TAMPSS system points to correct sealing solution
The ubiquitous sealing devices that keep plant equipment and systems operating reliably often do not receive the attention they warrant from operations and maintenance personnel. Typically, little thought is given to these low-cost, but high-consequence components until specifications are being developed for a plant-wide contract, or they begin to leak causing unscheduled downtime, lost product...
The ubiquitous sealing devices that keep plant equipment and systems operating reliably often do not receive the attention they warrant from operations and maintenance personnel.
Typically, little thought is given to these low-cost, but high-consequence components until specifications are being developed for a plant-wide contract, or they begin to leak causing unscheduled downtime, lost production, safety issues and potential penalties for noncompliance with environmental regulations.
Sealing devices come in a variety of materials and configurations to meet the requirements of specific applications. These include compression packing for valves and pumps, mechanical seals for pumps, bearing isolators for gearboxes in power transmission systems, gaskets for pipe flanges, hydraulic seals for air and oil cylinders, oil seals for rotating equipment, and flexible joints for piping systems subject to expansion and contraction. Add to these the special seals used in high-performance engines, turbines and numerous other applications, and it becomes clear that sealing devices play a critical role in virtually any process that involves, liquids, gases or bulk materials.
TAMPSS for success
Best sealing practices address two basic criteria: the application in which the sealing device is to be used and the service conditions to which it will be exposed. A simple acronym, TAMPSS (temperature, application, media, pressure, size and speed) provides useful guidelines for selecting the correct sealing device for a particular application.
Temperature. The first consideration should be the continuous temperature to which the seal will be exposed, including high/low excursions as well as any regular thermal cycling inherent in the process. Note the frictional heat generated by rotating equipment will increase the temperature of the fluid contacting the seal. Temperature data will immediately limit the number of viable seals for an application.
Application. Knowing how the seal is to be used and the function it is expected to perform are keys to making the right selection. This type of information points up the anomalies of an application and the special requirements for optimal seal performance. Defining the parameters of a particular application requires information about where the seal will be installed.
For example, selecting the proper gasket for a flanged piping connection requires knowing the type of flanges involved, their material structure and physical condition, the grade of bolts used to secure them, and whether collectively these factors can provide sufficient compressive force to affect a leak-proof seal.
This is extremely important, since more than 70% of gasket failures are attributed to insufficient load. If the application is a valve, selection of the compression packing will depend on the condition of the stem, whether its motion is reciprocating, helical or continuous, and whether a specific level of leakage must be attained to meet environmental regulations.
Media. Common chemical nomenclature or trade names are used to identify the media that will come into contact with the seal. Some processes employ secondary media that may not be addressed at this stage of inquiry. For example, a food processing line that is flushed once a day with a sodium hydroxide solution calls for a seal that is compatible both with this corrosive medium and the food being processed.
Pressure. This refers to the internal pressure a seal must contain. Most systems operate at fairly consistent pressure, but as with temperature it is important to know if the seal will be subject to pulses and other variations as a normal part of operation.
Speed. The speed of a rotating shaft or reciprocating rod must be taken into account when selecting oil seals, bearing isolators, mechanical seals or compression packing for dynamic applications. High speeds call for sealing materials that can withstand and effectively dissipate frictional heat.
Size. There are standard sizes for ASME flanges, API valve stems, ANSI pump shafts and bores, etc. Non-standard sizes are best conveyed to the sealing manufacturer in the form of dimensional drawings. Some applications may require field measurements.
After the best seal has been selected on the basis of these criteria, special attention should be given to the manufacturer's installation instructions. The right seal installed improperly will not provide the desired results.
Minimize inventory impact
With regard to specifying seals for an entire plant, the explicit need is to narrow the selection to the smallest number of viable options; the implied need is to minimize stocking and inventory costs.
Best sealing practices demand addressing needs beyond merely meeting the requirements of service conditions. Chief among these are cost-of-ownership and environmental regulations.
Cost-of-ownership is largely a function of the reliability of the sealing product itself, as well as its manufacturer. Failure of misapplied sealing devices can shut a plant down. Such failures can range from creating nuisance leaks to major catastrophes, resulting in hundreds of thousands of dollars per day in downtime, lost production and regulatory non-compliance, and even more if an employee is injured.
From an environmental perspective, the much over-used term "green" refers to the ability of a seal to meet the requirements of regulatory compliance in terms of ground contamination, fugitive emissions, and conservation of water and energy. Fugitive emissions from leaking valve stems in refineries and chemical plants are major offenders. The usual benchmark for levels of leakage is 500 ppm, varying from state to state and country to country. To meet such levels valve stem seals need to be qualified, as do pump seals and flange gaskets.
Adhering to the basics of TAMPSS, selection, installation, cost-of-ownership and environmental concerns will result in sealing solutions that perform as expected, meeting both users' explicit and implied needs, and minimizing costly downtime and safety incidents in the process.
Jim Drago, P.E. of Garlock Sealing Technologies has worked in sealing technology for more than 25 years, including engineering, applications, product development and management. He may be contacted at firstname.lastname@example.org .