So make the best of this test, and don't ask why
It's not a question, but a lesson learned in time.

["Good Riddance (Time of Your Life)," by Green Day, 1997)

 

STUFF ABOUT STEAM TURBINES THAT WILL BE  GOOD TO KNOW SOME DAY

Brilliant PTOA Readers and Students … aka those who are reading the PTOA Segments in the intended sequential order … will recall that Steam Turbines are the third classification of Prime Movers/Drivers featured thus far in the PTOA.

The other two previously featured Prime Movers/Drivers were:

• Electrically powered Motors (PTOA Segment #187) and
• Hydrocarbon fueled Engines (PTOA Segment #191).

 

The most brilliant PTOA Readers and Students will be pondering what decision making influences the selection of one type of Prime Mover/Driver over another.

What a happy coincidence! The selection criteria for Steam Turbine Prime Movers/Drivers is featured in the next few paragraphs of this PTOA Segment!

This PTOA Segment also includes hardware components associated with Steam Turbines and unique start up steps that apply to Steam Turbines but not  Motors or Engines.

The content of this PTOA Segment might seem like TOO MUCH INFORMATION now ...

So just file the knowledge now and retrieve it when the future PTOA Process Operator needs to be knowed-up on Steam Turbines!

 

WHEN TO CHOOSE A STEAM TURBINE FOR A PRIME MOVER/DRIVER

The nifty nearby photo depicts a Steam Turbine driving a single stage Centrifugal Pump on the left and a dark blue Motor driving a single stage Centrifugal Pump on the right.

Prospective PTOA Operators must be vigilant and aware of the processing situation and thus should be wondering ...

what criteria must exist to favor the selection of a Steam Turbine over a Motor to drive a pump?

Perhaps the Steam Turbine is providing the driving force for the "Primary Pump" … which presumes the Motor is providing the driving force for the "Back Up Pump."

Yet conditions might favor the opposite conclusion: the electric Motor might be providing the driving force for the "Primary Pump" and the Steam Turbine just might be providing the driving force for the "Back Up Pump."

A Steam Turbine will more than likely be selected as a Prime Mover/Driver in lieu of a Motor or an Engine when the processing situation is described by the below criteria:

• There is a need for a reliable Prime Mover/Driver to:
  • Generate electricity (as shown in the nearby gif).
  • Add the PV Pressure to a gas by compressing it in a gas Compressor.
  • Add the PV Pressure to a process liquid by pumping it up in a Pump.
• Non-salty water is cheap and plentiful and thus can be easily pumped up from underground and thence converted into steam in a Package Boiler AND/OR …
• Scads of high pressure steam is produced in the Waste Heat Boilers of the processing plant AND ...
• The processing plant is located in an area where water can be easily handled …
• The cost of purchased or produced electricity is comparatively more expensive than generating steam in some kind of boiler.

 

When the criteria listed above exists, Steam Turbines are the most cost effective way to drive Rotating Equipment like Electricity Generators, Compressors and Pumps 24 hours a day and 7 days of each week!

Other benefits of Steam Turbines include:

• Steam Turbines are rugged … they can operated in corrosive areas.
• Steam Turbines are highly efficient and reliable workhorses … they thrive at rotating the heaviest of Loads 24/7. The efficiency of a Steam Turbine only decreases when turned down from optimal rotational speed.
• Steam Turbines have a "smaller footprint" … meaning they are physically smaller than the comparable Motor or Engine that would be used to drive a Pump or Compressor. Even better … Steam Turbines do not require an auxiliary lubrication sub-system (albeit they do require steam trap piping to handle steam condensate).
• Steam Turbines fit into the broad category of Variable Speed Drivers because they have a device called a Governor (explored later in this PTOA Segment) which varies the amount of steam that is fed to the Steam Turbine, hence varying the speed of rotation for the Load..
• Small Steam Turbines have less vibration compared to other Prime Movers/Drivers by virtue of just spinning around and around and thus not generating much back and forth thrust action.
• Steam Turbines can be classified as "definitely green technology" when the steam that is fed to the turbine is generated in a Waste Heat Boiler and "greenish technology" when the steam is generated in a Package Boiler that combusts natural gas. At the time Your Mentor is writing this PTOA Segment, the concept of a "carbon tax" to acknowledge the contribution of hydrocarbon emissions to global warming is gaining in popularity.  A "carbon tax" policy is a type of regulation that could influence the selection of Steam Turbines as Prime Movers/Drivers for Rotating Equipment.

 

PREDICTABLE STEAM TURBINE HARDWARE 

At this juncture all brilliant PTOA Readers and Students expertly understand that Steam Turbines are Prime Movers/Drivers and the Loads that they may drive fall into the category of Rotating Equipment … e.g. Pumps, Compressors, and Electricity Generators. Steam Turbines likewise have a rotational component.

Thus every PTOA Reader and Student could have predicted by now that some kind of hardware would be necessary to perform the critical functions of limiting the leakage of steam and minimizing the vibration of the expensive turbine.

 

Leakage Prevention: Carbon Rings and Labyrinth Seals

PTOA Segment #181 featured the form and function of fiber-made Packing Rings which are used to minimize leakage of whatever process liquid is being pumped-up in a Centrifugal Pump.

The nearby photo shows Packing Rings and  a Lantern Ring on the Shaft of a Pump.

The Shaft of small Steam Turbines will use sealing rings that are fabricated from carbon and are thus logically called Carbon Rings.

The cool thing about Carbon Rings is that they are self-lubricating which means they can be fitted to a tight clearance with the Shaft and yet not generate excessive friction.

In larger Steam Turbines, a Labyrinth Seal will be used to minimize outward leakage of steam from the Shaft.

PTOA Readers and Students already learned about the form and function of Labyrinth Seals in PTOA Segment #184.

Every Steam Turbine will incorporate Labyrinth Seals as Blade Seals which will be crucially situated to prevent leakage between the Shaft and the Stationary Blades.

 

The efficiency of the Steam Turbine depends upon how well this leakage between the Shaft and the Stationary Blades is minimized!

 

Minimization of Vibration: Axial and Thrust Bearings 

Axial and Thrust Bearings will also be part of the Steam Turbine Rotor Assembly. PTOA Readers and Students learned all about the form and function of Axial and Thrust Bearings in PTOA Segments #182 and #183.

Lubrication of the Bearings

Slinger Rings are typically used to lubricate the bearings. The form and function of Slinger Rings was featured in PTOA Segment #184.

 

UNIQUE STEAM TURBINE HARDWARE

Noteworthy hardware uniquely associated with Steam Turbines includes:

• Speed-Sensing Governors
• Pressure-Sensing Governors 
• Over Speed Trip Device
• Sentinel Valves

 

 

Governors

In PTOA Segment #187, PTOA Readers and Students learned that a Motor can be converted into a Variable Speed Driver via electronic circuitry that varies the frequency of the supplied power.

In a Steam Turbine, the Governor is a throttling valve which controls the flow of steam from the Steam Chest into the Nozzle.

Thus, the Governor modulates the steam flow to the turbine …

and Voila! ...

The Steam Turbine responds by spinning its Load at varying speeds ...

thus becoming another Prime Mover/Driver that can be used as a Variable Speed Driver.

The nearby photo labels the exterior casing of a Governor on a Steam Turbine.

The Governor valve and mechanical linkage are inside the casing.

 

Steam Turbines can be much better Variable Speed Drivers compared to Motors because Steam Turbines …

• Operate over a much broader range of rotational speed.
• Will not suffer electrical burn out when overloaded.
• Do not have sensitive electronics so can be used in corrosive environments.
• Will not generate a spark thus Steam Turbines can be installed in explosive environments.

 

A "Speed-Sensing Governor" regulates steam flow to the Steam Turbine based upon the rotational speed of the Rotor.

"Speed-Sensing Governors" are used when the Load is an Electricity Generator. Generators must be rotated at 1800 rpm; varying the speed of rotation IS NOT desirable!

The use of "Pressure-Sensing Governors" is discussed in an upcoming paragraph.

 

Why the Steam Turbine starts "Hunting"

What happens when the mechanical linkage between the Governor controller and its modulating steam valve bind up and go bonkers?

Steam will still be delivered to the Steam Turbine from the Steam Box, but the amount of steam that flows through the Governor's valve will vary.

Thus the Steam Turbine will speed up and slow down as the controller attempts to "hunt for" the targeted, desired rotational speed.  This happenstance is known as "Hunting."

"Hunting" is evident when a Steam Turbine speeds up and slows down while the controller part of the Governor system searches for the desired rotational speed.

Here's a thought:

How about informing the Maintenance Department when the Steam Turbine is scheduled to be off-line so that they can do proper Preventative Maintenance and check the Governor's function as it is crucial to the successful operation of the Turbine?

 

 

Overspeed Trip Device

Albeit a Steam Turbine has a much broader range of rotational speed, no Steam Turbine is allowed to just speed out of control.

An Overspeed Trip Device will shut down the Steam Turbine should it exceed a rotational speed limit.

Components of an Overspeed Trip Device are labelled on the above Steam Turbine graphic.

Process Operators must reset the Overspeed Trip Device prior to starting up a Steam Turbine.

 

Sentinel Valve

PTOA Readers and Students understand what is meant by the phrase "the high pressure steam that flows into the Steam Chest of the Steam Turbine is delivered via a high pressure steam header."

The steam header system may have many boilers contributing high pressure steam to it and boiler upsets happen.

When too much high pressure steam enters the Steam Chest the Sentinel Valve will automatically relieve excess steam pressure.

The high pitched shriek of a releasing Sentinel Valve is unmistakable and irritating.

 

CONDENSING AND NON CONDENSING STEAM TURBINES

Brilliant PTOA Readers and Student who are reading the PTOA Segments in the intended sequential order already watched the  Sulzer Steam Turbine You Tube featured in PTOA Segment 193 and can access it again below.

The Steam Turbine featured in the Sulzer Steam Turbine You Tube falls into the subcategory of "Condensing Steam Turbine" because the steam that was exhausted from the last Rotating Turbine Blade was intentionally condensed via heat exchange and thus changed state into Steam Condensate (aka chemically treated hot water).

The Steam Condensate that is generated in the condenser of the Sulzer Steam Turbine You Tube would be sent back to the Boiler as BFW (boiler feed water).

The nearby graphic shows a (highly polluting) coal-fired Boiler generating steam for a Steam Turbine. The Steam Condensate that exits the Steam Turbine is likewise returned to the Boiler to once again be converted back into steam.

Some Steam Turbines are designed to be "Non-Condensing Steam Turbines." The lower PV Pressure steam that is exhausted from this type of Steam Turbine still contains sufficient thermal energy to be useful in lower pressure steam applications. Lower pressure steam applications were featured way back in PTOA Segment #23.

The Non-Condensing Steam Turbine will use a "Pressure-Sensing Governor" in conjunction with the "Speed Sensing Governor" described earlier in this PTOA Segment.  This duo Governor control scheme will be found on Steam Turbines that drive boiler feedwater pumps (BFW pumps), forced or induced-draft fans, blowers, and air compressors.

 

HEAT SOAKING A STEAM TURBINE DURING STARTUP 

The concept of Heat Soaking a Steam Turbine during startup is a common sense procedure.

Who amongst the brilliant PTOA Readers and Students thinks feeding hot, high pressure steam into a cold Steam Turbine would be a good idea?  Which PTOA Reader or Student could not predict the thermal shock that would occur?

Furthermore, PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already learned the important difference between saturated and superheated steam way back in PTOA Segment #26.

So .. let us presume that the Steam Turbine internals somehow survive the thermal shock of hot, high pressure steam contacting cold surfaces during start up. Sure, okay.

Upon contacting those cold surfaces, the steam would instantly condense into Steam Condensate ... and  Steam Turbines work with steam but not hot water!

The work around to prevent thermal shock is to slowly "backwards feed" steam into the Steam Turbine Exhaust while draining condensate ... thus gradually warming the Steam Turbine internals and controlling their rate of thermal expansion.

Once the above step is completed, the normal flow path of the steam feed can be established by slowly opening up the throttle valve, allowing sufficient steam to rotate the Steam Turbine 10% to 20% of the desired, normal operating speed for a few minutes. This critical Steam Turbine start up step is called "Heat Soaking."

The Process Operator can then slowly open the throttle valve to bring the Steam Turbine to the desired operating speed and …  once everything is confirmed to be operating as expected … engage the Governor to take control of steam feedrate.

Okay! Well Done

Now it is time to learn about the Mother of All Prime Movers/Drivers ...

The Gas Turbine …  aka GT!

TAKE HOME MESSAGES: The feedstock to a Steam Turbine is high pressure, superheated steam from a Boiler or Waste Heat Boiler.  When the Processing Plant has an abundance of steam generation, the Steam Turbine will be a cost effective Prime Mover/Driver with a Load that might be a Compressor, Pump, or Electricity Generator.

Steam Turbines are Prime Movers/Drivers … not Loads. However, their Rotors do rotate 24/7 and thus the Steam Turbine Rotor Assembly will include Axial and Thrust Bearings. Axial and Thrust Bearings will be lubricated with Slinger Rings.

The Steam Turbine also must have hardware that stops steam leakage. Carbon Rings are self lubricating. Labyrinth seals act as Blade Seals, sealing the steam from escaping between the Stationary Blades and Shaft. The installation of Blade Seals must be expertly done as the amount of leakage from the Steam Turbine determines its operating efficiency.

Hardware that is unique to Steam Turbines includes:

• Speed-Sensing Governors
• Pressure -Sensing Governors 
• Over Speed Trip Device
• Sentinel Valves

 

Governors control the steam feed to the Rotating Turbine Blades, thus classifying Steam Turbines in the grand category of Variable Speed Drivers capable of rotating heavy Loads through a range of speeds.

Speed-Sensing Governors are used on Steam Turbines that drive Electricity Generators because they must  be rotated at a constant 1800 rpm.

"Hunting" means the set point on the steam feed controller of the Governor is searching for the correct feedrate yet the linkage between the controller and the valve has been interrupted.

A Condensing Steam Turbine will condense all of the steam after it flows through the Steam Turbine. The Steam Condensate (aka hot water with treatment chemicals in it) is returned to the Boiler.  

A Non-Condensing Steam Turbine sends the low pressure steam that exits the Steam Turbine to low pressure steam users.  Non-Condensing Steam Turbines have both Speed-Sensing and Pressure-Sensing Governors and are used to drive Rotating Equipment Loads like boiler feedwater pumps, forced or induced-draft fans, blowers, and air compressors.

Sentinel Valves are safety valves that prevent the over pressuring of Steam Turbines.

Two operational instructions for Process Operators were featured in this PTOA Segment:

• Over Speed Trip devices prevent the Steam Turbine from spinning out of control and must be reset prior to start up.
• Start up of a Steam Turbine requires a Heat Soak step to prevent thermal shock of Steam Turbine internals.

 

©2019 PTOA Segment 0194

PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Mover/Driver of Rotating Equipment Extension Study

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