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Arrangement 1 mechanical seal[/caption]
Arrangement 1 defines the use of one mechanical seal in a seal chamber. Historically, this has also been called a single seal. The seal contains one set of seal faces and the seal is exposed to fluid in the pump seal chamber. While various piping plans are available to modify the environment in the seal chamber, the mechanical seal’s primary objective is to minimize the amount of process fluid in the seal chamber from reaching the atmospheric side of the seal. Under normal operation, a very small amount of process seepage will exit the seal. Arrangement 1 seals continue to be the most commonly applied arrangement due to their lower cost and simplicity. In most cases, the seals can be applied with a simple piping plan (e.g. Plan 11) and do not require the operator to monitor or maintain the seal or sealing system. Arrangement 1 seals are also generally small in physical size and can fit into virtually any centrifugal pump design.
This article contains excerpts from the paper, "Leakage detection and containment in arrangement 1 seals" by Michael Huebner of Flowserve Corporation at the 2014 Turbomachinery & Pump Symposia.
Arrangement 2 seals are designed with two mechanical seals in series (Figure 2). The inner seal is designed to seal
Arrangement 2 seal with liquid buffer fluid[/caption]
the fluid in the seal chamber. Any leakage past the inner seal will flow into a cavity between the two seals. This cavity is normally filled with a buffer fluid which is maintained at a pressure lower than seal chamber pressure. The outer seal is designed to normally seal the buffer liquid (and any accumulated process leakage) under low pressure conditions. If the inner seal fails, the outer seal is designed to operate under the full pressure and temperature conditions of the seal chamber. In this way, the Arrangement 2 seal provides full redundancy in case of inner seal failure.
Containment seals provide another common option for Arrangement 2 seals designs (Figure 3). The outer seal, designated as a containment seal, is designed to operate as a dry running seal without the need for any liquid lubrication. A containment seal will normally operate under low pressure conditions for the majority of its life. If the inner seal fails however, the containment seal will prevent excessive leakage from reaching the atmosphere. There are several standard piping plans which help support the use of these Arrangement 2 configurations (e.g. Plans 72, 75, and 76).
Arrangement 3 with liquid barrier fluid[/caption]
One obvious benefit of an Arrangement 2 seal is the redundancy of providing two seals. If either the inner or the outer seals fails, the seal design will be able to contain the process fluid. Another benefit is that a properly designed seal support system will allow the operator to monitor the seals performance with the instrumentation in the piping plan. For liquid buffer fluids, a Piping Plan 52 will allow the user to monitor liquid phase leakage through a level transmitter or vapor phase leakage through a pressure transmitter.
For Arrangement 2 seals with a containment seal, vapor phase process leakage is monitored with a pressure transmitter and liquid phase leakage is monitored by a level transmitter in a leakage collection reservoir. Both of these options require more sophisticated seal support systems than an Arrangement 1 seal. Liquid buffer fluids will require a seal reservoir to contain, cool, and monitor the buffer fluid. This may require external utilities such as cooling water. Containment seal configurations may require the use of external buffer gas flush. Both of these options require connecting the seal support system to a process leakage disposal system such as a flare or recovery system. The seal support systems, while reliable, do require attention in proper design, installation, commissioning, operation, and maintenance to ensure adequate seal performance.
Arrangement 3 seals are defined as two mechanical seals with the barrier fluid between the seals maintained at a pressure higher than the seal chamber pressure (Figure 4). In this arrangement, both the inner and outer seals are sealing the barrier fluid. This makes the seal less dependent on the fluid properties of the process fluid in the seal chamber. The seal can even be operated without any liquid in the seal chamber.
Another benefit of this arrangement is that no process leakage will migrate to the atmosphere since the barrier fluid is maintained at a higher pressure than the seal chamber pressure. Barrier fluid will leak across the inner seal into the process and across the outer seal to the atmosphere. The selection of a barrier fluid is critical in these applications since it will leak into the process. It must be compatible with the process fluid and not create any complications with downstream process operations in the plant or negatively impact pump performance. In addition, the barrier fluid must be environmentally acceptable since small amounts of leakage will migrate to the atmospheric side of the outer seal.
The real complication of an Arrangement 3 seal design comes from creating the higher pressure required by the barrier fluid. There are several piping plans which describe common methods for pressurizing barrier fluids such as Plan 53A, 53B, and 53C. Each of these piping plans not only creates the required pressure but they also provide a means for monitoring the loss of barrier fluid (seal leakage). This is accomplished by monitoring the liquid level or pressure decay in the system depending upon the piping plan. In addition, the sealing systems provide a means of cooling the barrier fluid through integral or external seal coolers. As an additional option, engineered support systems defined as Piping Plan 54 can provide a virtually unlimited set of options to monitor, condition, and circulate the barrier fluid for one or more mechanical seals.
Seal systems which support Arrangement 3 seals are generally the most complex systems for seal applications. The number of components, the requirement to pressure the systems, and the need to maintain barrier fluid levels make these the most operator intensive seal systems. Depending on the piping plan, external pressurization sources (most commonly, Nitrogen) must be available. Other external utilities such as cooling water may be required.
Arrangement 3 seal systems provide some unique benefits but also have some unique demands on the operators of the equipment. In addition to Arrangement 3 seals with liquid barrier fluid systems, Arrangement 3 seals can be provided with gas barrier fluid systems (Figure 5). These are commonly referred to as dual gas seals. While dual gas seals offer some unique benefits and require an arguably simpler support system, they do require a constant connection to a barrier gas supply system. The reliability of the seals is often tied more to the reliability of gas supply system than to the pump operation. This reliance on the barrier gas supply system also limits its use in remote locations.
In selecting the appropriate seal design and arrangement, the operator must consider all of the strengths and limitations of these options. Arrangement 1 seals provide a simple, cost effective sealing solution with a limited ability to control and detect excessive leakage. Arrangement 2 seals capture process leakage and allow for leakage monitoring at the expense of a more complex support system, external utilities and operator intervention. Arrangement 3 seal provide excellent product isolation and leakage monitoring often with the most complex support systems, use of external utilities, and required operator maintenance. There are some existing and new piping plans which can expand the use of Arrangement 1 seals by addressing some of their limitations. This may allow users to consider upgrading their current Arrangement 1 seals rather than switching to a dual seal solution. It may also allow users to specify Arrangement 1 seals in new applications where the capabilities of the new piping plans will meet the application requirements. Before examining these options, it is informative to consider leakage paths of process fluids during the failure of an Arrangement 1 seal.