Why 3 Way Ball Valves Cannot be Fire Safe
A ball valve with a soft polymer seat is used when a tight shut-off of flow is required. The excellent shut-off capability of polymer seats is tempered by a high-temperature service constraint. Depending on their seat and seal properties, soft seated ball valves can operate at maximum temperatures. These valves are designed to melt or vaporize in the event of a fire at a site or factory where they are installed, leading to massive pipeline leaks. Ball valves with fire-safety features were introduced to prevent such an event. The secondary metal seats come into play when the soft seats melt due to heat, caused by a fire. When the fire safe ball valve is closed at the time of the fire, the central part of the leakage is arrested. After passing a test specified by API 607, which is a standard for isolation valves, such ball valves are certified as fire safe.
The next topic we will discuss is 3 Way ball valves. A three-way ball valve diverts flow in one direction. Therefore, 3 Way valves function like elbows. There is no doubt that elbows do not stop fluid flow, but rather divert it through 90 degrees. We find that a 3 Way ball valve shall continue to allow fluid to flow through it in the event of a fire under the conditions of fire safe testing. Due to the temperature, the polymer seats will melt, but adding a metal secondary seat will not make the 3 Way ball valve fire safe.
Some manufacturers provide a fire safety certificate along with their 3 way ball valves. Fire tests can only be conducted on a 3 Way ball valve by closing one outlet and using the other two ports for pressurizing and testing leaks. Although this arrangement mimics actual usage conditions where one outlet cannot be shut off, it does not accurately reflect actual usage conditions. Under normal working conditions, one port shall continue to allow fluid passage. Although the arrangement for testing the fire safety of the secondary metal seat on the closed side meets API 607 standards, it does not test the fire safety of the open side.
3 Way L Port ball valves have one more unique issue that prevents them from passing fire safety tests. Whenever pressure is applied to a two-way valve, the ball inside the valve will float toward the downstream seat. During fire safety testing, this characteristic is exploited to the fullest extent possible. After the primary polymer seats are destroyed, the ball keeps floating under pressure towards the secondary metal seats and seals the leak. However, when a 3 Way ball is connected to an L port, it floats only in one direction. Rotating the valve 90 degrees prevents the ball from floating. The ball of a 3 Way ball valve with one outlet shut is arranged in such a way that it floats toward the metal seat during fire safe testing. In this configuration, the valve may pass the "fire safety test". When the ball is rotated 90 degrees, no floating is possible, and the valve will fail the fire safety test. As a result, the fire safe test is staged to suit the convenience of the manufacturer since it can only pass in one configuration.
It is only when sufficient pressure is built inside the ball valve that the ball can float and minimize leaks that the ball valve can pass the fire safe test. The fire safety test should mimic actual working conditions since the valve should prevent a catastrophe during a fire. During a fire incident, a 3 Way ball valve must continue to allow fluid flow to one port. This does not meet the standard's requirements. Based on whether the ball is floating or non-floating at the time of the incident, the flow to the closed port may be allowed or prevented. Even if someone argued that the valve minimizes flow through the closed port during a fire, it is evident from the above information that the valve is only fire safe 50% of the time. If a product fails half the time, it cannot be certified as conforming to a standard or engineering practice. Therefore, 3 way ball valves cannot be fire safe.
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