cold stack design 1

[pastyl]

I have sized a natural gas vent cold stack acc. API 521 considering stack tip velocity >150m/s, so as no flammable mixture to be created.
Acc. API 521,even so, the height of the stack shall be sized in the case of flammable mixture occurs.
But,by taking the above case into consideration, the height of the stack does not differs too much for a similar flare and I suppose that the same restrictions for people access near the stack are valid.
My question is that if in case of cold stack there is any anti-fire device or other technical mean that can prevent any flame creation allowing thus the stack to be much lower and people have access below the stack, as it would be a simple vent.

 

[RotatinSpecialist]

The gases from a vent stack may ignite, by lightning, by static electricity or by contacting any ignition source. This may typically happen some 30 seconds or more after the blow down process is initiated. That's because initially the velocity is too high and dispersion has not taken place in sufficient amount as to provide a gas-air mixture within the flammability limits of the vented gas.

At the usually very high flow rates of vent stacks (I'm thinking of stacks 8 to 20 inches or more, venting at high gas velocities, in the range of 0.2 - 0.6 Mach), it is unpractical to have any kind of flame extinguishing means that would be able to block the combustion. Usually, CO2 bottles are provided, but just to extinguish flames at low flow conditions, such as those that may happen by ignition of the purge gas, or at the end of the blow down process.

That's why the radiation must be taken into account, as recommended by API. During engineering design, the safety measures considering potential damage to operators and equipment by flame radiation are to be considered, such as: building the stack with sufficient height; placing the stack at a safe distance; isolanting the risck area around the stack; using more than one stack, placing them at a safe distances.

Performing a gas dispersion analysis is also a good tool for ptimized decision as to stack flow, height and position.

 

[pastyl]

Thanks for the valuable answer

I want to be more specific because the application is critical.
It concerns a natural gas pressure reducing station for transmitting the gas as a fuel to a gas turbine, part of a CCPP project.
At the pipeline and at the equipment (filters and heaters) there are relief valves and vents with different opening pressures.
Although the flow is appr. 85000Nm3/h all the vent and relief system is not sized for the whole flow but abound a 20%.
We usually lead each vent and relief of different opening pressure separately to atmosphere app. 4-5 m from top of the building without consider any flame case.
Now we must lead all the vents and reliefs to one stack and this stack should not be above 10m.
I wonder if that is a good engineering practice or we do something wrong

Best Regards

 

[RotatinSpecialist]

Dear friend:

Some basic concepts, I have been using and seen into practice in many natural gas pipeline installations:

1 - For NG vent systems, always consider ignition as a possibility. Therefore, flame radiation calculations (as per API) is a must. Gas dispersion study, for given stack dimensions, gas composition, and gas flow, taking into account 3 typical wind velocities (low, average, high), determining gas cloud dimensions, gas cloud flammability limit geometry, flame porportions and radiation over plant areas, is still better. They are the tools one use to determine stack geometry and position.

2 - As to pressure reduction station, API/ASME/ANSI standards recommend (and I fully agree) you not to rely on pressure control instruments and pressure reducing valve (PRV) as being sufficient protection against surpassing downstream design pressure. If the downstream system is to be constructed with a lower pressure rating than upstream system, then it must be protected by safety valve(s) set at downstream system design pressure, selected for the maximum flow that the pressure reducing valve may deliver, considering that it may fail, fully open. Do not rely on valve 'FAIL CLOSED' criteria. It is just a (usually good)criteria, but mechanical defects may cause the PRV to remain fully open independent of what instrument or even valve components (spring) system "orders" it to do.

3 - Fuel gas feed systems for gas turbines must be provided with two block valves (SDV)in series, with a blowdown valve (BDV)in the intermediate piping. The shut down sequence command shall close both SDV's and open the BDV. In addition, a pressure transmitter in the intermediet section send information to the PLC that is used to check (a) if the SDV's are working properly (b) if there is no system leakage, and (c) if the BDV is also working well. It's not difficult to develop a logic for that, if you agree. When the system starts, PLC checks if pressure at the intermediate transmitter is near zero (BDV and SDV's should be closed); then the upstream SDV opens and close. The pressure must go to the pre-set value and remain stable for some time (2 minutes?); then the BDV valve opens, and the pressure must go again near zero. Etc... In case of valve malfunction is detected by the PLC logic, then system startup is aborted.

4 - Normally, all BDV and Safety Valve discharges in a natural gas system hould be routed to a common blowdown header and stack.

5 - Don't forget to place a manual quick-closing shut-off valve in the fuel gas line, near the Gas Turbine skid. The local operator may need it in case of real troubles with turbine going out of control.

6 - If your vent stack cannot be above 10 m high, then radiation or dispersion studies will dictate its localization. If radiation at plant level is above the max. recommended by API (say, above 6.3 to 9 kw/m2-h), then the area submitted to such rad level should be encircled with a fence and warning signals.

I'm herewith attaching well-known pictures of a natural gas accidente in a pipeline. It's a 30 inches pipeline fully open to atmosphere, far a bigger a problem than yours, as I understand, but anyway, it gives us a hint on how bad natural gas flame radiation can be.

Good luck!

 

[RotatinSpecialist]

Unfortunately, there is no way of avoiding the flammable / explosive gas mix forming possibility when venting a flammable gas to atmosphere. I have watched lots of discussions around the subject, because some people always come saying that the probability of a flame ball or explosion occurence is very remote, and that it is not necesary so take many safety measures in a cold vent stack design.

Well, my opinion is that it depends on the kind of installation we are designing, and on the amount of vented gas, that is, on the calculations / dispersion study. I usually respond to those that think that API is too conservative, with a question: "suppose that you vent the gas and it ignytes, and a flame ball results in your plant. Suppose also that material and equipment are lost as a result. Supopose that operators are injured or dead as consequence of that improbable accident. What would you say to their families? what would you say to plant owners? What would you respond to the experts that will come to investigate the causes, when they demand you why didn't you follow the well known API recommendations?"

I prefer to have sound explanations to say, as to my cold stack design criteria, than having nothing to say to those experts.