Definition of Terms Related to PRESSURE

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A simple question always asked and debated by many engineers. What is the different between the following terms ?

• Maximum Allowable Working Pressure (MAWP)
• Design Pressure (P_D)
• Maximum Allowable Operating Pressure (MAOP)
• Maximum Operating Pressure (P_O,Max)
• Normal Operating Pressure (P_O)
• Minimum Operating Pressure (P_O,Min)
• Minimum Allowable Operating Pressure (MinAOP)
  

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A mechanical engineer may have different understanding than a process engineer. A process engineer from an organization may have slightly different understanding than a process engineer from another organization. A process engineer educated and/or practising engineering in a country i.e China may have different interpretation than another process engineer educated and / or practising engineering in another country i.e. USA, UK, etc. This always creates a lot of confusion, unnecessary argument and rework. In worst event could lead to hazard. Thus, first far most important thing is to define correctly the meaning of each terms so that everyone have same understanding.

Following are the simple definition of above mentioned terms. Whenever a post contains one or more of these terms, the following shall be referred.

Normal Operating Pressure (P_O) - System pressure as expected to be operated at during normal operation throughout the design life of the system.

Maximum Operating Pressure (P_{O, Max}) - Maximum system pressure as expected during normal operation, may occur in some process transient period or different operating mode or campaigns and it built into the design to cater for any uncertainties due to start-up, fouled, decayed, etc. It provides some level of flexibility for a proper operation of the system throughout the entire life of the system.

Minimum Operating Pressure (P_{O, Min}) - Minimum system pressure as expected during normal operation, may occur in some process transient period or different operating mode or campaigns and it built into the design to cater for any uncertainties due to start-up, fouled, decayed, etc. It provides some level of flexibility for a proper operation of the system throughout the entire life of the system.

Maximum Allowable Operating Pressure (MAOP) - Maximum system pressure that can be allowed to ensure a proper operation of an a device or system.

Minimum Allowable Operating Pressure (MinAOP) - Minimum system pressure that can be allowed to ensure a proper operation of an a device or system.

Design Pressure (P_D) - A pressure chosen / specified (normally chosen by process engineer) to have certain margin (i.e. 10%) above the P_O,Max (or MAOP). It is a maximum pressure in the system that :

- is NOT expected during normal operation
- may only occur during emergency situation such as fire, loss of utilities, valve failure, any abnormal operation corresponding to a short duration, mal-operation, etc

Design pressure becomes MINIMUM pressure that can be hold by any components within the system without mechanical failure. It is used to define the minimum MAWP of components within the system. For example, design pressure is used to calculate minimum vessel wall thickness.

Maximum Allowable Working Pressure (MAWP) - A maximum gauge pressure permissible by a equipment / device (at coincident temperature specified for that pressure) and is governed by code i.e. ASME, JIS, GB, etc

In many cases...

MinAOP <= P_O,Min <= P_O <= P_O,Max < MAOP < P_D <= MAWP

Example :

A pressure vessel contains instrument air is normally operate between 6 - 8 barg. The system would trip under Low-Low pressure of 5 barg to allow sufficient instrument air volume for safe operation of some critical valve during shutdown. The system is designed for 11 barg as specified by the process designer. A conventional spring loaded pressure relief valve (PRV) is provided to protect the vessel from overpressure. The minimum wall thickness required for 11 barg is 6.27 mm, plus 1.5 mm corrosion allowance as specified by process engineer, the total required wall thickness is 7.77 mm. Mechanical engineer has decided to provide 8.0 mm as wall thickness which correspondence to 12 barg.

(Note : all parameters have been selected arbitrary. Just for illustration only.)

• Maximum Allowable Working Pressure (MAWP) = 12 barg
  
• Design Pressure (P_D) = 11 barg
  
• Maximum Allowable Operating Pressure (MAOP) = 10 barg (~90% of 11 barg)
  
• Maximum Operating Pressure (P_O,Max) = 8 barg
  
• Normal Operating Pressure (P_O) = 7 barg
  
• Minimum Operating Pressure (P_O,Min) = 6 barg
  
• Minimum Allowable Operating Pressure (MinAOP) = 5 barg

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