Biogas velocity 1

[JohnWeal]

Good Evening All,

Is there a recommendation in any standards for the velocity range for biogas in stainless Steel pipelines?

If the moisture created during cooling is flowing in the same direction as the gas, the pipeline is laid with a 1 degree fall.
If the condensate is draining back against the gas flow, the pipe will be laid it’s a 3 degree incline.

So we have a wet gas which als has 400 ppm H2S.

The biogas is 69% CH4 and 37% CO2.

I don’t want to oversize the pipeline by having too low a velocity.

Regards
John

 

[georgeverghese]

For this case, with SS304/316, use 20-25m/sec, subject to pressure drop limitations. If you are looking for a standard / guide to fall back on, use the non erosive / non corrosive case guideline in API RP 14E - the earlier editions also state pressure drop limits for each operating pressure range.

 

[JohnWeal]

Many thanks for your reply George...

I did spot a mistake. Should have read CH4 as 60% not 69%.

I should also have mentioned that we only have an operating pressure between 20 and 26 mbar.

Regards
John

 

[georgeverghese]

At 20mbarg (200mmAq), using 20-25m/sec for nominally allgas lines should be generically okay for sour wet gas with high CO2 with SS304/316. At beyond 30m/sec, you may run into noise problems for general piping, which may be a HSE issue for your plant operators. You may have to reduce line velocity if lines are long and P2 limits apply. Since this is condensing gas, take care to avoid piping pockets that can induce slug flow, ie all lines should be free draining - add drip legs / KO pots where required if pockets are unavoidable.

 

[LittleInch]

Pipeline sizing as opposed to pipe sizing is a balance between competing forces and issues.

In short these are:

Pressure drop / friction for the fluid flow for the length and the pressure drop available
Static head changes ( topography)
Inlet and outlet pressures if fixed or known
Erosional velocities on bends and tees
Cost
Liquid hold-up for wet gas / 2 or 3 phase systems including slugging
Materials available for the pressure /temp ranges
Other factors - crossings, regulations, safety, standardization

There are so many variables that no one can say the velocity is between X-Y for all cases.

For long gathering systems the gas might only have enough driving force to "waft" along at 1-2 m/sec. Much shorter bits it could be 10-15 m/sec.

Biogas is a very low pressure gas collection system running at unbelievably low pressures. So sizing will be dominated by gas flow and arrival pressure. We know nothing about your system (size, gas flow, number of inputs, lengths, arrival pressure fluid compositon etc etc) so can't provide any guidance here unless you provide us with a bit more.

It sounds like the liquid hold up / no pockets issues is known and dealt with presumably as a number of drain points and over a longer section a slope down at 1 degree to a drain followed by a slope up at 3 degrees followed by a slope down.... all depending on your topography.

This is the work pipeline engineers do to design the most optimum pipe size taking into account all these issues.


For low pressure biogas I'm amazed you're not running low strength / wall thickness PE. Stl Steel sounds like an expensive alternative - care to share why?

SO in short no - you design each to its own requirements. That's the fun of Pipeline Engineering


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Assuming that this is for a wastewater digester project, the WEF Manual of Practice No. FD-8 recommends that piping should be sized with a maximum gas velocity below 12 ft/sec (3.7 m/s). This velocity limit will prevent liquid and solid carry-over which damages equipment. In addition, the number of bends and long piping runs should be minimized to reduce pressure losses.

Digester safety equipment

 

[JohnWeal]

Many thanks for your replies. The longest length we have is 70m and this is on a steady fall of 1 degree.

Yes bimr, it is a for a wastewater treatment works two AD's.

LittleInch, A large proportion of the 70m is indeed PE yellow gas pipe buried and rated at 2 bar. (we are well under that!!)
The stainless steel above ground is metric 'tru-bore' tube with a 3mm wall. So less than schedule 5s pipe wall. We will still do the leak/pressure tests in accordance with the IGEM/UP/1 standard.

Is the WEF manual FD-8 available online bimr?

 

[LittleInch]

Great response from bimr, thinking about it a bit more, the velocity of the gas will need to pretty low if you get any condensate running counter to the main gas flow. Even 3m/sec would probably be enough to "blow" liquid up the slope. 3 Degrees is pretty low.

So long as you can deal with any liquid going in a different direction to what you would like to happen away you go.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

https://www.amazon.com/Design-Munic...9D3CIVO/ref=mt_kindle?_encoding=UTF8&me=&qid=

 

[georgeverghese]

At these low pressures, it should be obvious that selection of a sizing case line velocity from some reference manual does not preclude the need for detailed pressure drop calcs to ensure that the minimum required final destination pressure is met.