Industrial Machinery Repair Part Episode 2 Part 6 potx

In order to provide a full under-standing of seal and packing use and performance, this manual discussesfundamentals, seal design, and installation practices.Fundamentals Shaft seal requ

is acceptable for each application The recommended range should includeadjustments for temperature, flow rates, mixing speeds, and other factorsthat directly or indirectly affect viscosity.

Troubleshooting

Table 18.1 identifies common failure modes and their causes for mixersand agitators Most of the problems that affect performance and reliabilityare caused by improper installation or variations in the product’s physicalproperties

Table 18.1 Common failure modes of mixers and agitators

Mixer/agitator setting too high • •

Mixer/agitator setting too low • •

Product temperature too low • • •Rotating element imbalanced or

damaged

Viscosity/specific gravity too high • • •

Proper installation of mixers and agitators is critical The physical location ofthe vanes or propellers within the vessel is the dominant factor to consider.

If the vanes are set too close to the side, corner, or bottom of the vessel,

a stagnant zone will develop that causes both loss of mixing quality andpremature damage to the equipment If the vanes are set too close to theliquid level, vortexing can develop This will also cause a loss of efficiencyand accelerated component wear

Variations in the product’s physical properties, such as viscosity, also willcause loss of mixing efficiency and premature wear of mixer components.Although the initial selection of the mixer or agitator may have addressed thefull range of physical properties that will be encountered, applications some-times change Such a change may result in the use of improper equipmentfor a particular application

All machines such as pumps and compressors that handle liquids or gasesmust include a reliable means of sealing around their shafts so that thefluid being pumped or compressed does not leak To accomplish this, themachine design must include a seal located at various points to preventleakage between the shaft and housing In order to provide a full under-standing of seal and packing use and performance, this manual discussesfundamentals, seal design, and installation practices.

Fundamentals

Shaft seal requirements and two common types of seals, packed stuffingboxes and simple mechanical seals, are described and discussed in this sec-tion A packed box typically is used on slow- to moderate-speed machinerywhere a slight amount of leakage is permissible A mechanical seal is used

on centrifugal pumps or other type of fluid handling equipment where shaftsealing is critical

Shaft Seal Requirements

Figure 19.1 shows the cross-section of a typical end-suction centrifugalpump where the fluid to be pumped enters the suction inlet at the eye ofthe impeller Due to the relatively high speed of rotation, the fluid collectedwithin the impeller vanes is held captive because of the close tolerancebetween the front face of the impeller and the pump housing

With no other available escape route, the fluid is passed to the outside

of the impeller by centrifugal force and into the volute, where its kineticenergy is converted into pressure At the point of discharge (i.e., dischargenozzle), the fluid is highly pressurized compared to its pressure at the inletnozzle of the pump This pressure drives the fluid from the pump andallows a centrifugal pump to move fluids to considerable heights above thecenterline of the pump

This highly pressurized fluid also flows around the impeller to a lower sure zone where, without an adequate seal, the fluid will leak along the drive

Housing

Suction

Back-headImpellerShaft

Balancing holesPumping vanes

Figure 19.1 Cross-section of a typical end-suction centrifugal pump

shaft to the outside of the pump housing The lower pressure results from apump design intended to minimize the pressure behind the impeller Notethat this design element is specifically aimed at making drive shaft sealingeasier

Reducing the pressure acting on the fluid behind the impeller can beaccomplished by two different methods, or a combination of both, on anopen-impeller unit One method is where small pumping vanes are cast onthe backside of the impeller The other method is for balance holes to bedrilled through the impeller to the suction eye

In addition to reducing the driving force behind shaft leakage, decreasingthe pressure differential between the front and rear of the impeller using one

or both of the methods described above greatly decreases the axial thrust

on the drive shaft This decreased pressure prolongs the thrust bearing lifesignificantly

Sealing Devices

Two sealing devices are described and discussed in this section: packedstuffing boxes and simple mechanical seals

Packed Stuffing Boxes

Before the development of mechanical seals, a soft pliable material or ing placed in a box and compressed into rings encircling the drive shaftwas used to prevent leakage Compressed packing rings between the pumphousing and the drive shaft, accomplished by tightening the gland-stuffingfollower, formed an effective seal

pack-Figure 19.2 shows a typical packed box that seals with rings of compressedpacking Note that if this packing is allowed to operate against the shaft with-out adequate lubrication and cooling, frictional heat eventually builds up to

the point of total destruction of the packing and damage to the drive shaft.Therefore, all packed boxes must have a means of lubrication and cooling.Lubrication and cooling can be accomplished by allowing a small amount

of leakage of fluid from the machine or by providing an external source offluid When leakage from the machine is used, leaking fluid is captured incollection basins that are built into the machine housing or baseplate Notethat periodic maintenance to recompress the packing must be carried outwhen leakage becomes excessive

Packed boxes must be protected against ingress of dirt and air, which canresult in loss of resilience and lubricity When this occurs, packing will actlike a grinding stone, effectively destroying the shaft’s sacrificial sleeve, andcause the gland to leak excessively When the sacrificial sleeve on the driveshaft becomes ridged and worn, it should be replaced as soon as possi-ble In effect, this is a continuing maintenance program that can readily bemeasured in terms of dollars and time

Uneven pressures can be exerted on the drive shaft due to irregularities inthe packing rings, resulting in irregular contact with the shaft This causesuneven distribution of lubrication flow at certain locations, producing acutewear and packed-box leakages The only effective solution to this problem

is to replace the shaft sleeve or drive shaft at the earliest opportunity

Simple Mechanical Seal

Mechanical seals, which are typically installed in applications where noleakage can be tolerated, are described and discussed in this section.Toxic chemicals and other hazardous materials are primary examples ofapplications where mechanical seals are used

Components and Assembly

Figure 19.3 shows the components of a simple mechanical seal, which ismade up of the following:

Stationary ringRotating ring

Force

Figure 19.3 Simple mechanical seal

material selection of the O-ring and seal materials The insert and insertO-ring mounting are installed in the bore cavity provided in the gland ring.This assembly is installed in a pump-stuffing box, which remains stationarywhen the pump shaft rotates

A carbon graphite insertion ring provides a good bearing surface for the sealring to rotate against It is also resistive to attack by corrosive chemicals over

a wide range of temperatures

Figure 19.4 depicts a simple seal that has been installed in the pump’sstuffing box Note how the coil spring sits against the back of the pump’simpeller, pushing the packing O-ring against the seal ring By doing so, itremains in constant contact with the stationary insert ring

As the pump shaft rotates, the shaft packing rotates with it due to friction.(In more complex mechanical seals, the shaft-packing element is secured tothe rotating shaft by Allen screws.) There is also friction between the spring,the impeller, and the compressed O-ring Thus, the whole assembly rotatestogether when the pump shaft rotates The stationary insert ring is locatedwithin the gland bore The gland itself is bolted to the face of the stuffingbox This part is held stationary due to the friction between the O-ring insertmounting and the inside diameter (ID) of the gland bore as the shaft rotateswithin the bore of the insert

Figure 19.4 Pump stuffing box seal

How It Prevents Leakage

Having discussed how a simple mechanical seal is assembled in the stuffingbox, we must now consider how the pumped fluid is stopped from leakingout to the atmosphere

In Figure 19.4, the O-ring shaft packing blocks the path of the fluid alongthe drive shaft Any fluid attempting to pass through the seal ring is stopped

by the O-ring shaft packing Any further attempt by the fluid to pass throughthe seal ring to the atmospheric side of the pump is prevented by the glandgasket and the O-ring insert The only other place where fluid can poten-tially escape is the joint surface, which is between the rotating carbon ringand the stationary insert (Note: The surface areas of both rings must bemachined-lapped perfectly flat, measured in light bands with tolerances ofone-millionth of an inch.)

Sealing Area and Lubrication

The efficiency of all mechanical seals is dependent upon the condition of thesealing area surfaces The surfaces remain in contact with each other for theeffective working life of the seal and are friction-bearing surfaces

As in the compressed packing gland, lubrication also must be provided inmechanical seals The sealing area surfaces should be lubricated and cooled

with pumped fluid (if it is clean enough) or an outside source of clean fluid.However, much less lubrication is required with this type of seal because thefrictional surface area is smaller than that of a compressed packing gland,and the contact pressure is equally distributed throughout the interface As

a result, a smaller amount of lubrication passes between the seal faces toexit as leakage

In most packing glands there is a measurable flow of lubrication fluidbetween the packing rings and the shaft With mechanical seals, the facesride on a microscopic film of fluid that migrates between them, resulting

in leakage However, leakage is so slight that if the temperature of the fluid

is above its saturation point at atmospheric pressure, it flashes off to vaporbefore it can be visually detected

Advantages and Disadvantages

Mechanical seals offer a more reliable seal than compressed packing seals.Because the spring in a mechanical seal exerts a constant pressure on theseal ring, it automatically adjusts for wear at the faces Thus, the need formanual adjustment is eliminated Additionally, because the bearing surface

is between the rotating and stationary components of the seal, the shaft orshaft sleeve does not become worn Although the seal will eventually wearout and need replacing, the shaft will not experience wear

However, much more precision and attention to detail must be given tothe installation of mechanical seals compared to conventional packing.Nevertheless, it is not unusual for mechanical seals to remain in service formany thousands of operational hours if they have been properly installedand maintained

Mechanical Seal Designs

Mechanical seal designs are referred to as friction drives, or single-coil springseals, and positive drives

Single-Coil Spring Seal

The seal shown back in Figure 19.4 depicts a typical friction drive or coil spring seal unit This design is limited in its use because the seal relies

single-on frictisingle-on to turn the rotary unit Because of this, its use is limited to liquidssuch as water or other nonlubricating fluids If this type of seal is to be used

with liquids that have natural lubricating properties, it must be mechanicallylocked to the drive shaft.

Although this simple seal performs its function satisfactorily, there are twodrawbacks that must be considered Both drawbacks are related to the use

of a coil spring that is fitted over the drive shaft:

● One drawback of the spring is the need for relatively low shaft speedsbecause of a natural tendency of the components to distort at high surfacespeeds This makes the spring push harder on one side of the seal thanthe other, resulting in an uneven liquid film between the faces Thesecause excessive leakage and wear at the seal

● The other drawback is simply one of economics Because pumps come

in a variety of shaft sizes and speeds, the use of this type of seal requiresinventorying several sizes of spare springs, which ties up capital.Nevertheless, the simple and reliable coil spring seal has proven itself inthe pumping industry and is often selected for use despite its drawbacks

In regulated industries, this type of seal design far exceeds the capabilities

of a compressed packing ring seal

Positive Drive

There are two methods of converting a simple seal to positive drive Bothmethods, which use collars secured to the drive shaft by setscrews, areshown in Figure 19.5 In this figure, the end tabs of the spring are bent at

Figure 19.5 Conversion of a simple seal to positive drive

90 degrees to the natural curve of the spring These end tabs fit into notches

in both the collar and the seal ring This design transmits rotational drivefrom the collar to the seal ring by the spring Figure 19.5 also shows twohorizontally mounted pins that extend over the spring from the collar tothe seal ring

Installation Procedures

This section describes the installation procedures for packed stuffing boxesand mechanical seals

Packed Stuffing Box

This procedure provides detailed instructions on how to repack centrifugalpump packed stuffing boxes or glands The methodology described here

is applicable to other gland sealed units such as valves and reciprocatingmachinery

Tool List

The following is a list of the tools needed to repack a centrifugal pumpgland:

● Approved packing for specific equipment

● Mandrel sized to shaft diameter

● Packing ring extractor tool

● Packing board

● Sharp knife

● Approved cleaning solvent

● Lint-free cleaning rags

Precautions

The following precautions should be taken in repacking a packed stuffingbox:

● Ensure coordination with operations control

● Observe site and area safety precautions at all times

● Ensure equipment has been electrically isolated and suitably locked outand tagged.

● Ensure machine is isolated and depressurized, with suction and dischargevalves chained and locked shut

Installation

The following are the steps to follow in installing a gland:

1 Loosen and remove nuts from the gland bolts

2 Examine threads on bolts and nuts for stretching or damage Replace ifdefective

3 Remove the gland follower from the stuffing box and slide it along theshaft to provide access to the packing area

4 Use packing extraction tool to carefully remove packing from the gland

5 Keep the packing rings in the order they are extracted from the glandbox This is important in evaluating wear characteristics Look for rubmarks and any other unusual markings that would identify operationalproblems

6 Carefully remove the lantern ring This is a grooved, bobbin-like spoolpiece that is situated exactly on the centerline of the seal water inletconnection to the gland (Figure 19.6)

NOTE: It is most important to place the lantern ring under the seal waterinlet connection to ensure the water is properly distributed within thegland to perform its cooling and lubricating functions

Figure 19.6 Lantern ring or seal cage