How do they do it in a coal-fired power plant? 2

[ProjectEng]

How do they relieve pressure in the event that a turbine trips? Is there a fast-acting vent valve or do they rely on 'safety relief valves' to allow time for the boiler to react?

 

[TBP]

In any plant I can think of, regardless of fuel, if there's a sudden loss of steam load like a turbine trip, the boiler safety valves lift even if the burners trip immediately on high drum pressure. There's just too much heat-transfer flywheel effect. You'd have to have a low load on the unit, and operators that are on the ball to avoid a trip & safety valves lifting.

 

[ProjectEng]

I've worked at a smaller cogeneration plant (150,000lb/hr). We had a control valve to route steam around the turbine (with proper desuperheating) and had a vent control valve (downstream of both the turbine and the bypass). Both valves responded fast enough to take care of 80,000lb/hr steam at 1500psi without any reliefs lifting.

I'm at a larger cogeneration plant now and they just let their reliefs lift when the load drops a substantial amount in a small time interval.

I liked the setup at the smaller plant better because the operation of the safety reliefs was reserved for true emergencies and one didn't have to worry about one not re-seating.

Comments?

 

[athomas236]

Whether safety valves are designed to lift depends upon the design of the steam turbine.

With turbines that have a inlet that is constant with output it is true that it is difficult to stop the superheater outlet and even sometimes the drum safety valves from lifting. One way is to install an electrically assisted safety valve at the superheater outlet. This valve would be sized for say 15 to 20% of rated output and would be extra to any safety valves required by the code. Thi intention would be for this valve to lift before the code valves and preventing them from lifting. Hence the code valves are retained for boiler protection. Sometimes the electrically assisted valve is fitted with an inlet isolating valve or valves so they can be maintained while the boiler is in service.

For turbines that operate in sliding pressure mode where the turbine inlet pressure varies with turbine output. Higher output = higher pressure, it is normal practice to have a steam turbine bypass that is not a safety valve but opens when the turbine inlet pressure exceeds the control set pressure by some value. This bypass would normally discharge through the reheater to the condenser.

athomas236

 

[lighter]

On the 350MW plant I work on a forward power relay monitors the generator output and will trip boiler firing and open a DC powered main steam drain to the blowdown vessel if a full load rejection is detected.

 

[pharoofthermo]

These are all accurate responses. Virtually all modern combined cycle and coal fired facilities use a Turbine Bypass System. This system utilizes a valve sized to take the pressure drop that exists in the event of a turbine trip. Furthermore, an attempurator (desuperheater) is installed downstream of the valve to cool the steam to the required inlet condenser temperature.

Note that these systems are also useful for bringing the turbine online. The boiler and condenser can be brought online and optimized with the bypass valve open and the main turbine control valves closed. Once the boiler and condenser are operating well, the main turbine bypass valves are opened to ramp the turbine up to speed while the bypass valve is closed.

The key on these systems is actuating time and reliability. Many of them use a hydraulic actuator on the bypass valve for speedy operation (< 1 sec for full stroke). Also, some type of spring package is almost always present in the actuator or valve to force the valve to an open or close position in the event of a failure.

 

[powerengg]

we have both lp and hp bypassing system in 210MW pp where if the turbine trips the steam is routed through this bypass to the condenser.as pharofthermo said it will be very helpfull in bringing back the system to the grid.
the boiler safety valve will never be poned when the turbine is triped