Operations Involving Enclosed Facilities

Operations Involving Enclosed Facilities

INTRODUCTION

Section 12 presents some additional recommendations that are unique to oil and gas producing and gas processing plant operations in enclosed facilities²³ and involving hydro­gen sulfide (refer to Par. 4.1). An enclosed facility may be as simple as a single piece of equipment within an enclosure or as complicated as complex onshore or offshore enclosed fa­cilities located in cold climates.

UNIQUENESS OF OPERATIONS INVOLVING
ENCLOSED FACILITIES

The uniqueness of oil and gas producing and gas process­ing plant operations in enclosed facilities is due to the poten­tial that escaping quantities of hydrocarbon gases containing hydrogen sulfide can be contained in an enclosed space²³, particularly if ventilation is inadequate. This space may be entered by personnel. A small leak of product containing hy-

drogen sulfide gas that would normally dissipate as it es­caped can be contained in the enclosed space surrounding the leak and increase the hazard to entering personnel unless adequate ventilation is provided to reduce the hazard.

DESIGN CONSIDERATION

The design and construction practices of Section 8 gener­ally apply to enclosed facilities, but there are areas of design that should be considered due to the unique operating situa­tions they present. Additional design considerations for en­closed facilities can include, but are not limited to, the following:

a. Means to prevent flammable liquids and gases from com­
ing into contact with surfaces hot enough to cause ignition.
The autoignition temperature for natural gas is approxi­
mately 900°F (482°C). The autoignition temperature for
other natural gas mixtures ranges from 700°F to 900°F
(371°C to 482°C). The autoignition temperature for hydro­
gen sulfide is approximately 500°F (260°C).

b. Ventilation.

c. Onsite respiratory protection equipment.

d. Electrical equipment (possibly Group С versus Group D
equipment required). Refer to Par. 3.5, "National Electrical
Code Grouping of Atmospheric Mixtures" of API Recom­
mended Practice 500.

e. Emergency relief and depressuring devices and their dis­
charge points.

f. Hydrocarbon vents from diaphragm valves, machinery,
and regulators.

g. Compressor depressuring and blowdown lines,
h. Floor drains.

i. Process drains, manual and automatic.

j. Vents from gas conditioning equipment (glycol and

amine).

k. Hydrogen sulfide monitoring system.

FIXED HYDROGEN SULFIDE MONITORING
SYSTEM

In many locations where personnel enter frequently, on a regular basis, or occupy enclosed facilities for relatively long periods of time, fixed hydrogen sulfide monitoring systems (with adequate alarms) can enhance safety. In some loca­tions, an alternative to fixed monitoring systems may be im­plementation of personnel entry procedures (refer to Pars. 9.18 and 12.5).

Fixed hydrogen sulfide monitoring systems should be in­stalled in facilities containing process equipment (vessels,

An enclosed facility (room, building, or space) is defined as a three-di­mensional space enclosed by more than two-thirds (²/з) of the possible pro­jected plane surface and of sufficient size to allow the entry of personnel. For a typical building, this would require that more than two-thirds (²/з) of the walls, ceiling, and/or floor be present. Refer to API Recommended Prac­tice 500.

machinery, etc.) handling gases or fluids containing hydro­gen sulfide when a release of these gases or fluids is capable of causing atmospheres with hydrogen sulfide in concentra­tions exceeding 10 ppm when the locations are both:

a. an enclosed area (room, building, or space) as defined by
Par. 12.1 and API Recommended Practice 500.

b. inadequately ventilated. [Inadequately ventilated is de­
fined as ventilation (natural or artificial) which is hot suffi­
cient to prevent the accumulation of significant quantities of
hydrogen sulfide-air mixtures in concentrations exceeding
10 ppm]. Adequacy of ventilation should be assessed on a
site-specific basis.

Fixed monitoring systems shall contain audible alarm de­vices (and visual alarm devices in high noise areas, refer to Par. 10.3.11) activated by hydrogen sulfide concentrations at preset levels (not to exceed 10 ppm), as required to alert per­sonnel. Hydrogen sulfide monitoring equipment should be calibrated in accordance with Par. 9.15.

It is recognized in specific instances that a fixed com­bustible gas detection system may detect the existence of a potentially hazardous atmospheric condition before a fixed hydrogen sulfide monitoring system that is set to alarm at 10 ppm of hydrogen sulfide would be activated. For example, in a release of a 300 ppm hydrogen sulfide in methane mixture, a combustible gas detector set to alarm at 20% lower explo­sive limit (LEL) would activate and sound an alarm at a hy­drogen sulfide concentration of approximately 3 ppm.

In such instances, a regular testing program should be es­tablished to monitor content of the process stream to ensure that the concentration of hydrogen sulfide has not increased. If an increase in the hydrogen sulfide concentration is con­firmed, the user should verify adequacy of the detection sys­tem in use. This verification should consider all variable criteria that will affect performance of the detection equip­ment as well as factors that could increase the concentration of hydrogen sulfide in the work atmosphere should a mal­function or equipment failure occur. CAUTION: This option is limited in its scope and should be used only when all applicable limitations and site specific parameters have been duly considered.

Fixed monitoring systems are also desirable to monitor the air intake for enclosed areas when the air is used for makeup or for pressurizing the enclosed facilities (refer to NFPA 496).

PERSONNEL PROTECTION TECHNIQUES

A method of protecting personnel from exposure to atmo­spheric concentrations of hydrogen sulfide exceeding 10 ppm should be provided in all enclosed facilities containing process equipment (vessels, machinery, etc.) handling hydro­gen sulfide bearing fluids and capable of causing hydrogen sulfide atmospheric concentrations in excess of 10 ppm. Ac­ceptable methods include:

a. Requiring personnel to wear proper protective breathing
apparatus (refer to Par. 6.4) before entering and when in the
facility.

b. Installing fixed hydrogen sulfide monitoring equipment
(refer to Par. 6.2, Section 10, and Par. 12.4).

c. Properly ventilating the facility to maintain hydrogen sul­
fide concentrations in the work atmosphere less than 10 ppm,
confirmed by monitoring with a fixed hydrogen sulfide mon­
itoring system. Recirculation of air is allowed, but recircu-
lated air streams should be monitored with fixed monitoring
systems to alarm when concentrations of hydrogen sulfide
exceed 10 ppm in the recirculated air.

d. Testing the facility before entry and continuously while in
the facility, using portable hydrogen sulfide detection equip­
ment (refer to Par. 6.3), to ensure that hydrogen sulfide con­
centration in the work area atmosphere does not exceed 10
ppm.

Note: Personnel shall either establish that entry without protective breathing (respiratory protection) equipment will be safe or shall wear appropriate personal protective breathing equipment (refer to Par. 6.4).

WARNING SIGNS

Legible warning signs, such as "HYDROGEN SULFIDE OPERATIONS—ENTER ONLY WHEN MONITORING SHOWS THE AREA TO BE SAFE" or "RESPIRATORY PROTECTION EQUIPMENT MUST BE WORN BE­YOND THIS POINT', shall be prominently posted outside all access doorways leading into enclosed facilities where produced fluids or gases containing hydrogen sulfide (refer to Par. 4.1 for applicability) are being processed or handled.

Note: Regulatory requirements for sign posting shall be observed.