Mechanical Seals

Introduction

Mechanical seals were originally designed to lend a greater sealing capability than could be achieved using common packing.

Before the advent of mechanical seals, pump users relied primarily on “rope” or braided style packing to achieve a “seal” around the shaft. A series of pieces or “rings” were installed into the pump “stuffing box” and they were compressed tightly so that they created a difficult leak path for the liquid to negotiate in order to leak to atmosphere.

Early packing styles did not seal very well. In fact, until recently, braided packing styles required varying amounts of leakage for lubrication. If leakage was not permitted to occur, the packing would literally “burn up” and often cause severe damage to the pump shaft. Even with adequate leakage for lubrication, pump shaft wear was a commonly expected occurrence and as the shaft wore it would in turn, causes poor shaft packing life.

As leakage becomes more excessive, the gland is tightened to reduce leakage. With the introduction of mechanical seals, this leakage could be controlled to a much greater degree.

The mechanical seal faces are obviously the most critical sealing point of a mechanical seal assembly. Although the faces can be manufactured from a myriad of different materials, one is typically carbon, while the other is usually a hard material. (i.e. Alox (Aluminum Oxide Ceramic), Tungsten Carbide, Silicon Carbide, etc.)

In order for a “seal” to be achieved, the faces must be very flat. This is achieved by machining the faces, then “lapping” them to a fine finish. Flatness is measured in “Light Bands”. After lapping, the faces are placed on an “Optical Flat”, a clear glass surface where a monochromatic light is shined on the face. This single wavelength light will produce an image of rings or lines on the face. Each ring/line is “One Light Band”. Each light band is equivalent to 0.000011” or eleven millionths of an inch. This refers to the variations in the surface of the face.

General Information

A mechanical seal provides a barrier between a rotating part, such as a shaft, and a stationary part to prevent friction wear and leakage. Mechanical seals see regular use in compressors, pumps, turbines and other rotating industrial machinery.

The usual "single inside" seal consists of a set of seal faces, one rotating with the shaft and the other remaining stationary. Various packing and mountings secure the seal between the shaft and the seal faces, while gaskets, coils and compression pins help prevent leakage from escaping to the outer part of the seal.

The two seal faces fit together in a "lapped" design that forces any leakage to follow a difficult path outward from the shaft. The faces rub together tightly when in motion to make leakage practically impossible. Other types of seals include single outside seals, which wrap around the outside of the pump, and dual seals that use additional gas or liquid barriers for extra protection against hazardous fluids.

Mechanical Seal Types

There are obviously many different types and configurations of mechanical seals. Shaft mounted and cartridge, balanced and unbalanced, pusher and non-pusher, single and multiple, etc., etc.

Shaft Mounted Seal

A shaft mounted seal requires the pump user or assembler to actually install individual seal components into the equipment.

Cartridge Type Mechanical Seal

A cartridge type mechanical seal is a pre-assembled package of seal components making installation much easier with fewer points for potential installation errors to occur.

The major benefit, of course is no requirement for the usual seal setting measurements for their installation. Cartridge seals lower maintenance costs and reduce seal setting errors

Pusher

Incorporate secondary seals that move axially along a shaft or sleeve to maintain contact at the seal faces. This feature compensates for seal face wear and wobble due to misalignment. The pusher seals' advantage is that it's inexpensive and commercially available in a wide range of sizes and configurations. Its disadvantage is that ft's prone to secondary seal hang-up and fretting of the shaft or sleeve.

Non Pusher

The non-pusher or bellows seal does not have to move along the shaft or sleeve to maintain seal face contact, The main advantages are its ability to handle high and low temperature applications, and does not require a secondary seal (not prone to secondary seal hang-up). A disadvantage of this style seal is that its thin bellows cross sections must be upgraded for use in corrosive environments

Unbalanced
They are inexpensive, leak less, and are more stable when subjected to vibration, misalignment, and cavitation. The disadvantage is their relative low pressure limit. If the closing force exerted on the seal faces exceeds the pressure limit, the lubricating film between the faces is squeezed out and the highly loaded dry running seal fails.

Balanced
Balancing a mechanical seal involves a simple design change, which reduces the hydraulic forces acting to close the seal faces. Balanced seals have higher-pressure limits, lower seal face loading, and generate less heat. This makes them well suited to handle liquids with poor lubricity and high vapor pressures such as light hydrocarbons.

MECHANICAL SEAL ARRANGEMENTS

SINGLE INSIDE

This is the most common type of mechanical seal. These seals are easily modified to accommodate seal flush plans and can be balanced to withstand high seal environment pressures. Recommended for relatively clear non-corrosive and corrosive liquids with satisfactory' lubricating properties where cost of operation does not exceed that of a double seal.

SINGLE OUTSIDE

If an extremely corrosive liquid has good lubricating properties, an outside seal offers an economical alternative to the expensive metal required for an inside seal to resist corrosion. The disadvantage is that it is exposed outside of the pump which makes it vulnerable to damage from impact and hydraulic pressure works to open the seal faces so they have low pressure limits (balanced or unbalanced).

DOUBLE (DUAL PRESSURIZED)

This arrangement is recommended for liquids that are not compatible with a single mechanical seal (i.e. liquids that are toxic, hazardous [regulated by the EPA], have suspended abrasives, or corrosives which require costly materials). The advantages of the double seal are that it can have five times the life of a single seal in severe environments. Also, the metal inner seal parts are never exposed to the liquid product being pumped, so viscous, abrasive, or thermosetting liquids are easily sealed without a need for expensive metallurgy. In addition, recent testing has shown that double seal life is virtually unaffected by process upset conditions during pump operation. A significant advantage of using a double seal over a single seal.

DOUBLE GAS BARRIER (PRESSURIZED DUAL GAS)

Very similar to cartridge double seals ... sealing involves an inert gas, like nitrogen, to act as a surface lubricant and coolant in place of a liquid barrier system or external flush required with conventional or cartridge double seals. This concept was developed because many barrier fluids commonly used with double seals can no longer be used due to new emission regulations. The gas barrier seal uses nitrogen or air as a harmless and inexpensive barrier fluid that helps prevent product emissions to the atmosphere and fully complies with emission regulations. The double gas barrier seal should be considered for use on toxic or hazardous liquids that are regulated or in situations where increased reliability is the required on an application.

TANDEM (DUAL UNPRESSURIZED)

Due to health, safety, and environmental considerations, tandem seals have been used for products such as vinyl chloride, carbon monoxide, light hydrocarbons, and a wide range of other volatile, toxic, carcinogenic, or hazardous liquids.

Tandem seals eliminate icing and freezing of light hydrocarbons and other liquids which could fall below the atmospheric freezing point of water in air (32? F or 0? C). {Typical buffer liquids in these applications are ethylene glycol, methanol, and propanol.) A tandem also increases online reliability. If the primary seal fails, the outboard seal can take over and function until maintenance of the equipment can be scheduled.

Advancements in Mechanical Seals

Here are some of the advancements that have been made that contribute to longer mechanical seal life.

v  The general acceptance of hydraulic balanced seal designs that eliminated a major source of unwanted heat at the seal faces.

v  Stationary seal designs that reduced the problems associated with the lack of stuffing box to shaft squareness.

v  The use of O-rings that reduced the problem of sliding dynamic elastomers

v  Self-aligning seal faces that made the sealing of horizontally split pumps practical.

v  Cartridge seal design that solved a lot of the seal failures caused by improper seal installation, shaft thermal growth and open impeller adjustment.

v  Unfilled carbon seal faces that eliminated most of the chemical compatibility problems we had sealing process pumps

v  A special grade of Dupont's elastomer Viton® that has a reasonable amount of water sealing capability

v  Chemraz and Kalrez. The wonder compounds of the 1970s that allowed mechanical seals to be chemically compatible with just about any fluid.

v  The creation of alpha sintered silicon carbide hard faces that are not only corrosion resistant to most fluids, but also excellent conductors of heat.

v  The elimination of Teflon in many original equipment seals. Teflon was the main contributor to shaft fretting.

v  Non fretting seal designs that eliminated the need for sleeved shafts.

v  Welded metal bellows designs that eliminated the problems of elastomers in cryogenic and non-petroleum, high heat applications.

v  Split seal designs that eliminated the last reason for using packing in pumps.

v  Finite element analysis techniques that allow us to design small cross-section seals with high-pressure capability.

v  The use of suction recirculation piping along with an oversized stuffing box to eliminate most of the problems associated with the sealing of slurries.

Future Of Mechanical Seals

v  The elimination of elastomers in process seals, not only because of elastomer temperature limits, but the more serious problem of chemical compatibility with both product and flushing fluids. Someone has to pick the correct elastomer and there is always room for error.

v  Seal designs that can take excessive axial movement without changing their face load.

v  The elimination of barrier or buffer fluid between dual mechanical seals. Present gas designs are not filling this need.

v  Temperature control in the stuffing box area without the use of water or steam. In many applications the fluid in the stuffing box must be kept within certain temperature limits to prevent it from changing into a sold or a gas.

v  Instrumentation to predict pump cavitation, excessive shaft deflection, high heat, etc.

v  Reliable non-stick seal surfaces to prevent solids from adhering to the sliding seal components

Reference:

Pump, centrifugal pumps, PD pumps, seals & mechanical seals data, Mc Nally Institute Dade City, Florida.

Technical Literature, Goulds Pumps Industrial Products