Root Cause Failure Analysis Part 9 pdf

However, much less lubrication is required with this type of seal because the frictional surface area is smaller than that of a compressed-packing gland and the contact pressure is equal

232 Root Cause Failure Analysis

Complete the equipment assembly, taking care when compressing the seal into the stuffing box

Seat the gland ring and gasket to the face of the stuffing box by tightening the nuts and bolts evenly and firmly Be sure the gland ring is not cocked Tighten the nuts and bolts only enough to form a seal at the gland ring gas- ket, usually finger tight and one half to three quarters of a turn with a wrench Excessively tightening the gland ring nuts and bolts will cause dis- tortion that will be transmitted to the running face, resulting in leaks

If the seal’s assembly drawing is not available, the proper setting dimension for inside seals can be determined as follows:

- Establish a reference mark on the shaft or sleeve flush with the face of the stuffing box

- Determine how far the face of the insert will extend into the stuffing-box bore Take this dimension from the face of the gasket

- Determine the compressed length of the rotary unit by compressing it to the proper spring gap

- This dimension, added to the distance the insert extends into the stuffing box, gives the seal-setting dimension from the reference mark on the shaft or sleeve to the back of the seal collar

- Outside seals are set with the spring gap equal to the dimension stamped

on the seal collar

Cartridge seals are set at the factory and installed as complete assemblies These assemblies contain spacers that must be removed after being bolted into position and the sleeve collar is in place

Installation of Environmental Controls

Mechanical seals often are chosen and designed to operate with environmental con- trols If this is the case, check the seal’s assembly drawing or the equipment’s drawing

to ensure that all environmental-control piping is properly installed

Seal Startup Procedures

Before equipment startup, all heating and cooling lines should be operating These lines also should remain in operation for a short period after equipment shutdown On

double-seal installations, be sure the liquid lines are connected, the pressure-control valves are properly adjusted, and the sealing-liquid system is operating before starting the equipment

Before startup, all systems should be properly vented This is especially important on vertical installations where the stuffing box is the uppermost portion of the pressure- containing part of the equipment The stuffing-box area must be properly vented to

avoid a vapor lock in the seal area that would cause it to run dry

When starting equipment with mechanical seals, make sure the seal faces are

immersed in liquid from the beginning so they will not be damaged from dry opera-

Seals and Packing 233

tion The following recommendations for seal startup apply to most types of seal installations and will improve their life if followed:

Caution the electrician not to run the equipment dry while checking motor rotation A slight turnover will not hurt the seal, but operating at full speed for several minutes under dry conditions will destroy or severely damage the rubbing faces

The stuffing box always should be vented before startup, especially with centrifugal pumps Even if the pump has a flooded suction, it is still possible that air may be trapped in the top of the stuffing box after the pump’s initial liquid purge

Where cooling or bypass recirculation taps are incorporated in the seal gland, piping must be connected to or from these taps before startup These specific environmental-control features must be used to protect the organic materials in the seal and to ensure proper performance

Cooling lines should be left open at all times This is especially true when hot product passes through off-line standby equipment, commonly done so

that additional product volume or equipment change can be achieved easily often by simply pushing a button

At the end of each day when hot operational equipment is shut down, it is best to leave the cooling water on long enough for the seal area to cool below the temperature limits of the organic materials in the seal

Before startup, face-lubricated seals must be connected from the source of lubrication to the tap openings in the seal gland For double seals, it is nec- essary for the lubrication feed lines to be connected to the proper ports before startup for both circulatory and dead-end systems This is very important because all types of double seals depend on the controlled pres- sure and flow of the sealing fluid to function properly Even before the shaft

is rotated, the sealing liquid pressure must exceed the product pressure opposing the seal Be sure a vapor trap does not prevent the lubricant from promptly reaching the seal face

Thorough warm-up procedures include a check of all steam piping arrange- ments to be sure that all are connected and functioning Products that solid- ify when cool must be fully melted before startup It is advisable to leave all heat sources on while the system is shut down to ensure that the product remains in the liquid state This facilitates quick startups and equipment switchovers that may be required during a production cycle

Thorough chilling procedures are necessary for some applications; for example, applications involving liquefied petroleum gas (LPG) LPG always must be kept in a liquid state in the seal area, and startup usually is the most critical time Even during operation, the recirculation line piped to the stuffing box might need to be run through a cooler to overcome fric- tional heat generated at the seal faces LEG requires a stuffing-box pressure greater than the vapor pressure of the product at pumping temperature A 25

to 50 psi differential is generally desired

234 Root Cause Failure Analysis

Lubrication and cooling can be accomplished by allowing a small amount of leakage

of fluid from the machine or by providing an external source of fluid When leakage from the machine is used, leaking fluid is captured in collection basins built into the machine housing or baseplate Note that periodic maintenance to recompress the packing must be carried out when leakage becomes excessive

Packed boxes must be protected against ingress of dirt and air, which can result in loss

of resilience and lubricity When this occurs, packing will act like a grinding stone, effectively destroying the shaft’s sacrificial sleeve and causing the gland to leak excessively When the sacrificial sleeve on the drive shaft becomes ridged and worn, it should be replaced as soon as possible In effect, this is a continuing maintenance pro- gram that readily can be measured in terms of dollars and time

Uneven pressure can be exerted on the drive shaft due to irregularities in the packing rings, resulting in irregular contact with the shaft This causes uneven distribution of lubrication at certain locations, producing acute wear and packed-box leakage The only effective solution to this problem is to replace the shaft sleeve or drive shaft at the earliest opportunity

Simple Mechanical Seal

As with compressed packing glands, lubrication must be provided in mechanical seals The sealing-area surfaces should be lubricated and cooled with pumped fluid (if

it is clean enough) or another source of clean fluid However, much less lubrication is required with this type of seal because the frictional 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 to exit as leakage

Most packing glands have a measurable flow of lubrication fluid between the packing rings and the shaft With mechanical seals, the faces ride on a microscopic film of fluid that migrates between them and results in leakage However, leakage is so slight

Seals and Packing 235

that, if the temperature of the fluid is above its saturation point at atmospheric pres- sure, it flashes off to vapor before it can be visually detected

Friction Drive or Single-Coil Spring Seal

The seal shown back in Figure 18-2 is a typical friction drive, or single-coil spring seal unit This design is limited in use to nonlubricating fluids (e.g., water) because it relies on friction to turn the rotary unit For use with liquids that have natural lubricat- ing properties the seal must be mechanically locked to the drive shaft

Two drawbacks must be considered for this type of seal Both are related to the use of

a coil spring that fits over the drive shaft Nevertheless, the simple and reliable coil spring seal has proven itself in the pumping industry and often is specified despite its drawbacks In regulated industries, this type of seal design far exceeds the capabilities

of a compressed packing ring seal

One drawback of the spring seal is the need for relatively low shaft speeds The com- ponents have a tendency to distort at high surface speeds This makes the spring push harder on one side of the seal than the other, resulting in an uneven liquid film between the faces, which causes 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 requires several sizes of spare springs be kept in inventory

Positive Drive

There are two methods of converting a simple seal to positive drive Both methods, which use collars secured to the drive shaft by set screws, are shown in Figure 18-1 1

In the Figure on the left, the end tabs of the spring are bent at 90" 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 drive from the collar to the seal ring by the spring In the right drawing of Figure 18-1 1, two horizontally mounted pins extend over the spring from the collar to the seal ring

Figure 18-11 Conversion of a simple seal to positive drive (Roberts 1978)

This part provides an overview of the more common failure modes for machinery found in integrated process plants Troubleshooting guides are provided for: pumps fans, blowers, fluidizers, conveyors, compressors, mixers, agitators, dust collectors process rolls, gearboxes/reducers, steam traps, inverters, control valves seals, and packing

19

PUMPS

Design, installation, and operation are the dominant factors that affect a pump’s mode

of failure This chapter identifies common modes of failure for centrifugal and posi- tive-displacement pumps

CENTRIFUGAL

Centrifugal pumps are especially sensitive to variations in liquid condition (i.e., vis- cosity, specific gravity, and temperature); suction variations, such as pressure and availability of a continuous volume of fluid; and variations in demand Table 19-1 lists common failure modes for centrifugal pumps and their causes

Mechanical failure may occur for a number of reasons Some failures are induced by cavitation, hydraulic instability, or other system-related problems Others are the direct result of improper maintenance Maintenance-related problems include improper lubrication, misalignment, imbalance, seal leakage, and a variety of other situations that periodically affect machine reliability

Cavitation

Cavitation in a centrifugal pump, which has a significant, negative effect on perfor- mance, is the most common failure mode Cavitation not only degrades a pump’s per- formance but also greatly accelerates the wear on its internal components

Causes

Three causes of cavitation in centrifugal pumps are change of phase, entrained air or gas, and turbulent flow

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240 Root Cause Failure Analysis

Table 19-1 Common Failure Modes of Centrifugal Pumps

Mochanlcal Def.cfs, Worn, Rusted, D & d h Beerho8

Source: Integrated Systems, Inc

Change of Phase The formation or collapse of vapor bubbles in either the suction

piping or inside the pump is one cause of cavitation This failure mode normally occurs in applications, such as boiler feed, where the incoming liquid is at a tempera- ture near its saturation point In this situation, a slight change in suction pressure can cause the liquid to flash into its gaseous state In the boiler-feed example, the water flashes into steam The reverse process also can occur A slight increase in suction

pressure can force the entrained vapor to change phase to a liquid

Pumps 241

Cavitation due to phase change seriously damages the pump’s internal components Visual evidence of operation with phase-change cavitation is an impeller surface fin- ish like an orange peel Prolonged operation causes small pits or holes on both the impeller shroud and vanes

Entrained Air or Gas Pumps are designed to handle gas-free liquids If a centrifu- gal pump’s suction supply contains any appreciable quantity of gas, the pump will cavitate In the example of cavitation due to entrainment, the liquid is reasonably sta- ble, unlike with the change of phase described in the preceding section Nevertheless, the entrained gas has a negative effect on pump performance While this form of cavi- tation does not seriously affect the pump’s internal components, it severely restricts its output and efficiency

The primary causes of cavitation due to entrained gas include two-phase suction sup- ply, inadequate available net positive suction head (NPSH,), and leakage in the suc- tion-supply system In some applications, the incoming liquid may contain moderate

to high concentrations of air or gas This may result from aeration or mixing the liquid prior to reaching the pump or inadequate liquid levels in the supply reservoir Regard- less of the reason, the pump is forced to handle two-phase flow, which was not intended in its design

nrbulent Flow The effects of turbulent flow (not a true form of cavitation) on pump performance are almost identical to those described for entrained air or gas in the preceding section Pumps are not designed to handle incoming liquids that have

no stable, laminar flow pattern Therefore, if the flow is unstable, or turbulent, the symptoms are the same as for cavitation

Symptoms

Noise (e.g., like a can of marbles being shaken) is one indication that a centrifugal pump is cavitating Other indications are fluctuations of the pressure gauges, flow rate, and motor current, as well as changes in the vibration profile

Solutions

Several design or operational changes may be necessary to stop centrifugal-pump cavitation Increasing the available net positive suction head (NPSH,) above that required (NPSHR) is one way to stop it The NPSH required to prevent cavitation is determined through testing by the pump manufacturer It depends on several factors, including type of impeller inlet, impeller design, impeller rotational speed, pump flow rate, and the type of liquid being pumped The manufacturer typically supplies curves

of NPSH, as a function of flow rate for a particular liquid (usually water) in the pump’s manual

One way to increase the NPSH, is to increase the pump’s suction pressure If a pump is fed from an enclosed tank, suction pressure can be increased by either raising the level

of the liquid in the tank or increasing the pressure in the gas space above the liquid

242 Root Cause Failure Analysis

The NPSHA also can be increased by decreasing the temperature of the liquid being pumped This decreases the saturation pressure, which increases the NPSH,

If the head losses in the suction piping can be reduced, the NPSHA will be increased

Methods for reducing head losses include increasing the pipe diameter; reducing the number of elbows, valves, and fittings in the pipe; and decreasing the pipe length

It also may be possible to stop cavitation by reducing the pump’s NPSHR, which is not a constant for a given pump under all conditions Typically, the NPSHR increases

significantly as the pump’s flow rate increases Therefore, reducing the flow rate by throttling a discharge valve decreases NPSHR In addition to flow rate, NPSH,

depends on pump speed The faster the pump’s impeller rotates, the greater is the

NPSHR Therefore, if the speed of a variable-speed centrifugal pump is reduced, the NPSHR of the pump is decreased

Variations in the Total System Head

Centrifugal-pump performance follows its hydraulic curve (i.e., head versus flow rate) Therefore, any variation in the total back pressure of the system causes a change

in the pump’s flow or output Because pumps are designed to operate at their best effi-

ciency point (BEP), they become more and more unstable as they are forced to oper-

ate at any other point because of changes in total system pressure, or head (TSH) This instability has a direct impact on centrifugal-pump performance, reliability, operating costs, and required maintenance

Symptoms

The symptoms of failure due to variations in TSH include changes in motor speed and

flow rate

Motor Speed The brake horsepower of the motor that drives a pump is load depen-

dent As the pump’s operating point deviates from BEP, the amount of horsepower required also changes This causes a change in the pump’s rotating speed, which either increases or decreases depending on the amount of work that the pump must perform

Flow Rate The volume of liquid delivered by the pump varies with changes in TSH

An increase in the total system back pressure results in a decreased flow, while a back pressure reduction increases the pump’s output

Solutions

The best solution to problems caused by TSH variations is to prevent the variations

While it is not possible to completely eliminate them, the operating practices for cen- trifugal pumps should limit operation to an acceptable range of system demand for

flow and pressure If system demand exceeds the pump’s capabilities, it may be nec-

essary to change the pump, the system requirements, or both In many applications, the pump is either too small or too large In these instances, it is necessary to replace the pump with one that is properly sized

For the application where the TSH is too low and the pump is operating in runout con- dition (i.e., maximum flow and minimum discharge pressure), the system demand can

be corrected by restricting the discharge flow of the pump This approach, calledfulsv

head, changes the system’s head by partially closing a discharge valve to increase the back pressure on the pump Because the pump must follow its hydraulic curve, this forces the pump’s performance back toward its BEP

When the TSH is too great, there are two options: replace the pump or lower the sys- tem’s back pressure by eliminating line resistance due to elbows, extra valves, and the like

Positive-displacement pumps are more tolerant to variations in system demands and pressures than centrifugal pumps However, they still are subject to a variety of com- mon failure modes caused directly or indirectly by the process

ROt8V TVpe

Rotary-type, positive-displacement pumps share many failure modes with centrifugal pumps Both types of pumps are subject to process-induced failure caused by demands that exceed the pump’s capabilities Process-induced failure also is caused

by operating methods that either result in radical changes in their operating envelope

or instability in the process system

Table 19-2 lists common failure modes for rotary-type, positive-displacement pumps The most common failure modes of these pumps generally are attributed to problems with the suction supply The pumps must have a constant volume of clean liquid to function properly

Reciprocating

Table 19-3 lists the common failure modes for reciprocating-type, positive-displace- ment pumps Reciprocating pumps generally can withstand more abuse and variations

in system demand than any other type However, they must have a consistent supply

of relatively clean liquid to function properly

The weak links in the reciprocating pump’s design are the inlet and discharge valves used to control pumping action These valves are the most frequent source of failure In

most cases, valve failure is due to fatigue The only positive way to prevent or minimize

244 Root Cause Failure Analysis

Table 19-2 Common Failure Modes of Rohry-Type, Positive-Displacement Pumps

Source: Integrated Systems, Inc

such failure is to ensure that proper maintenance is performed regularly on these com- ponents It is important to follow the manufacturer’s recommendations for valve mainte- nance and replacement

Because of the close tolerances between the pistons and the cylinder walls, reciprocat- ing pumps cannot tolerate contaminated liquid in their suction-supply system Many

of the failure modes associated with this type of pump are caused by contamination

(e.g., dirt, grit, and other solids) that enter the suction side of the pump This problem can be prevented by the use of well-maintained inlet strainers or filters

Pumps 245

Table 19-3 Common FQilure Modes of Reciprocating Positive-Displacement Pumps

I Law Vdumetric Eplidencv

20

FANS, BLOWERS, AND FLUIDIZERS

Tables 20-1 and 20-2 list the common failure modes for fans, blowers, and fluidizers

v p i c a l problems with these devices include output below rating, vibration and noise, and overloaded driver bearings

Centrifugal fans are extremely sensitive to variations in either suction or discharge conditions In addition to variations in ambient conditions (Le., temperature, humid- ity, etc.), control variables can have a direct effect on fan performance and reliability Most problems that limit fan performance and reliability are caused, either directly or indirectly, by improper application, installation, operation, or maintenance However, the majority are caused by misapplication or poor operating practices Table 20-1 lists failure modes of centrifugal fans and their causes Some of the more common failures

are aerodynamic instability, plate-out, speed changes, and lateral flexibility

Aerodynamic Instability

Generally, the control range of centrifugal fans is about 15 percent above and below

its BEP Fans operated outside this range tend to become progressively more unstable, which causes the fan’s rotor assembly and shaft to deflect from their true centerline This deflection increases the vibration energy of the fan and accelerates the wear on bearings and other drive-train components

Plate Out

Dirt, moisture, and other contaminates tend to adhere to the fan’s rotating element This buildup, called plate our, increases the mass of the rotor assembly and decreases

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