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Summary
Safety Shutoff Valves—North American Approvals
- Factory Mutual (FM) Global Class 7400 (Fire-Safe Class 7440)
- Canadian Standards Association (CSA Group) International Standards Specification Z21.21/CSA 6.5, Automated Valves for Gas Appliances
- Canadian Gas Association (CGA) 3.9-M94, “Automated Safety Shutoff Gas Valves”
Additionally, National Fire Protection Association (NFPA) standards for combustion safety—such as NFPA 85, Boiler and Combustion Systems Hazards Code; NFPA 86, Standard for Ovens and Furnaces; and NFPA 87, Standard for Fluid Heaters—emphasize the use of “listed” or “approved” valves, i.e., manufacturers who have received formal approval by FM or CSA for specifically constructed assemblies that meet the standards stated above.
National Fire Protection Association 85
NFPA 85 Section 5.1.3 states: “All safety shutoff valves, safety interlock devices, valve proving systems, and flame detection systems shall be listed or approved. A safety shutoff valve proof of closure switch shall be an original design component of the valve or actuator assembly and shall activate only after the valve is fully closed.”
Note: FM Approved “Heat-Activated Safety Shutoff Valves” and “Supervisory Cock Valves,” associated with FM Class 7422, are outside the scope of this document.
Approvals and Standards Can Be Consequential
However, many combustion systems, especially in the hydrocarbon processing industry, do involve a “covered” process. While the specific implications are appropriately left to the reader, an OSHA memorandum in June of 2015 provides guidance “on the enforcement of the process safety management (PSM) standard’s Recognized and Generally Accepted Good Engineering Practices (RAGAGEP), including how to interpret ‘shall’ and ‘should’ language in published codes, standards, published technical reports, recommended practices (RP) or similar documents…” OSHA updated this in May of 2016,[i] emphasizing that inspections and tests are to be performed on process equipment (equipment in a PSM-covered process or associated with hazard prevention), subject to the standard’s mechanical integrity requirements and in accordance with manufacturer recommendations.
OSHA specifically refers to provisions noted in consensus documents as “shall” in regards to RAGAGEP, which OSHA would consider for violation if a deviation occurs. OSHA also emphasizes the importance of following industry codes, standards, and recommended practices, such as NFPA documents (previously referenced) for combustion-related hazards.
A review of publicly available OSHA investigative records reveals several accidents related to combustion systems that were not PSM-covered processes. Should a hazardous incident occur, the follow-up investigation and analysis can include an evaluation of whether RAGAGEP, codes, and/or standards were originally followed in design, testing, and maintenance, with accordant liability implications.
What Is Unique about FM-Approved Liquid and Gas Safety Shutoff Valves?
FM Class Number 7400 details what is necessary to achieve approval, which sets performance requirements for liquid and gas safety shutoff valves used in commercial and industrial fuel supply lines to burners and ignitable liquid piping systems. A few of the more notable FM Approval elements and requirements are detailed below:
- The standard only applies to valves with automatic operation.
- Factory Mutual must satisfactorily evaluate the product and manufacturer, including the specific assemblies offered for approval. The process is sufficiently rigorous and not without appreciable cost for the manufacturer. The number of automated valve assembly manufacturers that have qualified for FM Approval is limited.
- Revisions in approved assembly construction are not allowed without proposed changes being vetted by Factory Mutual. To retain FM approval, field repair must be completed by an FM-approved entity.
- FM evaluation includes an examination of manufacturer facilities and quality control execution with regular periodic audits required for continued approval status.
- FM-approved SSVs will have certification marks, applied by the manufacturer, as authorized by FM Approvals, along with other required tag information.
- Valve position indication must be visible from at least five feet. The failure of an electrically operated valve indicator cannot imply an incorrect position. However, these requirements are waived for solenoid valves up to ¾-inch NPT in size/connection.
- Overtravel of the actuator stem is required if an electrically operated valve indicator (limit switch) is used as an interlock in a combustion safety circuit.
- Proof-of-closure switch must be factory set and sealed to prevent field adjustment (including FM approved assemblers).
- The valve assembly cannot have any hardware that is capable of being blocked open or closed.
- Operating temperature range of the assembly must be at least 32°F to 140°F. This is verified in sample assembly leakage testing at minimum and maximum temperatures.
- Valves designed such that upstream inlet pressure tends to open the valve or keep it open cannot be FM approved.
- The model/type identification of the FM-approved assembly must uniquely identify the product as FM approved and exclusive to this approval.
- For sample or type-tested valves:
- Upon loss of holding medium (typically, electricity or air), the SSV must return to normal position within 5 seconds or less under all applicable process conditions for the rated working pressure. This includes multiple tests at various inlet pressures for sample assemblies.
- Through-leakage shall not exceed 400 cc/hr of air or nitrogen for gas valves and 11.8 ml/hr of water for liquid valves for 5 minutes and shall be measured with a sample valve subject to multiple pressures. This leakage threshold must be met for all approved valves produced and tested at the rated working pressure of the valve.
- Approved valves must operate reliably with no significant changes in performance after 20,000 cycles at the rated working pressure.
- All electrical components shall be capable of withstanding a high potential between input terminals and ground for one minute without arc, current leakage exceeding 5 milliamps, or failure at a specified over voltage.
- A specified fire test is required for sample assemblies for fire-rated approval. This involves leakage testing after 60-minute fire exposure.
- All FM-approved assemblies must have a documented seat leakage test, external leakage test, and operation test, which includes confirmation of stroke speed. A special apparatus is used for this testing, which measures dwell time between initiation of signal and movement and open/close stroke times. FM-approved soft seat ball valve assemblies typically achieve “bubble tight” shutoff, which is superior to what is required by the FM 7400 approval standard. Metal seat/disc valves have more allowable leakage in the 7400 standard but generally meet ANSI/FCI 70-2 Class VI.
Additional Engineering Considerations for Fuel Gas Safety Shutdown Valve Specification and Selection
Original equipment manufacturer (OEM)-packaged combustion equipment may be designed to the minimum NFPA requirements unless otherwise requested by the purchaser. These minimum requirements may not meet those defined by the company or plant safety instrumented system (SIS) approach, based upon IEC 61508 and IEC 61511 technical standards for combustion applications. While the specific engineering implications are beyond the scope of this document, functional safety design practices commonly involve the use of safety integrity level (SIL)-rated instrumentation and safety instrument redundancy, as well as a plant-specific safety shutdown valve engineering specification. This may involve manufacturer or third-party certification for the SSV to a given SIL or can include analysis of the probability of failure upon demand (PFD) of a given SSV assembly.
API Recommended Practice (RP) 556, “Instrument, Control, and Protective Systems for Gas-Fired Heaters,” is also used as guidance, especially in hydrocarbon processing plants. RP 556 is sometimes regarded as not being as prescriptive as NFPA, in general, but RP 556 includes more helpful details particular to protective systems common to combustion systems used in petroleum production and downstream processing. Although a requirement for the use of listed or approved safety shutoff valves is not included in RP 556, the specifics on SSVs are notable.
Excerpts from API RP 556 regarding safety shutoff valves (SSVs) are listed here with some key portions noted in bold:
- SSVs are used to isolate fuel sources (fuel gas, pilot gas, or waste heat gas) to a heater after initiation of any of the protective functions, including manual shutdown.
- SSVs be fail-safe (spring return fail closed) and should remain closed until safe conditions are present (i.e., manual reset)
- SSVs should not have hand wheels.
- Solenoid-operated valves shall not allow forcing or reset to the normal position when de-energized.
- SSVs should provide tight shutoff (per ANSI/FCI 70-2 Class V or VI) or bubble-tight (per API 598). The criteria for resolving unacceptable seat leakage rates (e.g., valve proving systems) and valve maintenance intervals should be determined by the owner/operator.
- SSVs should not be used in lieu of manual isolation valve(s) and/or blind for extended shutdown.
- SSVs shall either be fire-safe per API 607 or API 6FA or be located in a fire-safe area.
- Unless otherwise noted in the Safety Requirements Specifications, safety shutoff valves shall have a maximum travel time as noted below:
- Up to 4 inch, 3 seconds
- 6 inch to 8 inch, < 4 seconds
- 8 inch to 12 inch, < 5 seconds
- Since safe state must be achieved within the available process safety time of 5 seconds to 10 seconds (per RP 556 3.4.4.1.2), the Safety Requirement Specifications may prescribe time to safe state not to exceed 5 seconds. This may require larger actuator connections (≥ 1/2 in. NPT) and quick exhaust valves (≥ 1/2 in. orifice) to expedite valve closure time.
- It is recommended that two valves in series be used to isolate fuel gas. This can take the form of double block safety valves (on/off) or a safety shutoff valve used in conjunction with a tight shutoff control valve.
- A double block valve (on/off) arrangement, for one-out-of-two (1oo2) voting, allows for higher performance (SIL) ratings and requires less proof testing than a single block valve.
- In many fired heater applications, the use of a bleed valve between two automated block valves has been discontinued due to environmental and safety implications of releasing fuel gas to the atmosphere. In the absence of a bleed valve, there may be increased concern for seat leakage of fuel gas into the heater. Since the automated block valves should maintain tight shutoff requirements, the purge cycle and sniffing a cold firebox with a portable combustibles analyzer prior to light-off minimizes the process hazard. If the owner/operator elects to implement a valve proving system to verify seat integrity, it is recommended that the automated block valves be proven at the scheduled outage instead of waiting until the startup sequence. This facilitates valve testing and repair in a more practical and timely manner. The basis for seat leakage flow rates at the testing pressure, the corresponding pressure setpoints, and the delay timers that define pass/fail criteria should be documented during the project design phase.
- Safety shutoff valves should be provided with proof-of-closure indication for shutdown verification and startup sequencing.
- A proof-of-closure valve diagnostic alarm is recommended if a safety shutoff valve fails to close within the prescribed time requirements (e.g., 5 to 10 seconds or twice the valve stroke time).
- The shutoff valve actuators should be sized with a safety factor of 25% to 40% more power in addition to typical considerations of the minimum instrument air pressure, operating conditions, and breakaway force or torque required to move the valve.
Going Above Minimum Specifications
While the FM 7400 approval standard requires rigorous testing of sample assemblies at various working pressures, a detailed actuator sizing analysis is not mandated. AWC’s Engineering Specifications and Quality procedures detail automated on-off actuator sizing for many applications.
An excerpt of AWC’s pneumatically actuated spring return actuator sizing documentation is shown below and includes six different torque values for the valve through the full stroke compared to six available torque values for the actuator, as well as a MAST evaluation.
The six required torque values are defined for the valve and actuator at the process conditions. For a spring return to close automated valve, these are as follows:
|
BTO |
Valve Start to Open Torque |
|
AST |
Air Start Torque |
|
RTO |
Valve Run to Open Torque |
|
ART |
Air Run Torque |
|
ETO |
Valve End to Open Torque |
|
AET |
Air Ending Torque |
|
BTC |
Valve Start to Close Torque |
|
SST |
Spring Start Torque |
|
RTC |
Valve Run to Close Torque |
|
SRT |
Spring Run Torque |
|
ETC |
Valve End of Close Torque |
|
SET |
Spring Ending Torque |
Note that the actuator torque data that corresponds to each of the six required torque values is compared to determine whether the required safety factor (see equation) is achieved.
Required safety factor
The generated actuator torque is compared to the maximum allowable shaft torque (MAST) value to ensure this value is not exceeded. Some customer engineering specifications mandate such an analysis for all automated on-off valves, and especially those in emergency shutdown or safety shutoff applications.
In addition, local engineering specifications may list the use of a certain type of solenoid, switch, actuator, etc., that has not previously been submitted to FM Global for approval. Given the expense and time required to obtain approval from Factory Mutual for such variances, choosing to purchase an assembly already approved can be compelling.
It is important to note that failure of an upstream pressure regulator can cause abnormally high fuel pressures at the safety shutoff valves. While it is not typical for such pressures to exceed the ANSI limitations of the valve body, there have been cases where SSVs have failed to actuate due to the higher-than-expected line pressure. For this reason, pressure relief (usually a pressure relieving regulator) is included in a branch line just downstream of the fuel gas regulator on the main fuel train. This relief regulator usually has tight shutoff and would only relieve in the case of a fuel train regulator failure. However, this option is sometimes not permitted by customers, especially if refinery fuel gas is used, due to the prohibition on allowing fuel gas to vent to atmosphere. In such cases, ensuring adequate actuator torque, even at an abnormal fuel gas pressure, is a preferable design choice.
In general, pneumatically actuated valves are less likely to have pressure-drop-related actuation limitations when compared to electric motor/spring-actuated safety shutoff valves. However, motor operated SSVs can produce faster stroke times when compared to standard, pneumatically actuated SSV assemblies. AWC utilizes a high-speed PLC equipped stroke-speed tester to ensure minimum actuation time requirements are achieved.
Given All Of The Above, Where Do We Start?
The engineer selecting or specifying fuel gas safety shutoff valves will typically want to first determine what requirements the Authority Having Jurisdiction (AHJ) mandates for combustion safety. The AHJ may be a regulatory agency or position, such as a local fire marshal or state boiler inspector, but could be a company’s loss prevention inspector, insurance carrier, or even a corporate engineering function. When the authority is FM Global, the installations must be “FM Global Accepted” and the use of products with FM approval may be one component of such acceptance.
Steam boilers, even those installed within process plants, may be subject to state inspection while other fired equipment, even with greater hazard potential, may not. In most cases in the United States, a combustion safety system is, at a minimum, designed to National Fire Protection Association standards, but there are notable exceptions in actual practice. Even NFPA allows for flexibility in some areas regarding retroactivity of newer revisions of the aforementioned codes.
Common Mode Failure: To reduce the potential of common mode failure for low fuel gas header pressure to both pilots and burners, it is recommended to separately source the pilots upstream of the fuel gas controller and safety shutoff valves to the main burners.
Maintenance and Reliability
