DYNATORQUE Valve Magazine Fall 2002 Article

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FALL 2002 1

MAGAZINE

FALL 2002

VOLUME 14, NO. 4

Advances in Valve & Actuator Technologies

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2 VALVE MAGAZINE

by Mike Mitchell There is a storm forming on the horizon for offshore

rigs, refineries, and all other industrial process

plants that have automated valves in their safety

systems. Like the rumble of distant thunder, they

know it is coming—but they’re just now cranking-up their radar

to find out how the storm will affect them. One thing is certain:

when this storm finally hits, no plant will remain untouched by

its fury....What’s causing the storm to brew?

In “the good old days” process plant operations managers had

the luxury of shutting down their facility every year or two for

what is now fondly remembered as the “maintenance shutdown”.

This was a period of time set aside to close down production and

perform maintenance on the plant’s equipment. So the produc-

tion stream was “turned off” and the maintenance crew was free

to disassemble, clean and/or replace equipment. The control

systems engineer, safety engineer, and reliability engineers were

able to test their process control and safety equipment without

the worry of dealing with process flow. That was when companies

could devote the time and resources to perform this task in the

most efficient and complete manner.

Times have changed, and to succeed in today’s aggressive

business climate, we must change along with the times. One of

the most obvious changes in the process industries in recent years

is the demise of the planned periodical maintenance shutdown.

Closing down the plant, for whatever reason, also shuts down the

revenue stream. The process industries are under incredible

pressure to increase revenues and these “shutdowns” of the

revenue flow are not acceptable to management and other

stakeholders. The result is an emphasis on running industrial

process plants on a 24-hour-a-day, 7-day-a-week basis for as many

years as possible-without maintenance shutdowns.

CONSIDER

Mechanical Partial S

FOR PROCESS ESD

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FALL 2002 3

Among the most critical valves affected by less frequent

maintenance are the Emergency Shutdown (ESD) Valves and

other fail-safe valves in safety related applications. These

valves may typically incorporate a spring-to-fail actuator to

stroke a valve in the event of an upset process condition. If

the ESD valve does not perform its function to close (or in rare

applications, open the valve) at the time of an upset event, the

consequences to property and life can be catastrophic. [Note:

For the purpose of simplifying this discussion, when referring

to pneumatically or hydraulically operated ESD and safety

valves, the convention of “air-to-open valve, spring-to-close

valve, de-energize to trip” will be used].

This new emphasis on continual process revenue genera-

tion has led the Occupational Safety and Health Administra-

tion (OSHA), insurance companies, other regulatory agen-

cies, and safety engineers to take a fresh look at how this new

operating philosophy impacts plant safety. These agencies

posed the question to the process plants: If the plant is going

to remain operational for an extended period of time, how can

we be assured the valve safety systems will function correctly

when called upon?

The industry has responded to this question with “accepted

industry standards” (essentially self-governing) such as ISA-

S84.01 (Application of Safety Instrumented Systems for the

Safety-Related Valves

Process Industries) and IEC 61508 (Functional Safety Of E/E/

PE Safety-Related Systems) to determine acceptable levels of

performance of these systems.

This article examines mechanical Partial Stroke Test

Devices as a preferred methodology for assuring compliance

with new standards. These standards define requirements for

Safety Instrumented Systems (SIS) and allow the end user to

establish Safety Integrity Levels (SIL) for specific applications.

One means of showing conformance is to test all the various

SIS components such as solenoid valves, quick exhaust valves,

relays, valve to stem integrity, and valve actuator without

actually closing the valve.

By the nature of the application, the ESD valve is subject

to flowing process fluid. If the ESD were to be full-stroke

tested while the plant is operational, the “flow of revenue”

(i.e., process flow) would come to a halt. One methodology of

overcoming this dilemma is to install a bypass valve and

piping around the ESD valve. When the bypass valve is open,

the ESD valve and its safety system may be fully tested

without affecting process flow. This option sounds effective,

and it is. However, piping dimensional constraints (particularly

on offshore drilling and production rigs) and the cost associated

with installing a dual system at every ESD valve make a

bypass system generally not preferred.

ERING

al Stroke Test Devices

&

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4 VALVE MAGAZINE

Figure 1: Typical example of a “sandwich-mount” product

installed between the pneumatic actuator and valve.

One of the terms associated with these ISA and IEC

standards is “partial stroke testing” (Reference ISA-S84.01,

paragraph 9.7.5.2.b). A “partial stroke” might be defined as, for

example, allowing the safety system to close a valve only 20%

(this would be the set point). In that case, all of the control

elements are being tested, but obviously the valve never fully

closes during the test.

The overall assumption is that a given number of “partial

strokes” can be applied as a statistical “credit” against “full

stroke tests”. Thus, the end user may be capable of either 1)

lowering the Probability of Failure on Demand (PFD) of a given

SIS by doing partial stroke tests between full stroke tests; or 2)

by doing partial stroke testing between full stroke tests, the end

user may be able to maintain or lower his PFD and enjoy a

greater interval of time between full stroke test intervals.

The background for safety testing has been explained in a

previous Valve Magazine (See “Getting Closure on Compli-

ance, Putting Valve-Related Safety Standards in Perspective”,

Valve Magazine, Summer 2002 Edition, Volume 14, Number

3). The technical analysis supporting the benefits of Partial

Stroke Testing have also been examined in detail at VMA

Seminars (Reliability, Safety, Integrity Today, March 7-8,

2002, Houston, TX), and ISA conferences (Safety Instru-

mented Systems for the Process Industry, May 14-16, 2002,

Baltimore, MD) and numerous websites (see, for example,

www.iec.ch/zone/fsafety/; www.instrument-net.co.uk/silansi.htm).

Thus, it would be redundant to cover that material here.

The purpose of this discussion is to examine the benefits of

mechanical Partial Stroke Test Devices as a preferred method-

ology for functional testing of the SIS when a full stroke of the

valve is not practical.

At both the consulting engineer and end user levels,

responsibility for process flow control and valve safety systems

generally fall under the authority of Control Systems Engi-

neering (Instrumentation) because the valves are automated.

Often times this engineering discipline attempts to affect a

Partial Stroke through the use of ancillary controls and

instrumentation. This often results in making the SIS more

complex, expensive, and subject to complicated software

programming, installation, and commissioning.

The following criteria should be considered when consider-

ing methodology for affecting the Partial Stroke Test of a SIS,

including the associated ESD valve:

Keep it a Simple System (KISS)

Because a mechanical Partial Stroke Test Device does not

require the addition of extensive extraneous controls, several

important user benefits are realized. There are no require-

ments to add additional power, wiring or control systems to

perform a partial stroke function test. The benefit is obviously

a reduction in capital cost and system complexity. Secondly,

mechanical interlock systems are generally considered a more

positive method of achieving the partial stroke. This option is

discussed further in “Mechanical Characteristics”, below.

Lastly, we can make these systems as complicated as we want

to, but more times than not, it is important to keep things

simple and safe. In the real world, often times the less

complex the system, the fewer things will go wrong.

Mechanical Characteristics

Perhaps the primary advantage of the mechanical Partial

Stroke Test Device is just that: It’s mechanical. A typical

installation of a mechanical Partial Stroke Test Device would

be to mount the device “sandwich” style between the valve

bonnet (mounting flange) and the drive face (or bottom) of a

pneumatic valve actuator. (See Figure 1). Within the device

there is a drive mechanism that, once engaged, prevents the

de-energized actuator from causing the valve to stroke more

than a specified percentage of full travel—thus accomplishing

the partial stroke. There is no mandatory device requirement

to integrate it into the control loop or add ancillary controls.

Therefore, there is no complexity added to what might already

be a rather sophisticated control loop or shutdown system.

Field Retrofits/No Calibration

Although Partial Stroke Test Devices are often supplied

with a new valve/actuator package, they are also field

retrofitable to existing valves. As mentioned previously,

typical installation of these products is to sandwich them

between the valve and actuator. Mechanical partial stroke

devices can be furnished with mounting surfaces premachined

to fit an existing valve and actuator interface. This makes the

installation of the device simple and cost effective. In fact,

depending on the specific valve and actuator, in many

190697klc 2/28/03, 8:33 AM4

FALL 2002 5

instances the sandwich-style device can

actually replace typical valve/actuator

mounting hardware.

Instrumentation personnel and

software programmers are not required to

install the devices, nor is commissioning

or routine calibration of controls

required, because there aren’t any. Most

process plants have qualified in-house

mechanics or Valve Automation Centers

nearby and, since there are no additional

controls required, installation cost savings

can be realized when compared to

electrical or controls-driven test systems.

Real World Automation

Although the point behind partial

stroke testing is to establish a statistical

analysis for lowering the Probability of

Failure on Demand, as one industry

speaker recently put it: “Calculations are

not necessarily reality!” Therefore, the

process operators must take care to assure

that real world conditions are considered

when selecting a methodology for partial

stroke testing.

ESD valves are often large and are

typically operated by pneumatic or

hydraulic piston cylinder actuators, or

electric motor operators. Due to the

critical nature of the application, one

would expect the ESD valve to seldom

operate. Although piston cylinder

actuators are considered extremely

reliable, in the real world these devices

are not always smooth-acting. The less

often they are stroked (as in the case of a

typical ESD valve application), the less

smooth is their operation. This might be

caused by the elastomeric piston o-rings

taking a set against the interior surface of

the cylinder.

Most of the non-mechanical Partial

Stroke Test Devices assume a relatively

smooth movement of the valve actuator

and that the SIS will act in a consistent manner, indepen-

dent of environmental conditions such as temperature and

humidity or how long the valve rests between test cycles.

This is rarely the case. The primary function of a mechanical

Partial Stroke Test Device is to stop valve movement at a

specified percentage or degree of valve

closure and will not give spurious SIS

trips (alarms) based on extraneous

conditions.

Further, when using non-mechanical

or controls-driven partial stroke test

systems there is an assumption that the

test device or system will actually

prevent the actuator from driving the

valve past the set point to the fully

closed position. In the real world, the

stored energy in an actuator may drive

the valve past the set point and actually

allow the valve to close, forcing a

process shut down. Mechanical devices

physically prevent the valve from

moving past the specified test point.

Once the test device is engaged, the

valve cannot move past the set point.

(Figure 2 - Figure 4)

Many process plants also use electric

Motor Operated Valves (MOVs) in

critical process applications. Although

most MOVs have internal mechanisms

that can be set to stop valve movement

at a specified percentage of stroke, plant

operators often select a mechanical

Partial Stroke Test Device to prevent

valve closure should the internal

mechanism malfunction.

Man-Machine Interface?

Partial Stroke Testing is receiving

increased attention as the aforemen-

tioned industrial standards are consid-

ered by industry. Progressive refineries,

process plants, pipeline companies, etc.,

are going through an evolutionary

process of developing the methodology

by which they will comply with the

standards and how those methodologies

will be implemented. One of the

considerations is, to what extent should

plant personnel be proactively involved

in performing the test of process valve

Safety Instrumented Systems?

One philosophy of methodology calls for a Man-Machine

interface. In the simplest format, mechanical Partial Stroke Test

Devices provide for a plant maintenance person to locally

engage the device using a controlled key (See Figure 5).

Figure 2: Fabricated body of a Partial

Stroke Test Device. Valve Stem would be

connected to the device through the bore

and keyway.

Figure 3: During normal operation, the

device is passive and will allow the valve

to ESD on demand.

Figure 4: When a partial stroke test is

required, the device is “engaged” and the

ESD valve will only travel to the specified

percentage of stroke. The device

mechanically prevents movement past the

interlock.

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6 VALVE MAGAZINE

Figure 5: For manual operation, a

controlled key is used to engage the

Partial Stroke Test Device. The key

cannot be removed from the mechanical

testing device while it is in the test

position. If plant operating personnel

know the key is in their control and not

in the test device, then they also know

the device cannot be accidentally

engaged.

By design, the key cannot be removed

from the mechanical testing device while

it is in the test position. If plant opera-

tions personnel know the key is in their

control and not in the test device, then

they also know the device cannot be

engaged. When it is time to test the

valve, the maintenance person inserts the

controlled key into the device, engages

the device and then informs the control

room that the valve is now ready to test.

When the SIS test is simulated, the

person witnesses and reports the event,

usually by radio, to the control room, and

then resets the system. Plant operations,

safety engineers and management are

assured the of the system’s reliability

because “the man saw it function”.

(Figure 6, Figure 7a, and Figure 7b)

Management philosophies at other

plants may hold a different view: They

want the entire system to be automatic

with absolutely no human interface. Many mechanical Partial

Stroke Test Devices provide for automatic operation as an

option to the Man-Machine interface. Under this methodol-

ogy, a remotely controlled mechanism is used to engage and

disengage the device and there is no key control system. The

ESD valve can then be tested either by a signal generated by

the control room or any number of other sources without field

personnel involvement.

Mechanical Partial Stroke Test Devices offer plant opera-

tions and maintenance management both manual or auto-

matic operating options as their philosophies dictate. In either

case, diagnostic devices can be integrated into the system to

report specific operating details of the SIS function.

Integrated Data Systems

Some end users are beginning to investigate diagnostic

technology for valve maintenance considerations. The data

acquired from these systems may be helpful in determining the

operational status of an automated valve. Even though

mechanical Partial Stroke Test Devices do not require ancil-

lary controls, such controls may be added if the end user

specifies them (Figure 8). For example, if the user would like

to have a data acquisition system to determine if the valve/

actuator package is experiencing a change in run time trends,

the actuator top works can use standard diagnostic technology

to report that information. But because the mechanical

device is engaged during the test, the end user has assurance

that the actuator will not stroke the valve

past the set point.

A mechanical sandwich-type Partial

Stroke Test Device may also come

equipped with its own data point device

(such as limit switches) for positive

annunciation to the control room

indicating whether the device is engaged

for testing the Safety System. Some may

want to use such a device to gather

information for the data acquisition

system such that auditable reports,

testifying to time and date of the test, can

be generated (Reference ISA-S84.01,

paragraph 9.8.1).

Controls can also be configured to

automatically disengage a Partial Stroke

Test Device in the unlikely event of an

ESD occurring during the test.

True Safety System Tests

As previously stated, a mechanical

Partial Stroke Test Device requires no extraneous controls or

devices to be added in the safety control loop. When the

device is tested, all the actual SIS components, controls and

elements used in an ESD or safety valve will be activated.

The user has real information about the exact controls that

will be relied upon to protect his plant and personnel.

Valverciser™

An additional feature of mechanical Partial Stroke Test

Devices is that they are not only applicable to safety related

applications, but can also be used to enhance the operation of

the valve. In many process applications the chemical

composition of the flowing fluid causes material to build-up

on the valve internal body and trim surfaces. Over time, this

build-up may cause the valve to “stick” in position and not

stroke. Partial Stroke Test Devices can be used to simply

“exercise” the valve by allowing it to partially stroke, keeping

the valve surfaces that are required to move free from material

build-up.

Many mechanical Partial Stroke Test Devices can also be

configured to accommodate maintenance lock-in, tag-out

requirements as well.

All Things Considered...

Now that we have fully explored the benefits of mechanical

Partial Stroke Test Devices it should be clear that the simplic-

ity, safety, reliability, and cost advantages of the devices far

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FALL 2002 7

outweigh the whiz-bang factor of adding more controls and

instrumentation to what is probably already a sophisticated

valve safety system. Consideration and use of the mechanical

devices will lead to “sunny days ahead” when battling the

oncoming storm of standards compliance!

The author is Vice President, Sales, DynaTorque Inc.,

Muskegon, MI; 231.739.1431; www.dynatorque.com;

mmitchell@dynatorque.com.

Figure 6: During normal

operation, key is kept in a

controlled environment

and device is free to

operate and is

transparent to the ESD

valve operation.

Figure 7b: ...the device is then “engaged”

and maintenance informs the control room

that the ESD is now ready to test.

Figure 7a: When

it is time to test

the ESD,

maintenance

inserts the

controlled key

into the device...

Figure 8: Although mechanical Partial Stroke Test Devices do not

require ancillary controls, they may be added if specified.

© Fall 2002 Valve Magazine and Valve Manufacturers Association of

America (VMA), Washington, DC. All trademarks, trade names, service

marks and logos reference herein belong to their respective companies and

the VMA. Valve magazine articles or content may not be reprinted or

reproduce in any format without written permission from the VMA. For

more information, please visit www.vma.org.

VM

190697klc 2/28/03, 8:33 AM7

8 VALVE MAGAZINE

Plants need to make sure ESD and other critical safety

valves function properly now that maintenance shutdowns

for “full-stroke testing” are less frequent.

The DynaTorque D-Stop™ (Patent Pending) allows safety

control elements to be tested, but does not allow the valve

to fully-close during the test so the “revenue flow” is not

affected. Partial Stroke Testing may lower the Probability of

Failure on Demand (PFD) or may increase the time interval

required between full-stroke tests.

The D-Stop™ covers actuator torque ranges from 3000 lb-

in to 4,000,000 lb-in. It is a reliable mechanical device. Di-

agnostics and ancillary controls can be added, but they are

not required. No software or commissioning, so capital

costs are low.

Most important? The D-Stop™ keeps the system simple

and safe. In the real world, the less complex the system,

the fewer things can go wrong!

Critical Valve Test?

D-STOP™ it!

For remote and/or auto-

mated applications the

key and safety release

are not required. The

top actuator controls

the valve, while the

smaller actuator on left

engages/disengages

the D-Stop™.

Limit Switches (left and bottom) may be added to provide

feedback to I/O system indicating the D-Stop™ has been

engaged and that the device is in the

test

position. Data

points can be fed to a data acquisition system for the Par-

tial Stroke Test audit trail.

Ph. 231-739-1431 Fax. 231-739-4514 www.dynatorque.com/dstop info@dynatorque.com

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