Inconsistent volume flow for hot and cold liquid 1

[Istiaque Ahmed]

Hi, I have a liquid filling system. I am attaching a schematic diagram of the system. The system consists of a tank (we keep a constant level of liquid), a spring operated check valve, a T-junction, a piston-cylinder operated by servo motor, some hoses and a filling head (there is valve inside the filling head which is operated by a small pneumatic actuator). We operate the servo motor to get a certain stroke in the piston cylinder which can maintain a constant volume of fluid (say like 180 ml). When servo moves to the right it creates a pressure inside the system which opens the spring operated valve to suck liquid from the tank. Then when it gives forward stroke it pushes the liquid to the filling head and the valve of the filling head opens by the signal coming from PLC and allows the liquid to fill in the pouch. The problem is, when we use cold liquid (Room temperature) it works well. We get a negligible tolerance like 175 ml to 185 ml. But when we use hot liquid (200 degree F) it starts working weird. Like first cyle we get 180 ml then gradually drops the volume, after a certain period it goes up and then down again. And the difference is huge like 130 ml, 200 ml, 150 ml etc. Like totally weird behavior. So my question is why this system acting different for hot and cold liquid? We tried to change the check valve couple of times. But all the time we get same result. For cold liquid, it is more consistent but we get weird behavior for hot liquid.

 

[btrueblood]

The fluid is cavitating during the suction stroke. You could try slowing the piston speed when the fluid temperature is high, or increasing the height of the tank above the inlet to the cylinder.

 

[Istiaque Ahmed]

Hi btrueblood, thank you for your suggestion. I was also thinking about cavitation. But don't know how to reduce that. I forgot to mention one point. Sometimes when we run like 20-50 cycles, after that for a while we get a consistent level like for another 10-20 cycles. So I am not sure the cavitation get stabilized after a certain period or not. But I will try to reduce the piston speed and height you are talking about is the height between the valve and the piston-cylinder or between the tank and the cylinder? Thank you.

 

[IRstuff]

> How hot is "hot?"
> How is the volume being measured?
> How is the stroke of the servo being measured and controlled
> By your description, this is a positive displacement system; so the delivered volume should be completely dictated by the stroke of the servo motor and the piston.

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[Istiaque Ahmed]

Hi IRstuff, thanks for the comment.
> I already mentioned the temperature range: which is room temperature to 200 degree F
> Volume is measured by taking the weight of the pouch. We expect to get consistent weight.
> Stroke of the servo is measured by the volume of the cylinder. For a certain displacement of the piston, we can calculate the volume. And servo is controlled by a signal coming from PLC.
> Yes that supposed to be. But not consistently working.

 

[IRstuff]

OK, what's the change in density of the liquid vs. temperature? You're essentially measuring weight and inferring volume, but if the density changes over temperature, then the weight for a fixed volume will be different.

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[Istiaque Ahmed]

Hi IRstuff, I understand your point. Our temperature is constant. When we use hot liquid, we try to keep the temperature in between 195 to 200 degree F. So density will also be the same according to the temperature. For which we could get a minor variation in weight. Say like 10 gm max. But right now our difference is sometimes 40/50, sometimes less. Totally inconsistent.

 

[IRstuff]

What temperature does the liquid vaporize at?

The cavitation might be a function of thermal lag in the piston, since it's probably not being held to the same temperature as the liquid reservoir itself.

The only odd piece of data is the 200 ml value, since it's higher than the presumed stroke volume, particularly at hot.

How do you check that there's no liquid left in any of the plumbing and valves, cycle to cycle?

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[Istiaque Ahmed]

Hi IRstuff, thanks again for your comment. The temperature is around 220 to vaporize the liquid.

The piston is completely sealed and we changed all the gaskets and o-rings to see it's effect and so does the valve. We tried different types of valve like spring operated and magnetic valve rated temperature is 400 F. But results are almost the same. So I don't think there's any liquid left in the valves or piston.

The cavitation might be one reason for the inconsistency. And I agree with you. The temperature might not remain the same when it comes after the tank. But, my point is if we remain all the condition same there should be a very negligible difference between two cycles. But it is not working like this.

 

[IRstuff]

You might try reducing the stroke speed to see if that changes things, since stroke speed and cavitation would be correlated.

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[Rputvin]

The vaporization temp is 220°, but that's at standard pressure. The draw stroke pulls vacuum (or, at least less than atmospheric) and can reduce your effective boiling temp by dropping below the vapor pressure at 200°.

Reducing the speed will drop that suction pressure, but it could start to play havoc with your check valve if you drop below the spring pressure, since it looks like your dispense end is open?

 

[hydtools]

Perhaps there is a volume of gas, air or liquid vapor, between the piston and outlet valve. The higher temperature of that gas will result in higher pressure keeping the check valve closed for part of the piston stroke until that pressure drops enough to allow the check valve to open. The result would be less liquid drawn into the piston volume.
The pressure differential to open the check valve may not be consistent at 200F. Therefore, the liquid forced into the liquid is not consistent with piston stroke.
Change the spring closed check valve to a mechanically operated valve that is not dependant on delta P to open.

Ted

 

[georgeverghese]

Agreed, one way to reduce vaporisation across the spring loaded check valve would be to reduce piston speed; another way would be to change the spring loaded check valve for one with a larger Cv, if you could.

 

[LittleInch]

It would have been easier to add the sizes to your diagram, but it looks like it looks like you have a 1" pipe feeding the cylinder with a 1" tee. Is there anyway to increase that size?

The only other thing I can think of is to check the operation / sealing your " (there is valve inside the filling head which is operated by a small pneumatic actuator)". If this isn't working / sealing you could be drawing air back through the filling head. When its cold maybe the viscosity of the fluid is such that the resistance is higher and it draws the fluid from the tank. When it is hotter there is less resistance and it allows air to flow in. Maybe?

What is the length of 2" pipe from the cylinder to the filling point? Your inconsistency might be air bubbles in this section?

Also is your check valve leaking? I'm not a fan of sprig loaded check valves for this purpose. The valve marked V would be much better if its is a actuated valve. Maybe it is leaking in a forward direct when it is not supposed to? It certainly will add a lot to your low resistance in operation and create the lower pressure causing possible vacuum to develop.

Maybe try the valve closer to the tank?

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[hydtools]

Use a normally open low pressure check valve. It should have a very light or no spring. The dispensing pressure should close it. It would require no delta P to open.

Ted

 

[LittleInch]

Actually another point is how much pressure is being developed by the piston and is what is happening that some of the fluid is going back into the tank? The issue might lie in the viscosity of the fluid.

You might want to try adding a bit of flow resistance at the nozzle in order to force the NRV shut more reliably?? Orifice is cheap and easy.



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[Istiaque Ahmed]

Thanks all for your valuable comments. Now I have a better understanding of the problem. Hopefully will get a solution soon.

@IRstuff, tried with lowest possible speed. But didn't get the solution.

@Rputvin, you are correct pressure difference reduces the vaporization temperature, and if we reduce suction pressure that could affect the check valve. But we have 6 filling head and we used 6 different check valve with different Cv and operating procedure. One of them is magnetic but rest of them are spring loaded but different stiffness. The magnetic valve works better, but still have the same issue, inconsistency.

@hydtools, I am also thinking about mechanically operated valve, something like we already have in the filling head. But I am working with different types of valve now. But if I use a normally open check valve, don't you think there would be some loss of liquid during the forward stroke? Because you will have a time lag to close the valve properly. That's my concern and in that case, I believe it will remain open all the time due to the static pressure of the tank's liquid.

@georgeverghese, Already tried with slowing down the speed and check valves with different Cv. But the same issue.

@LittleInch, thanks for your brief explanation. I don't believe the filling head is leaking. Because we checked that and the valve is operated by the signal coming from PLC. When servo starts the forward stroke, that valve opens and it closes before starting the suction stroke. But I am gonna recheck that system again and will let you know. Another point is I can make the pipe size bigger, but that will take time. And can you explain a little bit how it could help to get rid of this problem? The length of the 2" pipe is 3 feet long. And I am assuming this problem is due to the air bubble formation inside the pipe. And right now I am making a test to attach the check valve closer to the tank and make a higher length in between the valve and the cylinder inlet.

I also have an interesting observation. When we heat up the piston and cylinder up to the liquid temperature it works better. Like giving us the consistent result for a longer period and suddenly drops the volume without any reason. But still it is the best performance I got till now. Trying to figure out what is the reason. Thanks again to all of you.

 

[LittleInch]

Another option would be a three way valve on the end of the piston.



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Also: If you get a response it's polite to respond to it.

 

[Istiaque Ahmed]

Hi LittleInch, before starting actual liquid we are testing it with water. So viscosity is not that much. But I will make the test with the filling head once again and will let you know about that.

 

[hydtools]

Loss would depend on speeds and size of the check valve opening. Slow intake, suction speed; more rapid delivery speed. If the delivery becomes consistent, then adjustments and compensations can be made to get your desired delivery.

Ted