The level in a particular vessel is controlled by a dedicated level transmitter and LCV. In addition, a separate level transmitter gives a High High trip and the emergency shutdown system closes an isolation valve which is incidentally a fail closed valve. Can this be considered sufficient protection against gas blowby? If not, what further precautions should be taken?
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Gas Blowby Case 1
- Thread starter MrDaveL
- Start date
The level of protection needed against blow-by will depend on the service the vessel is in, as well as the downstream equipment.
I don't know much about the actual number crunching that goes on, but in general, this particular scenario will be given a severity depending on the consequenses of blow-by and downstream equipment overpressure. A LOPA (level of protection analysis) can then tell you whether a trip valve is required, as well as to what level of reliability this valve must be built to. The LOPA can usually be done in parallel with your HAZOP.
In high pressure blow-by cases, I am used to the downstream PSV being the main level of protection, with a trip valve added only if additional layer(s) of protection are needed.
I don't know much about the actual number crunching that goes on, but in general, this particular scenario will be given a severity depending on the consequenses of blow-by and downstream equipment overpressure. A LOPA (level of protection analysis) can then tell you whether a trip valve is required, as well as to what level of reliability this valve must be built to. The LOPA can usually be done in parallel with your HAZOP.
In high pressure blow-by cases, I am used to the downstream PSV being the main level of protection, with a trip valve added only if additional layer(s) of protection are needed.
rzrbk is on track. Some data to consider on your LOPA is things like time from normal level to full. If that time is say 10 minutes, you could monitor the level transmitter an give the operator a warning. You could monitor the output signal to the LCV andif the output is near maximum, give an operator warning. If the time to fill is 1 minute, you may consider adding a second level switch. If it is very criticle that any liquid in the vapor line would have a disastorous result, you may want a 2 out of three voting SD.
I remember on plant even had a dedicated video camera in the control room looking at the sight glass on a vessel.
I remember on plant even had a dedicated video camera in the control room looking at the sight glass on a vessel.
Many facilities no longer permit gauge glasses.
The failure of discrete devices such as level switches can remain undetected for long periods. Two transmitters using different measurement technologies are better to avoid common mode failure.
Also consider the restriction orifice.
The failure of discrete devices such as level switches can remain undetected for long periods. Two transmitters using different measurement technologies are better to avoid common mode failure.
Also consider the restriction orifice.
- Thread starter
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MrDaveL,
I assume by blowby you refer to a systems as follows:
HP Vessel --> Valve --> LP Vessel
If you lose level in the HP vessel, gas will blow into the LP vessel and overpressure it.
In this case:
- Ensure there is sufficient volume in the LP vessel to accommodate the liquid in the HP vessel without overfilling.
- Your PSV on the LP vessel must be sized for the vapor that can flow through a full open control valve. Instrumentation does not remove this requirement.
- The real difficulty is deciding what to do with the control valve bypass. Different companies handle it differently.
I assume by blowby you refer to a systems as follows:
HP Vessel --> Valve --> LP Vessel
If you lose level in the HP vessel, gas will blow into the LP vessel and overpressure it.
In this case:
- Ensure there is sufficient volume in the LP vessel to accommodate the liquid in the HP vessel without overfilling.
- Your PSV on the LP vessel must be sized for the vapor that can flow through a full open control valve. Instrumentation does not remove this requirement.
- The real difficulty is deciding what to do with the control valve bypass. Different companies handle it differently.
CJKruger is correct, the downstream equipment will be protected from over pressure by the downstream vessel PSV.
When calculating the size of the PSV on the downstream vessel the amount of gas that can pass through the failed open valve must first be calculated (from the valve Cv). Then this flowrate will set the PSV orifice area.
As far as I can see there will be no need for an orifce plate.
When calculating the size of the PSV on the downstream vessel the amount of gas that can pass through the failed open valve must first be calculated (from the valve Cv). Then this flowrate will set the PSV orifice area.
As far as I can see there will be no need for an orifce plate.
Refer to API RP 14C including sections 4.2.1.4, A.4.2.2.2, A.4.3.2 and the associated tables listed.
A low level signal can interlock a shutdown valve on the liquid outlet (or inlet) as the primary protection from gas blowby. Safety devices downstream provide secondary protection for the downstream equipment.
Table A-4.2-Safety Analysis Checklist (SAC)-Pressure Vessels
Table B-1-Composite Safety Analysis Checklist (SAC) (Continued)
A.4 Pressure Vessels
e. Low Level Sensor (LSL)
1. LSL installed to protect each liquid outlet.
2. Liquid level is not automatically maintained in the vessel, and the vessel does not have an immersed heating element subject to excess temperature.
3. Equipment downstream of liquid outlet(s) can safely handle maximum gas rates that can be discharged through the liquid outlet(s), and vessel does not have an immersed heating element subject to excess temperature. Restrictions in the discharge line(s) may be used to limit the gas flow rate.
A low level signal can interlock a shutdown valve on the liquid outlet (or inlet) as the primary protection from gas blowby. Safety devices downstream provide secondary protection for the downstream equipment.
Table A-4.2-Safety Analysis Checklist (SAC)-Pressure Vessels
Table B-1-Composite Safety Analysis Checklist (SAC) (Continued)
A.4 Pressure Vessels
e. Low Level Sensor (LSL)
1. LSL installed to protect each liquid outlet.
2. Liquid level is not automatically maintained in the vessel, and the vessel does not have an immersed heating element subject to excess temperature.
3. Equipment downstream of liquid outlet(s) can safely handle maximum gas rates that can be discharged through the liquid outlet(s), and vessel does not have an immersed heating element subject to excess temperature. Restrictions in the discharge line(s) may be used to limit the gas flow rate.
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