In the High Integrity Pressure Protection System (HIPPS), usually it is required to have a HIPPS valve that can close as fast as possible (in general it is 2 second closing time). Where this requirement comes from? Why 2 second, 3 second, 4 second…etc?
First of all, the HIPPS it self is selected to protect the plant from excessive pressure due to blockage or any other possible cause. This excessive pressure increasing is very fast thus the pressure source shall be cut in a fast way also by a HIPPS valve. How fast it is? It depends on the mass flow rate from pressure source and the delta pressure of the operating and design pressure of the plant downstream of the pressure source. If the mass flow rate is high and the delta pressure between operating and design pressure of the plant is quite small, then the required time to cut the pressure source is faster than if the mass flow rate is slow and the delta pressure is large. These rapid changes of pressure need to be calculated so that we get the required closing time of HIPPS valve to cut the pressure source.
In order to calculate the pressure increasing per time, we can use an ideal gas equation as follow:
Let’s take a case study regarding this so that we get a clear understanding. The following data was given:
Separator design pressure: 800 psig
Separator Volume: 60 m3 (with HHLL liquid filled as worst case)
Choke valve downstream pressure: 1900 psig (valve full open as worst case)
Choke valve flow at full open: 200 MMSCFD = 239056.4 kg/h = 239056404.5 g/h
Wet gas operating temperature: 360 K
Wet gas molecular weight: 24 g/mol
Gas constant: 8.31441 J/(mol.K)
Let’s say we neglect the volume of the pipeline from choke valve up to production manifold and up to production separator. So the volume that we consider is only the separator volume.
From above data, the pressure increasing can be calculated as follow:
Pressure increasing rate = (8.31441 * 360 * 239056404.5) /( 24*60) = 496903240 Pa/h
Pressure increasing rate = 72069.7 psi/h
Pressure increasing rate = 20.019 psi/second
Available time delay calculation before the separator reaches its design pressure when the outlet is blockage and the choke valve is fully open.
Separator Operating Pressure = 600 psig
PAHH = 750 psig (lets take this pressure as initial pressure for our delay time calculation)
Design pressure = 800 psig
Delta pressure = 50 psig
Required time to charge the separator = (50 psig) / (20.019 psi/second) = 2.49 second
This is the available time delay that we have for the HIPPS valve closing time. The HIPPS valve closing time must be equal or faster than 2.49 second so that sudden overpressure in the separator will not happen.
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