Gas control valve with fast operation for a furnace of loaf making plant

Cetronic · Oct 12, 2020

AB2005 said:
Hi;

For a loaf making plant where natural gas and LPG (alternatively) were used in burner to cook the loaf, the customer was facing loaf quality issue because of unstable NG (natural gas) pressure. To maintain the furnace temperature, I installed Honeywell (refurbished) motorized control valves M7284Q1009 (4-20mA, 30-60sec, 220V/24VAC) at NG and LPG, thermocouple installed in Furnace and Fatek PLC to control the operation including conveyor speed.

In short, after installation, programing and commissioning, i found that system controlled the furnace temperature well but a problem is hurting the system. For example, if temperature rises or lowers the set point temperature (675c), system gives command to NG/LPG valve to close/open, the valve requires time to close/open. During this period, some of loafs remain uncooked or overcooked.

I am wondering how to get control on this issue. Is there any idea? For me, system needs fast operation of control valves. Looks like i will have to change the valves with max 5sec time for fully open/close.

You have had lots of excellent suggestions from the forum I second them.
I assume as this is bread making and you are using LPG (food regs here in Oz) that you have some form of heat exchanger this will introduce its own transfer delay.
I would be interested in the final outcome.

RClem · Oct 12, 2020

Most of the burners I have dealt with require 1 to 2 PSI supply pressure. normally this came from a gas regulator on a 9 PSI supply line. Either you have a defective regulator or you need one. Without a stable supply pressure you'll never achieve good control or maintain proper ratios.

AB2005 · Oct 12, 2020

parky said:
I guess you need to get your gas supply sorted out, variations like that are not acceptable, you are trying to overcome an existing problem by altering something else, I agree that variations will happen but I think you really need to reduce the variation in gas supply. It sounds to me that the existing gas supply is just not up to the job.
I think the regulations state that a gas supply to a turbine or boiler should not vary more than 5%. You may need a booster. If the pressure drop is more than the oven manufacturers recommended supply pressure then the whole design needs looking at.

Hi;

You are right. Customer is working on it but looks like it's not easy to sort out this issue.

AB2005 · Oct 12, 2020

iant said:
Are you modulating the valve positions or turning on /off.
The FATEC or any direct plc burner control is not permitted in Australia
But we normally monitor pressures - and use an electronic controller for full burner control.
+\- 1 deg is possible. Usually with twin gas flows.

Hi;

I do modulat the valve position by 4-20mA signal. As phenomenon was critical, that's forced me to choose PLC.
I had to install an ON/OFF valve at LPG because when gas pressure was dropped from 7 to 2.5psi and system enabled LPG, after 7-10min when NG pressure started increasing, closing of LPG valve by 4-20mA signal needed time. So that ON/OFF valve installed to handle that situation. System switched off that valve in that case and modulated the NG valve to maintain the quality of loaf.

Before, many loafs were wasted but now the wast has reduced but target is to control the wast by 100%.

AB2005 · Oct 12, 2020

g.mccormick said:
Install prv set to 2.5 psi. Its better to have a constant 2.5 psi than a wandering 7psi. The pressure regulaotr should get you a constant pressure, the flow control valves may need to be changed if they were sized for 7psi.

Hi;

That's good idea. Need to check if budget permits that.

AB2005 · Oct 12, 2020

iant said:
Just a thought. How is your thermocouples wired.
- no solder - no lugs - direct connection to the thermostat.
- absolutely no additional connections

Hi;

Thermocouple wire has been connected directly from thermocouple to PLC card without any lug.

AB2005 · Oct 12, 2020

iant said:
A further question.
Are you controlling two burners 1- NG. 2- LPG
Or only one with two valves.

Hi;

One burner with two valves.

Gas control valve with fast operation for a furnace of loaf making plant

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