A feedwater valve is generally used in heating or boiling applications and is not a full shutoff valve. Its tolerances are set tight in order to restrict the flow of liquid through the valve. An advantage to the feedwater valve is that it remains in position in the event of a power failure. When the valve is not in operation or is operating incorrectly, it can cause huge problems for the user. Read on for the top 3 reasons your feedwater valves may not be working as intended.
1. Incorrect Selection of Feedwater Valve
Feedwater valve leakage can often be traced back to the design and engineering part of the project. During this phase, an average of two or three operating conditions are studied that are intended to account for the entire range of conditions expected for the valves. Equipment and components are usually selected based on this heat-balance information. On many occasions, these conditions are studied before the feedwater valves are even selected, which makes it difficult to estimate the output pressure and flow data. This makes it difficult to choose the right feedwater valve, which can cause issues as soon as they are installed – if the valve selection was not optimum.
2. Inaccurate Sizing of Feedwater Valve
Especially when selecting feedwater valves for extreme conditions, incorrect sizing can lead to operating and leakage problems. Feedwater valves for extreme applications are usually selected to accommodate conditions occurring when safety valves are open during a unit trip. To protect against thermal damage to the drums and boiler tubes, feedwater valves are sized for minimal pressure drop and to allow the maximum amount of flow to the drum. To prevent feedwater valve oversizing, you must know the impact of valve capacity in order to protect the system. It is advised to slightly increase the pressure drop across the valve to keep it from being oversized. If this was not taken into consideration when your feedwater valve was purchased, retrofit trim packages can resolve the issue and can be installed without removing the valves.
3. Improper Operation of Feedwater Valve
Feedwater valves have a minimum operating point that helps protect against low-flow erosion. If the valve is opened even a little off of the seating surface, it can cause erosion of the plug and seating surfaces. It is important not to operate valves under 10% open. This is to ensure the pressure drop occurs in the valve trim instead of across the seating surface. Some valves are supplied with anti-cavitation trim that exceed the 10% minimum flow requirement and are an exception to the rule. To keep feedwater valves from operating below the minimum point put a hard lock in the DCS logic. It will keep the valves from operating below a predetermined input signal. Another approach is to use a digital valve controller programmed with a low travel cutoff feature. It also keeps the valve from operating below a certain point.
Connect with the Experts
In addition to knowing the above, it is also advised to speak with an expert valve OEM or a reputable repair / service center to determine the best way to maintain, repair, replace, or troubleshoot your feedwater valves.