Introduction Case 1 : Benfield Stripper Control Valve. Case 3 : Hydrocracker 3-Way Control Valve Case 4 : Crude Heater Control Valves

Transcription

1 WISC 2008 Control Valves Problems ; Case Studies Fawaz A. Al-Sahan Instrument Engineer Saudi Aramco / Riyadh Refinery

2 OUTLINE Introduction Case 1 : Benfield Stripper Control Valve Case 2 : Boiler Feed Water Control Valves Case 3 : Hydrocracker 3-Way Control Valve Case 4 : Crude Heater Control Valves Case 5 : SRU High Pressure Steam Valves Conclusion

3 Case 1 : Benfield Stripper Control Valve

4 Case 1 : Benfield Stripper Control Valve BACKGROUND: Frequent internals and body wear out. Stem Vibration FINDINGS: Corrosion & cavitation of valve body and trim ANALYSIS: ROOT CAUSE: RECOMMENDATIONS Service becomes solid if it leaks or temp. drops Control valve sizing calculation is showing the existing valve is running too close to the severe conditions (flashing). Trim repair (machining, lapping) for double-acting valves is very critical (keeping the distance between the upper and the lower seats). Service + Valve design + aging Replace by a single port (ET model Fisher control valve) with no Cavitation trim

5 Case 1 : Benfield Stripper Control Valve ET design standard trim

6 Case 1 : Benfield Stripper Control Valve Single port design is better than double-ported valve design: Compact. Better leakage class. Trim repair (machining, lapping) for double-acting valves is very critical. Lower price

7 Case 2 : Boiler Feed Water Control Valves BACKGROUND: Repeated Failure of HCU BFW control and bypass valves. FINDINGS: Operating process data is different than design data. ANALYSIS: For the original design data : o P1= PSI o P2= PSI no cavitation exists. For one operation scenario: o P1= PSI o P2= PSI cavitation exists. ROOT CAUSE: RECOMMENDATIONS Deviation from the unit operation design basis and hence driving the control and bypass valves into cavitation condition. To replace the control/bypass valves by anti-cavitation valves design.

8 Case 2 : Boiler Feed Water Control Valves

9 Case 2 : Boiler Feed Water Control Valves ET design anti-cavitation trim

10 Case 3 : Hydrocracker 3-Way Control Valve Background : Minor Leak from packing gland. Leak Location Manufacturer recommendation: repack the valve (HCU shutdown ) 3-Way Valve Specifications: Masoneilan made, Diverting Type Model AB 10 Size ANSI Class 2500 RTJ Service Conditions: Temperature = Deg.C Inlet Pressure = Kg/Cm2

11 Case 3 : Hydrocracker 3-Way Control Valve 1. Greasing bars mixed with packing rings material 2 R i h i h ki b hi 2. Retightening the packing by pushing down the packing follower

12 Case 3 : Hydrocracker 3-Way Control Valve 3. Injectable Graphite Packing (Stuffit) (351 kg/cm2 pressure, 650degC temperature) INJECTION LOCATION Results: Hydrocarbon Leak Stopped No HCU shutdown

13 Case 4 : Crude Heater Control Valves Problem 1: Control valves 3 size (typical trim design). During the heaters commissioning, i i piping i vibration problem appeared. Root Cause: Incorrect process parameters used to select and size the control valves.

14 Case 4 : Crude Heater Control Valves Problem 2: New Control Valves: 6 size with anti-cavitation trim. Repeated valves plugage, Lab analysis : 19.05% coke & 26.5% iron, remaining crude Root Cause: Valve design (cage-guided, anti-cavitation trim)

15 Case 4 : Crude Heater Control Valves

16 Case 5 : SRU High Pressure Steam Valves BACKGROUND: Repeatedly stuck FINDINGS: The valves are OK out of service but stuck when in service. Reported twice to vendor ANALYSIS: ROOT CAUSE: RECOMMENDATIONS Galling Design Deficiency : thermal expansion which caused the plug to stick to the guide bushing. 1. Plug/seat hard facing 2. increase the plug/guide bushing clearness).

17 Galling Failure mechanism caused by two metal surfaces of similar chemical composition or surface hardness. As the surface undergoes load, galling results in a cold welding type of failure that causes the two surf aces to stick together.

18 Case 5 : SRU High Pressure Steam Valves

19 CONCLUSION Correct process data is the key for correct valve selection and reliable valve operation. Control valves problems can have simple or very sophisticated solutions. Most applications in the refinery are dirty, care when choosing: Cage guided valves Anti-cavitaion trim Anti-noise trim Liquid packing is innovative solution for packing gland leak

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