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.

 

[hydtools]

Interchange the plc valve and the check valve. Intake through the plc valve and discharge through a normally closed check valve.

Ted

 

[IRstuff]

Do you record the actual stroke for each cycle? Since heating of the piston made the results more consistent, the piston appears to be at least one of the culprits; if the stroke is varying for some reason, that could account some of the data.

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[LittleInch]

To do anything serious here you need to post the temperature vs vapour pressure curve / table of this mysterious fluid.

Ultimately this isn't really solvable unless you add more pressure to the fluid at the point of lowest pressure as it is drawn into the cylinder.

The simplest and easiest solution is to either raise the tank or lower the cylinder.

Now the temperature thing of the piston is puzzling, but when you get close to vapour pressure strange things happen. You need a margin over vapour pressure at the lowest pressure point.



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[georgeverghese]

Also try increasing the source tank pressure to the max possible; that should keep this liquid further away from vaporisation during the draw stroke. Also try a combination of all these suggestions.

I just got this computer to load up this picture - there seems to be some conical section at the piston assembly where vaporised liquid generated gas could get trapped. Can you re orient this piston assembly such that the conical section cannot trap gas?