Since 1970 we operate a VCM plant w

[abigail1]

Since 1970 we operate a VCM plant with a nameplate capacity of 550.000 MT/year.
The reliefvalve headers are connected with a 100 m high flare built by John Zinc.
The flare is steam assisted (steampressure = 12.5 barg).
Up to 5 years ago the ventgases of the plant was continuous burned in the flare. At the moment, we burn the ventgases in an incinerator and the flare is only used as a backup for the incinerator and for emergency reasons.
During the past 33 years and up to now, we keep the steamline warm by continuous limited steaminjection (bypass over the steamvalve with a restriction orifice).
Nobody in the company knows why we are doing this, but one of the possible reasons could be to prevent steam condensation or possible flow back of hydrocarbons in the steamline.
Is it good common practice to keep the steamline warm all the time with continuous steaminjection? What will happen if we stop the injection. The burned gases contain HCl and are higly corrosive.

 

[July]

Are you located in a cold climate?

 

[abigail1]

July,

Duriong summer we have max.temps of 30 °C and in winter we have minima of -10 °C. A colloegue of Borealis told me that John Zink (flare manufacturer) recommends to keep the header always above 100 °C to prevent condensation during startup phase. Condensate could exstinguish the pilots and provoke water hammer. There may be other raisons and I wonder if there are other solutions.

 

[TD2K]

THere are several reasons you might want to keep a minimum steam flow up your lines.

Valves will always pass to some extent. You can therefore expect to get condensate built up on the downstream side of your control valve manifold at the flare stack where the lines turn and go up the stack, that water can freeze so when you need steam, you can have problems. If the piping is heat traced, that may not be a concern (make sure you have a trap to drain any water, where you drain it can create an icing problem if there isn't a drain system at the flare stack. You won't be able to trap this point to your condensate system, if there is one, as there normally won't be any pressure on the steam side of the trap).

The second one is to prevent condensation in the flare line. Condensate can be blown out of the top which can create problems with your flare stack (I worked in a plant where we plugged up the molecular seal this way. When you have a plant running at full rates and NO flare is available, the pucker factor goes way up).

The steam tips can also be overheated by the 'usual' flame that occurs at many flare stacks. That flame licking on the steam injection tips can overheat and damage them, the minimum steam flow can be therefore set by the vendor as a cooling flow.

 

[abigail1]

Dear TD2K,
Thank you for your usefull input. I highly appreciate this.
Abigail1