Mechnical Seal MAWP

[ShahabAPI]

Is it required to consider MAWP of the mechanical seal same as pump casing based on any applicable code or standards?

Shahab Zardynezhad

 

[War_Eagler]

In the petroleum industry, API 682 only requires the seal pressure casing to be the same as the pump casing. There is a note that it is not normal practice for mechanical seals themselves to be rated for the MAWP for the pump in which they are installed.

In my experience, this is because the stuffing box pressure of a pump, and therefore the pressures that a mechanical seal will see, are much lower than the discharge pressure. It would be impractical to design the seals for high MAWP when the operating pressure is much lower. Also, pump cases are typically hydrostatically tested with the mechanical seals removed.

 

[SB1976]

From a purely technical perspective, designing a mechanical seal for the entire MAWP of the pump is absolutely not required. MAWP for the mechanical seal should be designed for the pressure rating as experienced inside the sealing chamber (know as the box pressure), which never goes till the pump discharge pressure. Even with seal plan 11 wherein tap-off is taken from pump discharge and put inside the sealing chamber, the box pressure assumes a value somewhere in excess of pump suction pressure.

Hence, a design with seal MAWP same as pump MAWP can only be construed as redundancy.