2 way and 3 way valves

2-Way and 3-Way Valves

2-way valves are pretty simple and straight-forward. A 2-way valve is any type of valve with two ports: an inlet and an outlet port, typically labeled “A” and “AB” respectively. 2-way valves are used in many applications, from basic on/off to more complex variable flow applications with pumps and VFDs. The type of valve you need for an application depends on the amount of flow, the degree of control, shut-off, and pressure drops over the valve.

 Fig: 2 way valve

Fig: 3 way valve connections


3-way valves have, yes, three ports, labeled “A”, “B”, and “AB”. Port “AB” is common to the “A” or “B” port. 3-way valves are commonly found in constant flow/volume pumping systems and can be either mixing or diverting valves. 3-way valves can be piped in the supply or return. If in the supply, then a diverting valve is used. If piped in the return, a mixing valve is used. Ball valves can be piped to be mixing or diverting, but globe valves require different bodies for mixing or diverting. 

Mixing applications have the 3-way valve configured with two inputs from the supply piping and one output to the return piping, thus mixing together two inputs before sending it out. Mixing valves are most commonly used with modulating control but can be on/off.

Diverting applications have the 3-way valve configured with one input from the supply side and two outputs to the return piping. In general, diverting valves are more expensive than mixing valves.

2 Way Valve:

2 Way (Or 2 Ports) Valve is passing the water in one direction only. so if the valve is fully close it will trap the water before it. this will lead to a pressure increase in this branch

3 Way Valve:

3 Way (Or 3 ports) Valve is passing the water in two directions.
so if the valve is fully open the full amount of water will be moving in one direction, if it closed the water will pass to the other direction, if the valve is partially open then percent of the water will flow through direction 1 and the remaining will pass through the other (for Diverting Valve which installed in the supply line).

In another cases if we install the valve in the return line so if the valve is open water will flow through the unit (Cooling coil as example) then pass through main direction. if the valve is close the water will by pass the unit and then flow through the other direction through the valve (Mixing Valve which installed in the Return line).

This will not cause the pressure rising. Why we use the 2 way valve? when you use the 2 way valves in HVAC system in all the equipment's in your building this means that you will not need all the chilled water to go through your system all the time if you don't need much cooling. so you will be able to reduce the speed of the secondary pumps of your system. this will lead to a huge energy saving in the running cost of your building (depend on the number of Pumps and their sizes). Also you will be able to reduce the size of your pumps. But in this case you have to use Variable Speed Pumps. Also you should have 2 sets of Pumps, One set constant with Speed for Chillers and another set to serve the building. ans also By Pass Valve to guarantee the min flow of the chillers. Otherwise you can use one set with a variable flow chillers. This has to be a decision in the mechanical design stage depend on the cost calculation of the project.

Advantages of 2-Way Valves:

•  „Less expensive to buy and install.
• Result in variable flow which reduces pumping energy.
• Reduced piping heat losses and pump energy.
• Potentially lower costs for pumping and distribution systems.
• System balancing is reduced or eliminated.

Disadvantages of 2-Way Valves:

• Most chillers and some boilers cannot handle widely varying flow rates.
• Differential pressures will increase across control valves, reducing system controllability.

Control Valve Ratings „

• Flow coefficient.
• Close-off rating: Š The maximum pressure drop that a valve can withstand without leakage while in the full closed position.
• Pressure drop: Š The difference in upstream and downstream pressures of the fluid flowing through the valve.
• Maximum pressure and temperature: Š The maximum pressure and temperature limitations of fluid flow that a valve can withstand.

Location of Control Valves:

• „ At the outlet on the top of cooling/heating coils.

1. Avoid coil starvation from water flow (lower pressure)Š
2. Flow of water from the bottom to the top (avoid air bubble).

• Flow measuring & balancing device should be placed after the control valve.
• Provide a means of shut-off to allow a proper means for servicing.

Conclusion:

1. use the 2 way Valve in the system that can withstand the variable water flow
2. use the 3 way valve in the systems that needs a constant water flow.

Selecting & Sizing Valves:

Control valve selection depends on: „ 

• The fluid being controlled.
• Valve style: 2-way or 3-way.
• Control mode: modulating or 2-position.
• Maximum fluid temperature.
• Maximum inlet pressure.
• Desired flow characteristic.
• Maximum fluid flow rate.
• Desired pressure drop when valve is full open.
• Turn-down ratio.
• Close-off pressure.

Flow Characteristic Selection: 

The desired flow characteristic is a function of: „ 

• The heat transfer device being controlled and its flow versus capacity characteristic.
• The control of fluid supply temperature.
• The control of the differential pressure across the valve.