Isolation rings are very effective devices that protect pressure instruments from contaminant clogging and corrosion in several industries. However, there are certain circumstances where alternate methods of instrument isolation are recommended.
As a global leader in pressure and temperature instrumentation, customers rely on the depth and breadth of our experience to educate and guide them through the process of finding the best solutions for their specific needs. In this article, you will learn why isolation rings are a good option for several very specific types of applications, but would not be recommended for others.
As you read, look for links to resources that can help provide additional information to deepen your knowledge on this particular topic.
Where isolation rings are most effective
Isolation rings, which are typically used in water/wastewater applications, mining and pulp/paper plants, do a great job preventing clogs. This is especially true for these industries because they all have three common factors:
- The process consistency is viscous and/or has large particulates moving through the pipe
- The pressure tends to be lower than 160 psi
- The temperature is usually reasonable, often at ambient
For these types of systems, one or more instruments can be assembled to the isolation rings, which are installed in line with the piping. The inner diameter of the isolation ring has a liner that allows the pipe pressure to deflect and displace a fill fluid (usually silicone) to drive the instrument(s).
Figure 1.
Isolation ring limitations
Although isolation rings are designed for clog prevention, there are several instances when this type of isolator will not be effective. Here are the top three:
1. Applications with harsh chemicals
Isolation rings have limited chemical resistance. They consist of two components that contact the process: the liner and the end plates. The liners are restricted to non-metallic materials only, such as PTFE, Viton, Buna, EPDM, and Natural Rubber, offered by Ashcroft. The end plates, typically made of stainless steel or painted carbon steel, are also available in plastics like Acetal, CPVC, PTFE, or PVDF.
While these materials offer some chemical resistance, exceeding the options listed above is challenging and expensive.
Diaphragm seals are typically more suitable for applications involving harsh chemicals because they can be constructed from a wider variety of materials with a greater resistance to aggressive media. These materials include both metallic and non-metallic options such as Hastelloy C276, titanium, tantalum, Monel, and Hastelloy B.
2. Applications with extra large pipes
The benefits of using isolation rings over traditional diaphragm seals can be significant, especially from a maintenance standpoint. The robust sensing elements and flow-through design minimize the maintenance attention required to keep the instruments running smoothly. However, these benefits can be quickly outweighed and outmatched for systems with larger pipe sizes.
The negative aspects associated with larger isolation rings (over 14 inches) include:
- Cost of the isolation ring. As the size of the isolation ring increases, so does the amount of material needed, leading to higher costs. More importantly, larger rings are not commonly bought, necessitating a more specialized acquisition of components, which further raises the price and extends the lead time.
- Material, flange class and design limitations. Because the volume is lower, the likelihood of materials other than 316L or Carbon Steel being available is lower. Wafer isolation rings only reach 20 inches so beyond that size you are limited to bolt-through options.
- Cost of installation as well as repair/replacement. As pipe sizes grow, the installation costs become more challenging, often necessitating crews and heavy equipment. Whenever instrument configurations require repair or replacement, these installation/removal expenses recur.
To reduce these costs, you can use a safety removal connection like the Ashcroft® Safety Quick Release (SQR), which allows instruments to be repaired or replaced without detaching the ring from the pipe. However, eventually, the ring itself will need to be replaced, and that will involve considerable effort and cost. - Cost and concerns of shipping. The primary goal of these assemblies is to integrate instruments like transmitters/transducers, pressure gauges and switches into piping systems. These instruments are inherently delicate, as they are carefully metered and calibrated, and typically weigh only a few pounds. Transporting them while attached to a 25-pound isolation ring can be challenging. While the SQR allows for shipping these components separately, which helps protect the instruments, not all manufacturers offer this capability.
Other strategies can be used to isolate instruments in these larger pipe systems:
- Tap off the main piping with a 3-inch pipe flange connection and install a flush flanged seal. The large diaphragm handles the very low pressures (or low set points on a switch) and the construction accommodates higher pressures depending on the class flange that is plumbed into the pipe system.
- Use a traditional flanged diaphragm seal with a flushing connection (available in ¼, ½, single or dual) that would allow periodic flushing if required.
- Consider a saddle seal diaphragm. These feature a lower housing a lower housing welded directly to the pipe and a top housing bolted onto the lower section, ensuring the diaphragm aligns flush with the pipe's inner diameter. This design reduces cavities and simplifies removal, replacement, or repair.
3. Applications involving high-pressure and extremely low pressure
The design of an isolation ring is meant for heavy slurries with pressures lower than 200 psi. Higher pressures, particularly when using gas or air can expose containment problems with the isolation rings. This situation usually arises during bench testing before installation rather than during the actual application.
Calibrating instruments with pressure spans below 15 psi can be challenging. This is especially true for pressure switches with set points under 6 psi. In these cases, these instruments might not function reliably in the field, even if they are calibrated by the manufacturer. Low-pressure spans and set points demand sufficient process effort to operate the instrument correctly and are also vulnerable to factors like temperature variations.
When dealing with high or extremely low pressures, traditional seals (flanged, saddle or threaded) are more effective solutions because they can be tailored to specific applications. For instance:
- Traditional diaphragm seal designs are more suitable for pressures over 1000 psi, and in some cases, up to 10,000 psi.
- Viton diaphragms: Provide flexibility for instruments with low-pressure spans.
- High displacement seals: Use larger diaphragms to displace more fill fluid.
There are also many instances when projects call for pressure switches on isolation rings requiring set points as low as 1 psi. Isolation rings are not as responsive as diaphragm seal assemblies and mechanical pressure switches do not work reliably with set points that low.
In summary, isolation rings have incredibly durable flow-through designs that can effectively protect your instruments under the right circumstances. However, if the installation is exposed to harsh chemicals, high pressures (above 200-psi), low pressures (spans less than 15, set points less than 6-psi) or if they're being installed on piping larger than 14-inches it may be better to consider a traditional diaphragm seal design instead.
In my experience, it would always be best to consult the manufacturer of the instruments as well as the manufacturer of the isolation device for input on what configuration would work best when you are pushing on any of these parameters.
Ready to learn more?
Now that you have a better understanding of the benefits and limitations of isolation rings, you may want to gather more information on the subject. The following resources may be of interest:
- Reasons to Use an Isolation Ring or Diaphragm Seal
- Best Practices for Installing Flanged Diaphragm Seals
- When to Use a Welded Instrument Assembly
You can also contact us directly. Our industry experts can help answer your questions. In the meantime, feel free to download our Complete Guide to Pressure Instrument Assembly.