Alb Mod Mechanical Seal Flashcards by Brandon Hatt

Mechanical seals are designed:

Not to leak, or to minimize leaking

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When pumps are used for fire safety, laws state that mechanical seals are prohibited. Why?

When the seal fails, it fails completely

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Mechanical seals are made up of four basic types of components: (4)

Primary sealing faces
Mechanism for holding faces together
set of secondary seals
metal components, set screws, springs, etc.

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What are the 6 styles of secondary seals?

Square/gasket packing
v packing
u cups
PTFE (teflon) wedges
delta rings
O rings

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What are the 5 sealing points on a mechanical seal?

Primary seal faces
Secondary sealing faces around stationary face
Secondary sealing faces around rotating face
between gland and pump
If a shaft sleeve is used, the point between shaft and sleeve

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Sealing at the primary faces is accomplished by the two faces, being extremely flat and smooth, being pushed together by ____ force

axial

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The primary sealing faces are pushed together via: (3)

Springs
Bellows
Product pressure

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What are the 3 factors that govern the amount of friction at the sealing faces? (3)

Seal ring material
smoothness and flatness of seal faces
closing force

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Mechanical seals may be lubricated by what sources: (2)

Pumped product, if compatible

- External supply

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Gas lubricated seals are designed so the faces never come into contact with each other while running. This is accomplished via:

Gas grooves

Note: Only a few microns deep

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The gap of a gas lubricated seal is due to aerodynamic lift off, provided via the rotation of the shaft, and is typically ____” to ____”

0.0001” to 0.0002”

Note: Clearance is set when the closing mechanism pressure is equal to the forces of the gas film

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Primary sealing faces are lapped and polished to what degree?

between one and two light bands

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In emergencies, sealing faces can be hand lapped to:

3 light bands

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It is common practice to have the sealing faces made of dissimilar materials to prevent:

Galling/adhesion

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The softer sealing face usually has a ____ seal face area

smaller

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What are three considerations to take into account when selecting a material for a seal face? (3)

corrosive properties of the liquid
temperature of the pumped product
Lubricity of the material

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This secondary seal can be used in both static and dynamic applications. Their ability to flex and slide axially allows them to tolerate slight misalignment and shaft endplay. Can be used for both pusher and non pusher seals.

O ring

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This secondary seal is designed to be pushed against the rotating carbon face without over stressing or breaking the primary seal. Used on pusher seals. Can only take pressure in one direction and cannot be used in a vacuum situation. Not tolerant of any misalignment or axial movement.

PTFE wedge

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This secondary seal is usually made of PTFE. It is an automatic type of packing, as in the pressure in the seal chamber aids in the sealing. Uses multiple springs and can be used where space is a concern. Should not be used in vacuum situations, unless turned around for an exclusively vacuum situation

V-rings

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This secondary seal is used mostly on pusher-type mechanical seals and are automatic, as in, they use the system pressure to aid in sealing. Uses an energizer/spreader ring to keep the seal loaded. Cannot be turned around to accommodate a vacuum situation.

U-Cups

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A pusher seal is so called because its secondary seal is pushed back and forth axially. This movement:

maintains constant contact at the seal faces

Note: to compensate for wear and misalignment

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The most commonly used pushing mechanisms are: (2)

Single spring

- Multiple springs

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With pusher seals, the constant axial movement of the secondary seals allows dirt to imbed in the packing, which:

causes the packing to act as an abrasive, fretting the shaft or sleeve

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Single spring closing mechanisms incorporate a single coil spring of a large diameter wire. This style is: (3)

Easy to use
resists corrosion
less susceptible to clogging

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Single spring closing mechanisms are at a disadvantage because: (3)

- More axial length is needed to accommodate the spring - Different springs needed for different size shafts - Uneven face loading and distortion at high speeds

What are 3 advantages of multiple spring closing mechanisms? (3)

- Require less operating length - Face loading is more evenly distributed - # of springs can be varied to alter the face loading

What are two disadvantages of multiple spring closing mechanisms? (2)

- More susceptible to corrosion | - Tendency to clog

Non pusher seals are also called:

Bellows

Non pusher seals differ from pusher seals in that:

The secondary shaft seal is static and does not move axially on the shaft

Bellows seals are inherently:

Hydraulically balanced Note: Meaning an increase in fluid chamber pressure does not lead to overloading of seal faces

Rubber (elastomer) diaphragm seals are the most common and use the rubber as a:

friction drive Note: the rubber swells in contact with the liquid (3-5%) and vulcanizes to the shaft

Diaphragm/bellows provides the drive and acts as a secondary seal. A single spring is used to keep the seal faces loaded in low pressure as well as to:

help position during assembly

_____ seals are used in conditions where no available metals can handle the mediums corrosive environment. Made from virgin material, glass fillers are added to provide rigidity and reduce cold flow characteristics. Used on outside mounted seals.

PTFE bellows seals

_____ occurs when PTFE extrudes when clamped and slippage occurs between the seal and the shaft. TO prevent this, a metal clamping ring is used.

Cold flow

With any outside mounted seal, seal failure causes the product to:

Leak out from the rotating seal

____ seals are made up of a series of metal plates welded together at the inside and outside diameters. Can be made of any weldable metal. Loading of the seal faces is accomplished by the spring action of the bellows. Bellows capsule is one piece and provides even pressure along the primary seal ring.

Welded metal bellows seals

Some of the advantages of welded metal bellows seals are: (3)

- Used in high pressure high temperature - do not require a secondary dynamic seal - are hydraulically balanced

Elastomer bellows are limited to lower rotational speeds of the shaft. Excess speeds cause the bellows to:

Deform

What are the two methods of providing drive for the rotating ring?(2)

- Mechanically | - Friction

Mechanical drives use ___ or ___ to engage the seal ring and the locking collar. The locking collar is held in place by set screws

Pins or tabs

Friction drives rely on what to drive the rotating ring?

Vulcanization of the rubber and shaft

Friction drives are normally used for pumping:

non lubricating materials such as water.

AM350, Alloy 20, Hastelloy, Inconel, monel and titanium are all:

Common materials for welded metal bellows seals

____ mountings, for friction drives, make use of gaskets and a clamping force

clamped mountings

____ provide a cavity for holding a circulating a cooling or heating fluid near the seal. This is accomplished via heat exchangers.

Gland jackets

_____ allows circulation of fluid around the seal faces for the purposes of lubrication, cooling, heating and cleansing. When corrosive or hazardous products are sealed, an external fluid can be piped in.

Flushed gland seals

_____ uses the pumped medium from the discharge to remove heat or to maintain a positive pressure in the seal chamber. Helps carry away heat and prevents contaminants from building up

Bypass flushing

_____ uses a compatible fluid that is piped into the seal chamber at a pressure greater than the pressure within the seal chamber. The downside of this is that the product experiences some dilution. Throat bushings, throttle bushings and lip seals can be used to minimize dilution.

External flushing

_____ are fitted with drains and vents for the introduction and draining of fluids. These are introduced to the low pressure side of the double seals, which is the primary seal that is exposed to atmosphere.

Quench glands

When steam is used for quenching, a ____ is normally recommended to prevent the bearings from being affected by the steam and to cut down on the volume used.

Throttle bushing

The low pressure side of the seal could be exposed to quenching for these reasons: (4)

- Adds heat to the product - Removes heat from the product - cleanses and build up due to coking or crystalizing - Used as a wetting agent to wash away leakage

Barrier systems are normally found on:

Double seals Note: uses a barrier fluid to isolate the pumping medium from the seals and atmosphere. Must be higher pressure than pumped medium

Buffer systems, unlike barrier systems, are introduced between the seals at a ______ in order to isolate the product from the environment

Lower pressure

When pumpage contains suspended solids, and a bypass flush is used, the seal can be severely damaged. Employing a ____ in the flush line can reduce the chances of premature failure. Common materials include cottons and fiberglass

Filter Note: when selecting a filter, consider quantity and size of particles, and pressure/flow

When there is a constant introduction of particles into a system, instead of a filter, you could use a:

cyclone separator

Unbalanced seals are ______ common inside seals

very common Note: higher seal face loading and therefore less chance of leakage, more tolerance to misalignment, cavitation and vibration

The advantages of unbalanced seals are they are less expensive and more adaptable to standard stuffing boxes, one major disadvantage is:

As the pressure in the stuffing box increases, so does the load on the seal face The lubricating film that is needed to separate the sealing faces is forced out.

Unbalanced seal chamber pressure cannot exceed:

300 PSI

Balanced seals optimize seal face loading so that increases in hydraulic force are:

Cancelled out

Balanced seals have hydraulic forces cancelled out using Pascals law by changing the area on which the pressure acts. This can be done by:

Machining a small step onto the shaft or sleeve to reduce the area that is exposed to the fluid pressure and expose an area that tries to offset the extra closing force

Rotating seals have the spring on the rotating seal and rotate with the shaft. When the increase in rotational speed is such that distortion takes place, the use of ____ is necessary.

stationary seals

Stationary seals are used when shaft speeds exceed 4500 ft/min. When the springs are allowed to move, the stationary seal should be able to to:

float

Single seals can be installed in two configurations: (2)

Inside mounted | outside mounted

If the pumpage comes in contact with the metal components of the seal, and the outside diameter of the seal is exposed to pumpage, the seal is classified as:

inside mounted seal | most common

If the metal components do not come in contact with the product and the inside diameter is exposed to the product, the seal is classified as:

outside mounted seal

What are four advantages of inside mounted seals? (4)

- Chamber pressure aids sealing - Inside seals can clean themselves (centrifugal force) - Can come balanced for higher pressures - Can be modified for environmental controls

The major downside of an inside mounted seal is:

It cannot be adjusted without dismantling, unless the seal is a cartridge type Note: also, centrifugal forces can throw abrasives into the seal faces, causing premature wear

Outside mounted seals are recommended when: (2)

- Stuffing box is too small | - Pumpage is corrossive

Outside seals can normally only handle pressures of 15-75PSI but can be engineered to to be balanced and handle pressures up to:

400PSI

______ have an arrangement that combines two seals with two rotating seals

double mechanical seals Note: Used for non lubricated products, carcinogenic, toxins and hazardous fluids

On a double mechanical seal, the seal closest to the impeller is called:

Inboard or inner seal

On a double mechanical seal, the seal closest to the atmosphere is called the:

Outboard or outer seal

______ double mechanical seal arrangements have two rotating seals that face away from each other. A barrier fluid is introduced into the cavity between them (lubrication, barrier)

Back to back

On a back to back double mechanical seal arrangement, barrier fluid pressure is recommended to be ___ % higher than the fluid pressure on the inner seal and a minimum 25 PSI delta P

10% This is to keep the inboard seal faces lubricated and closed

The main function of a back to back arrangement is to:

-Seal the barrier fluid

Applications of a back to back double mechanical seal arrangement are limited by:

Its inability to handle changes and reversals of pressure

____ arrangements are used when the stuffing box is too small to use a back to back double mechanical seal. Has two rotary seals that face each other and commonly share a stationary ring.

Face to face

If on a face to face double seal arrangement you have fluid chamber pressure lower than the product pressure, it is called ____ and the inboard seal acts like any other seal with the outboard seal acting as a backup.

Buffer fluid

If the fluid in the seal chamber is ____ than the pumpage, it is considered a true double seal.

Higher

______ mechanical seals are most often used for volatile, carcinogenic, toxic and hazardous fluids and are often called environmental seals

Tandem mechanical seals

Tandem mechanical seals have both the inboard and outboard seals facing:

away from the product. Note: system pressure helps seal the faces

The inner seal of a tandem mechanical seal must handle the full temperature and pressure, and the outer seal:

Must be able to do the same, and act as a backup if the first fails

When mechanical seals come from the manufacturer, they are lapped to within 3 light bands. one light band is equal to:

11.6 millionths of an inch

Seal faces should be checked for flatness using: (2)

- Optical flat | - Monochromatic light

A monochromatic light uses a ____ lamp

helium

If a mechanical seal has signs of heat cracking, or wear in excess of ____ ", it should be replaced.

1/32"

Mechanical seal surfaces should be brought within _____" before they are lapped

0.005"

Lap the mechanical seal face on a lubricated diamond plate in what pattern?

figure 8

When checking the flatness of a seal face with an optical flat, besides cleanliness, you should ensure:

proper optical flat orientation

When reading an optical flat, several straight bands or one curved band indicates:

A flat surface of 1 light band

When reading an optical flat, curved lines indicate:

The surface is not flat Note: the number of crossed lines indicates the degree of flatness

Mechanical seal faces must be handled with:

great care

Examine seal faces for chips caused by the seal faces separating and slamming together. This is caused by:

liquid flashing to a vapour

Examine seal faces for radial cracks, these are caused by:

high heat