Valve Magazine - Fall 2011

It’s easier to express probability in terms of failure, rather than in terms of proper performance. As published in the aforementioned standards and some product brochures, four levels of SIL are listed, enumerated 1-4—the higher the SIL level, the higher the associated safety level, and the lower probability that a system will fail to perform properly (Figure 1).

2. For a WPS that involves carbon and
alloy steels postweld heat-treated
(PWHT) below the lower transfor-
mation temperature (LTT), Section
IX only requires the WPS to indicate
the PWHT must be below the LTT.
Most WPSs for non-impact-test-
ed carbon steels and alloy steels will
actually state a PWHT temperature,
but many omit the tolerance on this
temperature and any soak time
requirements.
To ensure that proper strength,
ductility and hardness requirements
are met, a PWHT temperature
range should be stated on the WPS.
That temperature range should be
MATERIALS Q&A BYDONBUSH

Detail
Q: My customer has rejected
my welding procedure
specification (WPS) and has
asked me to address items
that are not required by

ASME code section has verbiage in the
foreword warning the user that merely
following the rules in that particular
section will not ensure an adequate
design. Such wording is phrased in
ways like: “The user of the Code should
refer to other pertinent codes, stan-
dards, laws, regulations or other rele-
vant documents,” and “it is not intend-
ed that this Section be used as a design
handbook; rather, engineering judg-
ment must be employed in the selection
of those sets of Code rules suitable to
any specific service or need.”

0.01

ASME Section IX. Is this
reasonable?

These various SIL levels might be correlated to the above mentioned examples of consequences of failure. A: Yes, this is a reasonable request. In many cases, it is a good idea to address items not covered by Section IX1.

0.001

For purposes of this article, it is appropriate to think of SIL as “the degree of likelihood that our system will work when we want it to.” (Generally,

Time

Many people view any given ASME code section as a handbook, and assume that if they follow everything in that code section, they have met all necessary requirements. However, each

Graphs reprinted by permission from ANSI/ISA-TR96.05.01-2008. ©ISA 2008
PFDAVG
Full Test

0.1

For example, Section IX does not impose enough controls to ensure reliable weld joints because it does not address the proper choice of filler materials. Section IX would allow welding nickel alloys or copper alloys together using carbon steel filler, provided the qualification specimen passed the appropriate mechanical tests.

Partial Test
Partial Test

SIL 4 is beyond the scope of what we
see in the process industries.) Again, we
might want to think of a “function” as
an emergency shutdown valve system,
typically consisting of a sensor of some
type (pressure, level, temperature) and a
logic controller that will send a signal to
an automated valve. The automated
valve package might consist of an
actuator (pneumatic, electric, hydraulic,
etc.), solenoid valves, quick exhaust
valves and the final control element, the
valve. The “system” may consist of
many functions; it might have five emer-
gency shutdown valves protecting a
pressure vessel cooking a process. Or, it
might have only one function (the SIF)
making the entire SIS.

Full Test

The SIS system will be independent from the BPCS and will act to prevent

Figure 3. PFDAVG is decreased due to PST combined with full cycle testing

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Many variables are not required to be addressed in a Section IX WPS. But in many of these cases, a WPS that does

1. There are supplementary essential variables not required to be addressed for a WPS to be used on materials that are not impact-tested. One such essential variable is heat input, which is calculated based on current, voltage and travel speed. If heat input does not need to be addressed, Section IX does not require including voltage or travel speed values. Some of the software packages for creating WPS documents will even omit this information automatically. On the other hand, if no voltage or travel speed values are specified, it is possible to follow the WPS and still produce unacceptable welds. Therefore, even though not required by Section IX, it is beneficial to include reasonable voltage and travel speed values on the WPS—even for materials that are not impact-tested.

To summarize, the HAZOP team will determine SIL levels based on determined PFD. Michael Young of General Monitors has summed this issue up nicely in his paper, SIL 101: How Safe Do I Need to Be?

New &
Improved

A simple example will help illustrate the concepts of SIS, SIF, and SIL. Consider the installation of a pressure vessel containing flammable liquid. It is maintained at a design operating pressure by the Basic Process Control System (BPCS). If the process control system fails, the vessel will be subjected to an over-pressure condition that could result in a vessel failure, release of the flammable contents and even fire or explosion. If the risk in this scenario is deemed to be intolerable by the facility owner, an SIS will be implemented to further reduce this risk situation to a tolerable risk level.

not address those variables may not produce consistently reliable weld joints or provide adequate guidance to the welder, both of which should be the primary purpose of the WPS.