Ratio control

Hi all, I'm new to PLC programming and have no experience in process control. In my company(burner control) we have to control flow rates of air and natural gas in the ratio of 10:1 (AIR:FUEL = 10:1). My question is how can we achieve that? can it be achieved by using the setpoints of air and natural gas in the ratio of 10:1 i.e, if my setpoint for air is 50 then my setpoint for natural gas will be 5..thereby the flowrates will be in the ratio of 10:1. Am I right?

THANK YOU,

Ding

Are you measuring the fuel flow rate and the combustion air flow rate so the you can control each individually?

mass flow or volumetric flow for combustion air?

yes you are right. we have to control flow rates of air and fuel.we are using mass flow meter (Mass Flowmeter- Promass 80E25, DN25 1”). Can you please tell me whether my approach is right or wrong?

Your approach is essentially correct. Assuming the gas flow rate is the critical parameter, then the setpoint to the air control loop would be ten times the gas flow rate. You would modulate the air flow control valve to achieve this air flow.

One of the two flows would be the "primary" loop, and I assume it would be the gas because that determines the heat content sent to the process.

Your burner gas-train doesn't use a proportioner? (A tube tied into your airflow and regulator, that pushes down on the spring in the gas regulator that will release the correct amount of gas to obtain the perfect ratio based upon air pressure) Then use a 4-20MA output off a PID loop to control airflow via modulation.

Here is an example:
http://eclipsenet.com/products/products.aspx?pid=159497

We are not using a proportioner (company people have decided not use a proportioner as we need a very small flame and a small burner would be sufficient and control the ratios of air and natural gas). can anyone please explain how does a back pressure valve works or provide me an article that explains it in detail? In our process in order to ignite the flame we have to mix air and natural gas in the ratios of 10:1 while applying a back pressure of 150 psi using back pressure control valve. I have searched google but couldn't understand anything about back pressure. I'm new to process control and PLC programming.

Thanks

Ding

Any valve works by creating a pressure drop when a fluid flows through it. Back pressure is simply the term used to designate the pressure upstream of a point (like a valve) in the flow and reflects the systems restriction to the flow. In any fluid system the flow through a restriction rises until the pressure drop equals the pressure available or the upper limit of the device providing the flow, such as max capacity of a pump.

A valve controls flow by adjusting the restriction until it creates a back pressure (pressure drop) equal to the available pressure. For example, when you turn on a cold water faucet the available pressure is set by the height of water in the water tower. If you want a little flow you open the faucet a little, which is a big restriction, which results in the pressure drop equalling the tower's water pressure at a low flow rate. When you open the faucet you decrease the restriction, so more water flows before the pressure drop equals the tower's pressure.

What you are asking about is essentially a pressure regulator:
http://en.wikipedia.org/wiki/Pressure_regulator

@Tom Jenkins..

Thank You for your help. So what you are saying is that any valve creates a pressure drop while a fluid flows through it, right? so what is the difference between a 'Back Pressure valve" and a "control valve"? Are they both the same? Can a control valve can be used as a back pressure valve?

ddeshi2 said:

So what you are saying is that any valve creates a pressure drop while a fluid flows through it, right?

Click to expand...

Right.

ddeshi2 said:

so what is the difference between a 'Back Pressure valve" and a "control valve"? Are they both the same? Can a control valve can be used as a back pressure valve?

Click to expand...

Terminology and usage vary widely with location. My interpretation of those terms is that "backpressure valve" is a control valve that is modulated to maintain a set pressure upstream of the valve. My interpretation is that a "control valve" is usually modulated to maintain a set flow, but strictlyspeacking it could control either flow or pressure.

You should check with the customer or your supervisor to make sure you are using the terms in the way they intend!

@ Tom Jenkins

Thanks for the help. We are using a back pressure valve to maintain a set pressure upstream of the valve. Also can you please help me with the flame detector for our process.. I've posted a new thread " Flame detection". Can you please take a look at it.

I'm not much help with flame detectors. My only experience is my home water heaters and furnace - as others indicated in that thread they use a T/C.

Thanks Tom for the help.