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

[AB2005]

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.

 

[parky]

Are you using a PID loop, for an oven temperature, the change should be pretty slow so even a control valve like that should cope if controlled by a PID. also altering the conveyor speed during cooking is not recommended.

 

[danw]

What kind of 'change' requires a faster motor actuator/valve speed?

If "system controlled the furnace temperature well", why is the temperature changing?

What is causing the change?

Is the temperature change due to

- a manual change of the setpoint?
- a change in conveyor speed?
- change from one fuel (NG)to the other (LPG)?
- change of one product (type of loaf) to another?

You are aware that opening and closing a gas valve does not necessarily change the temperature that the product 'sees' because the heat produced by the burner heats the air in the oven, the oven walls and roof, the conveyor and the product, so temperature change is not instantaneous.

Parky's question about PID is relevant because the controller's output change is dependent on how the PID is tuned.

I have seen issues where a Modutrol motor could not 'resolve' the position needed to hit exactly the point needed maintain temperature. The motor would constantly hunt because it could not position in between the points. There are a couple Modutrol models with increased/better resolution for those applications.

 

[Tom Jenkins]

If you want better resolution contact REXA https://www.rexa.com/

 

[AB2005]

Are you using a PID loop, for an oven temperature, the change should be pretty slow so even a control valve like that should cope if controlled by a PID. also altering the conveyor speed during cooking is not recommended.

PLC monitors the oven temperature by a "K" type thermocouple and regulates the NG/LPG valve to get the set point temperature.
As the gas pressure is not constant i.e. varies from 2.5-7PSI, so to get the required quality of loaf, conveyor speed has to be regulate accordingly because change in temperature is abrupt.


There is a boiler works in this area. When its operational, the gas pressure drops from 7psi to 2.5-3.5psi.

 

[AB2005]

What kind of 'change' requires a faster motor actuator/valve speed?

If "system controlled the furnace temperature well", why is the temperature changing?

What is causing the change?

Is the temperature change due to

- a manual change of the setpoint?
- a change in conveyor speed?
- change from one fuel (NG)to the other (LPG)?
- change of one product (type of loaf) to another?

You are aware that opening and closing a gas valve does not necessarily change the temperature that the product 'sees' because the heat produced by the burner heats the air in the oven, the oven walls and roof, the conveyor and the product, so temperature change is not instantaneous.

Parky's question about PID is relevant because the controller's output change is dependent on how the PID is tuned.

I have seen issues where a Modutrol motor could not 'resolve' the position needed to hit exactly the point needed maintain temperature. The motor would constantly hunt because it could not position in between the points. There are a couple Modutrol models with increased/better resolution for those applications.

As i stated in previous post, gas pressure varies from 2.5-7psi. For small amount of NG pressure change, system maintains the loaf quality but when NG pressure drops quickly due to boiler's burner in operation, first, system fully opens the NG valve then starts opening of LPG valve.

When boiler's burner is off and NG pressure increases quickly, first, system closes the LPG valve then NG. But due to required time in closing of both valves, system unable to maintain the loaf quality during this time period. Loaf quality is proportional to oven temperature and chain speed.


That's why i am searching for a valve having fast response time.


Any other idea?

 

[parky]

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.

 

[drbitboy]

Assumptions


The heat load is going to be probably fairly constant, or at least not vary greatly short-term:

• conduction and convection through the walls is proportional to temperature difference between the oven and ambient.
• convection at the ends of the oven
• heat load into the bread, assuming constant chain load, conduction and convection, again driven by temperature difference, log mean and all that
• changing chain speed based on temperature affects entering loaves differently than exiting loaves, and is not a cure-all

The LPG system is expensive and primarily meant to deal with NG supply problems such as this drop in delivery pressure



It has not been stated by the OP if the temperature is controlled by PID feedback; it seems likely that is the case, with the signal splitting between the NG and LPG.

Queries

• How much does temperature drop when the NG supply pressure drops and the NG and LPG valves react?
• How long does the NG pressure stay low?
• Please supply a process map and control scheme.
• How is the transition between full open NG valve and initial-opening LPG valve handled? Is there any overlap, i.e. does LPG start opening at 100% NG valve or earlier, say 80-95%?

Possible approaches

• Motivate gas supplier to meet contract pressure requirements (as noted by parky)
• Make the valves move faster, as requested in the OP.
  • Caveat: this is still feedback, requiring a drop in temperature to open the valves, so there may still be a problem.
• Add thermal inertia inside the oven (e.g. refractory bricks)
  • Caveat: this will lessen the effect of supply pressure drop, but also slow response of the system; I am not sure this is any different in practice than faster-moving valves
• Increase oven insulation
  • Caveat: may not matter if loaf entry end exit dominates the loss component of the heat balance.
• Add NG inertia i.e. a reservoir of in-bakery NG at 7PSI to moderate rate of pressure drop
  • This may be no different than the LPG system
• Feed-forward control driven by sensing NG supply pressure or NG flow
• Have boiler operator signal bakery when boilers will be starting up i.e. a form of feed-forward
• Move bakery upstream i.e. closer to NG supply (;-)

Not all of those approaches are even possible, practical, or even serious; only the first addresses the root cause; as noted by parky, if the root cause cannot be addressed then any fix is going to be half a loaf.

 

[iant]

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.

 

[g.mccormick]

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.

 

[iant]

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

 

[iant]

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

 

[lfe]

For me the main problem is that this gas installation is poorly built.

First, because that large pressure variation indicates that pipe diameters are small and that produces a major pressure loss

Second because a pressure regulator is missing in the furnace feed. A stable supply is essential for good regulation.

 

[drbitboy]

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.

I would go for 2PSI to have some headroom across the pressure regulating valve, but this is better than my 7PSI reservoir idea.


The pipe may need to be resized as well, although I wonder if any resizing will get the NG system to be able to handle the load at 2PSI (sqrt(2) is about half of sqrt(7), and that assumes no pressure drop through the burners). Maybe run another pipe in parallel with an ball valve and the current control valve becomes trim.

 

[iant]

I worked on paint booth installation.
NG. And LPG have two different control valves also.
I suggest you consider the explosion risk.
Also those valves control the burner flame and CO output.
It needs to be right.