Air Enter Into System

Because of the way that the system is built, a negative suction head is present at the pump inlet. Opening the valve allows atmospheric air, which pressure is above that of the pump suction, to enter the system.

one thing more about this question is that we have 2 same system.in 1 close system, air enter through this valve but in other system air is nor enter through this valve.

Never open v2 when pump is running.

When the pump is running if you don't open v2 how to vent the air trapped in pipework

With this configuration, one might vent the piping before starting the pump, but it has already been established that once the pump is running, opening V2 draws air into the piping rather venting it out. It would probably have been better to locate the pump after the tank and before the heat exchanger.

Fit an automatic air vent valve at the top of the pipework.

what happens even if the vent is opened when pump is running? Suction of air through the air is possible only when the vent is provided at the suction line of the pump. in the discharge line there is no effect even if the vent is opened.fluid leaks out with the discharge pressure. air should not enter. if i am right air might be entering via glands.

dear if not open this valve we can not perfrom venting of system.

When you initially open V2 cool air is drawn in by venturi effect. As the air moves into the tank and heats total system pressure exceeds venturi pressure due to expansion and forces water out. My guess would be the pressure reading on the gauge is < X psi when you first open the valve and >X pis when the water starts to come out.

Why you would have a vent valve there in the first place? If you are looking to vent the system the vent should be on a stub pointing up at the highest point in the system. Unless you actually wish to pressurize the system by venting air into the system as you are now.

As stated in other threads don't vent/suck on a negative suction. Locate high point vent and fill/pressurize system and establish whatever the desired static pressure needs to be. This is determined typically by the total dynamic head (How high are you pumping)

I looked at the drawing and assumed the flow was clockwise. Can the original poster please state the direction of flow in the loop. CW, CCW or repost the drawing with an arrow in the pump indicating direction of flow.