Mechanical seals are devices installed on pumps, mixers or other equipment having rotating shaft, that can operate under very severe conditions and handling highly toxic, flammable and lethal fluids, where emissions are not allowed.
In addition to avoiding emissions to the atmosphere, in pharmaceutical and food plants, mechanical seals must prevent contamination of the process by external agents coming from atmosphere. In these cases, the mechanical seal must be dual and it can be flushed or pressurized by an auxiliary fluid, which prevents the process liquid from leaking to the atmosphere or the atmosphere from entering into the process fluid (see Fluiten’s alpha seminar – dual seals).
Fluiten has been producing mechanical seals for a wide range of industrial sectors since 1962; its Technical Department has developed a series of auxiliary systems, selected according to the specific application and the plant technical specifications where these devices must be installed.
The different types of system are identified through a code defined by the API 682 standard, the Oil & Gas sector is very concerned to the specifications and the selection of these important devices. The most common ones are described in the table on the left side of this page.
CHOICE OF AUXILIARY LIQUID
Barrier liquid: pressurized auxiliary liquid in a double seal.
Buffer liquid: non-pressurized auxiliary liquid in a dual seal.
Quench: fluid washing seal atmospheric side, carried out at atmospheric pressure normally with water, dry steam or nitrogen.
CHARACTERISTICS OF THE AUXILIARY LIQUID:
It must have sufficient lubricating properties; this parameter is influenced by temperature and pressure variations; it must not evaporate or solidify, so it must be selected in relation to the operating conditions and the calculation of the heat generated by the sliding surfaces.
It must be compatible with the process fluid and selected with the approval of the end user and finally must be compatible with the construction materials of the machine and mechanical seal.
The auxiliary liquid must not be harmful to people and the environment, dangerous, toxic, flammable.
Instruments installed on the auxiliary systems are fundamental for monitoring the correct mechanical seal operation parameters.
An example is the temperature transmitter on the 53B accumulator, which is used to prevent errors on the barrier oil pressure that may occur in relation to the change in ambient temperature. In the desert, for example, the daily temperature range is considerable: this could cause an appreciable variation in the pressure of nitrogen in the bladder, influencing the barrier pressure very much.
The temperature transmitter then reads the nitrogen temperature in the bladder (which is approximately equal to the ambient temperature) and, through the DCS, communicates with the pressure transmitter in real time, modifying the pressure alarm thresholds accordingly. These thresholds are provided by the seal manufacturer, who processes them by means of an algorithm indicated by the API 682 standard.
This algorithm also defines the bladder pre-charge and oil volume values for a refilling in accordance with the temperature read on the transmitter by the operator.
It is worth noting that the temperature transmitter does not have an alarm function, but rather reads the environmental conditions in order to suitably modify the pressure transmitter alarm threshold.
The API 682 standard specifies that, as an alternative to this method, a fixed alarm strategy can be used (even with a simple pressure switch), avoiding the installation of the temperature transmitter, but it has to be noted that this selection involves a reduction in the useful oil volume between maximum operating pressure and alarm point.