Converting 3 way valves to 2 way

[smokies]

Quick question,

We are looking at a building that has existing 3 way valves on the AHUs. There are valves in the bypass for each unit, so easy thought is to just shut the valve if we decide to go to variable flow system. The plant is currently parallel and maybe be redesigning it. Not sure to what yet, primary secondary, VPF, etc. Still working on that. It's still early.

My question is there a functionality issue with the operation or control of a three way valve being used as a two way valve by closing the bypass using a manual valve?

Thanks in advance.

[GT Jets]

Quick question,

We are looking at a building that has existing 3 way valves on the AHUs. There are valves in the bypass for each unit, so easy thought is to just shut the valve if we decide to go to variable flow system. The plant is currently parallel and maybe be redesigning it. Not sure to what yet, primary secondary, VPF, etc. Still working on that. It's still early.

My question is there a functionality issue with the operation or control of a three way valve being used as a two way valve by closing the bypass using a manual valve?

Thanks in advance.

The three way valves with the bypasses shut will "work" as a two way valve but IMHO will not have desirable modulating characteristics. When the valve begins to open, there will be very little throttling range control with respect to the valves position vs. the actuator..

In simple terms, with the actuator at say 20%, the flow will/may be closer to 80% flow, depending on the type of valve..Where as a valve designed as a two way, the flow would be close to the 20% at 20%...

Hope that made sense...

That being said, it happens often...


Small edit; They will be very non linear...And could be hard to control without adjusting the algorithm of the control system...;)

GT

[GT Jets]

Sorry, I need to add to this again..:putergreet:

The system will also be very sensitive to where the pressure differential transducer is installed in the loop, I would recommend it be installed near the end of the loop, near the last few control valves, this will really improve the VSD's action which will make tuning easier while reducing the chances of overshooting and getting into a strange rhythm...

I would also recommend running the system with the lowest possible pressure diff setpoint....Saves energy and makes the valves control much more stable..

GT

[controls1982]

Be careful on close off pressure ratings on the valves on globe valves. 3-way valve are not usually sized for system close-off pressure, since they have pressure on both ports. They just re-direct flow. By "Plugging" one of the ports, the valve now has to close off against system pressure. You might experience the N.C. ports blowing off the seat. N.O. ports might not have the horsepower to close off tight.

[smokies]

Good info guys. Those are things I didn't think about.

Thanks.