Last November, FM Approvals released a revised standard for safety shutoff valves that strengthens the requirements for these components and consolidates nine sub-classes of valve standards into one comprehensive standard. Approval Standard 7400, Liquid and Gas Safety Shutoff Valves, establishes new performance requirements for both non-fire safe valves (FM 7400) and fire-safe valves (FM 7440).
Safety shutoff valves are important elements in automatic emergency shutdown systems designed to help control and mitigate the damage caused by fires involving ignitable liquids and gases. These valves close automatically when exposed to fire or can be triggered remotely, cutting off the supply of ignitable liquid or gas and helping to reduce fire severity. Some safety shutoff valves may also open automatically (or when signaled) to release fire extinguishing agents or provide storage tank vacuum protection (in a fail-open configuration).
A non-fire safe shutoff valve is intended to be located in an area remote from where the fire exposure is anticipated. A non-fire safe valve will, typically, leak—from the stem and/or seat—if exposed to fire for a significant length of time.
Fire-safe shutoff valves are valves that, when exposed to fire, will not only close or open automatically, but also remain leak-free or exhibit low rates of leakage through valve seats, gaskets, and stems even when fully engulfed in flames. Fire-safe shutoff valves may be held open or closed electrically, pneumatically, hydraulically, mechanically or by a combination of methods.
A fire-safe thermal shutoff valve is held open by a fusible link that automatically triggers at a predetermined temperature (e.g., 165°F or 74°C), allowing a spring-powered mechanism to close the valve. An electro-thermal fire-safe valve is also equipped with a spring-powered closure device that reacts when subjected to either heat or a low-voltage electrical impulse. The electrical impulse may be triggered by heat, smoke, gas, flame energy, maximum temperature or rate of temperature rise detectors.
Chemical and petrochemical plants, oil and gas drilling operations, on-shore and off-shore installations, oil refineries, tank farms, power generation, diesel-fired boilers, and electrical discharge machining are just some of the systems and facilities that might rely on fire-safe and non-fire safe shutoff valves. Automatic shutoff valves can help reduce the environmental and financial impact posed by an uncontrolled release of ignitable liquids, toxic chemicals, and gases.
"There was no specific situation that moved us to revise this standard—it was a combination of factors," notes Joe Looney, an FM Approvals advanced engineer and author of the revised standard. "However, there was clearly a need to clarify and consolidate multiple valve sub-standards. Our goal was to be clear about the scope of the standard, standardize the terminology we use and provide a more rigorous and realistic fire exposure test."
According to FM Approvals technical team manager Stan Ziobro, development of the revised standard was closely tied to ongoing research. "The previous version of this standard used a pan fire test that simulated a pool fire that might occur as a result of fuel leakage," Ziobro notes. "However, this test primarily relied on temperature and didn't provide the full story, such as the impact of radiant heating. We launched a research effort in 2015 to determine the best way to test the valves and we've incorporated the findings in the revised standard."
New fire test protocol emerges
To determine the best test to replace the legacy heptane pan fire used in the original version of the valve standard, FM Global Research conducted a series of fire tests in mid-2015 using a representative selection of fire-safe shutoff valves, including both FM Approved and non-FM Approved products.
"Early on in our testing we determined that these valves need to be under pressure to work correctly," explains Stephanie Thomas, FM Global senior research engineer. Many fire-safe shutoff valves use dual-seat sealing construction to contain through-seal leakage before, during and after a fire. The first—primary—seat is typically an elastomer material and the secondary seat is metal.
The elastomeric primary seat provides leak-free normal operation. During a fire, however, the elastomeric seat melts away leaving the metal seat to provide an adequate seal to prevent excessive leakage. Some leakage typically occurs, but should not be enough to adversely influence fire behavior.
According to Thomas, a key challenge in developing a test for the fire-safe shutoff valves was to provide a pressurized circuit that would simulate an adequate working pressure, but not over pressurize the valves during the fire test; and, at the same time, capture or quantify any leakage from the valves during the test.
Thomas developed a fire exposure test apparatus (Figure 1) based on the parallel panel test (PPT). The PPT is used by FM Approvals in fire testing a wide range of building and fire protection products. The PPT apparatus consists of two (non-combustible) panels held 21 in. (0.5 m) apart in a metal frame with a propane sand burner at the base (Figure 2). The test valve is mounted 18 in. (46 cm) above the burner.
The valve test, now incorporated in Approval Standard 7400, delivers a uniform heat flux of 125 kW/m2 for 60 minutes at the valve's rated working pressure (Figure 3). Stainless steel tubing and a positive displacement pump are used to pressurize the test valves with water. A water jacket is provided downstream from the valve to capture, condense and measure any leakage using a container on a load cell.
"The parallel panel test delivers an increased fire load compared to the old pan fire test," Thomas says. "This now aligns the fire test methodology for fire-safe shutoff valves with that used to evaluate other equipment for ignitable liquid service across the applicable FM Approvals' standards."
Ultimately, the research study confirmed the effectiveness of the PPT and the valve test apparatus. It was determined that the allowable total leakage from valves undergoing the fire test will be limited to 17 oz./min (500 ml/min) over the 60-minute test period.
The bar has been set
The revised FM 7400 standard includes the following performance requirements:
- Seat leakage. Through-the-seat leakage in the closed position shall not exceed 24 in3/hr (400 cc/hr) of air or nitrogen for gas valves and 0.4 oz/hr (11.8 ml/hr) water or water glycol-based hydraulic fluid for liquid valves.
- Body leakage. Safety shutoff valves shall be able to withstand an internal pressure equal to 200 percent of the rate working pressure without external leakage or damage.
- Durability. Safety shutoff valves shall operate reliably after 20,000 operational cycles or the specified maximum allowable cycles (whichever is greater) at the rated working pressure. The total operational cycles may be reduced to 1,000 for heat-actuated valves (i.e., spring-loaded and fusible link-activated).
- Electrical insulation. All electrical components of a safety shutoff valve, if so equipped, shall be capable of withstanding a high potential between input terminals and ground, for one minute without arcing or breakdown.
- Voltage variation.
- Actuator pressure range.
- Ambient temperature effects.
- Actuator control valves.
- Fire exposure test. To be classified as fire-safe, safety shutoff valves must not exceed an average combined internal and external leakage rate of 17 oz./min (500 ml/min) while at rated working pressure (+/- 10%) during a 60 minute fire exposure at a uniform heat flux of 125 kW/m2 (+/- 5%). Visual observation of external valve leakage in the form of a spray shall be considered a failure.
- Operational requirements include: demonstrated quality control program, initial and surveillance audits, installation inspections, change notification, and manufacturing and productions test.
Manufacturers of current FM Approved fire-safe shutoff valves have until September 1st, 2018 to comply with the new requirements of FM 7400. "I can tell you that the FM Approved fire safe valves we tested during our research performed substantially better than the non-FM Approved valves," Joe Looney notes. "By consolidating multiple sub-classes of valve standards into this newly revised standard, we aim to make it easier for anyone using the Approval Guide to find the right FM Approved valve for their purposes. While we've set the bar high, we believe that the new requirements are attainable and, ultimately, the end users will benefit from products that have been tested to FM Approval's highest standards, and designed to offer the utmost in quality and fire protection performance."