Guidelines for Evaluation and Selection of Continuous Hydrogen Sulfide Monitoring Equipment

Guidelines for Evaluation and

Selection of Continuous Hydrogen Sulfide Monitoring Equipment

INTRODUCTION

Section 10 is provided to make users of hydrogen sulfide monitoring equipment aware of some equipment limitations and certain desirable features of such equipment. There are a number of detection principles and analytical procedures available for monitoring the concentration of hydrogen sul­fide in ambient air where the potential exists for exposure to levels that may be hazardous to health. These guidelines are intended to aid in the selection and application of continuous monitoring equipment for use in production operations in­volving hydrogen sulfide. The term "continuous hydrogen sulfide monitoring equipment" as used herein is defined as equipment capable of continuously measuring and display­ing the concentration of hydrogen sulfide in ambient air. Sec­tion 10 is not applicable to personnel monitoring badges or length-of-stain or color-comparison type detector devices (refer to ANSI/ISEA 102).

GENERAL

All monitors, both portable and stationary, shall be de­signed on sound engineering and scientific principles and constructed of materials suitable for the application. Their design and construction should allow for ease of mainte­nance and repair. Instruments should be verified by a na­tional recognized testing laboratory (NRTL) as meeting the minimum performance requirements of ISA-SI2.15, Part I. Equipment should be installed, operated, and maintained in accordance with ISA Recommended Practice 12.15, Part II.

It generally is recommended (and frequently required) that electrical controls for safety systems such as hydrogen sul­fide monitoring equipment and other gas detector systems be installed normally-energized ("fail-safe"). This means that power is supplied continuously during normal operations to devices which provide alarm(s) and corrective action if con­centrations corresponding to specific alarm set points are reached. Under these conditions, interruption of power due to either deliberate safety device actuation or loss of power will initiate corrective action. It is desirable to provide a test means that will allow the system to be tested (and calibrated) without shutting in producing or gas processing plant oper­ations (or other corrective action), but it should be evident to operating personnel that the system is in the test (bypass) mode.

 

To better ensure proper application, it is recommended that an environmental and application checklist (similar to the example shown in Appendix 1, ISA Recommended Prac­tice 12.15, Part II) be provided to prospective vendors by the

user.

CONSTRUCTION CHARACTERISTICS

The following construction and useability characteristics are desirable for hydrogen sulfide monitoring equipment.

Portability

Portable monitors, including all required parts and acces­sories, should weigh a maximum of ten (10) pounds and have a maximum volume of one (1) cubic foot.

Power Supply, Portable Monitoring
Equipment

Portable hydrogen sulfide monitoring equipment is de­fined as self-contained, battery-operated, carriable or trans­portable instruments capable of operating within speci­fications from integral batteries for a period of eight hours minimum, including a IS minute period of maximum load conditions (with alarms, lights, etc. activated), while exposed to clean air at a nominal temperature of 14°F (-10°C). Appli­cations requiring in excess of eight hours of continuous op­eration or operation at lower temperatures should be specified by the end user.

Readout

Monitors should provide a direct readout of hydrogen sul­fide concentration in parts per million (ppm) by volume.

Recorder Output

For certain applications, it may be desirable for monitors to provide an output signal (e.g., 4-20 ma) proportional to hydrogen sulfide concentration for use in connection with recorders or for other purposes.

Simplicity of Operation

Monitoring and detection equipment should be readily op­erable by personnel without scientific background or training in instrumentation.

Instruction Manuals

An instruction manual should be provided by the manu­facturer with each instrument The instruction manual should contain complete operating instructions, including proce­dures for startup, warm-up time, zero checks, calibration, alarm setting and testing, preventive maintenance, perfor­mance checks, and trouble-shooting. Monitors with rechargeable power supplies should be furnished with in-

structions for charging, storing, and maintaining the power supply. Information also should be included regarding instru­ment recovery time after the exposure of sensor(s) to hydro­gen sulfide. The manufacturer should provide response time data and a list of interfering, desensitizing, or contaminating substances or water vapor concentrations known to the in­strument manufacturer which may adversely affect proper operation and performance of the instrument (refer to Par. 10.4.7). Instruction manuals should include wiring diagrams and estimates of the life expectancy of all consumables. The manual should include a complete parts list suitable for iden­tification of all replaceable parts and sources for procure­ment of these parts.

Electrical Approval

Any portion of a stationary hydrogen sulfide monitoring instrument intended for installation or use in a hazardous (classified) location and all portable monitoring instruments shall be approved for use in such a hazardous (classified) lo­cation and marked accordingly. Refer to NFPA 70, Article 500-3.FPNNO.2.

Ruggedness

Portable monitoring units should be sufficiently rugged to withstand routine transporting, handling, and use in the field environment, as specified by the user. Refer to 1SA-S12.15 for details of a recommended "drop test" to evaluate portable unit ruggedness and a "vibration test" to evaluate ruggedness of fixed and portable monitors.

Calibration Equipment

All accessories required to calibrate the instruments should be made available by the manufacturer. The life ex­pectancy and any special handling required of any hydrogen sulfide test concentrations should be provided by the supplier.

Zero and Span Adjustments

Zero and span adjustment controls should be readily ac­cessible for field adjustment, and the monitor design should include provisions to apply zero and calibration gases to the sensor(s) in a non-laboratory environment. All accessories for calibration and zero should be made available with the monitor and should be useable under field conditions.

Alarm Systems

Fixed monitors shall have provisions for external alarms. Portable monitor units should contain integral audible, vi­sual, or physical presentation (e.g., vibrator signal) alarms as specified by the user. Hydrogen sulfide alarms should be unique to the location.

Testing Alarm Circuitry

Provisions should be included for the testing of alarms and alarm outputs. The test procedure should be included in the equipment instruction manual.

Remote Sampling

A remote sampling accessory (such as a probe) may be desirable for use with a portable monitoring unit. CAUTION: Optional probe attachments for portable moni­toring units, which allow the user to manually draw samples from remote locations, inherently prevent continuous moni­toring of the immediate local environment. Users should consult the manufacturer's instruction manual to determine the proper number of bulb strokes required to draw samples when non-continuous monitoring attachments are used. The remote sampling attachment should be removed after use to restore the instrument to the normal continuous monitoring mode.

Equipment Trouble (Malfunction) Alarm

A trouble (malfunction) signal (indicator or output) should be provided for all monitors.

Detection Range Indication

The range(s) of detection should be conspicuously marked on the instrument.

PERFORMANCE GUIDELINES

The following recommended performance parameters are applicable to fixed and portable hydrogen sulfide monitoring equipment.

Accuracy

Instruments should meet requirements of the accuracy test specified in 1SA-S12.15, Part I. Users are cautioned that the class of instruments suitable for field use are not "laboratory-grade" instruments, and an equivalent degree of precision should not be expected.

Zero Drift

Instruments should meet the requirements of the "Long-term Stability Test" specified in ISA-S12.15, Part I. Exces­sive zero drift is undesirable and can require instrument calibration at unreasonably short intervals.

Warm-up Time

The minimum warm-up time when power is first applied should be stated in the equipment instruction manual. A monitor ready-status indicator is a desirable feature.

Response Time

ISA-S12.15, Part I specifies monitoring equipment mini­mum response time to input step changes. The toxicity of hy­drogen sulfide requires that monitoring equipment have rapid response time to alert personnel of potentially danger­ous concentrations. Hence, response time of monitoring equipment is an important parameter for consideration in evaluation and selection of such equipment.

Operating Humidity Range

Monitoring equipment should meet the "Humidity Varia­tion Test" specified in ISA-SI2.15, Part I. Users should ad­vise equipment manufacturers of the humidity ranges expected for specific equipment applications.

Operating Temperature Range

Monitoring equipment should be suitable for viable use in an ambient temperature range of 14°F to 122°F (-10°C to 50°C). Applications requiring equipment exposure to temperatures outside this range should be specified by the user.

Interferences

A list of interfering, desensitizing, and contaminating sub­stances (e.g., carbon monoxide, sulfur dioxide, aromatic mercaptans, methanol, oxides of nitrogen, aldehydes, carbon sulfide, monoethanolamine, carbon dioxide, benzene, and methane) known by the manufacturer should be listed in the equipment instruction manual. Also, water vapor concentra­tions which may adversely affect proper equipment opera­tion should be included in the instruction manual. CAUTION: Monitoring and detector equipment and sensors should be protected from exposure to liquid spray or wash down. Such exposure can affect equipment performance and reliability.

Functional Field Test

Functional field testing of monitoring equipment should be conducted under "as installed" or "as used" conditions. All instrument and system accessories normally used with this equipment should be installed and operated during func­tional field tests. Functional field tests may include, but are not limited to, exposing the sensor to a sample containing sufficient hydrogen sulfide to cause response of the system. Functional field testing does not necessarily include zero and span adjustments. The hydrogen sulfide concentration used in functional field testing of monitoring equipment should not exceed the maximum operating range of the equipment being tested.

Air Velocity

Monitoring equipment should meet the "Air Velocity Vari­ation Test" specified in 1SA-S12.15, Part I. Accessories are

often available and may be desirable for use with detector equipment installed in areas of high air velocity.

Electromagnetic Interference (EMI)

Some monitoring equipment may be susceptible to elec­tromagnetic interference (EMI), particularly radio frequency interference (RFI). Caution should be taken when using monitoring equipment in close proximity to a radio transmit­ter or other EMI generators. Refer to ISA-SI2.15, Part I, Sec­tions 9.5 and 11.6.