Bypass necessary for a whole station?

[shanghaigas]

Hi,

I'm dealing with the contractor for city gate station designing and coming with a argument of bypass. It's quite common to equip bypass parallel to installations like valve, filter, meter and regulator etc in pipeline system. However, is a special bypass necessary for a whole station (normally a city gate station) with parallel lines in it? The bypass here means the pipeline linking the input and output point to skip the station. Somebody suggests that a bypass with valve control will play as a temporary substitute for the station in emergency to secure gas supply. But I can hardly image a station with parallel lines may fall into malfunction simultaneously. My personal feeling about it is little bit over-designed. In Chinese designing code, the bypass of this kind is optional. I'd like to get some ideas from the forum about your practise. Does any code in your context require the bypass as compulsory?

Thanks!

 

[CostasV]

shanghaigas,

I do not know of any code requiring bypass for a station.

I only know that in Greece, there is a provision for a bypass of the border metering station (BMS). This BMS has 4 parallel metering lines.

Regarding city gates, in Greece there is no provision of bypass line because here the city gates are metering and regulating stations. So if the bypass line is not able to regulate the pressure then it is useless and may be catastrophic. If the bypass line is able to regulate the pressure the it not a bypass line; it is a parallel line.

But, we do have 2 city gate stations for each city. So one city gate station plays the role of the bypass of the other one.

Until now, with our 8 year experience of gas transportation and distribution, we did not use the bypass of the BMS.

Hope this helps

Costas

 

[1969grad]

My first thought is that if you bypass the city gate station that you do not meter the gas and hence someone gets the gas for free.

Second thought is overpressure protection for the downstream distribution system if it is not designed for the same MAOP as the pipeline.

For single meter stations a meter bypass is often installed but not the full station. Usually, multiple meters are installed which do not have a bypass. You can also install parallel regulators with different set points.

Finally, what about odorization if you bypass the whole station.

 

[ICman]

I agree with 1969grad. We've been designing city gates for several decades, and I have yet to see one with a bypass for all of the reasons cited.

 

[vzeos]

I agree with 1969grad and ICman. Bypasses can be very dangerous. About 20 years ago, there was a fatal gas explosion in Chicago when the mechanic who was operating an emergency bypass left the bypass unattended. Based on this incident my company, along with much of the industry, began a bypass replacement program. Our design philosophy changed from installing bypasses to installing full redundancy. I don't know if this was ever incorporated in code.