2 replies to this topic

[mastergii]

Hi guys,

 

I'm looking into blow down valves and how to activate a total blow down of an offshore oil and gas platform in case of an emergency.

 

So I'm just wondering about how much time it will take before I get the desired pressure on different segments.

 

For instance, Lets just say I have a segment of a platform, gas with a mass-flow rate of 20 000 kg/h. The pressure is 22 bar. I have to perform a blow down and send the gas to the flare. If I want a pressure level of 7 bar in my segment, how do I calculate how long time this will take and what mass-flow I would have when I reach 7 bar?

 

Hopefully you guys understand my case and my question, even though the explanation is a bit messy.

 

Hope you can help me!

 

Thanks

[trotter]

I think you may be looking at this backwards. You don't usually size your blowdown system based upon a set flowrate. You size your blowdown system based upon a set time that you deem safe to prevent loss of containment. A general rule-of-thumb previously stated in API 521 (but no longer) was that you should depressure your system to 6.9 barg (or 50% of operating pressure whichever is lower) in 15 minutes. This was noted as a suitable time that a vessel exposed to fire would take to heat to a point where it may fail. It is left to the designer to choose a time which is suitable for the specific system/area of plant based upon likely emergency scenarios.

 

If you do want to calculate the time taken then you need to perform iterative calculations as a function of time. Most blowdown flowrates are 'controlled' by an orifice downstream of the blowdown valve itself. As a result of this the flowrate will decrease over time as the upstream pressure drops. First determine the mass of gas that will be require to be removed to reduce your pressure to the desired level. Perform an orifice calculation to determine the flowrate when the valve first opens. Use this flowrate for say, 10 seconds and then recalculate what the vessel pressure is from the mass that has been removed. Then do another orifice calculation with this pressure as a starting point and follow the same procedure. Keep iterating this until you have removed the mass required to depressure the vessel to your selected level. 

These calcs can get very messy - HYSYS and UniSim have a utility which allows you to do this quickly. 

 

Hope that helps

PS: if your blowdown is maintained at a constant rate through a control valve rather than an RO, this make the above procedure a lot similar. Just work out how much mass you need to remove, and divide this by the flowrate for your blowdown time. 
 

[shan]

Usually, BDV (Blow Down Valve) of offshore platform is an on/off valve without throttling capability to control the blow down rate.  A blow down is a dynamic process.  The instant blow down flow rates are determined by combination Cv of BDV and RO (most cases) and  are varied with upstream and downstream conditions (pressures and temperatures) of BDV with time.  Although, there are calculation spreadsheets around nowadays, people use the process simulators (Hysys) to obtain time to reach certain pressure value (7 barg for your case) with certain initial conditions (22 bag and 20,000 kg).  Please notice the initial mass content should be 20,000 kg not mass flow 20,000 kg/hr because the shutdown valves upstream and downstream of BDV should be closing when a blow down process starts.