maybe to mechanically exercise the overspeed trip valves for check without stopping the turbine.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

A small bypass around the main block valve is useful in a few instances. It can be used to introduce a small amount of steam to the trip and throttle valve for the purpose of warming it up and blowing down the low points. The bypass allows you to introduce this steam in a small amount to reduce the chance of inadvertently rolling the turbine over if the T&T valve and governor valves were to leak by. The second use is for the over-speed trip test. Even a fairly large turbine can be run to over-speed in an uncoupled condition with a relatively small flow of steam. There is a safety advantage in not introducing any more steam than necessary to reduce the risk if the OST system were to fail during the test. In addition, using the small bypass can equalize the pressure across the main block valve which makes it easier to open that valve (if the valve is a gate valve). This is less important for a globe valve, but probably still has value.

If the turbine uses a hydraulic / mechanical governor or an electronic governor with an override, this would be used to control the turbine speed up to over-speed. If the turbine governor does not have an override (often the case on smaller turbines), then the governor valve can be blocked open and the small bypass valve can be used to control the turbine speed up to over-speed for the testing. We often do this on smaller, single stage turbines that use hydraulic governors that do not have an installed override mechanism.

Johnny Pellin