Process mode for the U7 example simulation
- Last UpdatedOct 04, 2024
- 3 minute read
We use only Process mode, because this model exclusively performs steam balances. We make the following assumptions and design requirements for process simulation:
-
Set all headers to use balanced inlet pressures. Balancing the inlet pressures works much better than setting the minimum inlet pressure because there are many flows into and out of each header.
-
Model the condensers as heat exchangers and set the vapor fraction of the shell side (VFso) to zero.
-
Specify at least one OutRatio element for each Header. This must be a flow that will never be zero. If you specify the OutRatio corresponding to the letdown valve at any positive values, then when the flowdown valve is closed, all outer OutRatio values will be infinite and the system will not solve.
Sequence logic approach
To accommodate alternative refinery operating conditions, we use sequence logic for the material balance to control the letdown valves, turbine extractions, and some of the steam turbines. For example, if the LP steam demand decreased by more than the excess MP steam in the MP-to-LP letdown, the model would not solve. However, in this situation, you could reduce the turbine extraction to the LP header when the letdown valve is fully closed. The sequence logic allows you to define these relationships.
The HP header does not need sequence logic, since one of the boilers can have a calculated (floating) flow rate and can move up and down accordingly.
The sequence logic that controls the material balance for the MP header (MPSQ) has the following priority sequence as you increase MP steam demand.
|
Priority |
Device |
Description |
Min Flow (lb/hr) |
Max Flow (lb/hr) |
|---|---|---|---|---|
|
1 |
PV7 |
Open the T3 governor valve (HP-to-MP) |
0 |
60,000 |
|
2 |
PV4 |
Open the valve for T1 MP Extraction |
0 |
94,000 |
|
3 |
PV1 |
Open the HP-to-MP letdown valve |
0 |
400,000 |
For example, if PV1 is open, PV7 and PV4 are completely open, as they are in the base case operation. However, if the LP demand falls and HP-to-MP steam is not required, then the MP extraction decreases to balance the MP header.
The sequence logic that controls the material balance for the LP header (LPSQ) has the following priority sequence as you move from high LP import to high LP demand.
|
Priority |
Device |
Description |
Min Flow (lb/hr) |
Max Flow (lb/hr) |
|---|---|---|---|---|
|
1 |
PV3 |
Close LP atmospheric vent to LP letdown valve |
0 |
100,000 |
|
2 |
PV8 |
Close the valve for the LP bypass condenser |
0 |
47,619 |
|
3 |
PV5 |
Open the valve for T2 LP Extraction |
0 |
39,000 |
|
4 |
PV2 |
Open the MP-to-LP letdown valve |
0 |
400,000 |
Optimization approach
Alternatively, you can use optimization instead of sequence logic to find the appropriate steam balance conditions. We use the Optimization Set in the following figure. The objective function minimizes the boiler feed water in stream S60 by adjusting the letdown valves, PV1 and PV2, and the steam turbine extraction valves, PV4 and PV5. The optimizer opens the extraction valves and closes the letdown valves as much as possible to prevent steam from venting through valve PV3 while still meeting the live steam and heat steam demands from the HP, MP, and LP headers. You can open the U7 - Refinery Steam Balance with Optimization.simx file in the Examples folder to explore this optimization approach.
