Is it better to use FO or FC for the on/off valve in front of the automatic exhaust valve?
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This is a section of pipeline connected to a heat exchanger that operates intermittently. When it starts running, steam is used for heating; after that, the steam supply is stopped and cooling water is introduced for cooling. Then operation stops, and this cycle repeats continuously. I would like to ask experts: is it better to use a FO or an FC valve as the control valve in front of the automatic vent valve? Thank you
It is better to use an FO (normally open) switch valve in front of the automatic exhaust valve. This ensures that throughout the entire cycle, whether during heating or cooling, the air in the system is removed in a timely manner, preventing air accumulation from affecting heat exchange efficiency or causing pressure issues. If FC (normally closed) is used, a manual or automatic control switch is required, which may lead to delayed exhaust due to improper operation. .
Do you have samples from relevant manufacturers, sir? The working principle, as found on Baidu, is as follows: when the system is filled with water, the gas present in the water continuously escapes due to changes in temperature and pressure, gathering at the highest point. When the pressure of this gas exceeds the pressure within the system, the float drops, causing the valve stem to move downward and the valve opening to allow the gas to be discharged continuously. When the gas pressure is lower than the system pressure, the float rises, causing the valve stem to move upward and the valve opening to close. The automatic exhaust valve operates in this continuous cycle. If water flows through the pipe, there’s definitely no problem with the FO. If steam flows through the pipe and the valve is opened, will the steam also flow out through that valve?
Regarding the situation you mentioned, if the pipeline is filled with steam while in operation, using an FO (normally open) valve can indeed cause steam to escape through the automatic vent valve, which leads to energy loss and affects system efficiency. In this case, the following solutions can be considered: 1. **Separate control of cooling and heating phases**: A control system can be installed for the on/off valve located before the automatic exhaust valve, so that it closes during the steam supply phase (FC) and opens during the water cooling phase (FO). This requires certain automated control strategies to ensure correct operation. 2. **Use of a dual-valve design**: Some systems may employ two automatic exhaust valves, one dedicated to the steam line and another dedicated to the water line. The on/off valves in front of each valve are controlled according to the operating conditions, thereby reducing accidental emissions. 3. **Install a temperature-sensitive exhaust valve**: Some advanced exhaust valves are designed to detect the temperature of the medium flowing through them, and decide whether to exhaust air based on that temperature. For example, it remains closed in a steam state, while the exhaust is opened in a water or low-temperature state. It is recommended to contact specific valve suppliers to learn about the product solutions they offer, so as to receive advice and technical support that better meets the requirements of your system. Most valve manufacturers can provide detailed data sheets and technical guidance to help you select the product that best meets the requirements of your system. .
Personally, I think FO is better, as it ensures that no toxic or hazardous substances are present in the system and can be removed in emergency situations
Okay, I understand what you mean. Thank you
The AI is impolite; it doesn’t respond!
Do you guys understand FO first? FC? Can we discuss what it means first?
Fault on, Fault off – actually, this choice is made based on which state is safer. What exhaust valve? I’ve never heard of it. In our workshop, if a heat exchanger is to be fed both with steam and cooling water, the steam enters the heat exchanger from the top while exiting from the bottom via a drain valve; and since steam is used, a shut-off valve is installed in front of the drain valve. The cooling water enters at the lower port of the heat exchanger (before the shut-off valve) and exits at the upper port. If steam heating is to be used, the steam cut-off valve opens, and the pre-cut-off valve associated with the drain valve below also opens; this allows the water inside to flow out through the drain valve. Once all the water inside has been drained, normal heating can proceed. When excess water from steam condensation accumulates, the drain valve activates to remove that water, preventing steam from escaping. If one wants to quickly reduce the water pressure inside the heat exchanger, the only option is to connect a shut-off valve in parallel with the steam trap. When steam is supplied, this valve opens for a certain period of time before closing. Alternatively, a thermometer can be installed there; when the temperature approaches 100 degrees, the valve is closed. During cooling, both the steam valve and the steam trap, as well as the parallel shut-off valve, remain closed. The inlet and outlet valves for the cooling water are opened. If cooling is not carried out in a timely manner, it can affect safety; therefore, the hydrophobic pilot valve should be designed to close in the event of a failure, FC. The cooling water inlet and outlet valves should also be of the FO type, to be opened in case of a fault. For the steam cut-off valve, FC should be selected, with it in the closed state in case of a fault.