What’s Your Temperature?
For decades, valve manufacturers have provided the maximum recommended working pressures and temperatures for their products, based on the materials used in the pressure-containing parts.
#standards #pressure-relief #materials
Until the demise of asbestos and the advent of asbestos-free graphite-based packings in the late 1980s, the listed maximum operating temperature requirement for packing and gaskets, as stated in the 1973 8th edition of the American Petroleum Institute (API) API 600 gate valve standard (the primary refinery valve standards document), was 750oF (399°C). This maximum temperature rating covered about 75% of the valves in refinery service, a percentage that is still close today.
Since the theoretical, non-oxidizing operating temperature for graphite is closer to 2000oF (1093°C), the refinery valve standards developers felt comfortable in raising the operating temperature of these packings and gaskets to 1000oF (538°C) in the 1991 9th edition of API 600. The 1000oF (538°C) packing and gasket requirement was mostly an afterthought until recently when the physics of low-emissions packings began to be extensively discussed in industry meetings and forums. For the past several years, most in the valve community ignored the fact that polytetrafluoroethylene (PTFE), or another lower melting point organic compound, was the key ingredient in graphite packings that made them seal fugitive emissions so effectively. PTFE serves an important role in binding the graphite fibers or flakes together, plus providing some valuable lubricity to the packing. The elephant in the room was the fact that PTFE starts to deteriorate at around 550oF (288°C). Burning the PTFE or any other organic material results in a weight loss that corresponds to a volume loss and subsequently, a packing load loss. Which means that if a packing set is exposed to a temperature above the melting point of the PTFE, its ability to seal effectively is severely compromised.
The issue with stating that today’s valve packings must be able to seal effectively up to 1000oF (538°C) is that they cannot repeatably contain fugitive emissions at an EPA and industry-accepted rate of less than 100 PPM at that temperature. As a result, we have a multitude of refinery valve standards in use today that require unattainable performance from the packing.
Although most refinery processes are well below the PTFE melting point, the question becomes whether the valve standards should drop their maximum packing temperature requirement to an effective or realistic 500oF (260°C)? That is not likely to happen, as there are some applications in a refinery where temperatures go much higher than 550oF (288°C), although 500oF (260°C) does match the test temperature of API valve fugitive emissions testing standards RP624 and RP641. So, we are stuck with a major incongruency: The valve packing temperature requirement in the valve standards is unrealistic because today’s state-of-the- art fugitive emissions packing will not seal either effectively or repeatably, at the currently mandated higher packing temperatures.
There is one other factor that is not clearly defined in either the valve or FE testing standards: Does the packing really see the same temperature as the fluid? The answer is: that depends. There are many factors that affect the transfer of heat from the fluid area of the valve up to the center of the packing area. These factors include:
- Insulation around the valve
- Ambient temperature
- Design of the packing chamber bonnet/yoke area
- Valve orientation—is it in the open or closed position most of the time?
- Presence of a double packing set with a lantern ring
- Ceramic spacers below the packing
- Frequency of operation of the valve
- Fluid type
- Velocity of the fluid through the valve
- Fluid state—liquid, gas, or multi-phase
- External heat sources near the packing area
Acknowledging and defining the differential temperature between the fluid and the packing could result in fundamental changes to refinery valve standards, as the valve packing temperature requirement could be lowered from the process fluid temperature.
Some initial testing to determine the actual packing temperature, compared to the fluid temperature has been performed by refinery end-users using non-contact temperature measuring devices. These tests so far are inconclusive, as there are too many variables preventing the accumulation of highly accurate data.
As mentioned earlier, the test temperature for the API valve FE testing standards is 500oF (260°C). The issue of test temperature is being considered as part of the current revision of these standards. An optional high-temperature FE test at 750oF (399°C) has been discussed by the revision workgroups. In conjunction with this revision work, three testing facilities are performing 750oF (399°C) fugitive emissions tests on several similar sample valves. The goal of these tests is to try and determine the viability of a higher test temperature along with the procedure and acceptance standards for such a test.
The results of this testing will hopefully be presented at the next API meeting, April 2021. At that time, the deci- sion to consider lowering the 1000oF (538°C) packing requirement that is resident in several API valve standards and possibly increasing the FE testing temperature to 750oF (399°C), either as a requirement or as an optional test, will be discussed.
It is important that packing manufacturers not only work towards creating packings that pass the API laboratory testing requirements but provide the best high temperature (>550oF [>288°C]) sealing possible. However, the consequences of higher temperature FE leakage might be so insignificant, and not a major real-world issue, that trying to create a new “super” packing to handle this leakage may not be cost-effective or necessary.
Significant thought and discussion are required in considering a change in either the requirements of the testing specifications or the valve design documents, as these documents tend to become de facto “law” and can affect end-user specifications and more importantly governmental regulations for years to come.
Greg Johnson is president of United Valve. He is a con- tributing editor to VALVE Magazine and a current Valve Repair Council board member. He also serves as chairman of the VALVE Magazine Advisory Board, is former chairman of the VMA Marketing & Communications Committee, and is a founding member of the VMA Education & Training Committee. He is past president of the Manufacturers Standardization Society.
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