North America relies on chlorine for many industrial processes and for making the ingredients that go into many of the nation’s chemicals. But using it carries a certain amount of risk. Among the solutions for handling those risks is the bellows seal metal-seated globe valve. This article outlines the hazards and how this solution was born.
About 13.6 million metric tons of chlorine is produced every year in North America with the greatest volume (40%) going into the production of polyvinyl chloride (PVC). PVC is a low-cost, versatile plastic used to construct everything from water pipes and home siding to appliance parts and food storage containers. Another 37% of chlorine produced in North America is used in other organic compounds, including basic chemicals needed for manufacturing, and solvents for metalworking, dry cleaning and electronics. About 4% is used for water treatment, and other, inorganic uses include producing hydrochloric acid for chemical processes and for making titanium dioxide, a popular white pigment.
Among the risks of using chlorine are:
Hydrogen: Hydrogen (H2) is a co-product of all chlorine manufactured by the electrolysis of aqueous brine solutions. Mixtures of chlorine and hydrogen are flammable and potentially explosive within a certain concentration range. The negative reaction of this mixing can be initiated by direct sunlight, other sources of ultraviolet light, static electricity or sharp impact.
Nitrogen Trichloride: Small quantities of nitrogen trichloride, an unstable and highly explosive compound, can be produced in making chlorine. When liquid chlorine containing nitrogen trichloride is evaporated, the nitrogen trichloride may come to hazardous concentrations in the residue.
Oil and Grease: Chlorine can react, sometimes explosively, with a number of organic materials, including oil and grease, which may come from sources such as air compressors, valves, pumps, oil-diaphragm instrumentation or pipe thread lubricants. Equipment and piping using chlorine must be cleaned to remove this oil and grease. Non-reactive lubricants can help.
Fire: Chlorine itself is neither explosive nor flammable, but it will support combustion under certain conditions. Many materials that burn in oxygen (air) atmospheres will also burn in chlorine atmospheres.
Chemical Action/Reactions: Chlorine has a strong affinity for reacting to many substances. It will react with both inorganic and organic compounds, usually triggered by the evolution of heat. Chlorine reacts with some metals, for example, under a variety of conditions. Chlorine will react with steel and other metals at temperatures above 300°F (149°C). For these reasons, welding piping and other equipment without properly evacuating and purging chlorine from the equipment is avoided at all cost. It is also vital to avoid using any titanium in dry chlorine service.
Corrosive Action on Steel: At ambient temperatures, dry chlorine, either liquid or gas, does not corrode steel. Wet chlorine is highly corrosive, however, because it forms hydrochloric and hypochlorous acids. To avoid this, precautions should be taken to keep chlorine and chlorine equipment dry. Piping, valves and containers should be closed or capped when not in use to keep out atmospheric moisture such as precipitation or humidity. Also, materials of construction must be chosen carefully, depending on the expected conditions. If water is used on a chlorine leak situation, for example, the resulting corrosive conditions will make the leak worse.
Volumetric Expansion: The volume of liquid chlorine increases with temperature. Precautions should be taken to avoid hydrostatic rupture of piping, vessels, containers or other equipment that is filled with liquid chlorine. Any time liquid chlorine can be trapped between two valves, an expansion device should be in place.
Auto-Refrigeration: When a chlorine container is punctured and chlorine is released, it initially will escape rapidly. However, as the chlorine is released, the container that was under pressure will attempt to equalize in pressure with the atmosphere. Once the liquid level drains below the puncture point, auto-refrigeration takes place and the rate of release will significantly decrease, although there still is a considerable amount of liquid chlorine remaining in the container.
Liquid chlorine is a challenging application for valves because of these factors:
Hydroscopic: This condition is the ability of a substance to attract and hold water molecules from the surrounding environment. When the water content in liquid dry chlorine is greater than 50 parts per million (ppm), hydrochloric acid (HCL) and ferric chlorides (CL3Fe) are formed. HCL is highly corrosive and damaging to piping systems made of carbon steel. In addition, CL3F3 scale can be problematic for seating surfaces made of Teflon.
Toxic by Inhalation (TIH): Liquid chlorine is classified as a TIH. To put this in perspective:
- One pound of liquid chlorine will create a lethal chlorine gas cloud equal to 6,440 cubic feet.
- 1000 ppm can be fatal (that equates to two deep breaths).
This illustrates the criticality of this chemical and the need to keep it inside a valve with no leaks from packing, body bonnet joint and castings.
Fire: Liquid and gas chlorine reacts with hydrocarbon-based lubricants, sometimes explosively. Valves must be cleaned and prepared for chlorine service. In addition, proper packaging for transport and storage is imperative. The norm for bellows seal globe valve makers and authorized repair shops is to ship valves with the gasket plus metal blind flanges that have studs and nuts.
Dual Phase Flow: Dual phase flow causes flashing and cavitation so it is problematic for valves because of the energy generated inside the valve that damages trim and stems. When applications involve any throttle, it is critical that end users communicate with valve manufacturers on potential issues. Opportunities exist here for bellows seal globe valve manufacturers to provide contoured discs or an open bonnet design.
BELLOWS SEAL GLOBE VALVES
In 1974 Dr. Roger Papp, engineering manager for UGINE KUHLMAN (now Arkema) asked a valve maker to design and build a bellows seal globe valve for two halogen services, which included chlorine and fluorine, from the ground up. Before this, valve manufacturers just changed the metallurgy on commodity valves for those services. Papp selected a small valve manufacturing plant in central France to execute his vision.
Today, what was created is reflected in what is globally recognized as the Euro Chlor Standards. Although Euro Chlor standards are also recognized in the United States, especially by multi-national firms, the Chlorine Institute and its guidelines are widely used here as well.
The standards include:
- GEST 06/318 – (Flanged Bellows Seal Globe Valves) – Requirements and Design for Use on Liquid Chlorine (42 pages)
- GEST 86/128 – Procedure for Approval of Process Valves for Use on Liquid Chlorine (14 pages)
- GEST 86/129 – Procedure for an Independent Assessment of Process Valves for Use on Liquid Chlorine, Prior to Consideration for Euro Chlor Approval (19 pages)
- GEST 96/220 – Specification for Weld Repairs during Manufacturing of Cast Valves for Liquid and Dry Gaseous Chlorine (10 pages)
To this date, the Euro Chlor membership does not recognize quarter- turn soft-seated plug and ball valves for liquid dry chlorine.
The Chlorine Institute sets safety guidelines for chlorine producers and consumers in the U.S. and Canada. Pamphlet 6 provides the guidelines for piping systems for liquid and gas chlorine.
In the mid-1960s, conventional gland-packed commodity globe valves were used in North America as primary isolation valves at chlorine-producing and consuming sites. To ensure durability, ANSI Class 600-pound valves were installed even when the application only required ANSI Class 300-pound. The trim was historically all Monel. At this point in time, Stellite was not widely accepted for industrial uses.
Starting in the mid-to-late 1960s, all new construction for chlorine production made a switch to quarter-turn ball and plug valves with Teflon seats. The switch was made because:
- The valves require less low torque to operate than rising stem valves.
- The initial purchase price is lower than rising stem valves.
Monel was the material choice for ball valves and stems. Because liquid chlorine could be trapped inside the ball or plug when in the closed position, the downstream-side of the ball or plug had to be drilled for venting.
An interesting note is that one ball valve maker made an ANSI 300-pound valve in a short pattern. It was a great marketing idea because no other company made one. When the valves failed, the user was forced to replace them with the same brand.
In the mid-1980s, the Euro-Zone makers of bellows seal globe valves that were Euro Chlor-approved came to the U.S. looking for market growth. They quickly found out that here, only chlorine producers and consumer sites built before the mid-1960s had ANSI 300-pound and ANSI 600-pound globe valves. The new valves were welcomed and very quickly replaced the commodity globe valves as they were needed for maintenance, repair and operations.
More than 70% of potential users had quarter-turn balls and plugs in place. The problem for the bellows seal globe valve makers then was the 300-pound ball and plug face-to-face was shorter than the 300-pound globe. Meanwhile, the producers and consumers of chlorine were not willing to cut pipe so they could install globe valves. For example, a 2-inch, 300-pound ANSI globe valve face to face is 10-and-a-half inches and a 2-inch, 300-pound ANSI ball and plug valve is eight-and-a-half inches.
Because of this, Euro-Zone makers of bellows seal globe valves in the late 1980s made new patterns for a 300-pound globe valve with a ball and plug valve face to face.
Because of superior performance and lower cost of ownership (see sidebar), bellows seal metal-seated globe valves are growing rapidly in use. From the early 1990s to present, bellows seal globe valves have come to represent 55-70% of the market share for the current installed base. VM
The reasons bellows seal metal-seated globe valves are becoming commonplace in chlorine applications are:
- The bellows seal design eliminates packing leaks by offering a metal barrier between the process and the packing (Figure 2). The HC 276 bellows allows the stem to move up and down acting as an expansion joint. Typically, the bellows are multi-ply with a guarantee of 10,000 cycles at maximum pressure and temperature for the ANSI class.
- Metal seats made of Stellite or HC 276 provide tighter shut-off over time because the sharp edges and hard surfaces cut through without damaging the ferric chloride scale, which does typically cut the soft seats in plug and ball valves (Figure 3).
- Level II castings, which are standard because of the Euro Chlor specifications, deliver confidence to chlorine producers and consumers in North America that the process can be contained.
- Quality trained valve repair companies have driven the total cost of ownership to less than a ball or plug because of the long-lasting castings. It is not uncommon to see valves 30 years old that continue to be rebuilt.
- These valves have several safety benefits including bi-direction vs. quarter turn with vented plug or ball, which makes them single-directional. Also, some manufacturers provide needle roller bearings in the yoke assembly as a standard. This allows for much lower torque output than traditional quarter-turn valves, resulting in fewer injuries for operations personnel.