Mechanical Seals - Balanced Seals

• When speaking of “Balance” in reference to mechanical seals, we are not talking about Mechanical or Rotational Balance. Instead, we are referring to Hydraulic Balance.

• Since mechanical seals are subject to stuffing box pressure, this pressure is utilized to achieve and maintain seal face closure in a non-balanced seal.

• If stuffing box pressure is very high, typically over 100psi., then the closing force may be too great to allow the “Boundary Layer Liquid” that lubricates the faces to be sufficient and the faces will wear prematurely.

• A balanced seal compensates for higher pressures by locating the seal faces such that stuffing box pressure has less effect on face closure.

Why Mechanical Seals Fail

Mechanical Seals failures seem to fall into four broad categories 1. The seal motion was restricted and the faces opened.

2. Heat caused the 0-rings to deteriorate. 3. The seal materials were attacked by the fluid sealed. 4. The seal was installed incorrectly. 5. 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.

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.

SPECIFYING MECHANICAL SEALS Mechanical seals have become increasingly popular in industry. Many companies develop a set of mechanical seal specifications to make it easier for their people to

select one or more seals for an application. The lack of a company specification can cause problems with inventory, installation and interchangeability of seals. Plants are accepting seal components with unidentified materials, making troubleshooting an impossible task for them. A good company specification for mechanical seals will include the following:

-- Mechanical Seal materials with the widest range of chemical compatibility, -- Mechanical Seals that are easy to install, -- Mechanical Seals with features that make seal repair easy and low cost, -- Mechanical Seals that require the least amount of inventory and spare parts, -- Mechanical Seals that operate over a wide range of temperature, pressure and speed, and -- Mechanical Seal designs that allow the easiest conversion of packed pumps to mechanical seals.

A typical plant will use some or perhaps all of the following five types of mechanical seals:

• A single cartridge-mounted balanced seal, • A dual cartridge-mounted balanced seal, • A balanced outside seal for low pressure applications, • A balanced seal installed inside the stuffing box, and • Metal bellows mechanical seals for chemical and high temperature applications

Mechanical Seals - Metal Bellows

Metal bellows mechanical seals feature a bellows core made of one of several types of bellows. The most common are AM350 and Hastelloy C. Usually with Stainless Steel end fittings permits these mechanical seals to be used for services beyond the range of mechanical seals with synthetic rubber sealing members. Typical applications include: cryogenics, heat transfer fluids, severe chemicals and corrosives at temperatures from —350° to +650°F (—212° to +343.33°C) with standard seal construction. (For higher temperatures—our engineering department.)

Custom MB Seals

MBFG

MB

Metal Bellows Mechanical Seals Design Features 1. Wide service range from cryogenics to heat transfer liquids and corrosive chemicals. 2. Long service life—only one moving part. 3. Flexible bellows construction eliminates seal hang-up due to solids or dirt in pumpage. 4. Metal sealing wedge eliminates temperature restrictions of elastomers or plastics. 5. Built-in hydraulic balance eliminates step in shaft or sleeve*. 6. Available for either inside or outside mounting. 7. Stabilizer tabs prevent possible damage due to vibration and excessive shaft run-out.

Nested Ripple design with advanced welding techniques insures maximum bellows strength and fatigue resistance. It all but eliminates stick-slip failures. Hastelloy “C” is used in both the MB and the MBFG bellows. It is far superior to 316SS, AM350, and Alloy 20 in corrosion resistance, stress corrosion cracking resistance, and yield strength. Alloy 20 has a yield strength of 35,000 psi while Hastelloy C has a yield strength of 58,000 psi, a 66% increase! That is where it counts; in the bellows. Drive set screws are Hastelloy “C” on both the MB and MBFG Hastelloy “C” set screws reduces galling problems that result with 316SS set screws. The 66% greater yield strength also insures that your Allen wrench will not “strip out”. Metal bellows mechanical seals will not fret the shaft or sleeve because it has no dynamic (sliding) “0” ring. All pusher seals (seals with springs) have dynamic “0” rings. This means that you can mount either metals bellows seal directly on the shaft and save the cost of a sleeve. Because the bellows spring does hot have to overcome “0” ring friction drag, as does the pusher seal, the spring load can be lighter in the MB and MBFG mechanical seals bellows then in the pusher seal design. Some common pusher seals have spring loads of 35 to 45 psi, while the metal bellows load is approximately 25 psi. This can result in up to a 50% decreased seal face wear rate. The bellows produces even face loading, superior face tracking and less frictional heat. *Built-in Hydraulic Balance: balance diameter lies within the bellows itself. This provides the ability to handle pressures to 300 PSI (21.1 KG/CM2) and, in certain environments, up to 500 PSI (35 KG/CM2) without the time and cost of cutting a step in the shaft or sleeve.

Metal bellows mechanical seals are also made into single and double cartridge configurations.

If you have any questions please contact us at: E-Mail about your mechanical seals Mechanical Seals - Outboard

If an extremely corrosive liquid has satisfactory lubricating properties, an outside mechanical seal offers an economical alternative to the expensive metallurgies required for an inside seal to resist corrosion. The illustration below shows a typical outside seal arrangement in which only the insert, seal ring and secondary seals are exposed to the product. All these components can be non metallic. The metallic rotary unit parts are exposed to the atmosphere. Outside mechanical seals can also be used in equipment with stuffing boxes which will not accommodate inside seals, and they are easier to access for adjustment and trouble-shooting. The outside mechanical seal does have disadvantages. Because it is exposed, the seal is vulnerable to damage from impact. Of greater importance, however, is an outside mechanical seal’s pressure limitation. The hydraulic pressure on an unbalanced outside mechanical seal works to open, rather than to close, the seal faces. The seal relies upon the springs to maintain face contact. Even though a degree of balance can be designed into a hydraulically balanced outside seal, all outside seals are limited to applications having moderate pressures (150 PSIG maximum, depending on shaft size and speed).

PTFE Outboard Mechanical Seals with a Ceramic Seat. The Seal face on this Seal is GFPTFE

Inboard mechanical seals:

The term Inboard seals simply refers to seals that are mounted inside of a pumps stuffing box, to a shaft or sleeve and are mounted independently as individual parts. Unlike Cartridge seals which is a unitized set of rotating, stationary, sleeve and gland components. Inboard seals are mounted to an existing sleeve or shaft. The stationary the inboard seal runs against is independently press fit (usually), into a bore and can be purchased separately from the inboard seal.

Mechanical Seals - Cartridge Seals

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Cartridge seals are available in both single and double configurations. If you have any questions please contact us at: E-Mail Phone: 714-593

Mechanical Seals - Split Seals

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Mechanical Seals - Double Cartridge Seals

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Some products are not compatible with a single cartridge mechanical seal. Examples include toxic liquids whose leakage into the environment would be hazardous, liquids whose suspended abrasives would rapidly wear the faces, or corrosive liquids requiring seals made of costly materials. There are two ways to design for such products.

One method is to incorporate environmental controls. This method is the more economical solution than the second option of incorporation high priced metals that may be compatible with the corrosive liquids being pumped. The use of a double seal allows the containment of clean fluid between the sets of faces. This allows the seal faces to only see clean lubricating fluid, greatly extending the life of the seal. There are two basic double seal arrangements: tandem and face-to-face. A double cartridge mechanical seal can be configured in either configuration. It is necessary to understand the differences between these arrangements in order to select the best design for a specific application.

DOUBLE FACE-TO-FACE SEALS

The double face-to-face seal is usually cartridge mounted with one seal inside the stuffing box and one seal on the outside. This arrangement is ideal for pumps with shallow stuffing boxes. Both seals rotate against common or separate stationary inserts.

The face-to-face arrangement may be used as a double seal. If the liquid between the seals is at a higher pressure than the product in the stuffing box, the inner seal is lubricated by a sealing liquid. This creation of an artificial environment for the mechanical seal is the role of a true double seal, if the liquid is circulated between the seals at a lower pressure than the equipment stuffing box pressure, the purpose of the inner seal is the same as any single seal, and the outer seal simply serves as a back-up.

The double face cartridge, assembly, is compact easy to install and can be bench tested with using air as a barrier fluid.

The weakness of a face to face arrangement is the exposure of the inner seal to abrasive or corrosive materials.

TANDEM SEALS

A tandem seal increases on line reliability and is considered the safest environmental design. If a primary seal failure occurs on a critical piece of equipment, the outboard seal can take over and function until maintenance of the equipment can be scheduled. Alarm systems can be installed with censors to determine an increase in the pressure between the two sets of seal faces. This is easily established by adding a pressure gauge on the line coming from the seal pot to the double cartridge seal. Tandem seal systems are used for products such as vinyl chloride, carbon monoxide, light hydro carbons, and a wide range of other volatile, toxic, carcinogenic, or hazardous fluids. Tandem seals eliminate icing and freezing of light hydrocarbons and other fluids which could fall below the atmospheric freezing point of water in air (32°F or 00C) because both seal faces are inside the stuffing box.. Typical buffer fluids (contained in the seal pots ) in these applications are ethylene glycol, methanol, and propanol.

Cartridge seals are available in both single and double configurations.

Mechanical Seals - Tungsten Carbide

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. It comes in two primary types:

Nickel Bound (Ammonia applications should be avoided) Cobolt Bound (May be used in Ammonia)

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. The most common complaint of Tungsten Carbide seal faces is "Leaching". Leaching is caused by chemicals like Ammonia. It causes minor leakage and a large increase in the wear of the opposing seal face (especially carbon).