United Statesp
Environmental Protectionp
Agencyp
United Statesp
Occupational Safety andp
Health Administration
EPA550-F-97-002Fp
September 1997p
SHAFT BLOW-OUT HAZARD OF
CHECK AND BUTTERFLY VALVES
The Environmental Protection Agency (EPA! and Occupational Safety and Health AdministrationH
(OSHA! are issuing this Alert as part of their ongoing efforts to protect human health and theH
environment by preventing chemical accidents. Under CERCLA, section 104 (el, the Clean AirH
Act (CAA), and the Occupational Safety and Health Act (OSH Act!, EPA and OSHA haveH
authority to conduct chemical accident investigations. Additionally, in January 1995, theH
Administration asked EPA and OSHA to Jointly undertake investigations to determine the rootH
causels! of chemical accidents and to issue public reports containing recommendations to preventH
similar accidents. EPA and OSHA have created a chemical accident investigation team to workH
Jointly in these efforts. Prior to the release of a full report, EPA and OSHA intend to publishH
Alerts as promptly as possible to increase awareness of possible hazards. Alerts may also beH
issued when EPA and OSHA become aware of a significant hazard. It is important that facilities,H
SERCs, LEPCs, emergency responders and others review this information and take appropriateH
steps to minimize risk.H
Certain types of check and butterfly
valves can undergo shaft-disk
separation, and fail cata-
strophically or "blow-out", causing toxic
and/or flammable gas releases, fires, and
vapor cloud explosions. Such valve
failures can occur even when the valves
are operated within their design limits
of pressure and temperature.
ACCIDENT
Tn a 1997 accident, several workers
sustained minor injuries and millions
of dollars of equipment damage
occurred when a pneumatically assisted
Clow stub-shaft Model GMZ check (non-
return) valve in a 300 psig flammable gas
line underwent shaft blow-out. The
valve's failure caused the rapid release
of large amounts of light hydrocarbon
gases which subsequently ignited,
resulting in a large vapor cloud
explosion and fire.
The check valve was designed with a
drive shaft that connects the internal
valve disk to an external pneumatic
cylinder (see diagram on next page). The
valve failed when a dowel pin designed
to fasten the drive shaft to the disk
sheared and a key designed to transfer
torque from the drive shaft to the disk
fell out of its keyway, disconnecting the
drive shaft from the disk. System
pressure was high enough to eject the
unrestrained drive shaft from the valve,
carrying with it the external
counterweight assembly, weighing over
200 Ibs., a distance of 43 feet away.
The absence of the drive shaft left a hole
in the valve body the diameter of the
shaft (3.75 inches) directly to
atmosphere, and initiated a high-
pressure light hydrocarbon leak. The
leak continued for approximately 2 to 3
minutes, forming a large cloud of
flammable light hydrocarbon vapor. The
vapor cloud ignited, resulting in an
explosion felt and heard over 10 miles
away. The explosion and ensuing fire
caused extensive damage to the facility,
completely or partially destroying many
major components, piping systems,
instruments, and electrical systems, and
requiring the complete shut-down of the
affected unit for cleanup and repair.
Minor damage occurred to nearby
residences and automobiles (mostly
broken glass and minor structural
damage due to the blast wave). Nearby
highways were closed for several hours.
Damage cost to the facility alone is
estimated at approximately 90 million
dollars. Fortunately, no fatalities and
only minor injuries to workers resulted
from the accident.
EPA and OSHA
^ Printed on recycled paper
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Shaft Blow-Out Hazard of Check and Butterfly Valves
September 1997
Previous malfunctions involving check valves
of the same or similar design occurred at
facilities in 1980, 1991, and 1994. In each case,
the affected check valve was located in a large
diameter (36-inch or greater) pipe in a
hydrocarbon gas compression system. Also in
each previous case, a dowel pin fastening the
valve's drive shaft to its disk sheared (in the 1980
case the pin was possibly never installed) and a
rectangular key fell out of its keyway,
disconnecting the drive shaft from the disk.
Although shaft-disk separation occurred in each
previous case, it did not result in shaft blow-out
or catastrophic failure. This may be because the
valves in these instances were installed in lower-
pressure service, or because the malfunctioning
valves were identified before shaft blow-out
occurred.
In the 1991 incident, the malfunction was
manifested by the erratic operation of the valve,
which was observed to operate independently
from its external drive mechanism. System
pressure was low enough (70 psig) that the
failure was detected before the shaft was
expelled out of the valve body. (At the time the
malfunctioning valve was identified, the valve
shaft was protruding about 0.75 inches out of
the valve body.) In the 1980 and 1994 cases, the
malfunction was identified when workers noted
that the external piston rod connecting the air-
assist cylinder to the drive shaft had broken due
to axial movement of the drive shaft.
systems containing chemicals leading to
hydrogen embrittlement.
Check and butterfly valves are used in
many industries, including refineries,
petrochemical plants, chemical plants,
power generation facilities, and others. Most
modem valve designs incorporate features that
reduce or eliminate the possibility of shaft blow-
out. However, older design check and butterfly
valves with external appendages such as
pneumatic-cylinders, counterweights, manual
operators, or dashpots may be subject to this
hazard. Shaft blow-out may be of particular
concern wherever these valves are installed in
Valves subject to this hazard may be designed
with a two-piece valve stem (sometimes referred
to as a "stub-shaft" design). In each of the cases
described above, the malfunctioning component
was a Clow stub-shaft Model GMZ
pneumatically assisted swing check valve (see
diagram below). In these check valves, one stem
piece functions as a drive shaft that connects the
internal valve disk to an external air-assist
cylinder and counterweight assembly. The drive
shaft penetrates the pressure boundary through
a stuffing box. The exterior portion of the drive
Simplified cross-sectional view of check valve (flow direction is into page)
Counterweight
Valve Disc (flapper
shown in open position)
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Shaft Blow-Out Hazard of Check and Butterfly Valves
September 1997
shaft is connected to a pneumatic piston and
counterweight, and the interior portion of the
shaft is coupled directly to the valve disk using
a cylindrical hardened steel dowel pin and a
rectangular bar key. This arrangement provides
a power-assist to close the valve during
compressor shut down, preventing reverse flow
of compressed gases. These particular valves
have probably not been produced since 1985. but
still exist in some process facilities constructed
before that date. Similar valves currently or
previously produced and sold by other valve
manufacturers may also be subject to this
hazard.
Factors In Valve Failure
A number of design and operational factors may
contribute to this hazard. These include the
following:
Factors
^ The valve has a shaft or stem piece which
penetrates the pressure boundary and ends
inside the pressurized portion of the valve.
This feature results in an unbalanced axial
thrust on the shaft which tends to force it (if
unconstrained) out of the valve.
•t-Thc valve contains potential internal failure
points, such as shaft dowel-pins, keys, or bolts
such that shaft-disk separation can occur
inside the valve.
4-The dimensions and manufacturing
tolerances of critical internal parts (e.g., keys.
keyways, pins, and pin holes) as designed or
as fabricated cause these parts to carry
abnormally high loads (e.g., in the 1997
accident, the dowel pin rather than the key
transmitted torque from the shaft to the disk).
•f'The valve stem or shaft is not blow-out
resistant. Non blow-out resistant design
features may include two-piece valve stems
that penetrate the pressure boundary
(resulting in a differential pressure and
unbalanced axial thrust as described above).
single-diameter valve shafts (i.e., a shaft not
having an internal diameter larger than the
diameter of its packing gland) or shafts
without thrust retaining devices, such as split-
ring annular thrust retainers.
valve is subject to high cyclic loads. In
all of the above incidents, the valve repeat-
edly slammed shut with great force during
compressor trips and shutdowns. Such re-
peated high stresses may cause propagation
of intergranular cracks in critical internal com-
ponents, such as dowel pins.
4- The valve is subject to low or unsteady flow
conditions, such that disk flutter or chatter
occur, resulting in increased wear of keys,
dowel pins, or other critical internal
components.
+ Valves in high-pressure service lines may be
more likely to undergo shaft blow-out (in the
1997 accident, system pressure at the failure
point was approximately 300 psig).
•t- Valves used in hydrogen-rich or hydrogen
sulfide-containing environments may be
more susceptible to blow-out due to hydrogen
embrittlement of critical internal components.
particularly if these are made from hardened
steel (as was the dowel pin in the 1997
accident).
Facilities should review their process
systems to determine if they have valves
installed that may be subject to this
hazard. If so, facilities should conduct a
detailed hazard analysis to determine the risk
of valve failure. Check valves or butterfly
valves which are subject to several or all of the
above design and operational factors are at
high risk for shaft blow-out. Detailed internal
inspections may be necessary in order to
identify high-risk valves. Facilities should
consider replacing high-risk valves at the
earliest opportunity with a blow-out resistant
design. Several blow-out resistant designs of
check and butterfly valves are available. If
immediate valve replacement is impossible or
impractical, facilities should consider
immediately modifying the valves to prevent
shaft blow-out. Valve manufacturers should
be consulted in order to ensure that any
modifications made are safe.
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Shaft Blow-Out Hazard of Check and Butterfly Valves
September 1997
ON VALVE
Some sources of information on valve safety
are listed below.
General References
Information on cases of valve failure can be found
in T. Kletz, What Went Wrong?, 3rd Edition, Gulf
Publishing Co., Houston (1994). This reference
contains general information related to check valve
failure (pp 127, 1.29, and 175) and cites one specific
case of check valve failure (page 124) similar to
those described in this Alert.
Information on hydrogen embrittlement can be
found in P.P. Lees. Loss Prevention in the Process
Industries: Hazard Identification, Assessment, and
Control, 2nd edition, Butterworth-Heinemann
Publishing, Oxford (1996), pp 12/82-83.
American Petroleum Institute
1220 L Street NW
Washington, DC 20005
Phone: (202) 682-8000
Web site: http://www.api.org
Relevant API standards include:
API 598-1996 — Valve Inspection and Testing
API 570-1993 — Piping Inspection Code:
Inspection. Repair. Alteration, and Rerating of
In-Service Piping Systems
API 941-1991 — Steels for Hydrogen Service at
Elevated Temperatures and Pressure in
Petroleum Refineries and Petrochemical Plants
Relevant API Recommended Practices include:
RP 574-1992 — Inspection of Piping. Tubing.
Valves and Fittings
RP 591-1993
Valves
User Acceptance of Refinery-
Codes,
Regulations
The American Society ofMechanical Engineers (ASME)
has a standard for valves.
American Society of Mechanical Engineers
345 East 47th Street
New York, NY 10017
or
22 Law Drive
Fairfield, NJ 07007-2900
Phone: (800) 843-2763
Web site: http://www.asme.org
Relevant ASME standards include:
ASME B 16.34-1996 — Valves - Flanged.
Threaded, and Welding End, an American
National Standard.
*
The American Petroleum Institute (API) has several
relevant standards and Recommended Practices.
Applicable regulations include:
29 CFR 1910.119 Process Safety Management
of Highly Hazardous Chemicals: Explosives
and Blasting Agents.
FOR MORE INFORMATION...
CONTACT EPA's EMERGENCY PLANNING AND
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MONDAY-FRIDAY, 9 AM TO 6 PM, EASTERN TIME
VISIT THE EPA CEPPO HOME PAGE ON THE
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WEB AT:
http://www.osha.gov/
NOTICEH
The statements in this document are intended solely as guidance. This document does not substitute for EPA's, OSHA's, orh
other agency regulations, nor is it a regulation itself. Site-specific application of the guidance may vary depending on processh
activities, and may not apply to a given situation. EPA or OSHA may revoke, modify, or suspend this guidance in the future,h
as appropriate.H
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