In any modern Power production facility, there are hundreds if not thousands of isolation valves of many types. In traditional thermal power production, many of these are in high pressure / high temperature service and draw the highest level of attention. These critical valves are often in thermally cyclic service and have a special set of demands associated with their applications
However, in these facilities as well as in Solar Thermal, Geothermal and small cogeneration facilities there are many low pressure (ASME Class 600 and below) valves in applications such as boiler feed water, condensate, cooling water, thermal fluids, chemical feed, water wash, etc. These valves often have more complex problems due to the variable nature of the materials inside and can be very costly in terms of lost production and expensive repairs during plan shutdowns.
By Barry Hoeffner – Director of Quarter turn Products, Ladish Valves
Issues with these low-pressure isolation valves can generally be broken down into three major areas:
- Inability to operate
- Poor Sealing
- Uncontrollable emissions to the environment from packing or seals
Importance of Material Selection
While there can be many reasons for the above, including the valve just being past its useful life, there are a number of applications where corrosion or coating failure is occurring due to poor material selection or changes in the operating conditions since the valves were initially specified. In the case of emissions, the packing and seal designs may be outdated, or were never really designed for use in a power generation environment.
While many valve types are used, it is very common to find low pressure valves with carbon steel (A216 WCB, A105, etc) bodies and stainless steel (316, 410, etc) internal parts. While this MAY be a good selection for the most benign conditions, there are many cases where the presence of oxygen, salts, acids, caustic, etc. may be attacking the base material. If this is true, simply continuing to replace the valve “in kind” with an inexpensive identical item is actually a very expensive approach. Often the cost of replacement is much greater than the cost of the valve itself. If corrosion is indeed an issue, a valve with a higher alloy body or at least trim could be a more cost-effective solution.
There are clearly many material options available in valves and there are many useful guides available. Particularly in ball valves it is possible to “mix-and-match” valve body and trim material to greatly extend the useful life of the valve in service. For example, if a system can see acidic conditions from time to time, the use of Alloy 20, Monel® or even Hastelloy® can reduce the corrosion rates significantly and are readily available. Also, if the valve utilizes some type of coating on the sealing surfaces, the better base material can extend the life of the valve and prevent the “sloughing” of that coating. Experience in plants with lines that are used infrequently often benefit from alloy valve materials since many chemical compounds (cleaners, water treatment chemicals, etc.) can degrade over time and become very acidic or caustic.
An interesting learning is that many plants utilize carbon steel piping with no corrosion issues, but valves in those lines can experience attack. This generally comes from the fact that the piping develops a dense surface layer that passivates the metal. As long as nothing disturbs this layer, corrosion is not much of an issue. However, inside the valve there can be turbulence that removes this layer and exposes the bare metal to corrosion. This same effect happens on the sealing surfaces of valves as the simple act of cycling the valve removes any passive layer. If this is happening, a move to a valve with a more corrosion resistant alloy will almost certainly improve reliability.
Mitigating the Risk of Leaks
In a recent example, a Combined-Cycle Power Plant in Texas had experienced repeated failures of small “Trim 12” (Stainless Steel) gate valves in a “water wash” service. The customer reported that the gate and body was being “washed away” with the valves lasting no longer than 60 days without leak-through and packing failure. After looking at the system it was realized that hot salt-rich water was flowing through the line followed by a cleaning solution. By moving to a carbon steel (like the pipe) ball valve with Monel internals, PTFE Packing and TFM® seats, the valves have been working without issue for over 18 months with no sign of attack.
If the failure mode of the isolation valve is packing or seal leaks, there can again be several root causes and opportunities for improvement. The first question to be asked for valves with a history of packing leaks is whether the packing is manufactured from the correct material. It is amazing how often a valve is ordered with Teflon / PTFE packing then is used in steam service above 500oF. It may last a short time but will quickly develop a leak. Other designs are found with packing that cannot be tightened, graphite that has degraded over time, or seal material that is incompatible with the service. If packing leaks are a continuing issue, you should consult with your supplier or the valve manufacturer about available options. The reality is that with the cycling nature of today’s power producers, the valves are subjected to stresses and conditions not expected when the specifications were written. Packing materials, including the new API 622 certified graphite blends, are far superior to what was available in the past, and when properly applied in a good valve design can make packing leaks a thing of the past in these low-pressure isolation valves.
The summary of this article is that there is no reason to accept poor performance, even in the most “lowly” isolation valve in your plant. In a time when personnel are in short supply and the demands on the plants are greater than ever, it should make sense to give some thoughts as to why a valve needs to be replaced or repaired. Like everything, parts, including valves do wear out over time, but proper materials, modern designs and good application engineering can improve reliability and reduce the number of four-letter words used to describe a leaking valve.
This article and more can be found at: https://www.valve-world-americas.com/webarticles/2021/11/05/improving-reliability-of-low-pressure-isolation-valves-in-power-production.html
About the Author
Barry Hoeffner, Director of Quarter Turn Products at Ladish Valves has over 30 years experience in the Chemical and Refining Industries. As a Chemical Engineer, Barry brings a unique perspective to the Flow Control space. His focus for the years in the Valve Industry has been in application engineering and development of severe service and engineered ball valves.