Revamps 2025 Issue

HP spray water MP spray water

EHP ringmain 125 barg

HP spray water

IHP ringmain 50 barg

MP ringmain 15 barg

TAHH

SDV

OT = 515 ˚C

415 ˚C

280 ˚C 230 ˚C

Design temp break 480˚C 280˚C

IHP- M P PRDS B

EHP-IHP PRDS B

MP steam export

IHP- M P PRDS C

EHP-IHP PRDS C

IHP- M P PRDS D

EHP-IHP PRDS D

IHP- M P PRDS E

EHP-IHP PRDS E

EHP steam export

IHP steam export

Figure 2 System overview

and could result in high-temperature creep, 3 reducing the metal’s fatigue resistance. If this continued for a prolonged period, there could be irreversible damage to the piping and support structures, with an increased risk of material failure leading to a loss of containment. • HP spray water failure (MP spray water continues). • MP spray water failure (HP spray water continues). In all scenarios, the high-temperature trip must respond and close the SDV at the inlet to the affected IHP-MP PRDS before the pipe wall temperature at the design temperature break at the battery limit reaches 280°C. It is also important to consider that when HP spray water fails, the EHP-IHP PRDS can also be affected, meaning that the inlet temper- ature to the IHP-MP PRDS is higher, in addition to losing its own spray water supply. It was known that the capacity of the MP spray water system was sufficient to desuperheat the MP steam to 230°C even in the absence of HP spray water. Therefore, if HP spray water was lost but the control system could respond by increasing MP spray water flow to maintain the temperature below 280°C, shutdown of the PRDS would be unnecessary, and the plant could continue to safely operate while the issue with the HP spray water supply was resolved. Due to the high operating temperature of the system, cooldown and warm-up cycle times were extensive in order to prevent thermal shock to piping and valves. It could therefore take several shifts to recover from an unneces- sary trip of a PRDS, meaning that avoiding an unnecessary shutdown was highly desirable for the steam plant and broader refinery operations. The following solution was proposed: There were three scenarios identified: • Total (HP and MP) spray water failure. • A higher set point trip that would instantly initiate shut- down of the PRDS in the event of a total loss of spray water. This also encompasses the non-governing scenario where MP spray water fails but HP spray water continues. • A lower set point trip that incorporates a time delay

before initiating a shutdown of the PRDS. This allows time for the control system to respond to bring the MP steam temperature back into its operating envelope if only HP spray water fails. Figure 3 shows how this would function in practice. The system operates at steady state at 230°C until the HP spray water fails. The MP steam temperature increases and reaches the lower set point trip at 280°C, starting the timer. The control system should respond by increasing the flow of MP spray water. If the steam temperature recovers to within the operating envelope before the timer runs out, the timer is stopped, and no trip is necessary. The pipe wall temperature at the battery limit stays within its design tem- perature at all times. To assess whether the proposed solution could be applied in practice, a dynamic simulation model of the PRDS and MP steam system was developed in Aspen HYSYS based on data from the 3D piping model. The HYSYS model could simulate total or HP spray water failure and calculate the system’s response over time, factoring in system hydrau- lics, control valve positions, and heat transfer through the pipe wall and to the environment. It is important to remember the following:

460

Higher trip setting

System recovers to within operating envelope. Timer stopped

Lower set point reached. Timer starts

TBD

Downstream system design temp Lower trip setting

280

Steam temp.

Steady state operation at 230˚C

Steady state resumed

230

Pipe wall temperature

Loss of HP spray water occurs

Pipe wall design temperature not exceeded

Time

Timer stopped No trip

Time TTT delay

Figure 3 HP spray water failure, control system responds

Revamps 2025