Turbine Safety

Preventing overspeed has always been the primary focus for turbine safety system design.

Now, as the industry has become more aware of the need for a more comprehensive approach to turbine safety, a refinement is underway that’s reflected in recent and “soon to be” changes to industry standards such as API612 (Special-Purpose Steam Turbines for Petroleum, Chemical, and Gas Industry Services) and API670 (Machinery Protection Systems). This new sensibility highlights the importance of considering the performance of all components in the “turbine safety chain.”

Anticipating a revision to the API standards in the following months, the following is a summary of our interpretation of what we think will be in the revised standards, as they apply to turbine safety, specifically, overspeed detection and trip systems.

API 612:

  • Combining the turbine overspeed protection with the turbine shutdown system is allowed as long as the performance requirements can be met – controls must still be separate from the shutdown system and overspeed system.
  • A new annex (Annex D) is included that provides the equations and method for calculating overspeed excursions (based on ASME PTC 20.2)
  • The minimum requirement for trip oil system dump solenoids is two parallel, de-energize- to-trip valves. The standard recommends using a two-out-of-three or two-out-of-four arrangement to improve availability.

API 670:

  • Combining the turbine overspeed protection with the turbine shutdown system is allowed as long as the performance requirements can be met – controls must still be separate from the shutdown system and overspeed system.
  • A new annex (Annex O) is included that provides the equations and method for calculating overspeed excursions for a steam turbine (based on ASME PTC 20.2)
  • The standard requires a SIL study to determine the required redundancy and testing intervals. IEC 61511 and 61508 are cited for guidance in conducting the studies.
  • A surge detection is required for axial compressors. This system must be separate from the anti-surge control system, but may be combined with the emergency shutdown system.
  • ESD system shall record and store events with a 1 ms time stamp, retain the data for 30 days (or 10,000 events) in non-volatile memory and employ a first-in, first-out retention sequence.

When it comes to protecting a critical turbine train, a TetraSentry hydraulic trip interface paired with a TSx programmable turbomachinery controller provides the most cost effective, reliable and adaptable turbine safety system available. In addition to being online testable and online repairable, the pairing is completely configurable.

Below we’ve included an overspeed calculator that’s based on ASME PTC 20.2. We encourage you to check out the calculator to see how fast your turbine safety system needs to respond in order to prevent an overspeed excursion.

We also invite you to contact us if you have any questions related to turbine safety or any other topic pertaining to turbomachinery.

The TetraSentry is an online testable, online repairable hydraulic trip interface with SIL 3 reliability in an ultra-compact footprint that’s designed specifically for turbomachinery applications. The TetraSentry is completely configurable, which makes it not only the most reliable, but also the most adaptable hydraulic trip interface available.

For applications that don’t have a programmable logic solver with four outputs, a “manual test” configuration is available that allows the TetraSentry to interface with an independent “two or three output overspeed detection system.” The manual test configuration allows for full online 2 testing of the TetraSentry. Optional block valves make this configuration online repairable.

For applications that have a programmable logic solver (we recommend the TSx), the TetraSentry can be configured with the instrumentation to allow for automatic testing.

If we’re providing a TSx for the electronic overspeed trip detection system, we’ll also program the TetraSentry testing logic. If we’re not, and the TetraSentry has been configured for automatic testing, we’ll provide a “narrative” that describes the test.

Learn more about the TetraSentry

The TSx is uniquely suited to meet the needs of critical turbine safety applications. As a turbine safety (electronic overspeed trip and surge prevention) device it can be configured to achieve SIL3 reliability with a 5 millisecond scan rate (12 millisecond screw to screw).

The TSx comes standard with a fully integrated suite of software applications that allows for as much customization as is required for a specific application. In addition to 1ms overspeed trip detection, the TSx can also be configured to detect and prevent surge events.

When used as for electronic overspeed trip detection, the TSx comes pre-configured to interface with the TetraSentry. This pre-configuration includes automatic online hydraulic trip testing, hydraulic trip performance monitoring as well as data archiving and storage.

The TSx employs a card-in-chassis arrangement with external termination assemblies. This arrangement provides the most concise and secure packaging possible. Removal and replacement of the I/O and processor modules can be accomplished without the risk of disturbing the field wiring. Chassis power is provided separately from the field power to assure that field faults will not affect operation of the logic system.

A hallmark of the TSx architecture is the ability to repair any redundant active component without interrupting the operation of the turbine or process.

Learn more about the TSx

Tri-Sen Steam Turbine Overspeed Calculator

Use the following calculator to determine how quickly your turbine safety system must respond to prevent overspeed excursions. Fields in light blue are editable.


Description Symbol Value Units Notes
Conversion factor, accelKrpm2-kg-m2 / (kW-s)
Turbine max net powerPgmaxkW
Turbine Rated SpeedNrRPM
Turbine Max Continuous SpeedNmcrRPM105% of rated
Trip Speed MultiplierKtrip%above Nmcr
Trip speedNtRPMAPI 612: 116% of rated
Turbine rotor inertiaWR_2kg-m2Turbine rotor only (coupling failure)
Complete train inertiaWR_2_ckg-m2Entire train (no coupling failure)
Acceleration turb, initialAlphaRPM/secTurbine only (no coupling)
Acceleration train, initialAlpha_cRPM/secEntire train (no coupling failure)

Rotor Kinetic Energy

Conversion factor, EtK_2kW-sec-min2/(kg-m2)
KE turbine at trip speedEtkW-sec
KE train at trip speedEt_ckW-sec

Control Time Delay Energy

Controller responseTs_1sec
Trip block responseTs_2sec
Trip Header translation timeTs_3sec
Pilot valve translation timeTs_4sec
Control time delayTssec
Control Delay energyDelta_EskW-sec

Stop Valve Closure

Valve flow characteristicffraction
Valve closure timeTvsec
Valve delay energyDelta_EvkW-sec

Trapped Energy

Conversion factor, Delta EeK_3kW-sec/kJ
Turbine isentropic efficiencyEtafraction
Trapped Steam Chest Steam
Steam VolumeVolume1m3
Starting pressurePress1skPa
Starting TemperatureTemp1sC
Starting massMass1skg
Starting Internal EnergyU1skJ/kg
Final PressurePress1fkPa
Final Internal EnergyU1fkJ/kg
Final EnthalpyH1fkJ/kg
Final massMass1fkg
Turbine isentropic efficiencyEta1fraction
Mass1s * U1sW1s_U1skW-s
Mass1f * U1fW1f_U1fkW-s
(Mass1s - Mass1f) * H1fDW_H1fkW-s
Trapped Extraction Piping Steam
Steam VolumeVolume2m3
Starting PressurePress2skPa
Starting TemperatureTemp2sC
Starting MassMass2skg
Starting Internal EnergyU2skJ/kg
Final PressurePress2fkPa
Final Internal EnergyU2fkJ/kg
Final EnthalpyH2fkJ/kg
Final MassMass2fkg
Turbine isentropic efficiencyEta2fraction
Mass2s * U2sW2s_U2skW-s
Mass2f * U2fW2f_U2fkW-s
(Mass2s - Mass2f) * H2fDW_H2fkW-s
Trapped Condensate 1
Condensate 1 VolumeVolume3m3Only use this section for condensate.
Starting PressurePress3skPa
Starting TemperatureTemp3sC
Starting massMass3skg
Starting Internal EnergyU3skJ/kg
Final PressurePress3fkPa
Final Internal EnergyU3fkJ/kg
Final EnthalpyH3fkJ/kg
Final massMass3fkg
Turbine isentropic efficiencyEta3fraction
Mass3s * U3sW3s_U3skW-s
Mass3f * U3fW3f_U3fkW-s
(Mass3s - Mass3f) * H3fDW_H3fkW-s
Trapped Condensate 2
Condensate 2 VolumeVolume4m3Only use this section for condensate.
Starting pressurePress4skPa
Starting TemperatureTemp4sC
Starting massMass4skg
Starting Internal EnergyU4skJ/kg
Final PressurePress4fkPa
Final Internal EnergyU4fkJ/kg
Final EnthalpyH4fkJ/kg
Final massMass4fkg
Turbine isentropic efficiencyEta4fraction
Mass4s * U4sW4s_U4skW-s
Mass4f * U4fW4f_U4fkW-s
(Mass4s - Mass4f) * H4fDW_H4fkW-s
Trapped energyDelta_EekW-s

Total Rotor Energy

Total Turbine Rotor EnergyEmaxkW-s
Total Train Rotor EnergyEmax_ckW-s

Maximum Rotor Speed

Conversion Factor, NmaxK_4
Max Attained Turb SpeedNmaxRPM
Max Attained Train SpeedNmax_cRPM
Turb Max Allowable SpeedNmxaRPM
MarginRPM27% over Rated Speed
Train Max Allow SpeedNmxcRPM
MarginRPM20% over Rated Speed

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