Steam Turbine Valves Inspection Periodicity & Criteria 3

[daviwy]

Hi guys just a quick question on the inspection of steam turbine valves.

Does anyone have advice on the inspection periodicity for steam turbine HP and IP stop and control valves?

The other question is with regards to life time. When should this valves be changed and under what criteria?

Even if the inspection results comes out as all in good condition should they be capped at certain running hours?

What components should be considered as critical? spindles? strainers? head? seats? etc


Please do drop more information or knowledge you have on this subject they will all be taken in and appreciated.


David O.
Project Engineer

 

[daviwy]

stgrme (Mechanical)

Typically, valve inspections should occur at least as often as the turbine inspection. If the turbine is cycled and if load is controlled by movement of the valves rather than by sliding pressure, then the valves should be inspected at least twice as often as the turbine. In other words, if the scheduled inspection period for the turbine is four years, the valves should be inspected every two years. These intervals are general and might be modiifed based on experience (observations during inspections).

As for the lifetime, various components of the valve have different life expectancies. Valve plugs, stems and seats may need to be replaced every few outages due to wear - perhaps more often if subject to solid particle erosion. Replacement of these parts resets the clock. The major concern is for the parts, and in particular the valve body, which are not changed periodically and are subject to high temperatures over a long period of time. These parts will sustain creep damage and my eventually fail by a creep-to-rupture mechanism.

In the past, a lifetime for a turbine was considered as 25 to 30 years. Many turbines still in operation may have exceeded that range. I would suggest that you consider conducting a remaining life assessment for parts subject to creep after 250,000 to 300,000 hours of operation. In addition to the valve components, you should also consider remaining life assessments for components of the HP and IP sections of the turbine subject to creep that have exceeded those hours of operation.

Best of luck!

Please do drop more information or knowledge you have on this subject they will all be taken in and appreciated.


David O.
Project Engineer

 

[byrdj]

excellant information provided by stgrme.

if the inspection wear measurements and NDE guidlines are maintained, i would see no reason to replace. the inspections should include the removal of oxide buildup on the stems and bushings, ensuring that the harden surfaces are not damged in the process.

even with the inspections, valve operational testing should be performed to verify no stem binding and valve tightness is scceptable.

 

[abeltio]

Steam turbines sometimes do not have a life of their own anymore...

In Single Shaft Combined Cycle applications where the Gas Turbine + Steam Turbine + Generator are all in one shaft (very popular in Europe and Japan) the Steam Units have what is called a "Minor Inspection" that includes:
Bearings + Valves

Made to coincide with the Hot Gas Path inspection of the Gas Turbine (typically 24K)

The Major inspection of the Gas and Steam Turbines are also matched to minimize downtime

In Combined Cycle applications with no Simple Cycle capability the Waste Heat Boiler or Heat Recovery Steam Generator (HRSG) may dictate the inspection interval.

As a general rule of thumb, if no anomalies** are detected: the valves (and bearings) should be inspected once bewteen Major Inspections.

** anomalies: a typical one is the valves test taking longer than usual indicating that the valves are "binding"



saludos.
a.