Simply put, a valve-proving system is a safety control used on gas fuel-fired industrial heating equipment that verifies the effective closure of two safety shutoff valves in series by detecting gas leakage.
TIP 1: Understand Valve-Proving Systems' Beginnings
In the 1950s and '60s, as the gasification of Europe began, the European code bodies adopted the U.S. safety standards and practices for gas combustion. Europe simply had little experience with natural gas at that time, and the U.S. safety practices, which applied redundant safety controls, were adopted. As a result, Europe adopted the "double block and vent" and "proof-of-closure" concepts as a means to validate the prepurge cycle.
As Europeans gained experience with fuel-fired equipment, incidences occurred that illustrated the shortcomings of proof-of-closure switches and vent valves, which could allow the system to operate under unsafe conditions. This provided the impetus for European manufacturers to look for other solutions, and the concept of a valve-proving system (VPS) was introduced in Germany in the early 1970s.
TIP 2: A Valve-Proving System Confirms the Effective Closure of Both Safety Valves
Currently, two basic types of valve-proving systems are available: the active, or pressure, system and the passive, or static, system.
The active valve-proving system detects gas leakage by verifying flow through an orifice of known diameter and proves both safety valves simultaneously (figure 2). The active valve-proving system confirms the effective closure of both safety valves by:
- Opening an internal safety valve, which typically is a 1 to 2 mm port diameter auxiliary safety valve that bypasses the first main safety valve.
- Starting an internal pump and simultaneously starting an internal timer. The pump pressurizes the volume between the two main safety valves using gas pressure from upstream of the first main safety valve.
- Monitoring positive differential pressure between the two safety valves within the specified time.
If the differential pressure between the safety valves reaches approximately 0.5 psi over the inlet pressure within the specified time, both safety valves are proven closed. If the pressure between the safety valves fails to reach the overpressure level within the specified time, the valve-proving system detects a leak and locks out the system.
Rather than proving both safety valves at the same time, the passive valve-proving system proves each safety valve separately (figure 1). When proving the No. 1 main safety valve, the passive valve-proving system detects gas leakage by monitoring for pressure rise between both safety valves; and subsequently when proving the No. 2 main safety valve, it monitors for pressure decay between both safety valves. The passive valve-proving system may have its own auxiliary safety valves that bypass main safety valves, or it may simply cycle the main safety valves during valve proving. However, to illustrate the valve proving sequence, it is easiest to show auxiliary safety valves. During valve proving, the passive valve-proving system proves the effective closure of the No. 2 main safety valve by:
- Opening the No. 1 auxiliary safety valve and allowing upstream gas pressure to fill the volume between both main safety valves.
- Closing the No. 1 auxiliary safety valve, and simultaneously starting an internal timer.
- Using the No. 1 pressure sensor to monitor the pressure between the two safety valves.
If the No. 1 pressure sensor does not detect pressure decay within the specified time, the second main safety valve is proven closed.
Essentially, the same process is repeated for proving the No. 1 main safety valve, but this time, the No. 2 auxiliary safety valve vents the volume between the main safety valves and the No. 2 pressure sensor monitors the manifold for pressure rise. If the No. 2 pressure sensor does not detect pressure rise within the specified time, the first main safety valve is proven closed.
If either pressure sensor detects a leak due to pressure decay or pressure rise, the valve-proving system locks out the system.
TIP 3: Valve-Proving Systems Are Included in Standards
A valve-proving system currently is used on gas-fuel-fired equipment in Europe, Australia, China, Russia and in South and North America (figure 3). The following codes and standards that apply the use of valve-proving systems can be used as reference:
- European Standard EN 746-2 for industrial thermoprocessing equipment.
- European Standard EN 676 for automatic forced draft burners for gaseous fuels.
- Australian AG 501 code for industrial and commercial gas-fired appliances.
In the United States, a valve-proving system can be used as an alternative to a vent valve in the following standards:
- NFPA 85 (when venting of gas is prohibited).
- NFPA 160, Standard for Flame Effects Before an Audience.
- Fuel-fired equipment insured by Factory Mutual (FM).
- Fuel-fired equipment insured by GE Global Asset Protection (GAP) Services (formerly IRI).
- Ford Motor Co. specification.
- General Motors specification.
Also in the United States, a valve-proving can be used as an alternative to proof-of-closure in the following standards:
- Fuel-fired equipment insured by FM.
- Fuel-fired equipment insured by GE GAP Services.
NFPA 86 currently is under revision, and the revision has provisions for either a valve-proving system or a proof-of-closure switch.
TIP 4: Understand Why Equipment Standards Require a Proof-of-Closure Switch, Normally Open Vent Valve, and/or a Prepurge
One major hazard with fuel-fired equipment is having an explosive mixture of fuel in an enclosed, or semi-enclosed, area that cannot safely relieve the expansion forces of the gas when ignited. Historically, the number of boiler and furnace explosions were reduced by requiring certain safety practices and incorporating certain safety controls into the fuel-fired equipment. This was done to provide an acceptable level of risk to the public.
For fuel-fired equipment, a high risk of incident occurs during burner lightoff. One important practice to reduce the risk of explosions is to prepurge the combustion chamber. The intent of prepurge is to remove all combustible gases from the combustion chamber before introducing an ignition source; the common four air-change prepurge is based on a worst-case scenario of having a burner chamber completely filled with fuel gas.
If both safety valves did not leak gas, there would actually be no reason to prepurge. However, loss experience has shown that safety valves can fail to close and can leak gas into the combustion chamber; therefore, the chamber is prepurged to provide redundant safety in the system in case both safety valves leak gas into the combustion chamber. The addition of proof-of-closure switches and/or a vent valve provides an additional level of safety to simply validate prepurge.
TIP 5: Proof-of-Closure Switches Are One Way to Validate Prepurge
Before introducing an ignition source, prepurge should be validated. Validate does not imply guarantee but rather implies a high degree of confidence that the risk to introduce an ignition source in the combustion chamber is considered negligible. For example, a prepurge is not valid if the safety valves were wide open during prepurge.
To validate the prepurge, a combustion air switch verifies adequate airflow. In addition, there should be some means to verify that gas is not flowing into the combustion chamber at a rate such that prepurge is not able to remove enough gas from the chamber to remain far below the lower explosive limit (LEL) during ignition. A proof-of-closure switch on a safety valve provides a degree of confidence in the "validity" of the prepurge by verifying that the valve is in its fully closed position. A proof-of-closure switch on each safety valve provides even a higher degree of confidence.
A normally open vent valve mounted in between both safety valves also provides a degree of confidence in the "validity" of the prepurge. This valve prevents the buildup of pressure between the safety valves and thus minimizes gas leakage to the burner if both the No. 1 valve leaks and the No. 2 valve leaks in the closed position.
TIP 6: A Valve-Proving System Is an Alternative to a Normally Open Vent Valve
In the case that No. 1 valve and the No. 2 valve leak in the closed position, the vent valve mounted in between two safety valves provides a degree of validity to the prepurge. The vent valve minimizes the potential for leakage into the furnace by diverting any leaking gas to a safer location. Unlike a vent valve, the valve-proving system does not allow for ignition if either valve is detected as leaking, thereby validating the prepurge cycle. Furthermore, a vent valve will not interrupt the limit circuits; thus, ignition is still allowed. By contrast, a valve-proving system locks out the ignition sequence when a problem is detected.
Unlike a valve-proving system, a vent valve allows ignition under conditions when far more gas can leak to the burner. Instances when a vent valve would allow ignition include:
- Both valves, which have severely damaged valve seats, leak in the closed position.
- The No. 1 valve fails to close and the No. 2 valve leaks.
- The No. 1 valve leaks in the closed position and the No. 2 valve fails to close.
- Both valves fail to close.
TIP 7: A Valve-Proving System Is an Alternative to Proof-of-Closure Switches
A proof-of-closure switch validates prepurge by proving that the valve stem is in the minimum (closed) position. The valve stem position is related to the valve seat position. If a switch indicates that the valve stem is closed, there is a high degree of certainty that the valve seat is not leaking gas into the burner.
Similarly, a valve-proving system provides a high degree of certainty by directly verifying the position of both valve stems. If a valve stem does not close all of the way, the valve seat is not in contact with the valve disc, and the valve-proving system detects an open valve due to gas leakage. The valve-proving system also verifies the valve seats. If the valve stem closes 100 percent but the valve seats are damaged, a valve-proving system will detect a problem and prevent ignition.
A proof-of-closure switch does not detect problems with a valve seat but only verifies the position of the valve stem. Figure 4 shows a relationship between valve seat closure and valve stem closure. Illustrated is a 0.25 mm dia. wire inserted below the valve seat of an energized (open) valve. The valve then is de-energized (closed), and the valve stem fully closes while at the same time the valve seat does not. The amount of gas leakage due to the 0.25 mm dia. wire is detected by a valve-proving system but not a proof-of-closure switch.
TIP 8: Don't Consider a Valve-Proving System an Alternative to the Annual Valve Seat Bubble Test
The valve seat "bubble" test is an annual maintenance requirement for installed safety valves in most fuel-fired equipment standards and generally a requirement or recommendation of safety valve manufacturers. The allowable valve seat leakage rates on installed safety valves are determined by the safety valve manufacturer, who typically applies the same requirements as specified in the safety valve standard ANSI Z21.21/CGA 6.5. This standard is applied to new safety valves leaving the factory. The allowable rates for installed valves typically are not more stringent than ANSI Z21.21/CGA 6.5 requirements.
A valve-proving system is not designed to detect leakage rates less than or equal to the ANSI Z21.21/CGA 6.5 standard; it is designed to detect leakages above the ANSI Z21.21/CGA 6.5 standard. Therefore, a valve-proving system, which is intended to prove the effective closure of both safety valves by detecting leakage, is not currently an alternative to the annual valve seat bubble test.
TIP 9: Don't Confuse the Valve-Proving System's Detection Limit with the Valve Bubble Test Standard
Generally speaking, a valve-proving system's detection limit is the maximum amount of gas leakage at which a prepurge can still be validated. A valve-proving system should take into account application-specific variables such as operating pressure, fuel gas type and the manifold volume. To simplify this, the European standard EN 1643, "Valve Proving Systems," adopts a maximum rate of 1.76 ft3/hr (50 l/hr) or 0.10 percent of burner capacity. In combination with a prepurge, this maximum rate still provides a high degree of certainty to allow burner ignition.
A common error in thinking is that a valve-proving system should detect leakage rates to the ANSI Z21.21/CGA 6.5 safety valve standard. This might be applicable if a valve-proving system were used as an alternative to the annual valve seat bubble test. However, no equipment standard requires a valve seat bubble test at every burner startup or shutdown.
TIP 10: Take Advantage of a Valve-Proving System's Safety Features
A valve-proving system provides several safety benefits. It actively checks the integrity of both safety valves on every startup or shutdown. It detects safety shutoff valve problems that other safety controls ignore. It is equally safe with heavier-than-air fuels as with lighter-than-air fuels. Also, it gives the user at least some idea of the condition of the safety valve seat leakage rate.
When a valve-proving system is used as an alternative to a vent valve, no gas is released to the atmosphere. As a result, it is environmentally friendly. Furthermore, unlike a proof-of-closure switch, a valve-proving system is a "self-checking" safety control, which means that the system faults and does not allow ignition if its internal proof-of-closure signal fails to change state.