Chapter 01

Seals & Packings for Rotating Shafts

Chapter # 1 Page. 1 of 45

CHAPTER 1

SEALS AND PACKINGS FOR ROTATING SHAFTS



INTRODUCTION

The purpose of this chapter is to introduce and describe seals and packings which are widely used to perform sealing for rotating shafts in fluid movers like pumps and rotary compressors (centrifugal compressors, axial flow compressors and screw compressor) We will discuss where and why seals are required, common types of seals, their variations and how each type perform sealing? Why do we Need to Perform Sealing? As a natural phenomena, fluids (liquids and gases) move freely from places where the pressure is higher to places of low pressure. This movement could be from places where they are wanted to places where they are not wanted. This movement of the fluids may be undesired for several reasons:

• A pressure may be needed which cannot be maintained if the fluid involved is allowed to move in certain direction.

• The presence of the fluid outside its container may involve waste,

expense, danger, contamination or other undesirable consequences.

This undesired motion is termed leakage, and our aim is to stop this leakage or minimizing it to the lowest possible level.

The kind of leakage that may occur and the technique used to prevent it, depends on the container construction. The container which may be a tank, a pipe, a valve, a pump, a boiler, or any of several other things-may be made of material which itself does not permit leakage. The leakage may happen through joints or holes.

Joints are necessary in many structures and must be sealed. Gaskets, pipe and thread compounds are used for sealing. The joining surfaces may be so perfectly mated that leakage is restrained.



The structure to be made leak-proof may have a hole with a shaft extending through it and the shaft may rotate or move in an axial direction. Categorization of Seals There are two basic kinds of seals:

• Static seals • Dynamic seals

A- Static Seals

1. Static seals are used to perform sealing between two stationary items, i.e., where no movement occurs at the juncture to be sealed. Gaskets and O-rings are typical static seals.

2. Another category of static seals are sealant, which are similar to

gaskets except that they are applied as a liquid or past.

3. This gasket is any device maintains a barrier against the transfer of fluids across mating surfaces of a mechanical assembly when the surfaces do not move relative to each other. Gaskets seal by being squeezed between the joint faces tightly enough so that they exert more pressure against the faces than does the fluid pressure tending to leak past them.

4. Joint and gasket design must be considered together. A joint is only

as good as its gasket, and the gasket may succeed or fail according to whether the joint makes the best use of the gasket material. Therefore, joint components must be thought of as a unit, or system, for effecting a seal. Otherwise, the end result may be a leaky joint.

5. A prime factor in any seal or gasket is minimum sealing stress. This

is the minimum stress necessary to make the seal material conform to the imperfections on the flange face (Figure 1.1) and, where necessary, to close the material's porous structure so leakage does not occur.



Figure 1.1 Conformation of gasket to flange Figure 1.2 Static Seals Gaskets would not be necessary if the flanges-or the two meeting surfaces were machined to mate perfectly and lapped to a good surface finish and if, in addition, the piping was in perfect alignment. Under such conditions, we might achieve a leak-proof joint but such procedures are obviously impractical and very expensive. Figure 1.2 shows another example of static seals (O-Ring).

B- Dynamic Seals

1. Dynamic seals are employed where surfaces move relative to one another. Dynamic seals are used, for example, where a reciprocating or rotating shaft transmits power or movement through the wall of a tank (Figure 1.3), through the casing of a pump (Figure 1.4), or through the housing of other rotating equipment, such as a filter or a screen.

Figure 1.3 Cross section of Figure 1.4 Typical centrifugal pump tank and mixer.



In general, dynamic seals can not stop the leakage 100% like static seals, but it is able to cut it (minimize it) to certain acceptable limits. The Various Methods Of Sealing Rotating Equipment There are several sealing devices which can be used to perform sealing around rotating shafts. In the case of centrifugal pumps, sealing devices limit liquid escape at the point where the pump shaft leaves the pump housing (See Figure 1.4). These sealing devices are:

1. Stuffing box packing. 2. Condensate injection sealing. 3. Floating seal rings. 4. Labyrinth seal. 5. Windback scroll or windback seal. 6. Segmented Carbon ring seals. 7. Lip seals. 8. Mechanical seals.

This chapter is divided into eight sections, each section covering one of the sealing devices. Section 1.1 Stuffing Boxes. Section 1.2 Condensate Injection Sealing. Section 1.3 Floating Seal Rings. Section 1.4 Labyrinth Seal. Section 1.5 Windback Scroll or Windback Seal. Section 1.6 Segmented Carbon Ring. Section 1.7 Lip Seals. Section 1.8 Mechanical Seals.



SECTION 1.1

STUFFING BOXES

To understand how such a seal functions, a quick review of pump fundamental is in order. Any pump converts the energy of a prime mover, such as an electric motor, into velocity or pressure energy of the liquid or gas being pumped. In a centrifugal pump, the product enters the suction of the pump at the center of the rotating impeller. (Figure 1.5).

Figure 1.5 Fluid flow in a centrifugal pump

As the impeller vanes rotate, they transmit motion to the incoming product, which then leaves the impeller, collects in the pump casing, and leaves the pump under the pressure through the pump discharge. Discharge pressure will force some product down behind the impeller to the drive shaft, where it attempts to escape along the rotating shaft. Pump manufacturers use various design techniques to reduce the pressure of the product trying to escape. Such techniques include:



1. The addition of balance holes through the impeller to permit most of the

pressure which acting behind the impeller to escape into the suction side of the impeller. (Figure 1.6).

2. The addition of small pumping vanes on the back side of the impeller.

(Figure 1.7). However, as there is no way to eliminate this pressure completely, sealing devices are necessary to limit the escape of the product to the atmosphere. Such sealing devices are typically either compression packing or mechanical seal.

Figure 1.6 Back wear ring and Figure 1.7 Back vanes balancing holes 1.1.1 Packed Stuffing Box Stuffing boxes have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump casing. If the pump handles a suction lift and the pressure interior stuffing box end is below atmospheric, the stuffing box function is to prevent air leakage into the pump. If this pressure is above atmospheric, the function is to prevent liquid leakage out the pump.



1.1.2 How it works? Early attempts to control the leakage of the product around reciprocating or rotating shafts consisted of merely restricting the clearance between the shaft and the wall of the vessel or pump casing by packing a soft, resilient material around the shaft within an extension of the tank wall or pump back head called a stuffing box. Figure 1.8 Shows a typical stuffing box sealed with square rings of compression packing. The compression packing rings, which must be carefully installed in a clean stuffing box, are held in place by a gland. As the gland bolt nuts are tightened, pressure applied to the gland is transmitted to the compression packing, forcing it against the shaft or shaft sleeve and effecting a seal. Because this pressure is not evenly distributed throughout the packing, most of the sealing and consequently most the wear occurs in the first few rings adjacent to the gland. (Figure 1.9). Frictional heat, which develops where the compression packing contacts the rotating shaft or shaft sleeve, is reduced by permitting the product to leak to the atmosphere at a controlled rate. This leakage is essential to carry away the frictional heat and as lubricant between the shaft (or shaft sleeve) as rotating element and the packing rings as stationary element.

Figure 1.8 Stuffing box with Figure 1.9 Pressure distribution compression packing



Figure 1. 10 Lantern ring Figure 1.11 Stuffing box with lantern ring 1.1.3 Lantern ring The lantern ring (Figure 1.10) is a device made from a rigid material such as bronze, stainless steel, nylon or TFE, and is of open construction to allow free passage of sealing liquid (or lubricant). Normally, the sealing liquid (or lubricant) enters the outside of the ring, and flows to fill the space between the packing rings and the shaft ( or shaft sleeve). The lantern ring usually has packing rings on either side (Figure 1.11) 1.1.3.1 Arrangements of the lantern ring to meet specific services The following figure (figure 1.12) shows three different arrangements. These arrangements are:



Figure 1.12 a When a pump operates with negative suction head, the inner end of the stuffing box (product side) is under vacuum, and air tends to leak into the pump. For this type of service, packing is usually separated into two sections by a lantern ring (seal cage). Sealing fluid is introduced under pressure into the space, causing flow of sealing fluid in both axial directions. This construction is useful to assure liquid for cooling and lubrication between the packing rings and the shaft (or shaft sleeve).

Figure 1.12 Arrangement of lantern ring to meet specific services

Figure 1.12.b This construction is useful for pumps handling flammable or chemically active and dangerous liquids since it prevents outflow of the pumped liquid. Figure 1.12.c If the product being pumped is too contaminated with abrasives, clean liquid flush injected to lantern ring to prevent dirty liquid to enter the stuffing box area. If the abrasives lodged between the packing rings and the shaft (or shaft sleeve) it will act completely like a cutting tool against the shaft or shaft sleeve.



1.1.4 Coopey packing design There is another design for packed stuffing box to overcome the problem of uneven distribution of the pressure throughout the packing, this is coopey packing design. (Figure 1.13). The designer added a helical spring between two washers where the lantern ring normally is used. External lubrication ( or sealing liquid) is introduced into the spring area and it is stated that "instead of the highest friction point being next to the gland (Figure 1.9), there are now two points at each end of the spring”. However, these are exposed directly to the lubricant. If its pressure is the same as the fluid pressure, the inner packing has just a slight differential across it and the packing acts very much as a diaphragm. There is almost balanced pressure. The outer packing is required only to retain the lubricant ( or sealing liquid) pressure. This is relatively easy to do because the lubricant is in direct contact with the point of highest pressure and friction. The spring compensates for any slight wear or compression of the packing.

Figure 1.13 Coopey packing design

This arrangement removes the human factor from the equation because when the packing is installed, the gland is pulled up to the face of the stuffing box and no further adjustment is required. The spring takes care of any thermal expansion or swelling of the packing.



1.1.5 Packing selection Factors that must be considered in selecting a packing involve:

• All of the fluid's conditions, such as temperature, lubricity and pressure. • All equipment parameters, such as speed, equipment condition,

material of shaft (or shaft sleeve); and miscellaneous factors, such as dimensions of the stuffing box area and the shaft O.D. in the stuffing box area, space available, continuous or intermittent service, and any combination of these conditions.

1.1.6 Conventional packing drawbacks The drawbacks of conventional packing are: • Packing operates on the principle of controlled leakage in dynamic

applications. They never attempt to totally prevent fluid from leaking from the equipment. This leakage will cause: a. Waste of product. b. Pollution.

• It requires regular adjustment of the gland.

• Pressure limits: Packing not suitable selection for high pressure working conditions like water injection pumps.

• Power consumption: The conventional packing consume more power. Packing rubbing on a shaft (or shaft sleeve) similar to driving an automobile with the hand brake engaged. This relatively high power consumption will increase the running cost.

• Maintenance cost: Most of the time, the shaft ( or shaft sleeve) should

be changed due to damage. The rubbing between the packing rings and the shaft will cause score marks and rough surface on the shaft in the stuffing box area. That means extra maintenance cost and more downtime. Beside this, most bearing failure is caused by contamination rather than overloading. The easiest way to contaminate a bearing is from the leakage coming through the packing. Stop this leakage and you will stop most of your bearing failures.



• Speed limits: Packing have limited speed, if you try to use it in

peripheral speeds higher than its limits, the failure will happen. Most of big pumps ( like water injection pumps and boiler feed water pumps) the rubbing speed is too high due to high R.P.M. and big shaft diameters.

The argument for packing usually centers around four statements: 1. You don't have to take the pump apart to change packing. 2 In an emergency, you can always add a ring of packing. 3. Packing is cheaper. 4. Packing is less complicated.

Let's look at each of these statements if it is true: Statement 1: You do have to take the pump apart to change sleeves and bearings. Shaft sleeve replacement is a normal part of repacking a pump. The fact of the matter is that you will have to dismantle a packed pump more than a sealed pump. Statement 2:

If you need reliability, use a mechanical seal with an auxiliary packing gland.

Statement 3:

Packing is cheaper if you consider the packing alone. Bicycles are also cheaper than automobiles.

Statement 4:

Packing is less complicated only to an inexperienced man. If you have ever tried to teach an apprentice how to inspect a stuffing box and shaft, cut packing, install it so as to align the lantern ring, tamp it in place, and adjust it properly so as to keep leakage to a minimum and not generate excessive heat (you have to do it by feel), then you know just how complicated packing really is.



SECTION 1.2

CONDENSATE INJECTION SEALING

Condensate injection sealing (sometimes called packless stuffing box) has been a very successful solution of the stuffing box problem for high-speed boiler feed water pumps (about 5000 RPM) where neither conventional packing nor mechanical seals provide a satisfactory answer. 1.2.1 How does it work? The construction of a pump with condensate injection sealing (Figure 1.14) involves the substitution of a serrated breakdown bushing for the conventional packing. This serrated breakdown bushing is stationary part and fixed to the machine casing. The pump shaft sleeve runs within this bushing with a reasonably small radial clearance. Cold condensate, at a pressure in excess of the boiler feed water pump suction pressure, is introduced centrally in this breakdown bushing. A small portion of the injection water flows inwardly into the pump proper; the remainder flows out into a collecting chamber that is vented to the atmosphere. From this chamber the leakage is piped back to the condenser.

Fig. 1.14 Condensate injection sealing (packless stuffing box)



Cold condensate at temperature from 80 to 100°F (26.7 to 37°C) is available at pressures in excess of the boiler feed pump suction pressure in closed cycles as well as open cycles in which the condensate pump discharge into a derating heater from which the pump takes its suction. The water for the injection in both should be taken immediately from the condensate pump or condensate booster pump discharge before it has gone through any closed heaters. 1.2.2 Factors affecting the water quantity The injection water quantity will depend upon:

1. The diameter of the running joint between the shaft sleeve and the pressure breakdown bushing (serrated bushing).

2. The clearance at that running joint.

3. The injection pressure.

To give some general idea of the values in question, if sleeve diameter is 5 inches (127 m.m.)and the diametral clearance 0.009 in (0.2286 m.m.), the amount measured in a 3,600 RPM pump will be approximately as follows:

1. Total injection per box, 8 to 10 GPM(30 to 37.85 liters per min.).

2. Leakage into the pump interior per box, 2 to 4 GPM(7.57 to 15.14 liters per min.).

3. Return to condenser per box, 6 to 8 GPM(22.7 to 30.28 liters per

min.). The injection supply must be absolutely clear and free of foreign matter. It is therefore necessary to install filters or strainers in the injection line to avoid the entrance of fine mill scale or oxide particles into the close clearances between the stationary bushings and the sleeves. Pressure gages should be installed upstream and downstream of these filters to permit the operator to follow the rate at which foreign matters clogs up the filters and to clean these when the pressure drop across them becomes excessive.



1.2.3 Control of the injection flow rate The variation in supply pressure to the condensate injection compared to the internal pressures at both ends of the pump makes it necessary to use a control for satisfactory seal operation. Three possible types of flow control are:

• Manual, • Pressure differential, and • Temperature.

1.2.3.1 Manual flow control A manual flow control requires hand-setting and re-adjusting a valve for each seal. This necessitates the availability of a man to recheck the setting, which varies with the load. While it is possible to use such a control it is not normally recommended. 1.2.3.2 Pressure – differential control The pressure-differential control senses the differences between injection pressure and boiler feed water pump internal pressure and, by sending a signal to an automatic valve in the injection line to each seal, maintains the differential at some predetermined setting, usually 10 psi. This system tends to be unstable because a change in pressure affects valve position which changes pressure, and so on. 1.2.3.3 Temperature control The temperature-sensitive control (Figure 1.15) operates on signals received from a temperature-sensing probe in each seal drain line. A controller transmits an air signal to a pneumatic control valve in the injection line. In addition to providing rapid response to variations in operating conditions, this type of control will use the least amount of injection water. It will always supply just enough injection water to keep the drains at the recommended temperature range of 140-150 °F. The injection valves are equipped with limit stops so that they cannot close fully regardless of the air signal.



Figure 1.15 Temperature control of condensate injection

1.2.4 Drains from condensate injection sealing

Two different systems are used to dispose of the drains coming from the collecting chambers:

1. The first system utilizes traps that drain directly to the condenser. 2. The second system collects the drains in a condensate storage tank

into which various other drains are also returned. As this tank is under atmospheric pressure, it must be set at a reasonable elevation below the pump centerline so that the static elevation difference will overcome friction losses in the drain piping. A pump then transfers the condensate drains from the storage tank into the condenser.

1.2.5 The effect of the clearances between the sleeves and the breakdown bushing The clearances between the sleeves and the breakdown bushings will double in a time approximately equal to the life of the pump internal wearing parts. With double clearances, the leakage will double. This factor should be considered when sizing the return-drain piping back to the condenser or to the collecting tank if friction losses are to be kept to a minimum in this piping.



The collecting chamber at the pump stuffing box is vented to the atmosphere; the only head available to evacuate it is the static head between the pump and the point of return. This head must always be well in excess of the frictional losses (even after the leakage doubles), otherwise the drains will back up and run off at the collecting chamber. 1.2.6 Drawbacks of condensate injection sealing

1. The injection of condensate requires a lot of piping and auxiliary equipment. There is also an amount of energy required to keep it going.

2. Pressure monitoring is also of great importance as the differential

pressure of the pump pressure must at all times remain equal. 3. Also of great importance is the radial clearance of the breakdown

bushing as increasing clearance through wear will require more and more injection flow thus reducing the effectiveness of the system.

The modern mechanical seal for boiler duties required only a cooler and a magnetic separators (or magnetic filters). As flow is induced by rotating part of the mechanical seal no further auxiliary equipment is required. (See Chapter 6)



SECTION 1.3

FLOATING SEAL RINGS The floating seal ring type of seal is a variation found made by some pump manufactures on the previous described sealing method. 1.3.1 How does it work? The serrated breakdown bushing shown in figure 1.14 is replaced by a number of individual solid rings. Each ring is mounted in a specially constructed holder and is spring loaded to produce a stationary seal face in an axial direction, and locked against rotation by a pin -and-slot arrangement. (Figure 1.16). A small radial clearance is provided between the rings and the shaft sleeve. The length of each individual ring varies with the diameter of the condensate injection seal, but is generally about 1/2 in. The individual seal rings are "floating" to a certain degree and can find their own position relative to the shaft. Their short length reduces the effect of angular displacement between the stationary and rotating components, whether this displacement arises from errors in original assembly or from distortions caused by temperature changes.

Figure 1.16 Floating seal ring design



The further principle of operations is much the same as with the serrated breakdown bushing, i.e., condensate injection, part of which enters the pump, the remainder of which enter an atmospheric collecting chamber. No direct contact between the floating rings (item #23-B figure #1-16) as a stationary part and the shaft sleeve as a rotating part because the condensate will provide a liquid film between them. The efficiency of this system as a sealing device depends upon the running clearance between the floating rings (item #23-B) and the shaft sleeve, surface finish of all meeting parts and the purity of the injection condensate.



SECTION 1.4

LABYRINTH SEAL

Labyrinth seals are the oldest type seals, simple to build and relatively trouble free. They are used universally at interstage seal point in centrifugal gas compressors, at the shaft ends of air blowers and compressors, and in other applications when a small loss of process gas or seal gas (air, nitrogen, etc.) may be tolerated. 1.4.1 How does it work?

1. The labyrinth seal is a set of metal rings or teeth that encircle the shaft. The teeth do not contact, or touch the shaft. (Figure 1.17).

2. The spaces between the teeth form a labyrinthian passage. The gas

enters the space between the teeth, it slows down and changes direction (Figure 1.17). The resulting turbulence resists the flow of gas.

Figure 1.17 Straight labyrinth seal Figure 1.18 Fluid flow and the

resulting turbulence

3. If the gas velocity is high, some of the gas does not change direction in the seal, but skips between the teeth and the shaft in a straight line (figure 1.17), such skipping increases leakage.



Figure 1.19 Interlocking labyrinth

4. When the gas velocity through a straight labyrinth seal is high, it becomes less effective in stopping leakage. To overcome this problem there is another design (Figure 1.19). The interlocking labyrinth seal is more efficient because the gas is forced to change direction, it encounters resistance to flow.

1.4.2 The leakage across a labyrinth seal Leakage rates are relatively high (1-2 percent of compressor flow on small units). Leakage across a labyrinth seal depends on the following factors:

1. Number of teeth, more teeth less leakage.

2. Diameter of the packing ring; big diameters more leakage.

3. Running clearance; the leakage rate increases with big running clearances.

4. Pressure to be sealed; the leakage rate increases in high pressure

applications.



There is always leakage and care must be taken that this leakage creates no hazards. The normal operating limits are about 200 psi maximum or 20 psi per inch of axial length for a straight pass labyrinth seal. This works out to be above 5 psi per tooth. 1.4.3 Labyrinth Seal Clearance The clearance usually provided for turbomachinery shaft seals, using labyrinth blading is maximum bearing clearance plus one mil per inch of diameter of labyrinth. Clearance is consider excessive when it exceeds two mil per inch of diameter plus the bearing clearance. Labyrinth seals on casing operating at pressures in excess of 50 psi have 8-20 blades. Lower pressure casings contain three to six blades. In general, the machine manufacturer will give the minimum and maximum clearances. The minimum value to avoid any direct rubbing between the rotating element and the labyrinth seal. The maximum limit to eliminate excess leakage or in other words to keep the leakage rate within the acceptable limits. Note: 1 mil = 0.001 inch. 1.4.4 Influence of labyrinth seal clearance on the compressor's efficiency It is easy to improve compressor efficiency by minimizing interstage labyrinth clearances. The new compressor will operate with minimum internal leakage, but when the rotor become slightly unbalanced or vibrations occur from another source, the labyrinth seals will wear rapidly, opening the clearance and allowing greater leakage with a commensurate decrease in efficiency. To prevent this situation, the minimum clearance should be equal to the bearing clearance plus rotor run-out at the seal due to its weight plus a given amount of mass unbalance.



1.4.5 Materials of labyrinth seals Labyrinth seal's materials should have the following properties:

1. Must be softer than the rotor material. If the rubbing happen between the rotor and the labyrinth seal, it is cheaper and easier to replace the seal than replacing the rotor.

2. Should be able to resist chemical attack of the fluid to be sealed. 3. Should be able (up to certain degree) to withstand the erosion effect

of contaminated gasses. If some moisture condensate during operation and this liquid droplets flows with the gasses, it could cause under certain conditions erosion in the machine parts including the labyrinth seals.

The common materials are: Aluminum, bronze, babbitt or steel. 1.4.6 Labyrinth Seal Designs Many successful design variations are in use. In this section we will cover different designs and applications: 1.4.6.1 The Straight labyrinth seal (Figure 1.20.a) This seal consist of a series of thin strips or fins which are normally mounted in a stationary ring which maintains a close clearance between the shaft and the tip of the fins. Since the labyrinth seal resembles a series of orifices minimizing the size of the openings is the most effective way of reducing the flow. The diameteral clearance is thus normally limited to 1 1/2 to 2 mils per inch of diameter



c - Rotating. Figure 1.20 Labyrinth seal designs

1.4.6.2 Stepped labyrinth seal (figure 1.20.b) This seal increases the resistance to flow by destroying the line of sight flow of the gas. A stepped tooth or staggered labyrinth design reduce leakage by over 40% compared to the straight pass type. One drawback to this type of seal is that the seal must be horizontally-split to facilitate removal. This usually means lifting the compressor top-half to replace the shaft seal. The second drawback of this design is that rotor axial movement would cause damage in the labyrinth seal teeth. For this reason, this design is very sensitive for rotor axial movement. 1.4.6.3 Rotating labyrinth seal (figure 1.20.c) Sometimes is called interference labyrinth seal. In this design, the fins are machined into a rotating shaft sleeve (hard material like steel) and allowed to cut their own clearance into the soft stationary ring (sleeve). This soft stationary sleeve materials is usually lead, which is inert to most gasses, and very ductile. The soft lead rubbing surface is normally bonded to steel ring for support, In some cases, the soft stationary sleeve may be bronze, aluminum, a lead-tin alloy, soft cast iron, carbon-graphite or (for high temperature) stainless steel.



1.4.6.4 Honeycomb labyrinth seal (figure 1.21) The use of honeycomb labyrinths offers better control of leakage rates (up to 60 percent reduction of a straight pass type). Honeycomb seals operate at approximately one-half the radial clearance of conventional labyrinth seals. The honeycomb structure is composed of stainless steel foil about 10 mils thick. Hexagonal-shaped cells make a reinforced structure that provides a large number of effective throttling points (figure 1.21). In addition, stainless steel honeycomb retain its strength at temperature and pressure levels which cause weakening of an aluminum labyrinth.

Figure 1.21Schematic of honeycomb labyrinth seal



1.4.6.5 Ported labyrinth seal arrangement

It is possible to alter the labyrinth seal for many gas applications. The packing can be expose to full discharge pressure and a port or lantern to bleed off leakage through the first section is connected to intake. Another sectionalizing port may be used to inject another gas at slight higher than intake pressure. This prevents process gas leaking out, but permits injection gas leakage both to atmosphere and to intake. The operation may be reversed and the port connected to an ejector, maintaining a pressure below atmospheric. In this case, the process gas and atmospheric air would leak to the port and removed.

Figures 1.22 through 1.24 show in schematic fashion in several types of ported labyrinth seal arrangements. Such seal system are often used on sour gas compressors, feed gas compressors, etc.

1. In the simple injection arrangement (Figure 1.22), a cool (approximately 100°F/40°C) clean sweet gas, which is not objectionable in small amounts in the compressed gas stream, is injected at 3-5 psi (0.2-0.35 bar) above gas pressure or at atmospheric pressure, whichever is higher. Naturally the gas will flow in both directions from the inject point, thus the sweet gas must also be non-toxic as well as relatively inexpensive.

Figure 1.22 Simple Sweet Gas Injection Figure 1.23 Simple Ejector

Seal



2. In the simple ejector seal arrangement (Figure 1.23), the seal port

pressure is maintained approximately 0.5-1.0 psi (35-70 m. bar) below either gas stream pressure or atmospheric pressure, whichever is lower. In this case, compressed gas plus atmospheric air are pulled out through the seal ejector. The ejector motive gas can be steam, discharge, or some other readily available gas.

These improvements are not attractive in these days of higher energy costs because even under the best of conditions the leakage rate is too high and the energy costs to operate the improvement are also costly.

3. While the above two mentioned arrangements have been used, the combination injection and ejector seal arrangement combines the good points of each and is thus used in the majority of labyrinth seal applications. With this arrangement (figure 1-24), the sweet gas or injection media is maintained 3-5 psi (0.2-0.35 bar) above the gas reference pressure and flows into the gas stream as well as toward the ejection point. At the ejection point, the sweet gas, as well as some air from the atmosphere or bearing housing, is pulled off through an ejector. In this way no product or compressed gas is lost, while at the same time the sweet gas is not allowed to contaminate the area outside the compressor.

Figure 1.24 Injection and ejector type seal



1.4.7 Labyrinth seal drawbacks

1. Labyrinth seal is never a positive seal, as all labyrinth seals are designed only to restrict the flow of fluid

2. As mentioned before, the efficiency of this seal to perform the

required job depends on the running clearance beside some other factors. Other problem also occur when using this type of seal. Thermal growth of the mating surfaces must be carefully controlled to maintain correct clearances under all operating conditions.

1.4.8 Areas of application The labyrinth seals suitable for rotating shafts. It is widely used in the following application:

1. Turbo compressors. 2. Gas and steam turbines. 3. In gear boxes, to seal around the input and output shafts in order to

stop oil leakage. 4. In some mechanical seals, where it is used as seal flange bushing

either for safety or for quenching. 5. In some designs of centrifugal pumps as inter-stage seal to minimize

the internal leakage from one stage to another.



SECTION 1.5

WINDBACK SCROLL OR WINDBACK SEAL

Figure 1.25 Windback seal These seals are also called pumping screws. Windback seal in its simplest form (figure 1.25) it is a sleeve with a helical (screw type) groove or grooves (multiple entry). These helical grooves are machined on the external surface of the sleeve. This sleeve is mounted over the pump shaft in the stuffing box area and rotates with it. Windback seals are dependent on the direction of rotation and the running speed (RPM).



1.5.1 How does it work? When the pump is in operation, this sleeve acts completely like a rotating element (rotor) of a screw pump. By suitable selection of the direction of the helical groove as (right hand) or (left hand) groove it provides a pumping action in reverse direction of the fluid leakage. This reverse pumping action slowing down the flow of the product to atmosphere. At certain R.P.M., the leakage rate will be zero. Variations can be found with a helical groove in both rotary and stationary members.

Figure 1.26 Multistage Centrifugal pump with pumping screw When a double bearing pump is sealed by this method, a right and a left hand thread is required. Figure 1.26.



1.5.2 Disadvantages of windback seal

1. The main disadvantage of this sealing method is that it is less effective at lower rotation speed when for example used on boiler feed water pumps running at partial load conditions. The reason of this is that this small screw pump (the sleeve with its helical grooves) is able to pump a fluid in reverse direction at specific speed. If it runs with speed lower than this specific value, its performance will completely changed.

2. Under stationary conditions (when the pump on standby, fully primed

and ready to run) the windback seal has practically zero effectiveness and then only acts as a regular throttle bushing.

1.5.3 Advantages of windback seal

1. No direct rubbing between moving and stationary parts i.e. no wear due to rubbing.

2. Sophisticated arrangements can be found (when sealing gasses)

when the scroll fed with oil provides a back pumping action to provide sufficient differential sealing pressure. Theoretically this type of arrangement has zero leakage as far as the gas is concerned, but as it is, once again, dependent on rotational speed it is sometimes backed-up by a mechanical seal to provide sealing at low R.P.M.

1.5.4 Areas of application Windback seals can be used in conjunction with mechanical seal to boost the circulation of coolant for single and double seals. (This point will be covered in details later on - Chapter #6)



SECTION 1.6

SEGMENTED CARBON RING SEALS

These seals are also called restrictive carbon ring seals. Figure 1.27 shows a restrictive carbon ring. In this seal the labyrinths are replaced by carbon rings, which provide a more tortuous path to the passage of gas along the shaft. As the carbon ring clearance is held to a minimum, the leakage is less than for a comparable labyrinth seal. But this close clearance increases the wear, and dry carbon seals thus normally require more frequent replacement than labyrinth seals. The rings are made out of low friction material such as carbon. It could be one piece, reinforced by a steel band (Figure 1.27.A) or segmented (Figure 1.27.B). If the rings are segmented, they are usually held together with a garter spring.

A- One piece, reinforced by steel band. B- Segmented ring with

garter spring. Fig. 1.27 Restrictive Carbon Ring



1.6.1 How does it work?

1. Refer to figure 1.28. The rings are held in position around the rotating shaft by stationary ring cups, these rings cups do not contact the shaft.

Figure 1.28 Restrictive carbon ring seals

2. The leakage over the ring is prevented by the vertical contact between the ring and the ring cup (in radial direction).

3. In this type of seal, the carbon rings actually do not contact the shaft,

so that some leakage will occur. The restrictive carbon ring seal can be ported for scavenging or inert gas sealing.

4. Restrictive carbon seals is also used in conjunction with labyrinth

type seals to further reduce leakage.



If the sealed gas contains some humidity to lubricate the carbon rings, radial clearances between the ring and shaft or sleeve may be very close and leakage rates substantially lower than those of labyrinth seals of similar space requirements may be achieved. The seal may be operated dry (as in the labyrinth seal), with a sealing liquid (as in the mechanical type) or with a buffer gas. An exception is where lube-oil is applied to carbon-ring seals by drop-feed lubricators. This is done to reduce the frictional heat of the higher pressure seal contacts and where the lube oil is not a nuisance. General operating limits are about 200 psi maximum or 35 psi per active ring. The basic restrictions and problems cited for the labyrinth seal also apply to this type. In addition, the carbon ring can shatter easily. Its basic advantage lies in reduced leakage if proper clearances are maintained (about two mils total at operating pressures and temperatures). 1.6.2 Areas of application

1. This seal can only be used in a relatively clean gas service. 2. In steam turbines to prevent steam from leaking to outside the

machine casing.



SECTION 1.7

LIP SEALS

Radial lip seals are used primarily for keeping lubricants in systems which have moving shaft. These seals are also called oil seals or shaft seals. A typical lip seal is shown in figure 1.29. All major elements are named (SAE Nomenclature). The basic parts are the outer metal case and inner flexible sealing element (soft lip).

Figure 1.29 Radial lip seal nomenclature



1.7.1 How it works ?

Figure 1.30 The lip seal working principle Sealing of a lip-type seal is normally the result of an interference fit between the inner soft lip (flexible sealing element) and the shaft. Usually spring pressure behind the lip is also added keeping the fluid in depends upon a precise amount of lip contact pressure. Figure 1.30 illustrates the working principle of lip-seal. The seal lip should ride on a thin film of lubricant. The film does the sealing so it must be controlled precisely by the mechanical pressure of the sealing element and the shaft finish. If the liquid film gets too thick, liquids leaks; if too thin, the lip wears and friction, heat, and lip oscillations can result.



Normally, if contact pressure increases the film gets thinner. Heat build-up can also reduce the liquid film. Never run lip seals without lubrication unless necessary-and then only for short periods. Unless defective or installed wrong, lip seals should leak only the thin lubricant film we discussed before. 1.7.2 Advantages Basic advantages of radial lip seals are:

1. Fit into small space. 2. Fairly low cost. 3. Easy to install. 4. Handle many variables while sealing. Variables include most oils and

hydraulic fluids over wide temperature ranges, moderate fluid pressures, some misalignment, some shaft run-out and variations in shaft speed.

1.7.3 Types of lip seals Radial lip seals are classified by lip types as follows:

a. Single lip (figure 1-31-A). Lip not spring - loaded. For containing viscous liquids like grease, at slow shaft speeds.

b. Single lip spring-loaded (figure 1-31-B). For retaining lower-

viscosity lubricant at higher shaft speeds in clean environment (atmospheres).

c. Double lip (figure 1-31-C). Lips face opposite, one spring-loaded,

one not. For retaining lubricant on spring-loaded side, while keeping out dirt on the other side.

d. Dual lip (figure 1-31-D). Lips face opposite, both are spring-loaded.

For containing lubricant on one side and excluding liquid on the other.



a. b c d

Figure 1.31 Basic types of radial lip seals Other special lip seals include:

1. Split Seals: for difficult installations in the plant and marine situs applications (figure 1.32). Split seals allow a seal to be assembled in situs around the shaft.

2. External Seals: for a fixed shaft and rotating bore (outer part of the

seal is the sealing surface). 3. Hydrodynamic Seals (Figure 1.33): Which have grooves or ribs

molded into the seal lip to direct oil flow back into the sealing area, reducing heat and wear. This seal can operate at less lip pressure than standard lip seals. Care must be taken when using these lip seals as they are “uni-directional” and is used for the wrong direction of rotation can actually encourage leakage.



SECTION 1.8

MECHANICAL SEALS

1.8.1 Mechanical Seals Overview The mechanical seals was developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level meeting the environmental standards. Mechanical Seal Construction All mechanical seals are constructed of four basic sets of parts. As shown in figure 1.34, these are:

1. A set of seal faces which are called sometimes primary sealing device. One that rotates (rotating face) and one that is stationary (stationary face).

2. A set of secondary seals known as secondary sealing device or gaskets such as 0-rings, wedges, U-cups and V-rings.

3. Spring (s).

4. Mechanical seal hardware including seal flange (gland ring), shaft sleeve. etc.

Figure 1.32 A simple mechanical seal



1.8.2 Sealing Points for Mechanical Seal

There are four main sealing points, (see figure 1.32)

1. The primary seal is at the seal face, point A.

The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all mechanical seals, one face is held stationary in a housing (stationary face), and the other face is fixed to, and rotates with the shaft (rotating face). These two faces are made from two dissimilar materials, one of them is usually a non-galling material such as carbon-graphite. The other is usually a relatively hard material. The seal faces are made from two different materials in order to help prevent adhesion of the two faces.

2. The leakage path at point B (between the floating seal face and the shaft or shaft sleeve) is blocked by floating seal face gasket (either 0-ring, U-cup, a V-ring, or a wedge).

3. Leakage paths at point C (between the seal flange and stuffing box face) is blocked by seal flange gasket which could be 0-ring or any other shape of static gaskets.

4. Leakage paths at point D (between the seal flange and the stationary face) is blocked by stationary face gasket or seat gasket.

1.8.3 How Does it Work?

1. The two flat seal faces are pushed together by axial force from the closing mechanism (spring or metal bellows) and by product pressure in the ^stuffing box cavity (seal chamber).

2. When the seal is in operation, the two seal faces are lubricated by the same product inside the stuffing box. It is known that, for the seal to work efficiently, it is necessary for a stable fluid film to exist between the seal faces. In the majority of cases this film is a liquid, although in certain applications a gas film may be induced between the faces. The function of this liquid film between the seal faces is for cooling (carry away the frictional heat) and lubrication. If this film stability is destroyed, excessive wear takes place leading to rapid seal failure.



3. Improperly positioned seals could allow a wide gap between the

faces, causing a leak path. The faces could also be squeezed so tightly together that no lubrication is present, causing rapid seal failure.

1.8.4 Advantages of Mechanical Seals Mechanical seals replace conventional packing in stuffing boxes where the fluid must be contained inspite of a substantial pressure head. These seals offer:

1. Reduced friction power loss. 2. Virtual elimination of wear on the shaft or shaft sleeve in the stuffing

box area.

3. Invisible or minimum leakage.

4. Ability to function in relative extremes of shaft deflection and end play.

5. Suitable for high working pressures and high rubbing speeds.

6. Relative freedom from maintenance.



1.8.5 Comparison Between Conventional Packing and Mechanical Seals

Conventional Packing Mechanical Seal

1. How does it work?

Conventional packing forced into a stuffing box around a revolving shaft seals by throttling the casing fluid trying to leak between the packing and shaft. The more packing forced into the stuffing box and the tighter it's jammed by gland, the less leakage. But packing wears the shaft and increases the power needed to rotate the shaft.

1. How does it work?

Mechanical seal has two rings (seal faces) at right angles to the shaft. One ring (seal face) is fastened to the shaft and revolves with it, while the other is stationary and is held against the machine casing. The wearing faces that seal have a small area compared to the area conventional packing seals against. Because of this small area, and the preloaded spring(s) forcing the two faces together at a few pounds per square inch of pressure, there's less friction at the seal faces. And of course there is no wear on the shaft because seal faces take it all, they can relapped or replaced when needed.

2. Shaft run-out Shaft run-out is one costly enemy of conventional packing. It beats out packing, making sealing problem tough. If the shaft run-out is over 0.003, it's impossible to seal properly, especially at high speeds.

2. Shaft run-out Mechanical seals can take more shaft run-out without leaking. Reason is that sealing faces are at right angles to shaft. The elastomeric gaskets and the spring(s) allow for some misalignment between the seal faces which could happen due to shaft run-out.



At extremely high speeds, shaft run-out may give trouble because of centrifugal force.

3. Shaft axial float

End play is common with most shafts especially when starting up or shutting down. Such shaft movement does not affect the conventional packing if shaft has no grooves in packing area. But usually shafts or sleeves do groove after a short while. Then shaft end play disturbs packing, open it up and causes leakage.

3. Shaft axial float

Shaft end play does not affect the mechanical seal if this end play within certain limits (about 0.003" for rolling element bearings as a thrust bearing and about 0.015" for slide surface bearing as a thrust bearing). The spring(s) will keep the seal faces close. The big values of shaft end play disturbs the mechanical seal.

4. Power consumption

It is relatively high (about three times the power consumption in the mechanical seal for the same shaft size and speed).

4. Power consumption

It is relatively smaller. The seal faces running with a lubricating liquid film between them.

5. The required time for replacing

5. The required time for installation



Can be packed in place. It needs adjusting several times after start up until it reach the normal running conditions.

Installed over shaft end. It needs more time for installation. It does not need any additional adjustment after the installation

6. Pollution Is relatively high because the packing must leak for cooling and lubrication.

6. Pollution In normal running conditions, less leakage i.e. less pollution. Double seals are able to stop product leakage 100%.

7. Cost of Product High- due to high leakage rates.

7. Cost of Product Low- due to very small leakage rates.

8. High pressures and big shaft diameter services Not suitable sealing device for big shaft diameter or high pressures.

8. High pressures and big shaft diameter services Mechanical seals can handle both vacuum and high pressures. Also it is suitable sealing device for big diameters.

On the next chapters you will find more details about mechanical seals.

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Chapter 01