Hi all,
We have an existing Methanol Tank on a FPSO. Details are as follows:
(The design is kind of dubious)
Code: ASME Section VIII
Code Stamp: No
Design Pressure: ATM
Design Temp: 65C
MAWP - -
Type: Cylindrical tank
Capacity: 5 m3
Material: SS316
Attached is a photo of the tank.
Methanol will be used only during startup for hydrate inhibition.
Currently there is a nitrogen line connected to the tank. The supply comes from N2 cylinder.
There is no PCV on the nitrogen supply line. The vent connection is blinded. No PVSV installed on the tank.
Clearly the design is not acceptable. I have the following questions, hope the professional can share their experience:
1. Since the tank is only designed for atm with no MAWP details, is it allowed to have nitrogen blanketing system? A checked with ASME VIII calculation (even though this is not a pressure vessel), the SS316 shell with 9 mm thk is good to withstand 0.9 MPa. Presumably in this case, the tank can withstand the nitrogen pressure.
2. It is proposed to add a pressure regulator on both the nitrogen supply line and also vent line and connect the vent to LP flare header. Proposed pressure to maintain in the tank 0.01 barg and outgoing pressure at 0.04 barg.
3. The N2 inbreathing rate is calculated to be 11.3 m3/hr based on the latest API 2000 equation. (Methanol pump out rate is 1.25 m3/hr). Given that the N2 is supplied from N2 cylinders, can anyone advise the qty of N2 cylinders required for 2 hours injection? I am not too familiar with N2 cylinders, is it liquified N2 that is stored in the cylinder? THe nitrogen manifold will be maintained at 500kPAg with a PRV.
4. There is no PVSV on the tank. Is it mandatory to add one ? Presumably the PVSV has to be good for emergency relief. How to work out the size? There is only one spare 1" nozzle that can be used in the tank and the existing 24" manway is not a hatch. It is not known if hte tank can handle vacuum condition, will have to work out the external pressure based on ASME VIII.
Instead of nitrogen blanketing system , is it allowed to have an open vent with flame arrestor installed on the methanol tank without any inert padding?