API plan 52 operation 2

[roker]

Hi all,

In our plant the reservoir of API plan 52 seal has a pressure switch (no level switch)and is vented through an orifice (2 mm) and a valve to the flare system, should the valve be close or open? how the seal malfunction is detected?

regards,
roker

 

[RGRAJANI]

In my earlier company we had similar plan, but the vent valve was solenoid operated. When pressure rises, it gives alarm and this valve opens up to flare.

If you have manual valve and 2 mm orifice, the valve should be open. When seal leaks, pressure will rise and pressure switch will generate alarm.

Problem of this arrangement is that, when outboard seal is leaking no alarm is generated. You have to depend on field operator observation to monitor outboard seal performance.

 

[roker]

hi RGRAJANI,
thanks for your reply, how do you observe outboard seal leaking? is plan 52 only for the seal near the impeller (or process fluid being pumped)?

regards,
roker

 

[JJPellin]

Plan 52 is a double seal. Leakage of the primary seal can be detected because product will be leaking into the seal pot. If the product tends to vaporize at flare pressure, the leak will be detected by pressure increasing behind the orifice in the vent line. If the product tends to remain liquid at flare pressure, the level in the pot can be seen to rise by the operators on normal rounds. If they fail to notice and the pot becomes full of liquid up to the orifice, then pressure will back up and be detected. A leak of the outer seal can be detected by visible leakage of the barrier fluid to the base. Also, the level of fluid in the pot could be seen to drop. However, in some services, depending on the barrier fluid used, the level could drop slightly just from barrier fluid vaporizing to flare. We see this with #1 fuel oil barrier fluid in not services. A loss of level in the pot is typically seen by operators on normal rounds checking the sight glass. A low level switch could also be used, especially for remote or unmanned installations to detect outer seal leakage.

 

[bingopin]

The valve to the flare should remain open. Usually the purpose of the orifice is to create enough pressure drop to activate a high pressure alarm when the inner seal leak rate is relatively large. This usally means the orifice is pretty small and can be prone to plugging. The orifice does nothing for a relatively small inner sealleak rate...the pot can fill without activating tha pressure alarm and either activate a high level alarm (if it is present)or the high level can be picked up by operator surveillance.
My experience on many of these installations has been that over time, the level in the pot will rise and will require intervention by the operator to drain the pot to an acceptable level. Declaring the inner seal failed depends on the frequency of pot draining. Draining a 3 gallon pot down every few days may be acceptable depending on the service. Leak rates high enoungh to activate the pressure alarm probably aren't.