A basic mechanical seal is not a complex device. It consists primarily of a rotary seal face with a driving mechanism which rotates at the same speed as the pump shaft, a stationary seal face which mates with the rotary and is retained using a gland or in some pump models an integral stuffing box cover, a tension assembly which keeps the rotary face firmly positioned against the stationary face to avoid leakage when the pump is not in operation, and static sealing gasket(s) and elastomers strategically located to complete the seal assembly.
The rotating and stationary sealing faces commonly referred to as primary seal members, are materials selected for their low coefficient of heat and are compatible with the fluid being pumped. Their extremely flat; lapped mating surfaces, make it extremely difficult for the fluid to escape between them. The fluid does however, form a thin layer or film between the faces and migrates toward the low pressure side of the faces. It is this boundary layer of fluid which is used and required to cool and lubricate the seal faces.

To prohibit leakage along the pump shaft through the inside diameter of the rotary and stationary seal faces the mechanical seal assembly uses o-rings, v -rings, wedges and packing. Commonly referred to as secondary sealing members these components of the seal are selected based on fluid compatibility, temperature, elastomeric qualities, and depending on the type and design of the seal they may perform in either a dynamic or static state. Mechanical seal hardware represents the components required to apply mechanical tension to the rotating and stationary seal faces. This hardware; depending on seal design, can include springs, bellows, retaining rings, and pins. Not to be overlooked, hardware materials must be constructed of suitable metallurgy compatible with the fluid. An appreciation of seal driving hardware is also extremely important when sealing viscous products as ample torque to rotate the seal must be made available when the fluid is at its standing viscosity when starting a pump, and effective viscosity at operating conditions.

Mechanical seal selection should never be addressed as simple, easy or standard, as it is this approach which results in inadequate performance. A mechanical seal will only perform as well as all the sealing components combined and any options and auxiliary systems which may be required. Failure to properly address any portion of the mechanical seal chain could result in catastrophic failure, down time, considerable damage and expense, and most importantly personal injury and possible damage to the environment.

Specific pumping application requirements will determine the complexity of the seal design to achieve optimum performance. Mechanical seal configurations and options are as vast as pump models and designs. Addressing all the application parameters and fluid behavior characteristics will result in long trouble free mechanical seal service and enhanced pump and process performance.

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Mechanical Seals failures seem to fall into four broad categories:

The seal motion was restricted and the faces opened.
Heat caused the 0-rings to deteriorate.
The seal materials were attacked by the fluid sealed.
The seal was installed incorrectly.

Mechanical Seals motion restricted:

The spring-loaded (dynamic) seal face constantly moves to maintain full face contact with the stationary seal face. The main reasons for this movement are
1. The stationary face is not perpendicular to the pump shaft.
2. The pump has bearing end play. This means that the shaft moves back and forth a few thousandths of an inch at frequent but random intervals.
3. There is some impeller unbalance causing shaft whip.
4. The pump is operated away from its BEP, causing side loads on the shaft.
5. There is thermal shaft growth and Pump vibration that affects the seal.

Here are the major conditions that can restrict.. movement of the spring loaded mechanical seals face:
1. Solids have collected in the seal or around the dynamic seal ring.
2. The fluid sealed has caused the dynamic 0-ring to swell.
3. The temperature limit of the dynamic 0-ring has been exceeded and the 0-ring has lost its elasticity (compression set) or become hard.
4. Spring compression is inadequate because of incorrect installation.
5. Solids in the stuffing box, gasket protrusion or other foreign material restrict the motion of the dynamic seal ring.

Thermal degradation of Mechanical Seal 0-rings:

0-rings are the one part of a mechanical seal that are sensitive to heat because of the way they are manufactured. The ingredients are mixed together, put in a mold and cured at high temperature for a specific time. The compound will then assume the shape of the mold and its hardness, or durometer, will increase. When the 0-ring is placed in an 0-ring groove in a seal and heated to a temperature beyond its recommended limit, the curing process will continue and the 0-ring will take a compression set. This means that the 0-ring has lost some of its resilience and squeeze, and fluid may leak past the 0-ring. The higher the temperature, the shorter the time before the 0-ring takes a compression set. When an 0-ring is exposed to high temperature for a long period, it will become hard and brittle, causing mechanical seals failure.

Since heat is often a problem and seldom helps the mechanical seal application, what can be done about it?
1. Use a balanced seal to minimize the heat generated by the seal.
2. Use low-friction face materials. Carbon vs silicon carbide is the best choice.
3. Use a clean liquid flush or product recirculation to carry away heat.

Mechanical Seal materials attacked:

When the correct materials are not selected,

1. The 0-rings may swell locking up the mechanical seal,
2. The mechanical seal faces may deteriorate rapidly, and
3. The metal seal components may corrode.
All can cause the mechanical seals to fail.

Mechanical seals installed incorrectly:

Many mechanical seals fail at initial start-up or prematurely because they were not installed correctly. Cartridge seals eliminate all measurement, protect the seal faces from contamination and are easy to install. With these seals, installation problems are minimized. The Outside seal is preset and requires no installation measurement. Only in-line seals require careful measurement to insure correct installation. By following the mechanical seals installation instructions ,step-by-step correct seal installation is easily achieved.

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Mechanical Seals – Face Materials

Mechanical Seals come in a wide range of designs however the most important factor when determining how long a seal is going to last, is deciding on the right seal face materials. The old adage “pay me now or pay me later” applies to seals. You must consider the environment the seal will be exposed to. Abrasive environments require seal faces that will hold up to the wear and tear of abrasive environments. It makes little sense to save money on the seal only to have to spend more money to replace the seal later. On the other hand if you are pumping very clean fluid that has a lubricating quality such as machine oil or Ethylene Glycol, save your money for better quality bearings.

The most common types of seal faces are: Carbon, Ceramic, Ni-resist, 17-4, Silicon Carbide, Tungsten Carbide, GFPTFE (glass filled PTFE…. often called Teflon (R)). Another group of seal faces would be those of coated seal faces. The coatings are “plasma coatings” and are generally a form or silicon or tungsten carbide sprayed onto a stainless steel seal head. They have been sold by various trade names by the major mechanical seal companies but in our opinion are not worth the money. We have found the coating will always eventually wear and once it has will need to be completely recoated. In contrast, if a Silicon Carbide or Tungsten Carbide seal face is worn, it can generally be re-lapped and polished , bringing it back to “like new” condition and allowing a second, third or even fourth use of the same seal head.

Seal Faces:

Ceramic – Generally a 99.5% aluminum oxide offering excellent wear characteristics due to it’s hardness. It is chemically inert and can be applied to nearly any product. Ceramic cannot however handle the thermal shocks that Ni-resist or 17-4 seats can. They will also shatter like a plate if dropped on concrete.

Silicon Carbide – Is a bluish-black material created by fusing silica and coke. It is in the same family as Ceramic (due to the Silica), but has much better lubrication qualities and is harder. The most common is reaction-bonded silicon carbide. In Chemical applications however, Alpha Sintered Silicon Carbide may be recommended. Alpha Sintered Silicon carbide does however sacrifice the Pressure Velocity ratios due to the lack of un-reacted free carbon. Either grade of SIC can usually be re-lapped and polished to be reused.

Tungsten Carbide – A very versatile seal face like Silicon Carbide. It is very hard like silicon carbide however it is very heavy to the touch, making it easy to distinguish from silicon carbide. Ideal for high pressure applications due to a high modulus of elasticity which helps prevent face distortion. Tungsten carbide also can be re-lapped and polished to be re-used.

Ni-resist – Is a form of Stainless steel which has had nickel added to it to lower the friction generated by the rotating face. it is an inexpensive seal face and ideal for fresh water applications.

GFPTFE – Say that one ten times…. Glass Filled PTFE (most people just say Teflon (R)). Gives the chemical resistance of PTFE however glass must be added to give the face hardness and to prevent cold flow issues associated with PTFE.

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Rotating Face Mechanical Seals

Mechanical seals are engineered for most pumps, mixer and agitator applications in maintenance. In many cases the designs have been proven to be workhorses over years of use. In others seals must be designed for evolving industrial demands.

Mechanical seals deliver a full range or rotary configurations and component materials – to handle virtually and fluid moved by any equipment. When you specify engineered mechanical seals you have the advantage of the most advanced mechanical seal technology, the latest in field proven design and when working with an outside sales force there is hands on technical support.

Rotating face units are the dependable mechanical seal answer for worn equipment when your shaft seals must be replaced. They are ideal for new equipment designs or to improve the usefulness of existing equipment by converting from pump packing to mechanical seals.

Mechanical seals feature:

• Invisible leakage
• Less friction/power loss
• No to little wear on the shafts or sleeves
• Flexibility – to accommodate shaft deflections and “End Play”
• No period maintenance
• Long Life

In practical use, the basic rotating face mechanical seal principle is adaptable to serve a tremendous number of sealing needs: standard mechanical seals can suit most requirements-including temperatures to 500 degrees F and shaft speeds to 3600 RPM through the choice of secondary seal and the combination of seal and seat face materials which are offered. Seals can be ordered in balanced configurations to seal pressures above 200 psi, or used in a multiple for extremely high pressures or especially severe fluid services….Special mechanical seals can be furnished to meet the most demanding of industrial applications considering pressure, temperature, speed or fluid.

Mechanical seal designs range from the work horse “single spring” mechanical seals, to highly engineered double cartridge seals for extreme applications. The basic of each type follows:

Single spring mechanical seals:

Utilize a single spring coiled in a right of left hand design to accommodate left and right hand turning pumps. They have the flexibility to accommodate misalignment, shaft deflection, and break away shock loading. It resists clogging in extremely viscous fluids.

All mechanical seals are constructed of three basic sets of parts:
1) A set of primary seal faces: one that rotates and one that remains stationary.
2) A set of secondary seals known as shaft packings and insert mountings, such as o-rings, rubber boots, PTFE or Grafoil wedges, or V-Rings.
3) Mechanical Seals have hardware including gland rings, collars, compression rings, pins, springs, retaining rings and bellows.

Mechanical Seals can be classified by Arrangement

Mechanical Seals can be classified by Design

A mechanical seal works though the use of two very flat (generally within 3 light bands flat) lapped faces which make it difficult for leakage to occur (beyond a vapor). As mentioned above one face is stationary and one rotates with the shaft. One of the two faces is usually a non-galling material such as carbon-graphite. The other will be a harder material providing dissimilar materials making contact and allowing one to be a sacrificial. The softer mechanical seal face usually has the smaller mating surface and is commonly called the “wear nose” of the mechanical Seal.

The Easiest seal to install as a replacement for pump packing is a cartridge type mechanical seal:

The Beauty of cartridge seals are they are self-contained. Holding all the elements of a mechanical seal set:

*Rotating Face Seal

*Stationary Seal face or faces

*Shaft sleeve

*Gland

Cartridge mechanical seals :

Are the easiest seal for a mechanic to install. Only being required to slide onto the pump shaft and bolt to the pump gland, the cartridge seal cannot be miss installed.

Cartridge seals must fit the stuffing box there is a form on www.americansealandpacking.com under mechanical seals with this and other information on it to help the customer make sure the seal fits there pump.

Installation of single spring, metal bellows or multiple spring seals require much more effort to be sure of an effective long lasting seal. Ask your American Seal & Packing sales representative for installation directions.

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A mechanical face seal is an important component of variety of pumps used in chemical, petrochemical and process industry. The primary function of a mechanical seal is to prevent leakage of the process fluid from the pump housing and shaft to the environment. The factors that affect the performance of a mechanical seal to leak are friction, wear and its thermal characteristics. Improving upon the thermal characteristics of a mating ring in a mechanical seal would enhance its performance. Implanting a heat exchanger in the mating ring hold great promise for improving the performance of mechanical seals from the viewpoint of reducing heat at the interface and hence enhance the performance of the mechanical seal. To reveal what affect the implanted heat exchangers can have on the thermal characteristics of different seals, in this thesis, three different designs of mating rings were tested in a test rig and the results were compared to a conventional seal in this thesis.

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