Q. How to determine the design pressure and design temperature of the equipment?
Ans: It is very important to determine the right design pressure of the equipment in order to prevent failure of the equipment in number of scenarios. Process Engineer plays a vital role in calculating the right design pressure of the equipment. For determining the design pressure of equipment, there are number of categories :
1. Design pressure for equipment containing vapour space
2. Design pressure of liquid filled equipment downstream
3. Minimum design pressure of equipment connected to flare system
4. Design pressure for inherent safer design
5. Design pressure of Shell and Tube heat exchanger
6. Design pressure in back flow scenario
7. Design pressure on interstage drum for multistage compressor
8. Lower Design Pressure
1.Design pressure for equipment containing vapour space
As per API 521 design pressure of equipment "The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature, and it is the pressure specified on the purchase order. The design pressure is equal to or less than the MAWP (the design pressure can be used as the MAWP in cases where the MAWP has not been established). Once the design pressure has been selected and then the vessel is designed for some thickness at corresponding design temperature. The actual thickness applied may be higher than calculated thickness. Then MAWP is established based on actual thickness.
To calculate the design pressure, first one need to know operating pressure of the system, which is determine by process engineer. Then one need to established maximum operating pressure. Which is as per the rule is MOP = 105% of OP ( P>20 bar) or OP+1 bar (P<20 bar). This margin is kept for control purpose.
Once the MOP is determined, then the design pressure is established using following rule :
DP = 110% of MOP (P>10 bar) or MOP+1 bar (P<10bar)
2.Design pressure of liquid filled equipment downstream
Design pressure of the liquid full system is determine based on the upstream equipment which can be pump. The design pressure of the liquid full system is equal to MOP. MOP is determined by upstream pump shut-off pressure. Pump Shut-off pressure is determined by Max upstream pressure of the pump+ Static height+ Pump differential head.
3. Minimum design pressure of equipment connected to flare system
Minimum design pressure for equipment connected to flare should be atleast 3.5 kg/cm2g, since even the operating pressure is less, during the relief, it should be able to discharge the content to flare system which will have some back up pressure (atleast 0.5 to 2 kg/cm2g).
4.Design pressure for inherent safer design
Sometime it is necessary to keep design pressure much higher than calculated by above formulas for emergency situation and keeping in mind inherent safer design. Example, due to abnormal situation such as runaway reaction, if the pressure rise is much higher than operating pressure and the relief devices may not be enough or practicable to implement then the equipment is design for maximum possible pressure that can be achieved during such situation.
Example, In the normal ethoxylation process ethylene oxide is polymerized using catalyst and operating pressure may be 5-6 bars. The vapour space of the reactor will have ethylene oxide vapour. It has been calculated that if the explosion in such system occur (ethylene oxide is very explosive and it does not need any oxygen as it contains its own oxygen), the expected pressure rise is about 12 times the operating pressure. Therefore the pressure in the system can rise upto 60 barg. In such a situation, the reactor may be designed for higher pressure i.e 60 barg to contained the explosion with the reactor.
5. Design pressure of Shell and Tube Exchanger
Design pressure of low pressure side of heat exchanger is equal to hydrostatic pressure of high pressure side of heat exchanger, if the low pressure side contains valve that can be isolated. In Industry it is know as 10/13th Rule (if the hydrotest pressure of high pressure side is 130% of design pressure) or 2/3rd rule (if the hydrotest pressure of high pressure side is 150% of design pressure). The logic behind this rule is that if there is a tube leak then high pressure side fluid will enter the low pressure side and if the low pressure side is isolated then it can attain the maximum possible of high pressure side (hydrotest pressure) upto the isolation valves. Therefore, isolation valves should also be able to withstand the high pressure side hydrotest pressure.