17 replies to this topic

[kangas]

Hello, 

 

Can I use the pressure reading, from a pressure gauge connected to a biogas pipe (of known diameter) to  calculate the flow rate of the gas flowing into a burner? The pressure gauge will be placed just after the valve to monitor the pressure flowing into a burner.

 

I have seen on some resources that the use of Hagen-Poiseluisse equation but  I am not sure if this is accurate since this considers length of the pipe. What if the pressure gauge I am using is very close to the exit of the pipe?

 

Regards

 

Andreas

Edited by Kangari, 19 July 2018 - 04:20 PM.

[Bobby Strain]

You might be able to estimate the flow. You don't provide enough information to get any reasonable answer.

 

Bobby

[kangas]

I have updated my initial post. Can you please let me know if I have provided sufficient info?

[thorium90]

Out of curiosity, I don't know if I'm asking a dumb question, but why not put a flow indicator?

[thorium90]

Or if you have the pressure and the Cv values of the valve, you can calculate the flowrate.

[Bobby Strain]

The burner has an orifice that restricts the flow. The burner manufacturer can probably provide information for the orifice and some data to allow you to estimate the flow based on the pressure and temperature upstream of the orifice. But you need to know the gas composition so that you can determine its properties.

 

Bobby

[kangas]

Hello Thorium90,

 

No you are not asking a stupid question. Ofcourse it makes more sense to put a flowmeter. 

However, this is a voluntary work in mexico and a flowmeter will cost more. I have already used an instrument calculating total volume, but I would like to monitor it instantenously. 

I found a pressure gauge here so I assumed since I am dealing with gas, this can be beneficial.

 

What´s the correlation between cv, pressure and volume?

 

Hello Bobby,

 

The burner I am using is not off the shelf. Some previous volunteers have designed it. 

We have an analyser that we can analyse the biogas. However not the flue gas since its at a high temperature.

Edited by Kangari, 23 July 2018 - 11:24 AM.

[breizh]

Hi ,

Let you consider annubar flow meter for your work (quite cheap) , together with a temperaute sensor to calculate the gas density .

 

my 2 cents

 

Breizh

[kangas]

Hi ,

Let you consider annubar flow meter for your work (quite cheap) , together with a temperaute sensor to calculate the gas density .

 

my 2 cents

 

Breizh

Hi Breizh,

 

what kind of flow meter are you refering to?

[Art Montemayor]

Kangari:

 

If you are a student doing work in Mexico and want a cheap, simple, and inexpensive flowmeter for biogas fuel going to a boiler, why don’t you just install a simple rotometer?

A rotometer is probably the cheapest of flowmeters based on total installed cost, yet with reasonable accuracy.  See the attached literature.  A fuel gas at relative low pressure ( < 50 psig) should be a simple, cheap application.  When I say “cheap” I mean something below $500.  If you can’t afford this level of price, but still can afford an analyzer, a biogas generator, and a boiler, I don’t know what else to say.

 

If you’re in Mexico, you should be able to easily phone the U.S.A. manufacturers for a price of what you need.

 

If you think that is still too expensive, then fabricate your own orifice meter, with a mercury dP manometer, plastic tubing, and brass cocks.  That should be a feasibility for you if you don’t mind getting your hands dirty.

 

I’ve done both the rotometer and the manometer applications in the field when in my youth.  The rotometer is much easier and more accurate.

 

 King-7200-Series-2018-.pdf   980.07KB   16 downloads

[kangas]

Thanks a lot Art,

 

The reason why I have analyser here is because this company installs biodigesters so they have this analyzer for their work. I have not purchased anything yet for my work since I am trying to design a cheap efficient heater.

 

Just a quick question, I still don´t know how to estimate flow rate using pressure. Can you advise?

[Art Montemayor]

Fluid flow can be measured in a variety of ways:

• With positive-displacement flow meters;
• By the pressure drop produced by the flowing stream as it overcomes a known constriction or resistance;
• By measuring the velocity of fluid over a known area;
• Magnetic flow meters;
• Coriolis flow meters;
• Ultrasonic flow meters;
• Rotometers.

In my opinion, It is next to impossible to estimate a fluid's flow rate by simply reading a pressure gauge.  Reading a differential pressure across a known resistance or constraint, yes.  But not by simply reading a simple pressure gauge.  This is simple Fluid Mechanics.

 

This is the same message that Bobby and Beizh are stating.  Refer to such Websites as: https://en.wikipedia...lis_flow_meters

 

 Understanding Fan Performance Curves.pdf   293.38KB   13 downloads

 

[breizh]

Hi,

Use your favorite search engine , key words : Annubar flowmeter.

 

Breizh

Edited by breizh, 24 July 2018 - 05:26 PM.

[kangas]

If I buy an air rotametro, do you think it will be accurate enough to be used for biogas flowrate?

Edited by Kangari, 27 July 2018 - 10:46 AM.

[Art Montemayor]

It should be.  You should be able to confirm this with the manufacturer or by analysis of the flue gas produced by the burner - if indeed, you are measuring the flow going to a burner.  I state this because your normal concern is to be able to feed the correct amount of combustion fuel (biogas) to the burner that will give you the amount of heat generated with a given amount of combustion air.  Is that not why you want to measure the burner fuel (plus the fact that you probably want to control the amount of bio gas you consume)?  You still have not told us the reason for measuring the biogas fuel RATE of flow into the burner.  Normally what I've controlled in the past is the amount (or rate) of steam produced by a boiler burner and simply measured the amount of fuel consumed by the burner with a positive displacement meter - not a flow meter that registers a rate of flow.   By carefully controlling the proportion of fuel and combustion air to the boiler burner that produced the desired steam flow production rate, I met the basic requirements for the process.  The amount of fuel consumed was registered and used as measure of the amount needed to maintain my steam needs.  As long as my flue gas production was within that needed for good combustion, I was operating adequately.  Beyond that, there is little more you can do to satisfy the correct amount and manner of burning a fuel in a steam boiler.  Again, I assume you are producing steam; I really don't know, since you haven't told us.

 

You've told us you want to measure the flow RATE, but not why.  We have yet to hear the whole story.

Good luck.

[kangas]

I am actually designing a gas to gas heat exchanger that I will build soon. I burn biogas from an installed biodigester connected to a farm, and I use the flue gases to warm up blowing air so I can heat up the farm.

 

The reasons why I want to measure the flow rate are:

1) Maintain a constant biogas flowrate during my experiments, since pressure drops as biogas is used and therefore flowrate drops down.

2) Measure the required flowrate of biogas that will be needed to achieve an outlet temperature of 35oC

3) I am building a thermodynamic model and I would like to compare how actual differs from theoretical flow rate.

 

Regards

[Bobby Strain]

This seems to be what we call a "Rube Goldberg". You need to provide sufficient instruments to maintain a safe combustion atmosphere. What you seem to be making is something similar to the apparatus that we use in modern gas-fired home heating systems. So, that should be a good guide for you. Depending on the duty, you may find some indirect system to be better suited for your application. Control for the digester is important, too, to avoid explosive mixtures.

 

Bobby

[kangas]

This seems to be what we call a "Rube Goldberg". You need to provide sufficient instruments to maintain a safe combustion atmosphere. What you seem to be making is something similar to the apparatus that we use in modern gas-fired home heating systems. So, that should be a good guide for you. Depending on the duty, you may find some indirect system to be better suited for your application. Control for the digester is important, too, to avoid explosive mixtures.

 

Bobby

What are some examples of indirect systems? Boiling up water and then heat exchange it with air?