Thermoplastic Valves Thermoplastic Valves
Pressure & Flow Control Pressure & Flow Control
Sodium hypochlorite possesses some unique qualities that have to be understood if you are going to be able to select a valve for that type of service. Because of their excellent chemical resistance, plastic valves are widely used in sodium hypochlorite applications. But in many cases, the wrong type of valve is put into this service and problems, some potentially dangerous, can develop.
Sodium hypochlorite is inherently an unstable compound. Two things can happen to it in a piping system that will affect the choice of a valve. Sodium hypochlorite can decompose over time, which results in the formation of crystalline salts, and as it decomposes, one of the decomposition by-products is oxygen gas.
Because they are the most common type of plastic valve, True Union Ball Valves are often used in sodium hypochlorite service. This type of valve may perform satisfactorily in some applications, particularly in 2" and under pipeline sizes, but in many other applications, it won't.
The main problem with ball valves, especially those over 2" in size in sodium hypochlorite service is the crystallization problem. When the valve is operated, the ball turns and liquid is trapped in the cavity between the ball and the valve body. The trapped sodium hypochlorite starts to decompose and crystals begin to form. The crystalline salts adhere to the surfaces of the ball and seals - causing a freezing of the valve and making it inoperable. When the valve is operated, the torque required to do so may be greater than the strength of the valve stem - causing it to break. But that is just the start of the problem. Now the valve has to be taken out of service to be repaired. If oxygen gas has formed and become trapped behind the ball, it is now pressurized. When the ball is freed-up and turned, the pressure of the oxygen gas will blow out any liquid sodium hypochlorite trapped behind the ball. A dangerous situation for operating personnel.
These types of problems are most likely to occur in valves larger than 2" that are operated infrequently. And also in valves installed at the bottom of tanks where precipitated impurities could get into the valves and cause accelerated decomposition of the sodium hypochlorite.
A plastic, PVC butterfly valve with FPM or Hypalon seals is often a better choice for sodium hypochlorite service in pipelines 2" and larger. Butterfly valves are less prone to freezing up than are ball valves because of the formation of crystallite salts on the sealing surfaces of the valve. For smaller size pipelines a plastic diaphragm valve may be a better choice than a ball valve for the same reasons. Remember that all applications are unique and these recommendations may, or may not, apply in all cases. Sodium hypochlorite is a potentially dangerous process media and Material Safety Data Sheets should be consulted for additional information and safety precautions considered before any final determination is made regarding the application.