Seal flush Plan

[maintennance]

For one of the pump ( BB2 type), handling diesel, the seal flush plan recommended by the seal vendor is 11,65,72.The max suction pressure is 4.5 bar and the working temp is 139 Deg C ( Flash point is 120 Deg C). The vapour pressure is 0.07 bar.
My question here is , what is the criteria to select such a combination of seal flush plans? For such high temp, why 23,65,72 seal flush plan was not recommended by seal vendor?
For the buffer gas system( Nitrogen), the seal vendor has recommended the working pressure as 2.5 bar. What is the basis of arriving at the buffer gas pressure for 72 seal flush plan.Thanks to clarify.

 

[longeron]

Curious. 2.5 bar for buffer gas? To clarify things: is this a single seal or multiple seal?

 

[flexibox]

Diesel at 139C is not high Temperature. You can use a Pusher seal as opposed to a Metal Bellows seal. Plan 23 not required VP = 0.07Bar suction press 4.5bar. Vapourization is not an issue. As a single seal you WILL experience Coking with diesel at that temperature.
Unusual for a seal company to recommend Plan 65. This is normally specified by End User. You need to decide if you really need it. Is this offshore application? If containment is an issue then Tandem seals may be better suited.
Plan 72 implies that you have a secondary containment seal. This may be a spiral groove type or contacting dry running seal. 2.5bar does seem high, consideration must be given to Reverse pressure. Ensure that when deciding N2 pressure, it does not cause a reverse pressure failure on the primary seal where the N2 pressure pushes the stationary seat off its housing resulting in seal failure on start up. I think 1.0bar is sufficient but check with Seal vendor.
In summary, for such a simple application i would recommend Option 1
API plan 11 + 62 (N2) Use a pusher seal with a floating carbon bush and LP N2 quench to prevent coking.
Option 2
If a single seal with bush is not acceptable then Tandem seal with Plan 52
Option 3
As above with Plan 11, 65, 72 (LP N2)
trust this helps

 

[maintennance]

Thank you very much flexibox for your reply. I will putforth all your queries to the seal vendor.

 

[maintennance]

In continuation with my earlier query, for one of the centrifugal pump handling naphtha the seal flush plan is 11,72,75. I am really confused. Both 72 & 75 does the same work. Is it not? why such combination?
In this case for 72, vendor has given 2.5 bar pr for buffer gas . For 75 plan the operating pressure is indicated as 1.7 bar and H alarm is indicated as 2.2 bar.
Please clarify for this combination of seal flush plans and the significance of this combination.

 

[longeron]

Each number is a different seal support plan.

Plan 72 is the introduction of a buffer gas between the process and containment mechanical seals. This is used to "sweep" leakage to the containment vessel or to a flare/ collection header system. Pressure specified here is for the Nitrogen or other buffer fluid sweep. In my experience 0.25 bar is adequate.

Plan 75 is the containment system, the vessel used to collect the leakage from the process seal. Pressure specified here usually relates to a pressure switch or transmitter set point for the alarm that indicates excessive leakage. In many cases the Plan 75 vessel is connected to a flare or collection header for safety. Does your flare header operate at 1.7 bar? (that seems high for a flare header)