VALVE SAFETY
A number of different organizations
have identified valve safety guidelines,
which cover considerations such as valve
type, materials, size, temperature ratings, pressure class, on/off functions,
throttling, diversion, media and application. The American Society of Mechanical Engineers (ASME), for example,
recommends rules, requirements and
limitations to help guide the safest product designs. ASME also provides guidelines on materials, material stress and
temperature limits, pressure classification, quality requirements and more.
Additional organizations that offer
guidelines include the American Petroleum Institute (API) and the Manufacturers Standardization Society (MSS).
There are a large number of international design standards as well.
FUGITIVE EMISSIONS
A fugitive emission can be defined as any
pollutant released into the air from leaks
anywhere on an industrial site. That can
include releases from equipment, pipe
lines, seals, valves, etc. This contrasts
with pollution released through chimneys, stacks and vents. In 1995, the EPA
estimated that 62% of fugitive emissions
from a typical plant come from valves.
Failure to properly handle fugitive
emissions can have multiple consequences—including product loss,
impaired safety for workers and the surrounding areas, EPA emission fees and
enforcement actions, and increased monitoring and repair costs.
In the U.S., fugitive emissions fall
under the purview of the Clean Air Act.
In the future, they will be covered by the
American Clean Energy and Security Act
of 2009, H.R. 2454, which as of this
writing, was making its way through the
legislative process. In Germany, emissions are regulated by the TA-Luft (
Tech-nische Anleitung zur Reinhaltung der
Luft or Technical Guidelines for Air Pollution Control).
Internationally, testing methods for
fugitive emissions for qualification of
valves are covered in ISO Standard
15848 (Jan. 1, 2006), though unlike the
other standards, this is not a government
regulation.
SAFETY VALVES: THE ULTIMATE BACKSTOP
BY PETER CLEAVELAND
Any discussion of valve safety is not complete without mention of the first device
designed to open and release pressure from a vessel to prevent explosions. This is
ascribed to Denis Papin, inventor of the pressure cooker (he called it a “steam
digester”) in the late 17th century. Reports say that the rhythmic up-and-down
motion of the valve in operation inspired the development of the steam engine.
Safety valves suffer from a certain ambiguity in nomenclature, with terms
like safety valve, relief valve, pressure relief valve and safety relief valve varying
with the industry and application. A general sorting out of these terms can be
found in Béla Lipták’s Instrument Engineer’s Handbook, which defines a relief valve
as one that opens gradually as pressure exceeds the opening pressure and is used
primarily in liquid service; a safety valve as one that opens rapidly (“pops”) when
its set pressure is reached and used for steam, gas or vapor service; and a safety
relief valve as “an automatic pressure-relieving device suitable for use as either a
safety or relief valve, depending on the application.”
Lipták points out that, while quality is important in any valve, it can be
especially important for safety relief valves, because such valves may sit for
years without ever opening, yet they must do so reliably when needed. For this
reason, Lipták continues, redundant valves may be used or valves backed up by
rupture discs or some combination of those two.
RESOURCES:
The Instrument Engineers’ Handbook, by Béla Lipták is available in hard cover from
various sources. Available free online are Selection and Sizing of Pressure Relief
Valves, by Randall W. Whitesides, P.E.; the Crosby Pressure Relief Valve Engineering
Handbook; the Anderson and Greenwood Pressure Relief Technical Manual from
Tyco; and a series of steam engineering tutorials from Spirax Sarco.
In addition, a number of companies provide software packages available to
help in the selection. You can also contact valve manufacturers for more information. Go to www.VMA.org > Product Finders > Valves/Actuators/Controls,
then use the pull-down Product Type to bring up a list of pressure relief and safety relief manufacturers.
ISO 15848 is in two parts: 1) “
Industrial valves—Measurement, test and
qualification procedures for fugitive
emissions—Part 1: Classification system
and qualification procedures for type
testing of valves” specifies testing procedures for evaluation of external leakage
of valve stem seals (or shaft) and body
joints of isolating valves and control
valves intended for application in volatile
air pollutants and hazardous fluids.
2) “Industrial valves—Measure-
ment, test and qualification procedures
for fugitive emissions—Part 2: Pro-
duction acceptance test of valves”
specifies test procedures for the evalu-
ation of external leakage of valve
stems or shafts and body joints of iso-
lating valves and control valves
intended for application with volatile
air pollutants and hazardous fluids.
