Dry-Running Sealing Technology: Pump Applications (P2)

Solutions to Common Problems

The operation of a single contacting seal would be difficult with any of the identified problems. A single contacting seal relies on cooling and lubrication from the process liquid being sealed. Any interruption in the cooling and lubrication processes will result in damage to the seal and leakage to the environment.

The non-contacting, dry-running seal is a solution to many of the problems identified by users. An installation to a pump is illustrated in Figure 2. This type of pump seal technology does not require the circulation of liquid for cooling. Instead, a static heat of an inert gas is used to pressurize the space between the seals. Nitrogen gas in normally used to create the barrier between the process liquid and the environment. The gas barrier pressure is normally 20 to 30 psi/1.4 to 2 bar above the seal chamber pressure.

The spiral groove geometry of the seal face is responsible for lift-off and separation of the seal faces during operation. Since the seal faces are not in contact when the shaft is rotating, the only heat that is generated is that of the shearing gas. For normal pump speeds, a temperature rise at the seal face would be 2ºF to 4ºF/-17ºC, compared with several hundred degrees for a contacting seal. This results in a very attractive sealing technology for heat sensitive liquids and those liquids being pumped near their vapor pressure.

This technology represents a solution for those problems that effect cooling and lubrication at the seal faces. These have been identified as loss of seal flush, dry run, start-up without venting, low NPSH and cavitation. Cavitation will also result in the vibration of equipment. The vibration limit for this sealing concept is 0.4 in./s (10 mm/s).

Non-contacting, gas lubricated seals can be used on a wide range of process liquids. If the process fluid contains more than 6% solids by volume, consult John Crane Engineering with specific details about the liquid being sealed. Designs are available for liquids with a concentration of solids to 20% by volume.

With a non-contacting, gas-lubricated seal, rubbing contact at the seal face has been eliminated, so the seal can be operated at the vapor pressure of the liquid being sealed. In addition, there is no limiting factor due to the pressure-velocity relationship and wear at the seal faces. The limiting factor in the application of the seal is pressure that has an effect on seal face deflection.

Dual pressurized, non-contacting, gas lubricated seals have been designed to fit oversized and small bore seal chambers. Oversized bore seal chambers allow for a larger cross section seal, which can handle greater pressures. This original gas seal for pumps, introduced over five years ago, is shown in Figure 2. Many existing pumps in the field have small bore seal chambers and do not require the same pressure capability as the Type 2800 seal. To meet this requirement, the Type 2800E shown in Figure 4 was designed. To meet the need for still higher pressures on pumping applications, with the ability to run dry, the Type 2800HP was designed. This seal fits the same space envelope as the Type 2800 seal and is being used in Petroleum Refining and other industries.

Benefits

Many users are still struggling to increase MTBPM. Plants that were constructed 20 to 30 years ago were not constructed to today’s more stringent specifications. Deficiencies exist in piping, foundation stiffness, proper NPSH, operations and installation. The competitive edge is being lost at older plants, due to higher costs of equipment ownership and lost production due to equipment down-time.

Non-contacting, gas-lubricated seals are a solution to improving the reliability and efficiency of rotating equipment. They have eliminated seal face wear, heat developed at the seal faces, and are designed to run dry. While trying to solve an emissions problem, this technology has revolutionized pumping systems.

The use of this technology and the absence of a liquid seal support system help to maintain a high degree of product quality, which is necessary in the pharmaceutical and biotech industries. The non-contacting, gas-lubricated seal technology is more forgiving of most operating problems than other seal technologies. The capability of this technology is substantially reducing the cost of ownership of equipment and reducing equipment down-time.

Seal designs shown are patent-protected.

Comments

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