If a valve in your thermal fluid heating system fails, catastrophe could result.
Your unit just got the call that your plant has an emergency situation. This is not a drill: Your thermal fluid (hot oil) system has to be shut down -- immediately!
The word goes out to close all block valves and shut down. As one of the 10" gate valves is closed, hot oil begins to leak at the stuffing box packing gland. The packing gland is tightened, and the leak appears to stop. Just as the maintenance guys are putting their gear away, the leak starts again. They put more torque on the packing gland and the leak is reduced, but it does not stop! A “cheater” bar is added to the wrench and a considerable amount of torque is applied to the packing gland. Just as the leak appears to stop, the wrench gives way and slips off the valve. Hot oil sprays under the boiler and ignites. It looks as if a flamethrower is loose in your unit. Luckily, everyone evacuates the area and the fire control team quickly prevents the fire from spreading to other parts of the plant.
The following day, as you survey the damage, you realize that every valve must be sent out and rebuilt; control systems replaced; wiring, tubing, gauges and instrumentation, gone. What should have been a quick one- or two-hour shutdown will now drag on for weeks or months. You cannot begin to calculate the production time and money lost. How could this have happened? What went wrong?
A fire very much like this occurred at a chemical plant less than one year old. It could happen at your plant. But, it can be prevented. I believe that valve selection during the design phase is more to blame than the crew that tried to stop the leak. Just as you should use the right tool for the job, you must choose the right valve for the service.
Chemical, petrochemical, textile, wood, paper, asphalt, plastic, rubber, food, metal working, waste, pharmaceuticals and other process industries are all hot oil users. Thermal fluid systems can have operating temperatures up to 750oF (399oC). Start with a liquid that has little viscosity, heat it up to 750oF and you have a “liquid” that is extremely difficult to contain.
The Valve You Choose Matters
Think about the valves in your hot oil system. Block valves are not cycled very often -- in some applications, maybe once a year. If you are lucky, depending on your system and operating conditions, you may only cycle them once in two or three years. The ideal plant situation is to not shut down but let it continue to run and produce product.
Stuffing box valves that “back seat” when fully open, have highly polished stems and high temperature graphite-type packing appear to be a good choice for block valves. They work great in all types of steam applications, and steam is the most popular method of thermal fluid heat transfer in this part of the world.
But think about the valves in your hot oil system. If a valve is open all of the time, foreign material may get on it and adhere to the exposed stem. In an emergency situation, you do not have time to clean and polish the stem above the packing. If you cut a path through the packing with the dust, dirt and grime on the stem, you will have packing leaks. If you cut a large path through the packing, you may not be able to stop the leak.
Thermal fluid energy transfer systems are not as well known in the Americas as in Europe and other areas of the world where energy costs are quite a bit higher. In those areas, industry was forced to look at more efficient, reliable and safe means of energy transfer years ago. They chose hot oil over the less efficient steam and developed specific and restrictive specifications for materials (equipment) used in hot oil systems.
For example, the European standard DIN 4754 requires that stop valves, controllers and rotary transmission leadthoughs use bellows-type seals with renewable safety backup packing. Pipe, valves and fittings preferably are permanently joined such as by welding. However, where this is not practical, flange joints are permitted (for example, as in DIN 2401-12, with bolts complying with DIN 2507), including flat-faced, raised-face and tongue-and-groove flanges.
At present, the European standards are not in place in the Americas. Some general guidelines will help you select a thermal fluid system that will be efficient, reliable and safe.
Screwed connections should be avoided wherever possible. Most hot oil systems operate from 45 to 65 psi. PN16 and PN25 flanged control valves of nodular iron are the accepted global standard for hot oil thermal fluid service if the system is put together properly, with the correct bolts, nuts, gaskets and other hardware. PN25 is more than enough for most, if not all, American hot oil systems.
Valves are a major source of leaks in thermal fluid systems, with most leaks occurring at the stuffing box. To reduce leaks, eliminate the stuffing box and weld everything in. It is not always practical to weld in your control valves, but you can eliminate the stuffing box. Think about the valves in your hot oil system.