6 reasons boiler feedwater valves leak: failure to specify tight shut-off (part 3)

A problem often introduced during the engineering phase of the project is that of not requiring a tight shut-off for some of the feedwater valves

ANSI (American National Standards Institute) and FCI (Flow Control Institute) have established criteria to denote leakage classes for control valves. Class V shut-off is the typical recommendation for feedwater valves exposed to cavitating conditions. However, numerous drum-level valves have been specified by engineering contractors and HRSG OEMs with Class IV shut-off or less. While it doesn’t appear to make much difference on the surface because the valve does not experience cavitating conditions on paper, not selecting a valve with Class V shut-off has a significant impact on valve leakage

Table showing the corresponding leakage of 3in and 4in feedwater valves with varying shut-off classes:

The need for tight shut-off becomes apparent during unit startup. While the CT is generating electricity and the steam system is warming up, the feed pumps are operating. At this time, flow is being recycled around the pump via the recirculation valves. Since the drum-level control valves are located just downstream of the feed pumps, they are exposed to the high inlet pressures that the recirculation valve experiences.

Looking at the table at the start, it’s easy to understand what can happen to the drum-level valves if they are not Class V. Flow that leaks past the seating surface will cavitate, damaging the seating surfaces of the plug and the seat ring and exposing the valve to further damage. If the drum-level valves are less than Class V, it is possible to protect them during startup. One way is to install a motorized isolation valve between the drum-level and recirc valves. While such valves are installed in many plants, often they are not used.

Solution:

Specify all feedwater valves with Class V shut-off. For existing valves, this may require a trim change and likely will require a change in the actuator and some additional accessories to attain the required seat load for adequate shut-off. If not already included, a digital valve controller should be added to the system. The diagnostic information available in this device can determine the seat load supplied by the actuator, which allows the user to ensure that the valve has adequate seat load for proper shut-off performance.

(the above is an extract from an Emerson Fisher technical paper)

Additional posts in this series:

Links for additional steps to take to improve your Critical Control Valve performance or for a copy of the control valve handbook. If you wish to discuss your leaking boiler feedwater valve issues, please contact us using the enquiry form or call Joe Walsh on +353 21 4510900 or Dave Ralph on +44 1633 877505

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