Flush plan for heavy oil pump

You should switch from your double seal to a single high temperature metal bellows seal. The problem with plan 54 is that it is susceptible to flow or pressure interruption from a variety of causes. I assume you are flushing with a cooler gas oil stream which is OK if the operator controls it correctly...however they often don't...The vast majority of these kinds of applications use single metal bellows seals with great success.
Seals on Atm: Bottoms are usually under low pressure unless you have a pump that allows discharge pressure in the seal chamber (and these are thankfully not so common) so a single metal bellows seal with Tungsten face and SiC seat with a plan 02/62 flush plan works well. The important things to remember are to warm up thoroughly before start up and use a constant dry steam quench to the atmospheric side of the seal. Lifetimes of 7 years plus have been recorded using this arrangement.

I agree with the comments above. We use single belows seals with heavy gas oil flush and a steam quench in this service. The problem with your existing seal may have more to do with the configuration than with the design. The original double seals in our vacuum resid and slop wax pumps were back-to-back. This placed the dirty product on the inside of the inner bellows. The solids that were inherent in the product stream would centrifuge outward and pack the bellows up. They probably installed the double seal in order to avoid having to flush gas oil into this product stream. Depending on the process you use downstream, it can be very expensive to recover that gas oil. If it is not practical to use a single seal with a flush, I would consider a redesign to a tandem configuration with the product on the outside diameter of the primary seal and keep the plan 54. Running this stream with a single seal and Plan 11 flush may not get very good reliability depending on the crude that you run.

High temperature metal bellow seal is the right answer for your application as suggested by some. Dual seal may still be required from safety standpoint. Check with your safety gurus if they can permit hot oil at 340 degC leaking and doing damage to personnel and asset. Note that single high temperature metal below seal can give good life as it is primarily designed for high temperature but the fact is that some day it will also fail and the question is would you permit leaking it ito the environment?

The issue of leakage on failure is usually addressed by a close clearance floating carbon bushing located on the atmospheric side of the seal. The space between the bushing and the seal is purged with low pressure (2-3 Psi) dry steam. The steam has two functions, 1) to prevent oxidation of the very small amounts of "normal" leakage through the seal faces. This oxidation clogs up the bellows and prevents the neccessary flexibility of the seal head. 2)In the event of failure the steam quench can be opened up to blanket the quench space minimising the danger of fire.

A more positive device is a dry running high temperature metal bellows seal which is used in place of the floating bush. As it's name suggests it will run without lubrication other than the low pressure steam quench. One caveat is that the steam pressure must be controlled to a maximm of 3 psi and the drain must be as open as possible. NTH is right though, you should run the single seal option past your safety people especially if the service is above auto-ignition temp. If in doubt have a look at the reccomendations of API682.