bently article over speed detection

[Vol.25 No.1 2005] ORBIT 17

Introduction

Original Equipment Manufacturers (OEMs) are generally well-versed in the

ramifications of overspeed as well as the numerous considerations inherent in a

properly engineered overspeed protection system. This is with good reason: they

supply such systems with their new machines and can often assist in retrofitting

existing machines in the field. The same level of understanding, however, does

not always exist with end users. This article is written primarily for the benefit

of end users and acts as a tutorial on basic overspeed concepts, focusing on retrofit

considerations and recommendations. The article also summarizes the economic

implications, safety considerations, and industry trends that are leading many

end users to replace mechanical overspeed detection apparatus with electronic

systems. Finally, the article provides a number of practical observations made by

the author over the last 15 years during projects applying and installing such

systems that will assist users who are in the midst of – or considering – an

upgrade to their older overspeed systems.

Barry Nurcombe, C.Eng.Sr. Engineer, Bently Nevada® Applications Design

[email protected]

18 ORBIT [Vol.25 N o . 1 2 0 0 5 ]

A P P L I C A T I O N S

What is overspeed?Some of the strongest forces acting on most turbo-machinery are the centrifugal forces exerted when massesare set into rotational motion. A simplified equation forthis force, assuming a constant speed of rotation, isshown in Figure 1.

The centrifugal forces are the outward-directed forcescaused by spinning the rotating masses that comprisethe rotor, while the centripetal forces are the equal andopposite restraining forces provided by the rotor thatkeep blades, windings, and other components from“breaking free” while they rotate. By spinning a rotorfaster, the centrifugal forces generated by these masseswill continue to increase, and the corresponding cen-tripetal “restoring” forces increase in an equal and oppositefashion. As can be seen from the equation in Figure 1,these forces increase with the square of the rotative speed,meaning changes in speed have a magnified effect in theforces generated (e.g., a 50% increase in speed resultsin a 225% increase in force). At some point, these forceswill exceed the limits of the rotor’s design. Yield pointsof materials will be reached and the rotor will be damaged.It may even fail entirely, potentially with catastrophicresults. In some circumstances, an overspeed event in amachine will cause blades – and even the entire rotor –to exit the machine casing, allowing the release of notonly high-velocity, high-energy projectiles, but alsosteam and other process fluids. The severity of economicand safety consequences arising from such failures areobvious.

While machines are engineered with a safety marginthat allows them to tolerate brief excursions above the

rated maximum operating speed, they are not designedfor sustained operation at such speeds and overspeedprotection systems are supplied for this reason. It isassumed by the manufacturer that the machine willnot operate above the rated maximum except under twoconditions:

1. Deliberate testing of the machine’s speed controland overspeed protection systems;

2. A mechanical or control system failure resultingin an overspeed condition.

Even during these conditions, the maximum speedachieved by the machine must be constrained and theduration of the elevated speed kept relatively short. Themanufacturer will generally have data for “yield speed”and “burst speed.” Yield speed is the speed at whichrotor components will be deformed or compromisedthrough excessive stress induced by the overspeed con-dition. Burst speed is the speed at which the machineis expected to fail catastrophically. As a machine andits components wear, these speeds may change.Clearly, a machine should never be allowed to approachits theoretical burst speed. Additionally, if a machineever exceeds the yield speed, an inspection is in order.Consequently, overspeed protection systems should beset to keep the machine from exceeding its yield speed,allowing it to be safely restarted if overspeed does occur,rather than necessitating an inspection. A “peak hold”mechanism for capturing the maximum speed achievedduring an overspeed event is very helpful in this regardand, for this reason, is a standard feature on Bently Nevadaoverspeed detection tachometers.

Fcp

Fcf

r

mω

Figure 1 – Centrifugal and centripetal forces on a rotating mass in equilibrium

WHERE Fcf = centrifugal force

Fcp = centripetal force

m = mass of rotating object

r = radius of mass m from center (axis)of rotation

ω = rotative speed

Fcf = Fcp = mrω2

[Vol.25 No.1 2005] ORBIT 19


As a rough rule-of-thumb, most overspeed protectionsystems are set to keep the machine from ever exceedingapproximately 120% of the rated maximum operatingspeed. The actual value may be higher or lower than120% and must be established by consulting the man-ufacturer along with pertinent industry machinerystandards (e.g., API 612) that the user may wish to apply.

Although overspeed can occur gradually (such as whena speed control system fails and the speed slowly creepsupward), it is more common for overspeed to occur veryquickly (just a few hundred milliseconds) because theforces acting on a machine can change very quickly.For example, consider a steam turbine driving a gen-erator whose breakers suddenly open: the turbine willsee an instantaneous loss of load. As another example,consider a gas turbine driving a pipeline compressorwhose coupling suddenly shears: this turbine will likewisesee an instantaneous loss of load, accelerating it withina fraction of a second to 120% overspeed. Speed controlsystems may not react to these kinds of sudden over-speed conditions; the overspeed protection system mustinstead be relied upon to trip the unit.

TerminologyThe words “detection” and “protection” are not usedinterchangeably in this article with respect to overspeedsystems. The distinction is important both in terms ofunderstanding the primary purpose and function of

each system, and in understanding the scope ofresponsibility assumed by the suppliers of each system.

t An Overspeed Protection System (OPS) is thecomplete electro-mechanical system (hydro-mechanical or electro-pneumatic) that senses theonset of an overspeed condition and automaticallyshuts the unit down by closing (or opening) valves,solenoids, and other devices necessary to bring theunit to a safe halt.

t An Overspeed Detection System (ODS) is one partof the larger OPS. It is responsible only for sensingthe onset of overspeed and providing a signal suitablefor triggering the rest of the OPS, which thenremoves energy from the machine and brings it toa safe halt. The ODS supplies this signal in theform of activation of one or more electrical relays.

The Overspeed MapThe time required to detect an overspeed condition andthen shut the machine down must be factored into thedesign of the OPS. As shown in Figure 2, the maximumspeed ever reached by the machine is Nos, the MaximumTemporary Overshoot Speed. For reasons previouslydiscussed, this speed must be less than the yield speedof the machine and should be determined through con-sultation with the machine OEM and consideration ofrelevant industry standards. To account for the time to

Figure 2 – Overspeed shutdown map

MA

CH

INE

SP

EE

D

T IME

OVERSPEEDSHUTDOWNDETECTION

OVERSPEEDSHUTDOWNEXECUTION

Maximum Temporary Overshoot Speed Nos

Overspeed Trip Speed Nost

Maximum Continuous Operating Speed Nmc

∆tod ∆toe

∆top

Not To Scale

∆tod Overspeed Detection Response Time

∆toe Overspeed Execution Response Time

∆top Total Overspeed Protection System Response Time

20 ORBIT [Vol.25 N o . 1 2 0 0 5 ]


detect an overspeed condition and then execute thevarious steps necessary to trip the unit, the OverspeedTrip Speed (Nost) is set at a lower level than Nos. Nostis generally set as low as possible to provide maximumtime for the overspeed protection system to respond,but without being set so low that normal speed fluctu-ations about the machine’s maximum continuous operatingspeed Nmc will trigger a false overspeed alarm andshutdown.

As an example, American Petroleum Institute Standard612 (pertains to Special-Purpose Steam Turbines) usesthe following values:

t Nost is 10% above Nmc

t Nos is 10% above Nost (i.e., 21% above Nmc).

Thus, an overspeed protection system designed to complywith API 612 must begin to act when turbine speedreaches 110% of maximum continuous operating speedand keep the machine from ever exceeding 121% ofmaximum continuous operating speed.

Reaction TimesIt is useful to think of the total time ∆top required forthe overspeed protection system to act as being com-posed of two parts: The detection time ∆tod and theexecution time ∆toe where

∆top = ∆tod + ∆toe.

The detection time is the latency for the overspeeddetection system to detect the onset of overspeed andgenerate a shutdown signal; the execution time is thelatency for the remainder of the overspeed protectionsystem to act on this signal. Table 1 summarizes typicalvalues for ∆top, ∆tod, and ∆toe along with some of theitems that would normally contribute to theselatencies in a typical system consisting of a fully elec-tronic overspeed detection system and an electro-hydraulicshutdown system. As shown in the table, a typical appli-cation will require a total OPS response time of between140 and 250 ms. ODS response time will generally be50 ms or less (API 670 requires that ODS tachometersbe capable of responding within 40 ms when an inputsignal of 300 Hz or greater is present).

The Economics of OverspeedShutdown EventsWith many operators striving to achieve an absoluteminimum of process interruptions, whether caused bymachinery failures or any other factors, traditionaloverhaul and maintenance intervals are being extendedwhenever possible. While this is being achieved suc-cessfully for many machines through appropriate conditionmonitoring and asset management strategies, it is neveradvisable to delay routine maintenance and inspectionsof safety-related systems, such as overspeed protection.As will be discussed next, fewer process interruptionsand enhanced safety are not mutually exclusive. Through

Table 1 – Overspeed protection system response times and factors

TO

TA

L O

VE

RS

PE

ED

PR

OT

EC

TIO

N R

ES

PO

NS

E T

IME

(∆

to

p)

(TY

PIC

AL

LY

14

0 –

25

0 m

s)

OV

ER

SP

EE

D E

XE

CU

TIO

N

RE

SP

ON

SE

TIM

E (

∆to

e)

(TY

PIC

AL

LY

10

0–

20

0 m

s)

OV

ER

SP

EE

D D

ET

EC

TIO

N

RE

SP

ON

SE

TIM

E (

∆to

d)

(TY

PIC

AL

LY

40

-50

ms

)

Time for multi-tooth speed wheel to generate sufficient pulses to measure speed change accurately

Time to compare speed inputs against alarm setpoints

Time to vote multiple channels against one another in 2-out-of-3 system

Time for alarm signal to be transmitted to relay contact circuitry

“Bounce” time for relay contacts to settle fully opened or closed

Input scan times for logic solvers or other safety instrumented systems

Hydraulic system latencies

Solenoid actuation latencies

Valve latencies

Time for entrained energy downstream of trip valve to fully discharge through machine

Interposing relay latencies and “bounce” time for contacts to settle fully open or closed

[Vol.25 No.1 2005] ORBIT 21


the use of electronic overspeed detection systems, ratherthan older mechanical “trip bolts,” reliability and safetycan be significantly enhanced while eliminating the needfor costly process interruptions required to test and verifythe overspeed detection circuity.

Mechanical Overspeed DevicesMechanical overspeed devices, such as trip bolts or triprings (which contain internal trip bolts), utilize movingparts. These bolts or rings are fixed to the rotor androtate with it. Under an overspeed condition, the cen-trifugal force of the rotating bolt causes it to move radiallyoutward against preset spring forces. At a pre-designedspeed, the bolt is designed to overcome these springforces and pop outward, far enough to strike triplevers. These levers are normally latched in place, butare released by the impact of the trip bolt/ring strikingthe trip lever surface. In turn, these levers are connectedto valves and linkages that commence the unit shutdownprocess. Figure 3 shows a typical trip ring from a steamturbine.

Unfortunately, these mechanical trip bolts can becomestuck by the ‘gelling’ or ‘lacquering’ of the sur-rounding lubricating oil. At minimum, this gives rise

to a delay in the instigation of the overspeed shutdownprocess. In other cases, the shutdown system will com-pletely fail to act.

To help ensure such systems continue to work properly,it is necessary to carry out an overspeed test at regularintervals. However, such testing can be potentially dan-gerous and is frequently very expensive for the followingreasons:

1. To test the system, the machine must be phys-ically oversped. If the bolt fails to operate duringthe test, it may be difficult to manually reactin time to prevent unconstrained overspeed andensuing results.

2. Such testing generally requires an interruptionof the production process – exactly the oppositeof the aforementioned goals to extend intervalsbetween outages and production interruptions.Often, these interruptions are measured in mil-lions of dollars per day due to the expensiveprocesses of which the turbomachinery is a part.

Ironically, although industrial insurers generally requireoperators to test their overspeed systems at regularintervals, it is precisely during these tests that many

Figure 3 - A typical trip ring relying on a spring-loaded bolt

22 ORBIT [Vol.25 N o . 1 2 0 0 5 ]


machines suffer overspeed damage because the bolt failsto act properly and the machine enters a runawayoverspeed condition.

Electronic Overspeed DetectionAn electronic overspeed detection system does not relyon mechanical parts or actuation. Instead, speed sensorsobserve a rotating toothed wheel on the machine. Whenthe sensors detect excessive rotative speed, a relay isactuated which causes the remainder of the overspeedsystem to trip the unit.

A properly designed overspeed detection system mustminimize both false trips and missed trips. False tripsrepresent expensive process interruptions. Missed tripsrepresent potentially very serious safety and economicconsequences arising from catastrophic machinery failure.Instrumentation with single-point failure modes cannotachieve the reliability and availability required by suchapplications. For this reason, a triple modular redundant(TMR) approach is used, whereby three separate sensorsare used as inputs to three separate tachometers, and thesystem energized by fully redundant power supplies.These tachometers then use logic to provide 2-out-of-3 voting (only when any two sensors/tachometers detectan overspeed condition will the system initiate a shut-down). This design ensures that a failure of a singlesensor or tachometer will not cause a shutdown to bemissed or initiated. It also allows users to isolate theindividual channels, simulate suitable speed inputs, andtest the system without physically placing the machineinto an overspeed condition.

A MigrationThe above benefits have resulted in a strong migrationof users (and manufacturers), switching from mechan-ically instigated overspeed shutdown to electronic systemsinstead. In addition, a number of industry standards(such as those from the American Petroleum Instituteand ISO) have undergone revision to reflect electronicprotection systems rather than mechanical bolts. It isour recommendation that all customers relying on amechanical bolt or ring consider replacing these systemswith an electronic detection system. These systemswill generally pay for themselves immediately becausetesting their proper operation does not require themachine to be physically oversped or production to beinterrupted, as already mentioned.

Even when an OPS uses fully electronic ODS tech-nology, there will generally be mechanical/hydraulic

To test or not to test…

This rotor is from a steam turbine

that was completely destroyed with

serious consequences due to an

overspeed situation when its mechanical

bolt did not function properly. The user

had stopped testing the system due to

significant problems with getting the

machine out of overspeed during

previous tests.

Electronic overspeed detection systems

avoid such scenarios by allowing func-

tional testing without the need to change

a machine’s speed.

[Vol.25 No.1 2005] ORBIT 23


devices within the overall shutdown loop. The properoperation of these non-electronic devices must also beassessed at regular intervals. Just as the detection portionof an overspeed system can employ triple modular redun-dancy to allow isolation and testing of circuits withoutthe need for a process interruption, so can other partsof the OPS benefit from redundancy. Figure 4 shows a2-out-of-3 arrangement for hydraulic solenoid valvesand the corresponding manifold. Such a design enhancessafety and reliability while simultaneously allowing easiertesting of this critical component within the shutdownsystem. Many users now consider such designs to be“best practice.”

ScopeThe scope assumed by the Bently Nevada team in anyoverspeed-related project is confined to the OverspeedShutdown Detection region of Figure 2. When a project’sscope includes retrofit of the entire OPS, the Bently Nevadateam works cooperatively with the customer’s choiceof provider which may be an OEM, the end user’s ownengineering personnel, or a third-party selected by thecustomer. In other instances, customers may want fullscope to be assumed by GE Energy. In these instances,the GE Energy Control Solutions team acts as lead con-

tractor with overall OPS responsibility, and they sub-contract the ODS portion to the Bently Nevada team.Regardless of where responsibility for the overall OPSlies, the Bently Nevada team must always conduct amandatory application review of the elementsaffecting the response time and integrity of the ODS,as discussed next.

ODS Application AuditsAn ODS must properly initiate the auto-shutdownsequence of a machine within the maximum allowabletime delay. If it does not, the result may be catastrophic,unconstrained overspeed. The ODS solutions deliveredtoday are based on the Bently Nevada® 3500/53 over-speed detection tachometer modules. The 3500system employs a TMR configuration utilizing 2-out-of-3 voting, and may be ordered with optional TÜVcertification, designating the system as suitable for SafetyIntegrity Level (SIL) 3 applications. It has been specif-ically designed to comply with the guidelines of relevantfunctional safety standards such as IEC 61508, IEC61511, and ANSI/ISA-84.01-1996.

There are many application variables that must be under-stood and addressed for an ODS to perform properlyand with the necessary response time. It is only by

Figure 4 Figure 5

i Control cabinet showing 2-out-of-3 hydraulicsolenoid valves and manifold used as part ofoverspeed protection system

i A turbine rotor undergoing retrofit for an electronicoverspeed protection system. Note mechanical bolton left, and existing castellated nut on right whichwas chosen as the speed sensing surface.

24 ORBIT [Vol.25 N o . 1 2 0 0 5 ]


Case One: the observed wheel is a true gear (i.e., it is designed to turn another gear)…

O/Dia. = _______

A = ___________

B = ___________

C = ___________

D = ___________

Case Two: the observed wheel is not a true gear (i.e., it is a toothed wheel designed for a magnetic pickup or proximity probe)…

O/Dia. = _______

A = ___________

B = ___________

C = ___________

D = ___________

Audit point Remarks / Options

Machine type Steam turbine Gas turbine Turbo expander

Manufacturer Name __________________ Model # _____________ Power _____________

Driven machine Compressor Generator Other ______________

Stored energy 1 Steam liquid gas Pressure ____________ Volume ____________

S/D System Electronic/electric Control oil Mechanical

S/D Valve Instantaneous Fast Slow

Bypass valve 2 Instantaneous Fast Slow

Speeds Operational _____________ Max continuous _____ Overspeed trip______

Sensor location 3 On driver shaft On driven shaft Other shaft _________

Sensor type Proximity probe Magnetic pick-up

Governor sensor Shared with ODS Separate from ODS

Speed wheel 4 True Gear type Pole wheel Profile/Dimensions

Events/Rev # of Teeth _______________ # of Holes ___________ a___ b___ c___ d___

Notes:

1. Differing media have different stored energy release capability.2. If fitted (e.g., expanders).3. ODS Sensor MUST be on the driver unit to properly protect against overspeed in the event of a coupling failure.

A sensor observing a gear immediately driven from the main shaft may be acceptable.4. Provide wheel dimensions using figures below.

Figure 6 - Partial list of ODS audit points

[Vol.25 No.1 2005] ORBIT 25


reviewing each application carefully and collecting necessary details such as geometries and location of thespeed sensing surface (see Figure 5) that the proper oper-ation of the ODS can be determined. For example, aspeed sensing surface must provide an adequate numberof pulses per shaft revolution to allow the ODS tachometersto detect speed changes rapidly enough. Consider a 1500rpm machine with a single Keyphasor® mark. It takes40 ms for the machine to complete one revolution,and multiple revolutions would be required to sense achange in speed. Clearly, if this machine required theODS to respond within 50 ms, the speed sensing surfacewould be totally insufficient. As another example, thespeed sensing surface should never be located on an aux-iliary shaft where there is an intervening gearbox orcoupling that could potentially fail, isolating the speedsensing surface from the actual machine speed.

Figure 6 shows a portion of the checklist used when con-ducting a Bently Nevada ODS application audit. Beloware examples of typical actions required as a result ofODS audit findings.

t A process train in a chemical plant had existingspeed control system sensors observing a portionof the turbine governor shaft which was coupledto the main turbine shaft by means of an inter-vening gearbox. If the gearbox or its couplingsbroke, the governor shaft (but not the actual turbine)would begin to decelerate. The governor wouldrespond by speeding up the turbine to compensatefor the falling speed observed by the transducers.To guard against such a failure mode, a differentlocation was chosen for the ODS sensors, allowingthem to observe the turbine shaft directly ratherthan the governor shaft.

t The existing speed sensing surface (i.e., “speedwheel”) for an ODS on one machine had beendesigned for use with a magnetic pick-up. Thecustomer wanted to retrofit proximity probes inplace of the magnetic pickups. The application auditrevealed that a modified speed wheel would berequired because use of the existing wheel wouldhave caused the probes to detect three teeth at a timeinstead of each individual tooth, resulting in incorrectand/or intermittent speed measurements.

On rare occasions, a customer will not permit an application audit to be performed, or will refuse toimplement aspects of the audit that ensure the ODS will

respond properly. In these cases, a Bently Nevada® ODScannot be supplied since the ability to confirm thesuitability of input signals is no longer within our control.

Supplemental FindingsIn addition to the mandatory ODS audit conducted,customers sometimes request that the Bently Nevadateam review the broader OPS rather than confiningthe audit to only the ODS. This scope falls outside theOverspeed Shutdown Detection region in Figure 2.While it can be provided upon request, and has beenperformed in numerous instances, the additionalscope covered by this review is intended merely to sup-plement – not replace – the review conducted by thoseresponsible for OPS supply and installation. In no cir-cumstances does Bently Nevada assume responsibilityfor the Overspeed Shutdown Execution portion of thesystem. The examples below illustrate typical supple-mental findings.

t A new high-speed, high-efficiency turbine at aEuropean facility incorporated several meters ofsteam piping between the emergency stop valveand the turbine. However, this length of piping wasfound to contain sufficient volume of pressurizedsteam to accelerate the turbine beyond Nos (referto Figure 2) even after the emergency stop valvewas closed. A modification to install a rapid-openingvent valve and duct was recommended, ensuringthat this energy could be quickly released duringa trip rather than expanding through the turbine.This allowed a sufficient reduction in the overallresponse time ∆top to meet the application’s require-ments.

t One facility had not tested the existing OPS ontheir steam turbine for five years. While the plantwas taking measures to install a better OPS solution,this interval between tests is considered totally unac-ceptable regardless of what type of system is installed– electronic or mechanical. Testing and mainte-nance of the new system was recommended at muchmore frequent intervals.

t A review of an OPS project at a large North Americanpetrochemical facility revealed that while the proposed ODS solution had been engineered torespond within 50 ms, the remainder of the systemcontained latencies of up to one second (the logicsolver into which the ODS would be fed was not

26 ORBIT [Vol.25 N o . 1 2 0 0 5 ]


programmed to scan its inputs more than once persecond). This was far in excess of the application’srequirement for ∆top ≤ 250 ms and wouldrequire extensive changes to the instrumentationdownstream of the ODS.

A Word About TachometersStandard tachometer modules (intended for indi-cation only, not as part of an ODS) and high-speedtachometer modules (intended for use as part of anODS) are both available as part of the Bently Nevadaproduct line. The high-speed 3500/53 overspeedtachometer modules are specifically designed for ODSapplications and can respond to an overspeed event with-in 40-50 milliseconds when properly applied andconfigured. They are designed for use only as part of aredundant voting configuration.

Customers with limited knowledge of overspeed and itsconsequences will sometimes propose as a cost-savingmeasure the use of standard indicator tachometer modules.There are several reasons why we cannot supply ourproducts for such applications. First, standard indicatortachometers do not provide the response times necessaryfor use in an ODS. Indeed, to help prevent misappli-cation by customers, these modules have been designedso that their alarm time delays can be set no faster thanone second – far slower than required by overspeed appli-cations. Second, these modules are “simplex” and arenot designed to be configured for the modular redundantvoting warranted by the critical nature of overspeedapplications.

Another cost-saving measure that is sometimes proposedis to use the process control system as part of the over-speed protection loop. However, numerous pertinentindustry standards, such as American Petroleum InstituteRP 554 (section 3.5) and ANSI/ISA SP84.01, specificallyadvise against such practices by calling for a segregationof basic process control systems from safety-relatedshutdown systems, such as overspeed protection. Thesestandards, and generally accepted engineering practice,recognize that the very conditions for which safety-related shutdown systems are installed can be causedby a failure of the basic process control system. Assuch, independent protection systems that are not affectedby process control failures are essential. This is the samerationale employed in standards such as API 612 thatcall for segregating the overspeed protection system fromthe machine control system.

Special Concerns for Turbo ExpandersTurbo expanders represent special consideration whenreviewing and specifying an OPS. These machines areused when the process stream pressure must bereduced prior to its introduction to another processstage. Expanders can also be employed to re-use theenergy contained in the process to drive compressors,pumps, or other machines.

On smaller turbo expander/compressor units, the pro-cess pressures can be significant, and this, coupledwith a low rotor inertia, leads to rapid acceleration inthe case of an inlet duct failure. Protection systems forthese units are required to operate with extremely fastreaction times to prevent serious mechanical damage.While API 617 (a widely used industry standard per-taining to these machines) permits a single-channeloverspeed detection system, it also provides an optionfor a 2-out-of-3 system. We strongly recommend thatcustomers choose the 2-out-of-3 option due to the criticalnature of the measurement and the consequences offailure. For the reasons discussed above, BentlyNevada tachometers may not be used in a simplex con-figuration for any ODS applications.

Large expander units likewise represent special concerns.On these machines, the pressure inlet duct can be onthe order of one meter in diameter (or greater), andthe associated control valves will be slow to operate.When designing an OPS for this type of unit, there isa strong case for installing a small diameter bypassduct with rapid-action valves. This reduces the inletpressure, allowing more rapid shutdown in the eventof overspeed.

RelaysThe relays provided from Bently Nevada overspeeddetection systems are generally limited to 5A of current.When replacing an older overspeed detection system,it is typical to find that the ODS drives a shutdown relaywith significant current draw. Leaving the old shutdownrelay in place requires the use of interposing relayswith higher current-carrying capacity than the 5A limitof the Bently Nevada ODS. Rather than using inter-posing relays, a better approach is generally to replacethe shutdown relay altogether, substituting a newer unitthat has less current draw. These newer relays generallyhave much faster switching times. In addition, they can

[Vol.25 No.1 2005] ORBIT 27


system to detect speed changes with adequate resolutionand response time. Customers sometimes propose touse a Keyphasor® mark (one-event-per-revolution dis-continuity on the shaft) for the ODS. Depending onmachine speed and ODS response times required, asingle event per revolution may or may not be adequate,as previously discussed. Most commonly, multiple eventsper revolution are required, necessitating the use of atoothed wheel (Fig. 7). The required number of eventsper revolution is carefully reviewed as part of the appli-cation audit. Other important considerations includethe geometry and location of this speed sensing surface.

During one recent ODS project, our design require-ments for the toothed wheel were – unbeknownst tous – not implemented by the customer. This led to inter-mittent false trips and costly loss of production, butfortunately not a missed trip. The customer has sincereplaced the wheel with one conforming to our recommendations, and no further incidents have occurred.

Speed SensorsAnother important consideration is the type of speedsensor. Magnetic pick-ups are often used for the basicspeed control system, and have historically also beenused for the electronic overspeed detection system aswell. A better alternative for most ODS applications isto use proximity probes as they offer the advantagessummarized in Table 2.

eliminate the need for interposing relays, which intro-duce their own latencies and add to the overall systemresponse time.

Speed Sensing SurfacesAn overspeed detection system relies on transducers thatobserve a multi-toothed wheel (speed sensing surface).This toothed wheel is one of the most important aspectsof the ODS and must be engineered properly for the

Figure 7

i View of a new speed wheel installed on the end ofa steam turbine overspeed stub, after replacementof the mechanical trip system of Figure 1.

Table 2 – Advantages of proximity probes over magnetic speed pick-ups*

Uniform, speed-independent response to zero speed.

DC gap voltage output, useful not only for establishing proper physical gap from the speed wheel, but also for enhanced transducer andwiring fault diagnostics.

Much longer linear range. The 80 mil effective linear range of a standard eddy current transducer is far greater than the typical 30 mil non-linear range of magnetic pickups. This increased range also offers a better physical “buffer” between the sensor tip and the rotating gearteeth. For machines with over 5 mils of mechanical runout or vibration, maintaining proper clearance can be a problem if magnetic pickupsare used.

Greater bandwidth when using longer cable lengths.

Less susceptibility to EMI (electromagnetic interference).

Interchangeability with vibration, position, and Keyphasor® transducers used elsewhere on the machine, reducing spare part requirements.

* Where the existing magnetic pick-up must be replaced with a similar device due to mechanical or cabling constraints, we can recommend suitable magnetic sensors to meet the requirements of our system as well as SIL requirements.

28 ORBIT [Vol.25 N o . 1 2 0 0 5 ]


When using proximity probes for speed measurement,special attention must be paid to signal characteristicsat operating speeds, not slow-roll speed. While thetachometers may trigger properly at slow-roll speeds,it is essential that they also trigger properly at oper-ating speeds, since these are the speeds at which protectivefunctions must actuate. As shown in Figure 8, the outputat slow-roll conditions may not always match the outputat operating speeds. The peak-to-peak value of thewaveform may decrease at operating speeds – a functionof transducer slew rate, frequency response, and othercharacteristics. In extreme cases, the output amplitudemay drop enough to result in an incorrect number ofteeth being detected, unless the proper threshold/hysteresis adjustments are made.

SummaryThere are significant advantages to the use of elec-tronic overspeed detection systems when compared tomechanical systems relying on trip bolts. There arealso significant considerations that must always beaddressed to ensure that both the overspeed detectionsystem and the larger overspeed protection system willwork properly and within the maximum allowableresponse time if an overspeed event occurs. This articlehas summarized several of those considerations,noting common pitfalls that can be encountered and

explaining the necessity of an application audit priorto supplying an ODS. This audit addresses only theODS portion of the total overspeed protection system.Users must also exercise good engineering judgementand practices in the remainder of the OPS, which isoutside the scope addressed by a Bently Nevada over-speed detection solution.

References

“Application of Safety Instrumented Systems for the Process Industries,”ANSI/ISA-84.01-1996, First Edition, The Instrumentation, Systems, andAutomation Society, Research Triangle Park, NC (1996)

“Axial and Centrifugal Compressors and Expander-compressors forPetroleum, Chemical and Gas Industry Services,” API Standard 617, SeventhEdition, American Petroleum Institute, Washington, D.C. (2002)

“Electronic Overspeed Detection Systems,” ORBIT magazine, Vol. 20 No.2, Second/Third Quarters 1999, pp. 44-45

“Functional Safety of Electrical/Electronic/Programmable ElectronicSafety-Related Systems,” IEC 61508, First Edition, International ElectrotechnicalCommission (IEC), Geneva, Switzerland (1998).

“Functional Safety: Safety Instrumented Systems for the Process Sector,”IEC 61511, First Edition, International Electrotechnical Commission (IEC),Geneva, Switzerland (2003).

“Process Instrumentation and Control,” API Recommended Practice 554,First Edition, American Petroleum Institute, Washington, D.C. (1995)

“Machinery Protection Systems,” API Standard 670, Fourth Edition, AmericanPetroleum Institute, Washington, D.C. (2000)

“Petroleum, Petrochemical, and Natural Gas Industries – Steam Turbines– Special-purpose Applications,” API Standard 612, Fifth Edition, AmericanPetroleum Institute, Washington, D.C. (2003)

i Output from proximity probe observinga toothed wheel at slow-roll speed(top) and running speed (bottom). Inthis example, if slow-roll output isused to set monitor triggering, runningspeed waveform will result inincorrect triggering.

Figure 8

bently article over speed detection

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