Training Functional Safety 08 - Partial Stroke Test Rev0.0

FUNCTIONAL SAFETY TRAINING

08 – PARTIAL STROKE TEST

1Dr. Ing. Carlo LebrunFunctional Safety Training


HOW TO INCREASE RELIABILITY?

The optimal ESD solution can be achieved by balancing:- Redundancy of components- Frequency of planned tests

• More redundancy leads to higher CAPEX (capital expense)

• More testing leads to higher OPEX (operating expense)


EFFECT OF A PERFECT PROOF TEST


EFFECT OF A NON-PERFECT PROOF TEST


ADDITION OF PARTIAL PROOF TEST


THE SUM OF ALL PFDAVG

The final element has normally the highest PFDavg.50% and more out of the total PFDavg

WHY TO PERFORM A PARTIAL STROKE TEST?


Advantage:Partial Stroke Test (PST or PVST) enables testing without production loss, and can be carried out more often with minor maintenance costs.

Disadvantage: Partial Stroke Test can detect only a fraction of the failures.


FAILURE RATES WITH PARTIAL STROKE TEST


COVERAGE OF PARTIAL STROKE TEST

Standard partial stroke testing will not test deterioration

ID FAILURE MODE FULL TEST PARTIAL TEST

1 Blockage/plugged Yes No

2 Control Failure Yes Yes

3 Electrical failure Yes Yes

4 Faulty position indication/alarm Yes Yes

5 Leakage Yes No

6 Material corrosion / deterioration Yes No

7 Mechanical failure – general Yes Yes

8 Out of adjustment Yes No




In the lack of detailed vendor data:

-Vendor summary data

-Approximate estimation based on conservative assumptions

- database like OREDA



Full Stroke case: 199/663 = 30%

Tight ShutOff case: 199/1844 = 10%

Open to trip case: 199/466 = 42%



Full Stroke case: 266/604 = 44%


Open to trip case: 266/391 = 68%



Spring Return case: 418/633 = 66%

Double Acting case: 762/1211 = 63%



De-energize to trip case: 186/188 = 99%

Energize to trip case: 562/568 = 99%



Combination of

SOLENOID


ACTUATOR


VALVE (butterfly)


Gives a final element PVST coverage

(186+418+199)/(188+633+1844) = 30%

To get a Tight Shut off using a butterfly valve



Combination of

SOLENOID


ACTUATOR


VALVE (trunnion ball type)


Gives a final element PVST coverage

(186+418+266)/(188+633+1269) = 41%

To get a Tight Shut off using a trunnion ball valve


FEASIBILITY OF PARTIAL STROKE TEST


PST is feasible only if:

-Valve is not too small

-Valve closure time is not too fast

-Valve PST impact on flow is negligible

- Valve allows an intermediate position detection


PARTIAL STROKE TEST TECHNOLOGIES

-Electrovalves Logics

-Mechanical Jammer

-Smart Positioner (Digital and Analog)

19Dr. Ing. Carlo LebrunFunctional Safety Training 02

CLOSE LIMIT SWITCH

PST LIMIT SWITCH

OPEN LIMIT SWITCH

IA SUPPLY

ELECTROVALVES LOGICS


Features:

•Enabling PST is simple in construction and operation (the simplest solution is to use the same solenoid which act the trip, for a shortened time interval)•No need for special software and hardware (just some additional logic in the logic solver)•It is not applicable to very small and/or very fast valves •Usually requires additional wiring field to/from ESD•Additional SOV can introduce spurious failures



CLOSE LIMIT SWITCH

PST LIMIT SWITCH

OPEN LIMIT SWITCH

IA SUPPLY

ENERGIZED

DE-ENERGIZED

NORMAL OPERATION

VENT

VALVE WILL OPEN



CLOSE LIMIT SWITCH

PST LIMIT SWITCH

OPEN LIMIT SWITCH

IA SUPPLY

DE-ENERGIZED

DE-ENERGIZED

VENT

VALVE WILL CLOSE

VENT

VENT

TRIP FROM NORMAL OPERATION



CLOSE LIMIT SWITCH

PST LIMIT SWITCH

OPEN LIMIT SWITCH

IA SUPPLY

ENERGIZEDPARTIAL STROKE TEST

VENTWITH REDUCED DIAMETER

VALVE WILL CLOSE

ENERGIZED


PLC WILL DE-ENERGIZE AS THE PST LIMIT SWITCH HIGHLIGHTS THE CORRECT MOVEMENT OF THE VALVE


CLOSE LIMIT SWITCH

PST LIMIT SWITCH

OPEN LIMIT SWITCH

IA SUPPLY

TRIP DURING PARTIAL STROKE TEST

VENTWITH REDUCED DIAMETER

VALVE WILL CLOSE

ENERGIZED

DE-ENERGIZED

VENT

VENT


PST MUST NOT HINDER THE PROTECTION LOGIC




-Mechanical Jammer



Features:

•Simple (and cheap) mechanical stopper inserted in the actuator path•Easily attachable to pre-existing valves through simple modification•Protection is not available during PST (if single valve I used)•Can be used locally or remotely (through additional ESD wirings)•There is some potential for human error (inducing spurious failures, or forgetting the jammer inserted)

MECHANICAL JAMMER


Picture taken from Netherlocks website

http://www.netherlocks.com

MECHANICAL JAMMER: FAITH


Pictures taken from Cameron website

http://www.c-a-m.com

MECHANICAL JAMMER: DYNATORQUE



-Mechanical Jammer




PST TECHNOLOGIES: SMART POSITIONER

Features:•Adjustable test operation range / speed settings•On-line monitoring and self-diagnosis function for valve opening / air pressure• Can be operated by ESD or by dedicated PC (via HART)• Test result well documented and properly archived• Requires some additional wiring field to/from ESD• Additional device/SOV can introduce additional failures/spurious failures•Not applicable to very fast valves•If a proper device and architecture are selected the protection is available during PST

Functional Safety Training 02 31Dr. Ing. Carlo Lebrun

I/PCONV

ZT

S

PI

VENT

IA SUPPLY


Functional Safety Training 02 32Dr. Ing. Carlo Lebrun

I/PCONV

ZSL2ZSH

S

S

PI

VENT

IA SUPPLY

ZSL1



Pictures taken from Siemens websitehttp://www.sea.siemens.com




Command to valve

Valve position

Delay

100%

80%

• PSTs using smart positioner analyze the “fingerprint” can detect also gradual degradation of valve performance

The pressure/position curve is called “valve fingerprint”


Which is the best technology?- Electrovalves Logics?- Mechanical Jammer?- Smart Positioner (Digital and Analog)?

The one with the best failure coverage

(if you are able to find detailed failures data)

and the lowest addition of new failure modes

(if you are able to find detailed failures data)



http://www.ecisgroup.it/

END OF PRESENTATION

Documents

Training Functional Safety 08 - Partial Stroke Test Rev0.0