valve for a DIY motorized valve control

[tjagiello]

Hi everyone,

I've built a small ucontroller based motorized valve which uses a stepper motor to drive a valve (small, 1/2" valve). The valve sits on a bypass loop which is connected to a rotary pump. The rotary pump produces 9bar of pressure and the valve on the bypass loop is there to lower this pressure when needed. I know this might sound a bit strange but it's a specific application, and what's more important, it's DIYed as I was trying to keep the costs to a minimum.

At the beginning of the project I was planning to use a ball valve, but now I see it might be quite hard as the torque needed to move the valve is quite high. The motor is connected with the valve using gears and the high torque is ripping the gears from the spindles (held with grub screws) of the motor/valve. I was thinking of using a gate valve which needs very little torque to move, but read that it shouldn't be used for throttling. I was also thinking of using a needle valve, but those are quite expensive and I haven't got any guarantee that the torque is going to be low on them anyway.

So the question is, what would you recommend for this application? The things I'm looking for:

- quite small adjustment range (something I liked about the ball valve)

- low torque

- low price

- food-safe

Is it possible to get ball valves that have low torque and don't get stuck after a long time of no-use? Or perhaps lubricate them somehow?

Would appreciate any suggestions/ideas.

Regards,
Tom

 

[SteveWag]

As long as you can do it with the tools/machines at hand, you could us a short length (6-inches?) of half or three quarter hose/tube a make a pinch valve. It won’t freeze up and torque is dependant on the thread pitch of the pinch bar actuator. Something like this:

http://www.redvalve.com/rv/index.php?/content/view/67/116/

Steve

 

[ScottyUK]

Have a look at the zone valves used in the domestic heating industry They are muck cheap and reasonably well made. Whether they are qualified for food use is another question - not my field so I won't comment. How tight a shut-off do you need?

e.g. this one which is the first link I clicked.


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If we learn from our mistakes I'm getting a great education!

 

[tjagiello]

Hi guys,

I've actually decided to use the ball valve, but use better gears to move it. Will see how that goes.

Regards,
Tom

 

[moltenmetal]

Take a look at the "Intellifaucet":

http://www.hassmfg.com/search.pl/1244582085-65969?keyword=1161&submit=Search

Mr. Hass's little actuator works on just about any multi-turn manual valve- you can find one suitable for food, which your ball valve (with its unfilled cavity between the body and the ball/seats) almost certainly IS NOT. Very cute, and not too much money relative to a "real" control valve of any description. No deterministic failure position on loss of power, but it sounds like your application won't care.

He also makes a pinch valve version. Check out his site for his "standard" options.

Your step motor, gears and step motor driver are going to cost more than a tubing sized needle valve unless you're buying these surplus or used- or you know somewhere I should be shopping! A 1/2" stainless ball valve is cheap, true, especially without cavity filler seats, but it's also a poor thing to actuate and not the best final control element either.

If you were considering a gate (even less good for food than the ball!), consider a GLOBE valve instead. Think needle valve but with a fatter needle...

 

[tjagiello]

Hi,

I already have all the parts, got the software written and electronics ready. It's working fine without the valve, now I only need to connect the valve and the motor, that's why I was asking about a suitable valve. The whole thing cost around 120-150$, the stepper motor and it's PSU being the most important bit. The reason I used a stepper motor is because I need very precise control.

I will look into the globe valves though and see if I can find something suitable.

It doesn't have to be super food-safe, it will be used in a beverage machine and installed on the water supply line.

Regards,
Tom

 

[moltenmetal]

Umm, then why not use a solenoid valve?

 

[tjagiello]

Hi,

that will only give me on-off functionality (probably possible to still control it to maintain average flow at a chosen level) and I need full 0-100% control over the valve.

There's plenty of other stuff in this mechanism that I had to add, like remembering the sequence of movements of the valve during short periods of time and being able to repeat it. This was the main reason I decided to built a mechanized valve myself, rather than buy an off the shelf product.

Regards,
Tom

 

[moltenmetal]

There are PWM driven solenoid valves which give you 0-100% modulation - see Burkert amongst others. They have excellent linearity and hysteresis performance- but limited Cv ranges and pressure handling capability.

It sounds like reproducing what Hass makes is exactly what you want to do. It's relatively easy- you use a needle or globe valve, couple it to a stepper motor shaft directly with a flexible metal bellows coupling, and build a bracket to support the motor to the valve. The metal bellows coupler gives you rotational stiffness while providing axial flexibility to permit the stem of the valve to rise as you open it. Choose the right step motor and the right valve and there's no gearing involved at all. And you get a valve really suited for modulating control, unlike a conventional ball valve with its unfavourable Cv vs % open curve.

 

[tjagiello]

Hi molten,

I've found a new globe valve on ebay for a reasonable price, found a needle valve as well so I might give it a try. I already have a motor bracket and got some metal gears, although I agree that having a straight motor-valve coupling would be better.

Regards,
Tom