Fuel gas pressure reduction

[janca]

Hi,
does anyone know how do I reduce fuel gas pressure from 67 bara to 7 bara???
Do I need any special led down stations, special valves, vessels...??

Thank you,
jana

 

[TD2K]

You will have chocked flow across the valve and with this dP, you will have noise issues you will need to consider. Give this dP, it's definitely a severe service valve, you might want to talk to a speciality valve company like CCI.

You'll need to run the valve calculations to see what Cv (or Cg) you need and whether you can have a single valve or need a staged pressure let-down (exit velocities from control valves are typically limited to about 0.3 mach). Again, the vendor should be able to guide you here with what options you have.

You also need to look at the fuel gas composition to see if you will get enough cooling to condense out any liquids which will mean a downstream KO vessel and some way of handling the liquids.

A pressure drop of 60 bar on a gas system will result in about 30 to 35 deg C of cooling (assuming no or minimal condensation). Is that sufficient cooling that you need to look at low carbon steel? That depends on what the range of inlet temperatures you need to consider.

 

[alsayed]

Hi,

I worked in a methanol company that used natural gas as feed source and for fueling as well.

The gas pressure was about 67 bar(g) and reduced to 22 bar(g). We had to first heat the gas up in a U-tube heat exchanger with L.P steam. This will ensure that the gas will always be above its dew point and will not condense and damage the equipment. Then reduce the pressure using three control valves in parallel. One of the valves was big so the pressure control was not that accurate (most of the time it was on manual) but the noise level was acceptable. The other two valves where smaller in size and used to fine tune the outlet pressure and with a smaller flow, noise levels where acceptable. The other valve was a spare. After that the gas inters a K.O vessel to remove any condensate if present.

That was our system. I hope all of this is useful.

 

[janca]

Hi guys, thank you very much. You both helped me a lot.
Till now I have already seen fuel gas pressure reduction but not that big one(from 67 bara to 7 bara).
The method of heating the fuel gas in the U-tube heat exchanger is a great idea. I was thinking about steam tracing of all the system, because of keeping the gas above it's dew point. But implementation of the heat exchanger seems better to me.
Jana

 

[MortenA]

Usually fue gas consumption is not that big. Unless you have the utilities nearby you could also consider an electrical heater for the fuel gas. Its easier to handle i beleive.

Best regards

Morten

 

[davefitz]

some things to consider:

a) the CCI valve is great for reducing noise but if you get condensation of hydrates I heard that they plug up the CCI drag trim a lot. This can be addressed by preheating the gas, but check the CCI users's list to confirm operating experience

b) different design codes have different rules on required relief valve capacity to be installed downstream of the PRV station. If the system is a small outdoor system ( ie can be relieved by a single 3" NPS releif valve) then full relief capacity makes sense. If it is indoors or is large, then most people take advantage of the code allowance to use 2 PRV's in series , soft seats, independent power supplies and controls, inlet fast acting stop valve with hardwired pressure switch, etc. to avoid the use of large releif valves.

 

[dhamo]

Safety in design is important. You may have 150# rating piping in the 7 barg downstream system. You should have a full flow relief valve in the downstream of pressure control valve for API-520.
If you do not have flare and the venting quantities (from relief) are high, then you can think of either over pressure protection system (OPPS-one slam shut valve and one active control valve and one monitor control valve) or HIPPS system (similer to OPPS, instrumentation reliability to be analysed).
Fischer (Emerson) or RMG, Germany are some of the vendors who can design and suplly the pressure reducing station for you.

 

[olef]

If you're using wet gas don't forget to check for hydrate formation.

We often tend to make things a little expensive. Going for CCI as the only option for the valve might be overkill in this service.

As for overpressure protection, a PSV is always best (API RP 14c). Just keep the full pressure rating for the piping past the last restriction before the vessel.

Regards
OleF