Thursday, November 27, 2008


What is cavitation?
Cavitation begins as the formation of vapor bubbles at the impeller eye due to low pressure. The bubbles form at the position of lowest pressure at the pump inlet (see Figure 1) which is just prior to the fluid being acted upon by the impeller vanes and then rapidly compressed. The compression of the vapor bubbles produces a small shock wave that impacts the impeller surface and pits away at the metal creating over time large eroded areas and subsequent failure. The sound of cavitation is very characteristic and resembles the sound of gravel in a concrete mixer.

Figure 1 Pressure profile at the pump entrance.

As you can see from Figure 1 the pressure available at the pump inlet which is the
pressure that we would measure if we put a gauge at that point, can be reasonably high but still drop considerably as it makes it way into the pump. The pressure may
be lowered enough that the fluid will vaporize and will then produce cavitation.
The same effect can sometimes be seen in control valves because they have a
similar pressure drop profile so that if the pressure is insufficient at the control
valve inlet cavitation will also occur.

Vapor pressure and cavitation
There is two ways to boil a liquid. One way is to increase the temperature while keeping the pressure constant until the temperature is high enough to produce vapor bubbles. In Figure 2 this is what happens when you move horizontally in the graph (that is at constant pressure) and increase the temperature. Eventually you hit the vaporization line of the particular fluid and the fluid starts to boil or produce
vapor bubbles.

Figure 2 Vapor pressure vs. temperature.

Figure 3 Making a liquid boil under low pressure.

We do the same thing every day when we boil water in a pot by using a stove top element for example.
The other way to boil a liquid is to lower the pressure. If you keep the temperature constant and lower the pressure you can make a liquid boil also. In Figure 2 this is what happens when you move vertically in the graph (that is at constant temperature) and decrease the pressure. Eventually you hit the vaporization line of the particular fluid and the fluid starts to boil or produce vapor bubbles.

If the pot were covered and you had a source of vacuum (see Figure 3) by lowering the pressure in the pot you would be able to make the water boil at a lower temperature. When the pressure is 7.5 psia or (14.7 – 7.5 = 7.2) or 7.2 psi less than the atmospheric pressure the water will boil at a temperature of 180 ?F and when the pressure is 1.5 psia the water will boil at 120 ?F. This is what happens at the pump suction when the pressure is low enough to make the fluid boil.

The pressure at which the liquid boil is known as the vapor pressure and is always specified for a given temperature because at different temperature the liquid will boil at different pressures. For example, the vapor pressure of water at 212 ?F is 14.7 psia the same pressure as the local atmosphere at sea level. The vapor pressure for water at 180 ?F is 7.5 psia and the vapor pressure of water at 120 ?F is 1.5 psia.
Read More

Wednesday, November 26, 2008

Mechanical Seal

Mechanical seals are being used increasingly on fluid pumps to replace packed glands and lip seals. Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time

Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts. The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively. The leakage path gap varies as the faces are subject to varying external loads which tend the move the faces relative to each other.
The mechanical seal requires a different shaft housing design arrangement compared to that for the other type of seals because the seal is a more complicated arrangement and the mechanical seal does not provide any support to the shaft.

In order for the mechanical seal to perform over an extended time period with low frictional the faces are generally hydrodynamically lubricated. The fluid film will need to carry substantial load. If the load becomes to high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick , or less. This thickness is critical to the required sealing function. Mechanical seals often have one face of a suitable solid lubricant such that the seal can still operate for a period without the fluid film.

Pressure Balance Seals
It is possible to reduce the seal contact pressure by using a pressure balanced seal design of off-set a proportion of the force generated by the pumped fluid pressure. This principle is illustrated in the sketch below.

Design Features
The mechanical seal generally includes a three static seals.
-The sleeve seal - this is usually an O-Ring
-The seal between the moving seal member and the shaft or sleeve. This is often an o-ring but can be a wedge or vee seal. This seal may not be used for bellows type mechanical seals
-The housing seal is generally an o-ring of a gasket.
All of these seal must be compatible with the fluid being contained and the associated environment. These seals may limit the design for high temperature applications. In this case the bellows type alternative may be the best option.
The sealing faces are generally pressed together using some form of spring loading. Several different spring loading systems are available.
-Single spring
-Multiple springs distributed around seal body
-Disc Springs
For conventional mechanical seals the single spring arrangements is used. The other spring arrangements are used in the space is restricted.
It is vitally important that the sealing surfaces perfectly flat and are parallel.
The seal faces are usually dissimilar materials with the softer face being the narrower surface. For abrasive applications similar hard materials are used e.g tungsten carbide. The seal surfaces must have sufficient strength to withstand the hydrostatic fluid forces and must be able to remove the heat generated by sliding action. Carbon is often used against bronze, cast iron, stainless steel etc.
The seal surface must be flat, smooth and square to the shaft. Both surfaces a normally lapped to a high quality finish. The harder surface is most important because the softer surface is designed to run-in over the initial operating period.

The shaft design is critical. It must be rigid enough to support the seal in the correct position and the shaft surface finish must be suitable to ensure good sealing on the static seals (0.4 micrometers CLA or better). The shaft Total Indicated Runout (TIR) should not exceed 0.125mm. There should be minimum shaft vibration. The shaft may be subject to fretting corrosion as a result of micro-movements of the seal and is is often desireable to have locally hardened surfaces or to use sleeves.

Assembly Options
There are a number of mechanical seal options
*External Seal.. This design is installed on the outside of the stuffing box with the sealed pressure inside. This provides good access allowing the seal components to be be cleaned.
*Internal Seal.. Generally mechanical seals are mounted inside the stuffing box with the sealed pressure outside the seal.
*Double Seals.. Mechanical seals mounted in pairs are used for sealing hazardous, toxic or abrasiv fluids and are often provided with clean flushing fluid between the seals. Double seals also provide an additional degree of safety were the pressure differentials are likely to reverse and/or there is a high risk of the sealing failing. There are a number of double seal assembly options as listed below
In Series - Used primarily to overcome the risk of failure of a single seal.

*Face to Face - Used when a cooling fluid interface is required . One seal is used for the process fluid the other seal is used for the coolant.

*Back to Back - Used when an abrasive fluid is being contained and both seals are flushed with a clean buffer fluid. The flushing fluid is introduced at a higher pressure the process fluid.

The are a large number of variant mechanical seals e.g split seals. Improved systems are constantly being introduced onto the market
Additional Equipment
The use of mechanical seals generally involve the use of additional equipment primarily for the flushing /coolant systems. This includes pumps, coolers, strainers, filters etc.
Read More

CAT Engine Course (Part 8) Valve Lash

The top bolt in the cover plate of the bore for the engine turning tool is the timing bolt. The timing bolt is used to find the location of No. 1 piston on top center (TC).
This is important to the serviceman because thereference point for all timing proce dures is with No. 1 piston put at top center on the compression stroke.
The pipe plug (arrow) is removed from the flywheel housing to install the timing bolt.

The optional engine turning tool fits in the bore in the flywheel housing and has teeth which engage with the flywheel gear teeth. The engine turning tool is a special tool used for rotation of the crankshaft of the engine when you need to make adjustments, time the engine, or turn an engine in storage.

A 1/2 inch drive ratchet is used to turn the tool and the engine crankshaft.

To find top center compression stroke for No. 1 piston, first turn the flywheel clockwise a minimum of 30 degrees. The reason for making this step is to be sure the backlash is removed from the timing gears when the engine is put on top dead center.

Next, turn the flywheel counterclockwise until the hole in the flywheel is in alignment with the timing bolt. When the timing bolt can be turned freely in the threaded hole in the flywheel, the engine No. 1 piston is on top center.

To check to see if No. 1 piston is on compression stroke, look at the valves of No. 1 cylinder. The valves will be CLOSED if No. 1 cylinder is on compression stroke. You should be able to move the valve rocker arms up and down with your hand.
If No. 1 piston is NOT on compression stroke, turn the crankshaft 360 degrees and follow the procedure again to put No. 1 piston on top center.

With the valve cover removed, we can see the three rocker arm assemblies. The rocker arm on the left (1) activates the bridge and the two exhaust valves. The center rocker arm (2) activates the unit injector for fuel injection. The rocker arm on the right (3) activates the bridge and the two inlet (intake) valves.

NOTE: On production engines, lower thread bosses are removed. EXHAUST and INTAKE are stamped on housing.

The valve arrangement is the same as other engine valvesactivated by push rods, except a valve bridge. The bridge activates the two inlet valves or exhaust valves at the same time (similar to the 3400 Series Engines). The bridge fits on a dowel which is the guide when the bridge moves up and down. The bridge is moved (activated) by the rocker arm. Adjustment of the valve bridge is done when the engine is assembled and when valve lash adjustment is made.

To adjust the bridge, loosen the locknut, push down on the rocker arm (or point of contact) and turn the adjustment screw clockwise until it contacts the stem of the valve. Turn the adjustment screw an additional 20 to 30 degrees (1/3 to 1/2 of one side of the nut) to position the guide of the bridge straight on the dowel.
Tighten the locknut to specifications.

NOTE: Normally, adjustment of the bridge will be donewith the rocker arm assembly off the cylinder head.

Valve clearance or valve lash is measured with a feeler gauge or 8T5207 setting gage, put between the rocker arm contact and the bridge wear seat. Valve clearance is changed using a screwdriver.

A 3/4 inch wrench is needed to loosen and tighten the locknut which holds the adjustment screw. To make the adjustment, turn the adjustment screw until you feel an easy pull on the feeler gauge as you move it backward and forward between the rocker arm contact and the bridge wear seat. Tighten the nut on the adjustment screw and check the adjustment to be sure it has not changed. The adjustment of all valves and the injectors can be done by putting the engine crankshaft in two positions. Refer to the Service Manual for these procedures.
Read More

CAT Engine Course (Part 7 ) Fuel Injection

The 3500 Series Engines use the direct injection combustion system. This system has the advantages of: low heat rejection (in comparison to precombustion); low fuel consumption; and easy starting.

A fuel injector (7) is in a central bore of each cylinder head. The position of the rack (6) of each injector is changed by a bellcrank and bracket (5) that is held to the top of the cylinder head by bolts. Each bellcrank
is moved by a control rod (4) connected to a hollow torsion shaft (1) through a lever (3).
Rotation of the torsion shaft (1) is done by the governor input shaft (10) and causes in and our movement of the rack (6).
The torsion shafts (1 and 8) are just below the camshafts of each bank of cylinders. A hollow cross shaft (9) at the front of the engine connects the right torsion shaft (1) and left torsion shaft (8) so they move together at the same time.

The control rods (4) have a “click” screw adjustment (11) at the bellcrank ends. There is one adjustment screw for each rack. This adjustment is used to synchronize all racks together. The adjustment sets the racks of the separate unit injectors so that they have the same reference position.
Also, there is a spring in the top end of the control rod. If one unit injector plunger will not turn (is STUCK) or the rack of that unit injector will not move, the control rod can still control the racks of the other injectors. This will prevent engine overspeed and the engine can be stopped. This design characteristic is for protection of the engine from damage.
Another protection for the engine: If the control linkage becomes disconnected from the governor, the WEIGHT of the control linkage can move the racks of the unit injectors to the fuel OFF position. The engine will STOP.
The torsion shafts (1 and 8) are marked with red and green colors on the inside diameter for assembly identification purposes. The left torsion shaft (8) has red and the right has green

In the inset we can see the power pad. The power pad has the power setting screw cover. The power setting screw cover has two bolts. The top bolt is the synchronizing pin and fastens the power setting screw cover. The bottom bolt also holds the cover on. With the cover removed, we can see the power setting screw and locknut.
The hole to the right of the power screw is where the collet and dial indicator is installed for measurement and adjustment of the power setting. The hole to the left of the power setting screw is for the synchronizing pin (the top bolt). This pin is used to put the fuel control linkage in the reference (fixed) position, when the synchronizing adjustment is made to the unit injectors.

NOTE: This illustration is not correct. The seal goes through the cover bolt, not the synchronizing pin.

In the highlighted area we see the power setting screw.
The power setting screw makes contact with the fuel stop lever. Adjustment of the power setting screw controls the maximum power setting of the engine. It controls the maximum movement of the control linkage and all injector racks.
Above the highlighted area is the governor lever which is connected to the governor output shaft. The governor lever, the fuel stop lever, the front end of the right torsion shaft and the power setting screw are in the front gear housing behind the power pad.

In this slide we see the fuel control linkage operation from the front of the engine. When the speed control of the governor is moved toward maximum rpm, the governor output shaft (black arrow) turns clockwise and moves the governor shaft lever to the left. A pin in the governor shaft lever is in the groove in the fuel stop lever and moves it to the left. The fuel stop lever turns the right torsion shaft counterclockwise as seen by the arrow. This counterclockwise movement moves the control rod up. This movement pivots the bellcrank and pulls the rack out of the injector in the fuel “ON” direction. The right and left torsion shafts always move together.
The ends of the shafts are connected to the ends of the cross shaft by a fork lever-ball lever arrangement (to understand better, see iron later).

This is the cross shaft on the front of the engine. It connects the right torsion shaft to the left torsion shaft.

The front housing is removed.

This shows the end of the right torsion shaft and cross shaft. You can also see the connection between the fork lever on the torsion shaft and ball lever on the cross shaft. This connection has a smalltolerance.
We can also see the fuel stop lever.

This shows the end of the torsion shaft and the left end of the cross shaft. The connection of the ball lever on the cross shaft with the fork lever on the torsion shaft can be seen.

The front housing is installed in this view.

Here we see inside the camshaft compartment. The camshaft is above. The torsion shaft of the fuel control linkage is below. The control rod of the torsion shaft (center) goes up to the bellcrank assembly of the unit injector.

This shows the rear end of the right torsion shaft. The support bracket for the shaft can be seen.

Also with the front housing removed, we can see the fuel stop lever clearly. With the front housing installed. . .

. . . we can see the power setting screw in contact with the fuel stop lever.
This also shows the notch in the fuel stop lever which connects with the pin of the governor lever.
We will learn about the power setting adjustment later; however, you can see how the power setting screw controls the position of the fuel stop lever by stopping its movement.

In this schematic we can see the injection components of this fuel system.
The components are:
1. injector cam lobe of engine camshaft;
2. a push rod
2a. a lifter assembly
3. an injector rocker arm
4. an injector clamp
5. a unit injector
6. a section of cylinder head; and
7. a piston in a cylinder

In this schematic we can see the control components of this fuel system.
The components are:
8. control lever on torsion shaft
9. control rod
10. bellcrank
11. injector rack
12. injector plunger

Here we see two unit injectors. The one on the left has been cut away for instructional purposes. The injector on the right is complete.
This slide shows the:
1. injector body
2. follower
3. follower return spring
4. rack
5. injector housing (nut); and the
6. injector nozzle (spray tip).

This is the unit injector designed and manufactured by Caterpillar. It is being used for current production marine engines. Remanufactured nozzles will be available. Service tools (to be announced at a later date) will permit some field service to be done on these nozzles. This injector has a removable cone on the end and a trim screw for bench calibration.

The injection of fuel is made by the rotation of the engine camshaft which causes the cam to lift the lifter assembly and push the rod up. When the push rod moves the injector rocker arm up, the contact of the rocker arm pushes the follower an injector plunger down. As the plunger moves down, fuel is injected into the combustion chamber. As the lower scroll on the plunger goes beyond the lower port, injection stops. When the rocker arm stops its downward movement, the follower return spring pushes the follower up with the plunger. The follower return spring also keeps a force on the rocker arm push rod and lifter. This force keeps the lifter in contact with the cam.

Looking at the cutaway of the injector, we can see the:
1. plunger
2. barrel
3. lower port
4. upper port; and the
5. spill deflector
The plunger position shown is at the top of the stroke.
The barrel (2) has an upper port (4) and a lower port (3). The relation of the scrolls to the ports:
(1) changes the length of the effective stoke and the quantity of fuel per injection stroke;
(2) permits the start of the effective stroke to be variable in relation to piston position.

The smaller the quantity (VOLUME) of fuel injected during the injection stroke, the later (NEARER TO TOP CENTER) injection takes place.

The larger the quantity (VOLUME) of fuel injected during the injection stroke, the earlier (FARTHER FROM TOP CENTER) injection takes place.
Movement of the control linkage and rack turns the plunger and changes the quantity of fuel injected and the point at which injection starts.
The action of the double scroll is a method of timing advance.
Older injectors are double scroll. Newer injectors aresingle scroll.

Let us look at the nozzle of the injector. We see the:
1. check valve
2. check valve cage
3. valve spring and seat
4. spring cage
5. needle valve
6. spray tip; and the
7. injector housing or nut
The spray tip has several small orifices. Each nozzle has two dowels in the body which puts it in the correct position when installed. This position puts the rack in the correct location with the bellcrank and the spray tip at the correct angle with the surface of the piston.
The plunger position shown here is on the downward stroke and the lower port is just closed.

The plunger position shown here is the start of the injection stroke. Both the lower port and the upper port are closed. It is at the start of the effective stroke.
During the effective stroke, the plunger forces fuel into the nozzle of the injector. The fuel goes around the check valve and through passages in the check valve cage. After fuel goes through the valve spring cage, it goes into the passages in the spray tip. The passages sends the fuel to the chamber around the needle valve. Here the fuel pressure lifts the needle valve off the seat and fuel flows through the spray tip and out the orifices into the combustion chamber. Injection of fuel continues until the lower scroll on the plunger goes by the lower port, the pressure of the fuel against the needle valve is less.

The valve spring pushes the needle valve closed. This stops the flow of fuel into the combustion chamber. Also, when the fuel is released through the lower port, the fuel pressure above the check valve decreases. The fuel pressure in the tip chamber then pushes the check valve up against the end of the barrel. With the needle valve on the seat and the check valve against the end of the barrel, combustion gases cannot get into the injector and cause damage between injection strokes.

NOTE: If the needle valve is held open by foreign particles between injection cycles,
combustion gases can come into the injector and cause damage.
Read More

CAT Engine Course (Part 6) Fuel System

The fuel transfer pump (2) pulls fuel from the tank through the inlet line (1) and forces it through a check valve and into the line to the fuel filters (3). After the fuel filters, the fuel flows to the fuel manifolds (6) along the inside of each cylinder bank. The top inlet passage of the manifold sends fuel through lines connected to each cylinder head. (Early engines had fuel filter screens in the connectors.) Fuel flows into a circular space around the injector (5). Part of this fuel is used for injection and part to cool the injector. (Later engines have fuel filter screens in the injectors.) The extra fuel that cools the injector is returned through lines to the bottom outlet passage of the fuel manifolds, through a pressure regulating valve (7) and then through a return line and to the tank.

The priming pump (4) has a supply line from the inlet side of the pump and sends fuel through the filters, into the fuel manifolds. The location of the pressure regulating valve is on the front of
the right fuel manifold (7). The pressure regulating valve is made to hold a constant pressure of approximately 415 kPa (60 psi). The valve makes a high resistance to the flow of fuel to 415 kPa (60 psi), but little resistance to air. In this way, air can be removed from the fuel system. A small orifice connects the inlet and outlet passages to make a siphon break when changing filters, thus making it less possible that the system will need to have the air removed after a filter change.

The fuel transfer pump is on the rear of the oil pump and is driven by the lower oil pump shaft. The pump sends fuel through the fuel filters and into the fuel manifolds located along each bank of cylinder. The pump capacity is approximately 21 litres/minute (5.5 gpm) or several times as much fuel as needed for combustion.

Here we can see the:
1. transfer pump
2. supply inlet
3. check valve fitting
4. fuel line to fuel filter housing base
5. fuel line to priming pump
6. fuel filter housing base and filters; and
7. priming pump
On the vehicular engine, the transfer pump (1) and filter housing base and filters (7) are on the right side of the engine. Fuel from the supply tank goes in the transfer pump at the supply inlet (2). A check valve (4) is in the fitting at the transfer pump for the fuel line (2) that goes to the filter housing base. The check valve prevents fuel flow back through the transfer pump when the priming pump (8) is used.

The transfer pump bypass valve (10) limits the maximum fuel pressure of the transfer pump.
The bypass valve will open at 520 kPa (125 psi) and send the extra fuel to the inlet side. This prevents damage to the fuel system components caused by too much pressure.

On vehicle arrangements, the spin-on replacement type fuel filter elements (the filter element and filter case are one unit) are located along the left side of the engine. On marine, industrial and generator set arrangements, the filters are located in a housing across the front of the engine as seen earlier.

Two (2) spin-on fuel filter elements are used for vehicular engines having 250 hour oil changes.
Each filter turns on the screw threads of the filter base. All of the fuel from the fuel tank goes through the filters for cleaning (a full flow filter).
There are five (5) fuel filter elements used on engine arrangements with 500 hour and 1000 hour oil change periods. In this way, the fuel filter elements may be changed when the lubrication oil is changed. When you remove the old filter, be sure the old gasket comes off with the filter.

Change the fuel filter elements at the recommended interval. Remove and discard the old fuel filter element. Clean the gasket sealing surface of the filter base. Make sure all of the old gasket is removed.

Apply clean diesel fuel to the fuel filter gasket. Install the new filter and tighten by hand until the filter gasket contacts the base. Tighten the filter 1/2 the 3/4 turn more with a filter wrench.

The optional priming pump is installed on the filter base/housing. It is used to prime a completely dry fuel system, or prime the system after the filters have been changed.

Here we can see:
1. fuel supply line
2. adapter with pressure regulating valve
3. fuel manifold (right side)
4. return line (from cylinder head)
5. inlet line (to cylinder head)
6. pressure regulating valve; and
7. connection for return line to tank

A fuel inlet line goes from the top passage of the fuel manifold to a fitting on the right side of each cylinder head.
A fuel return line goes from the left side of each cylinder head to the bottom passage of the fuel manifold where it goes through the return line to the fuel tank.

On Industrial, Generator Sets, Marine and Marine Auxiliary Engine Arrangements, and fuel filter housing (1) is on top of the oil filter housing (3).
Here we can see:
1. fuel filter housing
2. drain valve
3. oil filter housing
4. adapter with pressure regulating valve; and 5
5. fuel manifold (right)
The drain valve (2) is used to drain fuel from the filter housing when the filters are changed.
Also, we can see the adapter with pressure regulator valve (4) and the fuel manifold (5) on the right side.

Read More