4 replies to this topic

[Art Montemayor]

Annee:

I'm having difficulty reading and understanding your English, but I believe I understand your problem. You can't tolerate breaking a vacuum of 5 torr because you run the risk of polymerizing your product should it come into contract with atmospheric Oxygen.

I would not rely on a check valves (or multiple checkvalves) to save my product in case of vacuum failure. Check valves (or "non-return" valves) are notorious for leaking when not expected to. The system I would incorporate - especially if the system involved is relatively small as you indicate - is one where I would bleed in inert Nitrogen gas from a pressurized cylinder. Immediately after injecting the Nitrogen, I would activate a leak-tight, soft-seat, ball valve to block in the process and not admit atmospheric air into the system. This would "lock-in" the inerted product and avoid a polimerization to take place.

Art Montemayor

[Art Montemayor]

Annee:

When you experience a vacuum failure in your process equipment you have no time for manual override or controls. It happens spontaneously and you must respond correspondingly. The usual method I use and the description I gave you employs a very sensitive vacuum switch set at a setpoint slightly higher than your operating point. When the vacuum is lost, the switch activates a series of actions:

1) it instantly opens a solenoid valve that is connected to a sufficient Nitrogen source that is stored at a pressure that is designed to not elevate your equipment pressure beyond its MAWP (Maximum Allowable Working Pressure). This action instantly replaces the partial vacuum with a rush of inert nitrogen that sweeps through the equipment and doesn’t allow atmospheric air to invade the equipment due to the vacuum loss.
2) after a reasonable time to allow the nitrogen to sweep through the equipment, the main block valve is actuated and this action seals in the nitrogen inert atmosphere that you have introduced into the equipment. The equipment should be effectively sealed and inerted while the system is shut down and the vacuum failure is investigated and repaired or resolved.

Note that there are various safety precautions and steps that you must study and undertake – as in all process changes – in order to make sure that you have a SAFE and operable operation under this type of instrumentation. You should hold at least 2 Hazops among your operators and engineers prior to installing and operating under this type of instrumented shutdown in order to ensure safety and total agreement among all your operating personnel.

Your question about maintaining a vacuum after failure is not a logical and valid engineering alternative. Once you loose vacuum you are, in effect, totally shut down in your unit. You must resort to finding the cause of the failure and resolving it. The issue at hand is what do you do about the product that is exposed to potential polymerization, and this is taken care of by the introduction of an engineered, inert atmosphere that allows you to safely shut down and proceed with resolving the problem and proceeding to startup again after the resolution is found.

I hope this explains the logic and engineering reasons for the recommended procedures.