Sight Glass Taskforce Update
The initial findings of the Sight Glass Taskforce were presented at a special session at the 2010 IIAR Industrial Refrigeration Conference & Exhibition. The moderator of the session was Peter Jordan of MBD Risk Management Services, Inc. Three speakers shared the stage with Jordan:
- Doug Reindl, Industrial Refrigeration Consortium
- Rowe Bansch, Refrigeration Valves and Systems Corporation
- Bent Wiencke, Nestlé
Background: Recent Sight Glass Failures
In 2007, two catastrophic sight glass failures occurred in close succession at two separate end-user facilities. These sight glass failures were the catalyst for the formation of IIAR’s Sight Glass Taskforce.
The first sight glass failure released 28 lbs of ammonia. The origin of the release was a ruptured sight glass located in a hot gas driven Liquid Transfer Unit (LTU) or pumper drum. Ammonia was released into the air and an employee working in the area where the sight glass failed was engulfed in an ammonia cloud and suffered severe injuries. Findings from the incident investigation suggested two possible causes for the failure — faulty installation or surface damage to the sight glass from external impacts. The sight glass was in an area where workers used a push-out bar. They may have hit the glass with the bar and weakened it. This first failure was considered a fluke that was unlikely to happen again. And then lightning struck twice! A mere 6 weeks later, a sight glass mounted in a plate freezer’s suction line failed. Constant “bumping” of the sight glass is suspected to have weakened it leading to its eventual failure. Another possibility was that a gasket wasn’t mounted correctly. Decades of wear may have also played a part in that the sight glass in question was over 20 years old.
Rather than ignore these incidents, corporate management and management of the affected facilities decided that they needed to investigate sight glass failures to ensure a safe working environment. They inspected all of their sight glasses and changed out any that seemed damaged. In the process, they discovered that sight glasses used in pumper drums showed signs of cavitation and erosion on the interior side of the glass. This damage is believed to have been caused by the erosive effects of saturated liquid between the gasket and the sight glass housing flashing when the pumper is rapidly depressurized.
The end-user informed IIAR of the sight glass failures and of the results of their initial sight glass inspections. Based on this information, the IIAR Board of Directors created the Sight Glass Taskforce in 2009. The taskforce was led by Marcos Braz, of MRBraz & Associates. It was charged to evaluate the safety of sight glasses and formulate recommendations to improve them.
Sight Glass Basics
Sight glasses are used extensively throughout industrial refrigeration systems. They provide visual access to portions of a system. They give operators an indication of whether ammonia is in liquid or vapor state. They also help operators detect oil in a system.
Doug Reindl described a typical sight glass assembly as consisting of a housing that is installed on the component on which the sight glass is located, vessel, piping etc, a sealing gasket, the glass, a fiber gasket and a retaining ring. The two sight glass design types used in industrial refrigeration systems are bull’s-eye and linear. Bull’s-eye is the most widely used type. There are two glass composition types, borosilicate and soda-lime materials. Sight glasses made of quartz or sapphire are used in laboratories but are too expensive for industrial refrigeration applications.
Taskforce Findings
Although sight glass failures are rare, they can have a powerful impact on facilities. In most situations, sight glass failures can be avoided through the simple application of common sense and an understanding of the nature of glass. Glass is a material that likes to be in compression rather than in tension. Glass is susceptible to shocks that can weaken it and cause it to fail. The Taskforce found that when sight glasses fail, they don’t give much warning. They suddenly fail and that’s it. This abruptness is due to the nature of glass — when it breaks, it doesn’t do so by degrees, but all at once.
The taskforce also noted that chips or nicks in a sight glass surface will greatly reduce its ability to withstand pressure, and that some of the glass materials used in industrial refrigeration systems is susceptible to corrosion when exposed to alkaline environments including ammonia. When glass corrodes, it becomes cloudy. Glass is also susceptible to latent damage when improperly handled (e.g. glass is dropped) or installed (gaskets not properly arranged).
These findings suggest some basic rules:
- All sight glasses should be inspected regularly for any type of visible damage. Look for surface imperfections using illumination to provide back- and/or oblique lighting.
- Everyone who comes into contact with the sight glass should be strictly prohibited from using sharp, metal objects to remove ice from sight glass.
- Metal wires should not be used to install frost shields.
- Sight glasses should be eliminated where possible.
- Sight glasses should not be used in applications subject to hydraulic shock which includes hot gas driven liquid transfer units (“pumper drums”) and any control valve group subject to hot gas defrost. • The use of sight glasses in portions of the system where they will be subjected to extremes of pressure or temperature should be minimized.
- Proper alignment of sight glasses is critical; make sure glasses are properly aligned during installation.
- When installing sight glasses, strictly follow the manufacturer’s recommendations. If the sight glass appears to have sustained any damage, it should be discarded before installation even if it’s a brand-new glass just out of the box.
- Since, after an incident, it may not be possible to trace the origin of the failed sight glass, it’s a good idea to keep an inventory detailing the glass’ manufacturer, pressure rating, and date of installation.
End-User Survey
The Sight Glass Taskforce undertook a survey of 40 enduser plants. Of those 40 end-user plants, 38 percent included visual inspections of sight glass in their mechanical integrity programs. A total of four (10%) of plants surveyed had a refrigerant leak as a result of a sight glass failure in the past. It is important to note that none of the end-user reported failures were catastrophic. All reported failures involved small leaks from hairline cracks in the glass. The next step for the Taskforce is to administer a manufacturers’ survey intended to answer the following questions:
- What changes (if any) to the sight glass fabrication process are being investigated?
- What do they currently do in terms of testing and inspection at their facilities?
- What, if anything, is being done to date on the key issue of traceability?
- What preventive maintenance practices do they recommend?
Codes & Standards
A review of codes and standards and guidelines revealed little information regarding sight glass construction and the use of sight glasses in refrigeration systems.
The Taskforce recommended that the glass used to fabricate ammonia refrigeration system sight glasses comply with specific standards regarding the chemical composition of the glass itself, the process that was used for fabricating that glass, whether or not it was treated after fabrication by tempering or annealing and the required testing of the finished product. A required minimum thickness for maintaining mechanical integrity in an installation also should be established.
The IIAR Sight Glass Taskforce has delivered recommendations to SRC and the Code Committee for incorporation into our standards. It is possible that this will result in a new performance standard for sight glasses. The preventative maintenance standards being developed in IIAR 6 may be another area where these recommendations will be incorporated. There is money available from ARF for potential research projects to fill in gaps where additional testing is needed. A
SME Boiler and Pressure Vessel Code
Because sight glasses are often used in conjunction with pressure vessels, Rowe Bansch explained how the ASME Boiler and Pressure Vessel Code, (“B&PV Code”), treats sight glasses.
Some sight glass manufacturers claim that the sight glasses meet the requirements in UG 11(a) (1) of the ASME Boiler and Pressure Vessel Code, (Section VIII, Division 1). Another common claim is that the sight glasses are in compliance with the intent of the ASME Boiler and Pressure Vessel Code, Section 8, Division 1, and that the housings meet the material requirements for ASME for direct welding into pressure vessels. Moreover, each housing is marked for material traceability and that certifications are available upon request. These claims are of little value since the codes do not really address the actual glass in sight glasses.
Paragraph UG 11 of the B&PV Code actually states that, “pressure parts shall not require inspection, identification or partial data reports when all of the following apply:
- That the parts are wholly formed by casting, forging, rolling or dye forming;
- That the parts are made to a manufacturer’s standard with materials permitted by this division;
- That the materials are certified by the manufacturer to be suitable for service at the rating indicated;
- That the parts are marked with the manufacturer’s name or trademark so that it is traceable.”
In reality, this paragraph only applies to the sight glass housing itself.
The scope of ASME Boiler and Pressure Vessel code (paragraph U-1(e)) includes the first sealing surface for proprietary components “for which rules are not provided by this division” such as gauges, instruments and non-metallic components. In other words, the ASME code ends at the first gasket face. The glass is NOT included within the scope. The code’s scope does include the welding and connection for the first circumferential joint where external devices are to be connected to the vessel. This means that if the sight glass is welded to a pipe nozzle in the vessel rather than into the shell or head of the vessel itself, the code may be stopped prior to the sight glass housing at the joint where the sight glass is connected to the pipe nozzle. In this case, the entire sight glass including the housing can be excluded from the scope of the ASME code.
In paragraph UG-4 of the B&PV Code, the statement is made that “materials subject to stress due to pressure shall conform to one of the specifications given in Section 2.” Nonmetallic materials are not included in Section 2; therefore, glass is not included in the scope of the code.
Sight glass manufacturers provide a range of ratings for their glass: Manufacturer A gives the maximum working pressure of 500 psi with a temperature rating of –40°F to 250°F. Manufacturer B rates the sight glass at 1,000 psi maximum working pressure, suitable for low-temperature applications to –60°F. Manufacturer C rates the sight glass at a safe working pressure of 400 psi with an operating temperature range of –60°F to 250°F. Although the ratings vary significantly, the actual products are often used interchangeably.
In conclusion, sight glasses are not included in the scope of the ASME B&PV Code (Section VIII Division. 1). There are no specific requirements in ASME for the design, manufacture, inspection or testing of sight glasses. There is no common standard for the design, manufacture, inspection or testing of sight glasses used in industrial refrigeration systems.
Alternatives to sight glasses:
Capacitance probes and magnetic level indicators are technology alternatives that can be used to provide an indication of level in a component such as a vessel. Capacitance probes find widespread and successful use in sensing liquid in level columns connected to vessels.
Magnetic level gauges are a relatively newcomer to industrial refrigeration systems. A magnetic level gauge is an indirect level-indicating instrument whose main components consist of a float, float chamber and a float indicator. The float rests in the chamber and is magnetically coupled to the indicator. The float creates a magnetic field and magnetic flags turn color as the float travels up and down. They utilize a level column with a flag-type indicator that’s magnetically coupled to an internal float. These devices are available with optional level transmitters that send a continuous signal indicating where the level is to the control system.
Level switches are another alternative to indicate the presence of liquid. Level switches have no glass pressure retaining parts. They operate by sensing the presence of liquid by a change in capacitance. They can have alarms or level control signals incorporated into their design. One of the advantages of this type of switch is that it’s easy to fabricate level columns similar to sight glass columns; it just requires the installation of threaded couplings that the level switches are threaded into. The drawbacks are that wiring is required; you’ve got to power these devices. A level switch is slightly higher in cost than a typical sight glass. It adds about 25% to the cost of a typical level column with a probe.
Conclusions:
The sight glass failures mentioned earlier should inspire us all to closely examine the design of the sight glasses we use and how we apply them. Make it a habit to consider safer designs along with the development of enhanced mechanical integrity inspection procedures. Challenge when and where sight glasses are installed and consider omitting them from locations where they are not needed. Follow the sight glass manufacturers’ installation instructions and avoid possible latent damage that can occur to the glass from an accidental drop. Visually inspect sight glasses and immediately replace them when any external damage is apparent. Damage that should prompt replacement includes surface nicks and scratches.
One big safety consideration is that we are using sight glasses that are not always traceable, once a sight glass is out of the box, there’s no way to discover what its pressure rating is, who made it or what material it’s composed of. If it fails, it’s almost impossible to trace back to the manufacturer. Sight glasses for other industries include information such as manufacturer, pressure rating and model number inscribed on the part. Manufacturers of sight glasses in our industry are encouraged to include similar information on their sight glasses.
