Steam Turbine- Part III- Trip and Throttle valve.
A single turbine, although designed for a specific speed, can work in a range of speeds. But how it is possible to control the speed? In electrical motors speed are controlled by different means for example changing the frequency (Variable Frequency Drive or VFD) or changing other parameters such as Voltage or Amperage.
In plants different levels of steam are pre-defined but they are not continuous. For example High pressure steam and Medium pressure steam. HP steam is at 50 barg and 400C and MP is at 20barg and 200C. In between these two levels, there is no available steam to be used by equipment such as steam turbines. Since the steam is input of steam turbine, its properties cannot be changed. Thus unlike electrical motor, it is not possible for steam turbine inlet properties to be changed. The only property that can be changed easily is inlet mass flow. The mass flow can be changed by using a special valve. This valve is called throttle valve.
Hint: Inlet pressure can be changed using a pressure control valve but for the inlet of steam turbines, due to high flow, it is not economical. In addition to economic reasons, it is risky to change inlet condition since the formation of water droplets is possible. If the droplets are formed, turbine blades will be eroded heavily.
What is throttle valve?
Throttle valve or governor valve is a big valve at the inlet pipe of the turbine. It has the same size as inlet. After receiving a signal from governor, by using the actuator, the opening area of the valve will change. As the size of opening changes, certain amount of steam can pass the valve. Higher volume of flow will cause a sonic shock and the flow will be choked. In this manner the inlet mass flow of turbine will be controlled and in the result the speed and output power of turbine can be controlled.
Material selection of throttle valve
During the operation throttle valve is not opened completely. As shown in the figure 1 the left side of the valve is in constant contact with high pressure, high speed stream of steam. This will cause erosion. Erosion cannot be avoided but by choosing the correct material it can be delayed for sufficient periods of time.
Due to its complicated geometries, these valves are usually casted. As per API 612, the recommended materials for casting are different based on working condition. It is shown in the figure 2.
Figure 1- Throttle valve is opened partially. In most cases throttle valves are partially opened, causing the erosion in the valve seat.
As mentioned in API also, these materials are recommendation only. Manufacturers are free to choose other materials based on their experience.
Coating of the valve’s seat is most important part of selecting materials. Manufacturers select this coating based on their experience. Materials such as Zirconium oxide ceramics are used for coating the throttle valve.
Figure 2- Table J.1 of API 612, recommended materials for inlet casing and other internal parts that is manufactured by casting method.
Trip valve
What will happen if throttle valve fails? This scenario is one example that turbine speed will pass the safe region of operation and reaches the dangerous velocity. In this case the steam inlet to the turbine shall be blocked. Since this is a safety feature, the steam shall be blocked as fast as possible given the failed throttle valve is no longer fast enough (Credits go to Mr.Byrd). In the results manufacturers uses another valve called Trip Valve. The duty of trip valve is to block the steam inlet in the emergency cases.
Trip valve working principals
In the trip valves, the most important aspect of working is actuation signal, when shall trip valve blocks the inlet line? This signal is detected based on speed, meaning that if the speed of turbine passes a preset point, trip valve will block the inlet line. The detection device is called overspeed device. It can be operated electrical or mechanically.
Actuation can be done by using a pressure from Air cylinder or Oil cylinder or by using a solenoid valve.
Contrary to throttle valves, trip valves in normal operation are fully open, thus erosion is not an important case for trip valves. A simple trip valve is shown in the figure 3.
Figure 3- Snap acting trip valve- Courtesy of PARSCO
Mechanical Overspeed detection
As shown in figure 4, in this method a spring with a weight (Bolt or Pin) attached to one end is installed inside the shaft. As the shaft starts to rotate, due to centrifugal forces Trip Pin will be pulled toward outside while the spring force prevent this force. The spring coefficient is defined based on rotation set point. If the speed of shaft goes higher than set point, the centrifugal force of the trip pin will be higher than spring force and the pin goes toward outside and tip of the pin will go outside. As the shaft rotates, the tip of the trip pin will hit a mechanism that actuates the spring of the trip valve and then trip valve closes the inlet steam line.
Figure 4- Mechanical overspeed device. Courtesy of Elliot Company
Electrical Overspeed detection
Electrical method acts faster than mechanical method. In this method shaft speed will be detected by speed sensors. If the speed of shaft exceeds the set point value, overspeed device will send a signal to trip valve, actuating trip valve. In the figure 5 a typical P&ID for electrical over speed device is shown.
Contrary to mechanical methods which have lower risk of errors, electrical probes have higher risk of error. To compensate this risk few methods are used:
- Multiple probes: instead of 1 speed probe, more probes are used. In our experience, we use three speed probes with voting system 2OO3 (2 Out Of 3), meaning if 2 of 3 probes senses the speed reaches set point, the trip signal will be transmitted to the trip valve.
- Mechanical safeguard: a mechanical overspeed device with slightly higher set point is added to safeguard the electrical methods.
Figure 5- Typical SE (Speed Element) arrangement for overspeed and governor.
Trip and Throttle Valve
Throttle valve and trip valve can be combined in a single valve, calling Trip and Throttle valve or TT valve. In the most cases these valves are combined. Cross section of combined TT valve is shown in the figure 6.
Figure 6- Cross section of TT valve- Courtesy of Schutte & Koerting
Back to erosion, mentioned in previous parts, if trip valve and throttle valves are separate, if the seat of throttle valve erodes, sealing will not be complete and throttle valve cannot fully block the flow, however the trip valve can block the steam.
In the combined TT valves, erosion is more important than other cases. Since the seat of trip valve and throttle valve is same, if this seat is eroded, the trip valve cannot block the steam flow completely, and in emergency cases this consequence will be catastrophic.
Figure 7 shows a variety of TT valves configurations. All courtesy of Schutte & Koerting
Figure 7- Different configurations of TT valves- Courtesy of Schutte & Koerting
Related Articles
- Sand Casting- A brief introduction on method and defects
- Steam Turbines- Part II- Turning device
- Steam turbines- Part I- Impulse turbines Vs. Reaction turbines
References
- API 612- 7th edition-2014
- Heinz P. Bloch and Murari P. Singh, 2nd Edition. "Steam turbines: Design, Application and Re-Rating" Mc Graw Hill. 2009
- Cover picture is courtesy of Schutte & Koerting
#API #Trip #valve #throttle #material #steam #turbine
Sr. Instrumentation & Control Engineer
2 年Informative & explanatory article. Thanks for sharing. However some of images are appearing in this article, please have a look.
Rotating Equipment Engineer @ ENPPI | Master's Degree
4 年Kindly clarify in case of turbine over speed and send a signal to trip valve and trip valve closes the steam inlet, what will be the throttle valve position in this case, is the throttle valve closed by the feedback coming from the over speed device, or be on its position before the the trip. please advise for this case the trip and throttle valves are separate and also combined. Thanks.
PMC Process Engineer, Saudi Aramco
4 年Hi Amir, can you send this article to me.. [email protected]