number can be seen in two ways: gauge or absolute.
1) 4.6 psig < 0.5 *15.5 psig =>TRUE or
2) 4.6 + 14.7(superimposed) psia < 0.5*(15.5+14.7) psia => FALSE
19.3 < 15.1
In figure 30 from API 520, the backpressure correction factor only arrives until values of 50 (percent of gauge pressure), so I understand that the superimposed pressure refers to a pressure different to the atmospheric (say that that my dump tank was at 0.5 barg).
So I should use a balanced valve, but what would happen if my pressure drop was of 8 psig (>50%) what kind of relief valve should I use then, would it be a pilot operated?
It is one of my first calculations and I would appreciate your comments to have a correct vision of how to design relief systems.
Cheers.
Dear colleagues,
I am designing a pressure relief configuration for a vessel. The relief system consists of a bursting disk followed by a relief valve and it is designed basically to prevent the overpressure in the vessel in case of an external fire.
After doing the calculations I obtain the following results:
The vessel operates normally at 8.7 psia (0.6 bara).
The set pressure is 15.5 psig (1.07 barg) so for the calculations I use 15.5 *1.1 +14.7 =31.8 psia.
Reading from API 520 3.5.3 Fire Contingencies I understand that I could use a value of 121% of the MAWP instead of the 10%. Is this correct? The design pressure of the vessel is 17.4 psig (1.2 barg)
The pipe that discharges the vapours is a 2” pipe that vents into an atmospheric dump tank. The pressure drop in this line is 4.6 psig (0.32 barg)
The Critical flow pressure is
PCF=0.6 * (15.5+14.7) = 18.12 psig (1.2 bara)
As the back pressure (4.6+14.7= 19.3 psia) is bigger than PCF (=>critical flow) I use the equations of sizing for subcritical flow.
For the calculations I use a Kd=0.62 for the bursting disk and 0.975 for the relief valve (plus a 0.9 of combination factor) and the results are a diameter of 1.37” and 1.15 “ respectively.
So I take the next size which would be 1.5 in for both. However the pipe is 2 in.
Would the normal configuration be a reduction before the bursting disk and a expansion after the valve or are there any other possibilities (valve of 2” body but with an 1.5” orifice for example).
What about the kind of relief valve?
According to API 520, “in a conventional pressure relief valve application, built-up back pressure should not exceed 10% of the S.Pressure at 10% allowable pressure”, in my case 0.1 * (15.5*1.1) = 1.7 psig.
My pressure drop is 4.6 psig so I should use a balanced one.
A balanced valve can be used when
(Built-up pres. + superimposed) < 50 % Set pressure.
This number can be seen in two ways: gauge or absolute.
1) 4.6 psig < 0.5 *15.5 psig =>TRUE or
2) 4.6 + 14.7(superimposed) psia < 0.5*(15.5+14.7) psia => FALSE
19.3 < 15.1
In figure 30 from API 520, the backpressure correction factor only arrives until values of 50 (percent of gauge pressure), so I understand that the superimposed pressure refers to a pressure different to the atmospheric (say that that my dump tank was at 0.5 barg).
So I should use a balanced valve, but what would happen if my pressure drop was of 8 psig (>50%) what kind of relief valve should I use then, would it be a pilot operated?
It is one of my first calculations and I would appreciate your comments to have a correct vision of how to design relief systems.
Cheers.
