Rotary Valves Can Prevent Stem Seal Woes
Consider the potential for stem seal leakage when selecting a valve
Experienced process control engineers follow familiar, time-tested steps when presented with a new system to design and specify. Selecting a control valve for a process always requires the same information: collection of the process conditions, calculating required capacity and determining the materials that will survive in the process stream. But some process streams present more challenges than others.
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Challenging gases and liquids
Some processes resist staying within the confines of the piping system. Gases with small molecules, like hydrogen and helium, present many difficulties, such as being able to permeate through the actual metallic structure of diaphragms and thin foils, as well as sealing mechanisms like valve stem packing (for more on hydrogen applications, see Part 1 of this Feature Report, pp. 22–25).
However, even some common liquids can provide a difficult challenge when specifying valves. Heat transfer oil is notorious for leaking through seals, packing and threaded connections. Glycol is another problematic liquid that frequently finds its way through process seals. Petroleum refineries and chemical plants have many intermediate and end products that present similar concerns. These kinds of fluids can be challenging enough if they are only causing unsightly drips, stained insulation or messy floors beneath the pipes. But what if the process fluid is flammable, explosive, poisonous or contributes to climate change?
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Valve seals
Engineers that routinely select globe valves for process control often discover the packing that seals around the rising stem struggles to contain many process fluids. This has become a major concern at petroleum refineries and chemical plants, with government agencies focusing on fugitive emissions from valve packing. The “Clean Air Act” in the U.S. and the “Technical Instructions on Air Quality Control” (TA Luft) in Europe have given birth to a compliance industry to provide the regular inspections and emergency maintenance that their regulations require.
A bellows seal on a globe control valve can sometimes provide an effective solution if the process conditions allow. However, these can be very expensive if exotic materials like Monel or Hastelloy are required to be compatible with the process chemistry. The bellows can also add to the height of the valve and increase the required thrust and thus actuator size and price. They also have a finite cycle life, so they may require frequent replacement if the process requires constant modulation or large valve position changes.
Owners that select valves with rotary operation rather than rising stems have found that stem leakage is much less of a concern. With a rising stem valve, the stem seal is forced to resist the effects of the stem drawing process fluid into the process side of the packing on a rising stroke. Environmental debris can also be drawn into the packing on a lowering stroke. This back-and-forth attack frequently results in relatively quick increases in stem-seal fugitive emissions over the maintenance cycle of the valve. With rotary stem valves, the stem seals are always sealing against the same stem surface. External conditions have very little impact on the seal, which can be optimized to deal just with the process conditions.
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New types of rotary valves
Fortunately, there have been several new types of rotary valves introduced in the last few decades that offer improved performance in severe-service conditions. Many processes that previously could only be safely handled by globe valves now have rotary options. Rotary valves, like ball and butterfly valves, have long been used where high flowrates and minimal to moderate pressure drops were the order of the day. However, now there are rotary valves that handle low-valve- flow-coefficient (Cv) requirements, and noise and cavitation-resistant trims are available on several types of rotary stem valves. There are far fewer processes now that must use a globe valve.
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Figures 1 and 2 depict one example where a new type of rotary valve provides improved performance over a traditional approach. These pictures show a stark difference in the stem seal performance of a globe (Figure 1) and a rotary (Figure 2) control valve installed in the same process for the same length of time. The owner noticed his globe valves developed “fuzz balls” of process liquid that leaked through the packing and then crystallized. The process fluid in this case is both toxic and flammable, so he looked for another solution. The rotary valve showed no stem leakage over the time it took for this fuzz ball to form.
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By changing the type of control valve specified, you may eliminate an operational and maintenance headache for your plant. Remember to consider valves with rotary stem sealing for your problematic processes.
Article featured in Chemical Engineering Magazine
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