Centrifugal Pump

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Centrifugal Pump A centrifugal pump is one of the simplest pieces of equipment. Its purpose is to convert energy of an electric motor or engine into velocity or kinetic energy and then into pressure of a fluid that is being pumped. The energy changes occur into two main parts of the pump, the impeller and the volute. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute is the stationary part that converts the kinetic energy into pressure. Centrifugal Force Liquid enters the pump suction and then the eye of the impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and imparts centrifugal acceleration. As the liquid leaves the eye of the impeller a low pressure area is created at the eye allowing more liquid to enter the pump inlet. Centrifugal Pumps are classified into three general categories: CENTRIFUGAL PUMPS RADIAL FLOW MIXED FLOW AXIAL FLOW Radial Flow - a centrifugal pump in which the pressure is developed wholly by centrifugal force. Mixed Flow - a centrifugal pump in which the pressure is developed partly by centrifugal force and partly by the lift of the vanes of the impeller on the liquid. Axial Flow - a centrifugal pump in which the pressure is developed by the propelling or lifting action of the vanes of the impeller on the liquid. Positive Displacement Pumps are classified into two general categories and then subdivided into four/five categories each: POSITIVE DISPLACEMENT PUMPS SINGLE ROTOR MULTIPLE ROTOR

Transcript of Centrifugal Pump

Page 1: Centrifugal Pump

Centrifugal Pump

A centrifugal pump is one of the simplest pieces of equipment.  Its purpose is to convert energy of an electric motor or engine into velocity or kinetic energy and then into pressure of a fluid that is being pumped.  The energy changes occur into two main parts of the pump, the impeller and the volute. The impeller is the rotating part that converts driver energy into the kinetic energy.  The volute is the stationary part that converts the kinetic energy into pressure.

Centrifugal Force

Liquid enters the pump suction and then the eye of the impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and imparts centrifugal acceleration.  As the liquid leaves the eye of the impeller a low pressure area is created at the eye allowing more liquid to enter the pump inlet.

Centrifugal Pumps are classified into three general categories:

CENTRIFUGAL PUMPS

RADIAL FLOW MIXED FLOW AXIAL FLOW

Radial Flow - a centrifugal pump in which the pressure is developed wholly by centrifugal force.

Mixed Flow - a centrifugal pump in which the pressure is developed partly by centrifugal force and partly by the lift of the vanes of the impeller on the liquid.

Axial Flow - a centrifugal pump in which the pressure is developed by the propelling or lifting action of the vanes of the impeller on the liquid.

 

Positive Displacement Pumps are classified into two general categories and then subdivided into four/five categories each:

POSITIVE DISPLACEMENT PUMPS

SINGLE ROTOR MULTIPLE ROTOR

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VANEPISTON

FLEXIBLE MEMBERSINGLE SCREW

PROGRESSING CAVITY 

GEARLOBE

CIRCUMFERENTIAL PISTONMULTIPLE SCREW

SINGLE ROTOR o VANE - The vane(s) may be blades, buckets, rollers or slippers which

cooperate with a dam to draw fluid into and out of the pump chamber. o PISTON - Fluid is drawn in and out of the pump chamber by a piston(s)

reciprocating within a cylinder(s) and operating port valves. o FLEXIBLE MEMBER - Pumping and sealing depends on the elasticity of

a flexible member(s) which may be a tube, vane or a liner. o SINGLE SCREW - Fluid is carried between rotor screw threads as they

mesh with internal threads on the stator. o Progressing Cavity - Fluid is carried between a rotor and flexible

stator.

 

MULTIPLE ROTOR o GEAR - Fluid is carried between gear teeth and is expelled by the

meshing of the gears which cooperate to provide continuous sealing between the pump inlet and outlet.

o LOBE - Fluid is carried between rotor lobes which cooperate to provide continuous sealing between the pump inlet and outlet.

o CIRCUMFERENTIAL PISTON - Fluid is carried in spaces between piston surfaces not requiring contacts between rotor surfaces.

o MULTIPLE SCREW - Fluid is carried between rotor screw threads as they mesh.

Know how they work!

Soft packing stuffing boxes and pump glands appear so simple and are so common-place that we continue to cope without taking a few moments to understand why they fail or how to improve their performance.

When I first worked as a maintenance technician it did not occur to me to wonder why it was that the metal shaft sleeve wore away at almost the same rate as the fiber packing. Later, charged with the responsibility of advising maintenance engineers I was faced with worn out shaft sleeves on a daily basis. The type of fiber made no difference, the metal sleeve became damaged against hemp, cotton, aramid, teflon, and especially asbestos and, later, the asbestos substitutes. It appeared to make no sense to have a soft material damage a metal surface so badly as to cause grooves and ridges in what was to become a familiar pattern.

The packing fibers available come lubricated with a variety of greasy materials or the packings are made of inherently lubricated materials such as teflon, or carbon fiber, often loosely braided materials contain fillers such as the silicone filler in braided teflon packing, and the search for the answer as to why the shaft becomes worn starts here.

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Fillers, grease, graphite grease, tallow, and the other materials used serve two functions in packing materials.

The grease provides a lubricant at the shaft / packing interface. The material acts as a filler to prevent leakage occurring through the

interstices of the packing itself.

Now take the packing material and place it in the pump stuffing box.

Cut to size, the packing pieces are eased down the shaft to the neck ring. The cut ends are staggered to prevent leakage through them, the lantern ring is placed in position, the final three pieces of packing tamped in place by the gland plate and the work is done.

The next stage is to adjust the gland to ensure that it leaks. The leakage rate is controlled by the pressure exerted by the gland plate on the end of the packing set and the leak is allowed to develop along the shaft / packing interface to provide a cooling medium, removing the friction heat generated by the rotation of the shaft in the packing set. Two further things happen here. The grease in the packing melts slightly and is washed away by the flow of liquid along the shaft, and wear at the surface of the packing begins. A cycle is beginning which leads to the destruction of the packed gland as an effective leakage control device. The packing volume decreases as the lubricant is lost. Inevitably this causes the leakage rate to increase. As the rate increases more material is lost until the gland is tightened to reduce the leak to a minimum.

In very few cases can an engineer claim that the fluid passing through his pumps is not contaminated by dirt particles. Iron oxides, chromium oxides, grit, aluminum oxides, mica, and many other minute contaminants will exist in all system fluids. These solids, being denser than the pumped fluids, will be centrifuged and concentrated at the outer edge of the volute casing at just the point where the lantern ring tapping is sited. This contaminated fluid is then passed, at pressure, directly into the pump gland. Whatever material is used to seal a stuffing box if it is cooled by fluid contaminated by solids its surface will change producing an effective

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grinding surface. To improve the life of the stuffing box gland, contamination from the cooling water has to be avoided..

The gland packing has been wearing away. Through loss of lubricant it has lost volume, and the packing surface is exposed. The fluid passing through the gland, providing a cooling stream, is contaminated with various oxides and grit. The flow is increasing. A passing engineer notices it and takes appropriate action. The gland plate is tightened, pressure is exerted on the packing material to make it deform to reduce the clearance between it and the shaft. For a moment the flow of fluid is stemmed and the packing clamps down on the shaft trapping any solids moving through the gland at that moment. The interstices of the packing fill up with debris. The packing surface is now beginning to be converted from its original state into one consisting of oxides. The shaft sleeve itself may be contributing. Stainless steel ss316 or ss304, continually polished by the action of the packing replaces its surface of chrome oxide instantaneously, oxide which is taken up into the packing material. The build up of oxides on the surface of the packing changes the nature of the gland dramatically.

Our engineer has adjusted the gland plate and reduced the flow of fluid leaking out of the pump gland. The packing set has deformed to reduce the leak path. The deformation is not uniform. The action of the gland plate is to provide a force directed along the shaft which has to be translated into a radial force to effectively deform the packing. The friction at the outer edge of the stuffing box possibly supplemented by vulcanization of the packing material with the metal surface of the stuffing box, prevents the packing from sliding easily. Consequently, the first two rings of packing, experiencing the most force, are unable to transmit the axial force evenly down the length of the stuffing box and invariably this results in an over-tightening of this area of the gland in order to effect sufficient pressure throughout the gland.

Combine the over-tightening of the front end of the gland with the oxide impregnated gland packing and we are beginning to re-shape our shaft sleeve. But there is more to come.

As the rest of the packing set is adjusted by the overtightened first two rings the lantern ring is gradually pushed down the shaft. The packing

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pieces between the neck ring and the lantern ring are squeezed allowing the lantern ring to move further until in extreme cases it is cut off from its fluid supply. The gland is failing fast. Cut off from its coolant the gland can now overheat, causing rapid failure. Often before this occurs a partial repacking of the gland has taken place. New packing pieces have been put into the gland replacing the badly worn first three rings. But their life is limited because the rings placed into a worn stuffing box need to be deformed to accommodate the increased radial width of the stuffing box, and the cycle continues until the shaft sleeve is destroyed. .

 

The purpose of the lantern ring is to provide a balancing pressure within the packing set and to allow the cooling water to flow evenly around the gland. The pressure within the gland is greater than the pump suction pressure but less than the discharge pressure of the pump and is easily calculated from

SP+ (DP-SP)/4

Where

SP = Suction pressure

DP = Discharge pressure.

Fluid is taken from a tapping in the volute casing and piped directly into the lantern ring. This is a convenient pressure source readily to hand and self contained within the pump unit but consider the action of the pump impeller. Rotating at high speed the impeller acts as a very efficient centrifuge. Any dirt particles entrained in the fluid will be flung to the outer limits of the volute casing, leaving the less dense fluid clean until the streams re-unite at the impeller throat on their way out of the pump. As all the particles of dirt are at the periphery of the impeller clean fluid exists at the impeller center.

The stuffing box pressure is greater than the suction pressure of the pump, but less than the volute pressure. The state of the fluid within the volute casing at the back of the impeller is relatively clean having been centrifuged by the spinning action of the impeller. To prevent contamination of the gland is therefore, a simple matter of reversing the flow of fluid through the gland, using the volute casing pressure to produce a flow back through the gland to the suction side of the pump. Leakage will be controlled in the same way as before but the gland, being supplied with clean fluid, will no longer be subject to contamination to the same degree as before and a longer interval between adjustments and replacements of the gland packing can be expected.

 

M e c h a n i c a l S e a l s

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Why use a mechanical seal? After all is said and done, its easy to stuff a few extra bits of packing into a leaking stuffing box, and it doesn't require any skilled help to achieve this, does it? In this section we take a look at some of the reasons why you should be using seals. The Economic case and the Environmental case as well as considering some of the seal types available for general use.

It takes a lot of skill to pack a pump properly with soft packing.

There are two basic cases to be made out for the use of rotary, fluid sealing technologies.

The Environmental Case

We all have a responsibility to conserve and protect. Conserve scarce commodities and to protect the environment from pollution. A major spill is news because it is dramatic but every day, millions of glands leak chemicals into the environment. You can stop those leaks and avoid cleanup costs.

Have you thought about what that gland packing is doing to the shaft of your pump? It works as a brake, gripping the shaft and causing more power to be absorbed in the unit. The extra power consumption of the gland contributes to the "green-house" gases effect because more power has to be generated at the power plant to drive your pump.

As an experienced engineer you will know that the overall thermal efficiency of the power plant is much less than 35%, so if you can reduce your take-off by reducing the demand by replacing your glands with seals, less fuel is going to be consumed. Less fuel less emissions and less overall cost of running your plant. There is lots more to think about but space is limited so let's get on.

 

The Economic Case

If you are not convinced that environmental pollution is your problem, loss of hard cash from your pocket or that of your company should be!

Water is becoming a scarce commodity. Let me re-phrase that - clean water is becoming a scarce commodity. For example, boiler feed water has to be at a high standard of cleanliness and chemicals are added to it to ensure that the water quality remains high.

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Make-up water is usually cooler than the water circulating in the system so additional fuel is required to heat the makeup water. It all costs money. Water is paid for on amount used. More cost! But you'll look at the pump in the corner dripping away and think that doesn't seem too bad. Ever done the mug test? A coffee mug holds 300cc. It is a simple matter to collect the leakage and note the time it takes to fill one mug.

Leak Chart

One drip/second 5,256 Litres / year

3 mm stream 315,360 Litres / year

6 mm stream 630,720 Litres / year

One drip a second is the standard rate for a properly adjusted packed gland : it leaks water, chemicals, and heat. Leaks usually get worse so look at the chart and now tell me if a leaking gland is inconsequential! Let's do another sum - how many leaking glands are there in your plant? Not all packed glands hold back water ... there may be more costly fluids leaking away. Each leaking gland is contributing to hard cash overhead expense. Packing is cheap, to buy, to fit, but its running cost is hidden and can be very expensive.

A mechanical seal appears expensive to buy when compared with a packing ring, but properly installed a seal will run for many years. The optimum life of a seal is the period between major overhauls of the pump unit. A seal that fails early by this criteria is in need of investigation. The criteria for a failed seal is one in which the running faces are not worn down to their designed minimum. However, an engineer does not want to spend money on a super seal that will last virtually forever because that will also not prove to be cost effective. When a seal fails it is possible, with experience, or the aid of this web site, to determine the cause of failure and to rectify that fault. This I promise!

I was asked to select a seal for a water pump working in a quarry. The engineer had been plagued with seal failures for many years on this pump. His success criteria was that the seal should run from tear down to tear down (12 months). I selected a seal which was ten times more expensive than the one he had been using. It was fitted over the Easter Holiday 1982. Over a year later actually the week after the Easter holiday 1983 he rang me to say that the seal had failed. I reminded him of my promise that the seal would run for 12 months trouble free. He calmed down and started remembering, I told him that actually he had gotten an extra week over my promised 12 months! The increased price of the seal was around $400 but the saving in cost through not having to replace the seal several times in a year was over $1,500. The whole plant soon became converted to seals because it is possible to show a cost benefit analysis for every application.

It is often the thought that seals are expensive that prevents the engineer from opting for them. The same applies to pump manufacturers. Ever wondered why your plant is fitted with a particular pump make, each with a packing gland? In a word, competition. In the enlightened 1990's whole life costing is becoming the way to assess a particular project's initial cost, but in the real everyday world engineers are facing the consequences of short sighted least cost solutions to immediate

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problems. But now you do not have to continue living with these problems if you look at the situation of your plant leakage in a business-like manner.

This web site does not represent any one company manufacturing seals. There are good logistical and economic arguments for standardizing on one manufacturer so long as they are major enough to run through all the applications you are likely to need. Whilst working for one of these majors in Saudi Arabia I found that it was common to find whole refineries using one manufacturers' seal. Long way from home, gutsy job, $millions at stake in oil revenues every day, it made a lot of sense for the engineers concerned. Only one company to deal with, lucky for some of them it was mine and my expertise was part of the deal! But there are many designs of seal and some I would think of as cheap and not so nice could give some of you excellent service. So this is not about price, but very much concerns cost. Balancing the cost of the seal installation against the outcome compared with the alternative. There is a wide range of materials to choose from. The range encompasses small variations in generic materials such as carbon, or o-rings and different metals used to cope with the conditions that faces the seal. I am not encouraging you to experiment blindly but to think the problem through and choose your materials carefully.

We are not going to look at the materials in detail here. For that information pop over to seal troubleshooting

I have not listed all seal types, the contact-less gas seals for instance are not covered here, this is because they fall outside the general seal types I aim to cover. For details on highly specialized seals of this and other types contact your favored manufacturer for details. In the links section of this site you will find hyper links to some manufacturers.

Now go look at the various seal types that are available to you for general use. In these sections you will find explanations of seal types and some of the problems associated with them.

I n s t a l l a t i o n C h e c k s

Face the facts, seals fail. They do not wear out. Most often something comes along to disturb the smooth running of the pump and  you are facing a steady leak which has already destroyed your seal by damaging the seal faces. But there is another case. The seal that leaks on startup after maintenance. A seal that lasts a week without letting go is generally thought to be OK.   By the way that's a ROT (Rule of Thumb).  Running mechanical seals is an art form. There is a lot of science in it but either you have the knack or you do not (in which case you need this web site bad).

A seal that leaks after maintenance has been badly installed. It is very unwise to ignore the basic checks listed here because without these checks there is no certainty that your seal will perform at all, let alone give a reasonable running life. I hate having to go over a job again after having fitted it all back together... don't you?

Pre-installation checks.

1. You have the correct seal and all the parts needed for the replacement.

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2. You have the pump drawing to hand with installation dimensions or the seal manufacturer's drawing.

3. The pump stuffing box is clean

4. On split casing pumps the gasket does not extend into the stuffing box.

5. The shaft is free of scratches and burrs, threads are taped, and keyways are filled flush with the shaft surface to prevent seal elastomers from being cut on the keyway edge (a dummy wooden key insert is ideal).

6. All the seal parts are in their protective coatings at this stage.

Pump Checks

Shaft Run-out

Shafts get bent. The spinning impeller has unequal loading on in causing the shaft to deflect away from the volute throat. Constant deflection causes weakness and can lead to a permanent offset of the shaft leading to shaft run out. Shaft run out is bad for seals. It causes them to flex twice on every revolution of the shaft. At high enough speeds this can cause a vibration in the seal which allows the seal faces to OPEN. BANG failed seal.

So, look into the dark recesses of your lockers and pull out the Dial Test Indicator (DTI) or Clock Gauge that lurks there, unloved & unused and check the shaft of your pump for any damaging shaft deflections.

Single stage overhung pumps should be checked near the seal running position but multi stage pumps should be checked at suitable intervals along the shaft as well as at the seal running position.

 

The run out should not exceed 0.002 inches or 0.05 m.metres.

Shaft Sleeve Concentricity.

You have checked the shaft for run-out and because the seal elastomer has a tendency to wear a fret ring on the shaft a shaft sleeve is fitted to protect

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the shaft. When a new shaft sleeve is fitted, and this should be with every new seal, it is a good idea to re-run the shaft run-out check to ensure that the sleeve is concentric with the shaft.

The run out should not exceed 0.002 inches or 0.05 m.

A note about shaft sleeves. It is a false economy to omit to change the shaft sleeve when replacing a mechanical seal.

I was called out to a cooling water pump supplying a 100Mw Power station. The shaft size was 230mm and it took three men two days to strip and rebuild the seal box. The shaft sleeve cost $4,000 and the seal cost $10,000. The new seal had been fitted onto the old sleeve and leaked immediately on startup. The seal faces were intact but having been run for 24 hours in that condition another new seal assembly was required. On examination it was found that the o-ring contacting the shaft sleeve surface had worn a groove (Fretting damage) and the new o-ring was unable to seal against this damaged surface. The extent of the damage was not immediately obvious to the eye but by carefully measuring the surface the fault was found. Amount saved on first installation $4,000, total cost of seal change $25,500, and it should have cost $15,500. Believe me, skimping on the job is not the same as saving hard cash.

A x i a l S h a f t M o v e m e n t

Set up your DTI to measure the amount of axial movement of the shaft. The amount will vary according to the type of pump, its bearing configuration, and the type of thrust bearing in use.

Essen t ia l l y the re a re fou r t ypes o f th rus t bea r ings

Deep groove ball bearings Roller bearings Michell, Kingsbury, or thrust pad bearings, usually made of white

metal bearing surfaces. Balance piston thrust absorbing arrangement. This type is often

found on high pressure multi-stage water pumps where the hydraulic forces are partially balanced by the impellers and controlled leakage past a balance piston provides the final stage of rotating unit positioning.

The basic principle is that the shaft should be set to its running position before attempting to fit the seal.  In the case of cartridge seals, the seal cover plate should be fixed to the pump casing, the shaft positioned, and then the seal locking screws tightened to the shaft.  Non cartridge types need to have a datum mark scribed onto the shaft relative to the seal plate position and then the fitting dimension marked from this point.

A note about fitting position.  It is not good practice to fit a new seal by looking at the old set-screw marks and then lining up on them.  If you want good seal performance then start out right ... measure the distance required, don't take short cuts.   The last seal could have been fitted

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incorrectly, perhaps causing the rebuild that is now necessary.  You are storing up future trouble if you skimp.

Seal Housing Squareness

The seal stationary must be fitted at 90 degrees to the axis of the shaft.  Failing to achieve this will cause the seal head to move to take up any mis-alignment.  This movement offers an opportunity for the seal faces to open and for the ingress of dirt particles.  If you are changing out packing and up-grading your equipment to a mechanical seal you need to pay close attention to setting the seal housing closing plate in the correct position.  The basic check is as shown in the diagram.

It is also wise to check the bore of the seal housing at this point for concentricity with the shaft.  Put the sensing tip of the Dial Indicator inside the bore on the wall of the seal housing and rotate the shaft.  A small amount of misalignment is permitted but the important thing is to check that the seal body cannot touch the seal housing wall at any point of its rotation.

General Checks

While the pump unit is in the shop for maintenance take the opportunity to ensure that the cooling water jacket is clear of debris, that any other cooling water arrangement is cleared of any obstruction.  Orifice plates controlling the flow of water to a seal housing should be checked dimensionally correct.  A seal starved of its ration of cooling water will be very unforgiving and cause you lots of grief in a short time.   This kind of fault is very difficult to diagnose for the average engineer.   Even the best have trouble with this one, too!  So check it out now while the doing is easy.

Bearings need to be replaced if they have been running with any pump leakage around.   Moisture ingress into a bearing dramatically reduces a bearing's useful life.   If you are changing out soft packing for a mechanical seal replace the bearings on the unit too.  The leakage from the packing gland is more than enough to damage the bearings.

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Check the impeller for cavitation damage indicating a system problem that might go un-noticed during normal running conditions.  Cavitation can cause vibration in the pump shaft which will affect the seal 's performance.

I know you will ensure that the impeller sealing rings are replaced or re-bushed to keep the clearances within design limits.   Allowing recirculation within the pump volute is no way to keep the efficiency of your plant at the highest level, and it can increase the pressure inside the seal housing which will cause your seal to wear out faster!

 

W h y S e a l s F a i l

Seals fail for a number of reasons. Your job is to pinpoint the reason and fix it.

Here you are in a situation in which the seal has run for a period well beyond the installation period. Its leaking and now you have to make a decision.  Has the seal failed or simply worn out?  What you decide now will determine whether you fit a replacement seal or seek out an alternative type.  The basics are simple.  

A worn out seal will leak when the seal face has worn away completely.

 If we extend this criteria to all leaking seals it becomes sadly obvious that the majority of seals, perhaps 85% of process seals, fail long before they are worn out.

This section is devoted to the three main reasons why seals fail. Only three you say? Three main reasons and lots of routes to them.

Sea l s f a i l because . . .

The seal faces open. Heat causes a problem. The chemical environment causes a material failure.

OK so there is another category ... the installation failure, but that's covered in the installation section.

Sea l Faces Open

The shaft moves for many reasons, those that affect the seal operation are:

Axial

End play Thrust movement Temperature growth

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Impeller adjustment Radial

Bearing wear Bent shaft Shaft whip Shaft deflection (discharge closed)

Vibration

System NPSH incorrect causing cavitation Harmonic vibration, check the coupling, does it "hum" or "buzz".

Rubber couplings can operate with high degrees of misalignment without total failure but cause problems for the seal.

Impeller imbalance Slip-stick. Not surprisingly not much is known about what happens

between seal faces in service. There are theories. The faces acquire a film of liquid that lubricates the seal surfaces, the carbon face wears slightly depositing a layer of carbon on the stationary face so that the carbon face runs on carbon , but there is a condition that causes the faces to vibrate open when pumping non-lubricating fluids. Fluids near their vapour point, very hot water, can cause these conditions. The seal faces "chatter " against each other in a slip-stick motion slipping when the drive lug hits the seal head, bouncing round and momentarily stopping before being hit by the drive lug again. To be a sealman you have to believe.

Poor pump performance. This statement covers a host of sins. Consider running two or three pumps into one discharge line, the odds are that the pump performances will not be perfectly matched. Does it matter? Not really, unless you are concerned about your seal life, because what is happening here? One or other of the pumps, because of poor performance now combined with poor system design, will be experiencing discharge throttling, tending to over load the impeller at the throat, causing turbulent flow and shaft

bending. Look into other causes of poor pump performance. Other causes

The seal runs against a stationary component. The stationary is usually fitted into the seal plate which is bolted to the pump and sealed with a gasket. Now, I do not want to sound too pedantic here but you have to realize that the seal stationary has to be fitted square to the axis of the shaft and in proper alignment with the axis of the pump shaft. The stationary has to be fitted into the seal-plate square. None of this is easy to achieve and each error compounds the next. The rotating head has to follow any misalignment from square that the stationary carries. Every rotation of the shaft causes the rotating seal head to move back-and-forth twice. Interfere with that movement and the faces are open.

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Difficult as it is to get the stationary fitted correctly, should you achieve it then other factors come into play to limit the excellence of your work. Stress imposed by pipe strain, coupling misalignment, or plain thermal growth put the pump casing out of shape just enough to cause the seal to work harder.

All of the items described mean that the shaft and seal are in constant relative movement. If anything interferes with the free movement of the seal, the faces open.

When the faces open, dirt in the liquid penetrates the lapped surfaces, embeds in the soft face which gradually changes to a grinding surface to score and wear away the hard face of the stationary ring. Have you noticed this effect? Do you look at your failed seals? You should, because on those faces lie clues to help you find the faults opposing long seal life. Well when we have gotten through this section and onto the tell tale signs I bet you will take a bit more notice of your failed seal bodies.

The main reasons why seal faces open are:

The elastomer sticks to the shaft. Spring loaded elastomers will stick to the shaft, O-rings will flex by 0.005" (0.13mm) and then roll. O-rings will fret a shaft but spring loaded elastomers (teflon wedges, chevrons, etc.)  can cause serious surface damage to your shaft or sleeve leading to early seal failure. A leak under the seal head looks very much like a face leak.

The shaft is out on machining tolerance. Correct tolerance is +0.000" to -0.002" from nominal. A packing sleeve is not machined to any close tolerance, after all it is going to wear against the packing so its external dimension is not too important. An oversize sleeve or shaft will cause the seal to hang-up, an under size shaft or sleeve will prejudice the ability of the elastomers to seal the head to the shaft/sleeve.

The surface finish on the shaft/sleeve is too rough. A lathe finish is not good enough. The finish should be at least 32 RMS and for that a ground finish is required.

Have you got a hardened shaft on your pump unit? The seal set screws will not "bite" into the shaft and could slip causing the setting dimension of the seal to alter.

The pumped fluid changes state. Sea water, brine pumps, sugary solutions, cause crystallizing when the salts come out of solution or the sugars become caramelized. Other coking substances, heat transfer oil, tar, cause similar problems. You will see the build up of material around the leak site.

Solids can cause the seal head to stick to the shaft or restrict the o-ring flexibility. Take a look at the double seal arrangement, back to back version. Used on some services the O-ring could very quickly become clogged preventing the seal head from moving to accommodate wear of the faces.

Incorrect setting length at installation. You may never figure this one out. Just make sure that the fitting dimension is correct when installing the seal. Otherwise sometime in the future the seal will let go, usually after the pump is stopped, and the faces will look good but only partly worn. What has happened is that the spring pressure

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has reduced to the point where the seal leaks during idle periods. This can be difficult to spot, unless you know what to look for ... and when.

Fretting. Very small movements between components causes a polishing action. The polishing action removes the surface molecules. On pump shafts made of stainless materials the surface of the metal consists of chromium oxide. Elastomers moving very slightly against this surface wipe away the oxide which immediately reforms. The oxide is carried into the wiping surface changing its character completely. A rubber ring coated with chromium oxide becomes more efficient as a polishing, grinding surface and removes material at a faster rate. A "fret" ring is characterized by a polish mark on the shaft surface at the point where the seal elastomer seals against the shaft. If worn badly enough the fret ring can cause a new seal to fail on installation because the elastomer cannot seal effectively due to the damage on the surface.

Distortion of the stationary face. This is not common but the stationary could be badly fitted leading to over tightening, especially the silicon carbide grades which are designed with a lip to be clamped in the seal plate. Failure under these circumstances may be confused with cracking due to heat checking of the component. S.C grades of 99.9% only heat check if they are tightened un-evenly, so check out your grade and suspect poor fitting if its a high grade material failing by cracking. With other materials such as tungsten carbide, or plated surfaces, such as stellite, consider the distorting effect of poor clamping if no other solution presents itself.

Face Mis-centering or run-off. This is not common and is easy to diagnose. The faces are not concentric and the rotating head comes off the stationary track and picks up dirt. Scoring of the stationary and an off center running track gives you all you need to know.

Incorrect grade of O-ring material. Lots of things happen to elastomers so check out the ones on your seal, are they swollen, hard, squashed, shiny, cracking?

The seal hits something, it is prevented from moving to accommodate runout.

o Lots of possibilities here, so I list a few.

1. The shaft is bent and hitting the stationary face. You will notice this pretty quick, but bear in mind that the running clearance of the seal components and the shaft may be quite tight, so a small shaft displacement may not be obvious, the seal will show you what is happening.

2. Solids in the seal chamber hitting the seal. 3. Incorrectly fitted gasket extending into the seal chamber. Split

casing pumps can suffer this problem. 4. The shaft is not concentric with the seal chamber. 5. Insufficient clearance in the seal chamber. Check this out if you

are changing seal type or intend using different materials to cope with other problems.

6. A seal box recirc line is directed at the seal faces. Most seal chambers have a radial flow insert when most seal manufacturer's will tell you that a tangential flow insert is safer and causes less disturbance to the seal faces.

Page 16: Centrifugal Pump

Heat Causes Sea l Fa i lures .

Heat affects the elastomer. This the part most sensitive to extremes of temperature.

Heat can change the state of the fluid being pumped. Raising the temperature of corrosive liquids increases their potency.

A 16 deg F rise doubles the corrosion rate of most acids. Differential expansion rates can destroy plated seal surfaces. Low

grade silicon carbide will crack with sudden changes in temperature. Differential expansion of shaft and pump casing can change the face

loading by altering the fitting dimension.

We now have the over-view of heat related failures so let us look in more detail at what is happening.

E las tomers .

A wide range of elastomers are in use and many of them are rubber compounds. Teflon materials have a predetermined heat range of up to 226 deg C beyond which Teflon breaks down and burns making small amounts of phosgene gas. Teflon should not be used in temperatures close to its ultimate limit because it is a heat insulator and local heat production may cause it to reach its ultimate temperature.

Rubber compounds are made by baking the material until it is cured to a predetermined hardness or durometer. The various materials formed in this way, nitrile, viton, buna-n, and others, are commonly found in sealing applications. Less common is Kalrez a specialized compound with a high resistance to chemical attack. Formed in a heat setting process, these materials continue to be affected by the heat applied during the life of the seal. At temperatures beyond the range of the rubber seal the material continues to harden. As it hardens the shape of the seal takes on the shape of the groove if an O-ring or splits appear in rubber bellows as flexibility is lost. O-rings take on a "compression" set and appear oval and feel hard to the touch. O-rings are manufactured with a 10% tolerance oversize to allow for some thermo-setting in service. At higher temperatures the elastomer life to full compression set will depend upon the temperature and time at this temperature. The point for you is that exceeding the range of the rubber parts of your seal will shorten the working life of the seal and you need to bear this in mind.

An odd case, in Saudi Arabia I was called to a refinery that had been under construction for several years and pumps had been installed, but not run, for varying periods. Pumps under going test runs were leaking along the shafts. Investigation showed that over time in ambient temperatures of 55 deg C the seal elastomers had baked hard and vulcanized to the metal parts. All seals had to be changed.

Heat is generated from the friction running at the seal faces. Depending upon the type of face material and the seal box environment a rise of around 25 deg C above the seal fluid temperature can occur. Look at your seal types, where is the elastomer in relation to the seal faces. The nearer the elastomer is placed to the running faces the greater the additional heat

Page 17: Centrifugal Pump

it will experience. The use of low friction seal face combinations will reduce this effect. The carbon / ceramic combination has the lowest friction rating with hard faces such as tungsten / tungsten faces the highest.

Unbalanced seals, because the face weight is varying with the system pressure, can experience greater rises in face generated heat creating damage to the elastomer.

Excessive heat producing a temperature rise of 55 Deg C on a Viton O-ring will reduce its useful life to less than 1000 hours running time. For a seal that is expected to run for one year that is an 88% reduction in useful life. An 82 deg C rise will reduce the life of the seal by 97%.

Loss of water to a cooling water jacket, loss of any cooling arrangements puts your seals at risk.

I was called to a split-casing boiler feed pump that was experiencing out-board seal failure. Normally I would expect more problems with the in-board (coupling end ) seal due to less opportunity to dissipate the heat soak along the shaft. Examination of regular temperature recordings made of the cooling water system and seal box temperatures revealed that the out-board seal was being starved of cooling water flow. Dismantling the orifice plate controlling the flow to the in-board seal showed excessive wear enlarging the orifice and allowing through a larger proportion of the flow. Replacing the orifice plate solved the problem. All can seem well with your equipment but the seals will always let you know first when problems are arising.

Chang ing s ta te o f the f l u id

Liquid gases and other volatile fluids can vaporize and freeze water out of the air on the outside of the seal restricting movement. Shortly before I took up my post in Saudi Arabia a liquid propane pump blew its seal open due to a build up of ice around the seal faces. Liquid released into the atmosphere created a vast cloud of highly flammable gas. Fortunately no one was hurt and no explosion occurred but it was a close thing. It was thought appropriate to fit a double seal with a barrier fluid for future installations.

Liquids changing state to a gas experience enormous volume increases. Water increases in volume by 1700 times, so a small drop vaporizing across a seal face will explosively blow apart the faces. Boiler feed pumps and other hot water pumps can be heard "popping" or "puffing" if the seals are not working correctly. As the water droplets expand and open the seal faces more water rushes in to cool the area, collapsing the steam bubble and causing the faces to snap shut. Another small droplet penetrating the faces vaporizes and causes the faces to open again. Water treatment crystals, entrained oxides, other dirt particles are trapped between the faces as they close. Your seal is on its way to the scrap yard.

Some fluids crystallize with additional heat. Sea-water, brine, and similar fluids leaking past your seal and drying out around the seal plate can build

Page 18: Centrifugal Pump

up to affect the seal head and prevent it from moving. Crystals can also score the running surfaces of the seal causing damage leading to failure.

Hydrocarbons form coke as they partially burn or vaporize. Coking causes a hard solid to form around the seal effectively stopping it from moving freely. A similar effect is seen in food plants handling product containing sugar. Sugar escaping across a seal face can crystallize, or simply burn and coke. The signs are un-mistakable on the seal face.

Heat can cause impurities to come out of solution and plate onto seal surfaces, building up hard films or lacquers.

Heat can des t roy sea l f aces .

I have mentioned some of these effects but I think a defined list will help you.

Plated materials can experience differential expansion. Often materials such as stellite are plated over stainless steel. The expansion rates are poorly matched so operating outside of the design limits of the materials will cause strains to appear in the plating interface, causing cracks to appear. The cracks will cause the carbon face to wear dramatically fast.

The less expensive ceramic material (85%) will crack if cold shocked. Sudden changes in temperature of 38 deg C or more will destroy the seal face. The higher quality ceramic (99.9%) will cold shock if it is under distorting stress, properly fitted and evenly clamped it will survive sudden changes in temperature. Get to know which materials are being fitted into your seal installations.

Carbon rings using fillers and fitted into high temperature pumps can have the filler material melt out of the carbon causing them to become porous

Poor carbons with voids can blister and pit as the trapped air or gases expand and blows pieces off the carbon surface.

Lapped seal faces can distort, going out of flat. The effect of touching the lapped surface with a finger is to coat the surface with dirt and skin oils but also to distort the surface away from flat by the application of heat from your hand. Distorted seal faces leak.

Heat increases the corros iveness of most corros ive mater ia ls

The carbon part of the seal will show signs of being attacked. O-ring grooves can be damaged limiting their ability to seal

effectively. O-rings can become hard or start to crack, or become swollen and

excessively soft. Metal surfaces can be attacked and appear pitted which will

prejudice the seals ability to work properly.

Page 19: Centrifugal Pump

Springs and other highly stressed parts can fail due to increased corrosion.

Expans ion due to hea t ing e f fec t s .

All metals expand when heated. A stainless steel shaft 48" long by 4" dia will grow 0.138" in length when heated through 300 deg F. The working limit of most carbon seal faces is 0.125" . Seal compression is set at about 0.064" to produce the spring face weight. A seal mounted on a shaft moving by 0.138" with other expansion effects happening to the pump casing is in danger of opening. Apart from ensuring the accurate placing of the seal on the pump shaft there is little to be done to compensate for such movement. Tell-tale signs of inaccurate setting of the seal will be where you need to be looking.

The shaft diameter will expand too, by about 0.010". The seal material will expand also but under extreme circumstances this expansion can cause the seal to hang-up on the shaft. Over-compression of the elastomers will limit their effectiveness, as well as the other effects mentioned earlier.

Mater ia l Fa i l u re .

Failure of materials is usually a sign of a mis-match of material to environment. The substantial construction of seals excludes major failure of some main component, so we concentrate on the effects of environmental attack on sensitive components.

Chemical attack on the elastomer will cause it to swell. The carbon will appear pitted. Acid attack on carbon is directed

against the impurities. The reaction of the impurities to the acid solution cause holes and pits to form, weakening the structure and producing a porous carbon. A higher grade of carbon is required.

The springs can break. Stainless steel is known to fail due to chloride stress corrosion. Many single coil spring driven seals fail because the spring breaks. They are usually in-expensive and over-engineered, but they still fail.

Metals corrode. In seals where metal parts are designed to be thin due to flexibility requirements, metal bellows seals, welding techniques used in construction and material compatibility with mating components and pumped fluids are factors that affect the life of a seal.

Set screws clamping onto a hardened shaft material will not grip properly, allowing the seal body to slip, leading to a range of other effects, but ultimately to a seal failure.

Plated seal faces are not corrosion resistant, so the plating material can be removed from the surface.

This list is not exhaustive however comprehensive it may appear. You will find some new problem and when you do I want to hear all about it. So do all the other guys visiting this site. Look forward to hearing from you, I just know I will in time!

Page 20: Centrifugal Pump

G e n e r a l

Pump Problems can be either caused by:

1. Mechanical Problem with the Pump

or

2. Pump System Problem

Truth

The Great Majority of Pump Problems are with the Pump System

The Majority of Pump System Problems are on the Suction Side

Pump Problems are usually associated with Noisy Operation

Mechanical Problem

To determine if this noise is a mechanical problem with the pump drain it, close both suction and discharge valves and run the pump briefly.

If the noise continues you have a mechanical problem.

The mechanical noise can be caused by:

1. Debris in the Impeller

2. Impeller Rubbing

3. Impeller out of Balance

4. Bent or Twisted shaft

5. Bad bearings

6. Coupling Misalignment

7. V-Belt sheave Misalignment

8. Pipe Stress

 

Page 21: Centrifugal Pump

In any case the problem can be corrected by taking it apart and simply fixing it by either replacing the damaged parts or correcting the installation.

Items Required:

1. Pump Installation Manual

2. Pump Operation Manual

3. Pump Maintenance Manual

4. Parts List

System Problem

However if the Noise goes away after draining the pump etc. then the noise is caused by the Pumping System.

Pump System Noise is usually caused by:

1. Cavitation

2. Vortexing

Tools Required

In order to troubleshoot a pump system problem the following tools are required:

1. Combination Vacuum/Pressure Gauge - To Check Pump Suction Reading

2. Pressure Gauge - To Check Pump Discharge Pressure Reading

3. Amp Meter - To Check Horsepower Load

4. Tachometer - To Check Pump Speed

5. Pump Performance Curve - To Check all Readings Against the Expected Pump Performance

C a v i t a t i o n

Suction Cavitation

Suction Cavitation occurs when the Net Positive Suction Head Available to the pump is less than what is Required ------------ NPSHA < NPSHR.

Page 22: Centrifugal Pump

Symptoms

1. The pump sounds like it is pumping rocks!

2. High Vacuum reading on suction line

3. Low discharge pressure/High flow

Causes

1. Clogged suction pipe

2. Suction line too long

3. Suction line diameter too small

4. Suction lift too high

5. Valve on Suction Line only partially open

Remedies

1. Remove debris from suction line

2. Move pump closer to source tank/sump

3. Increase suction line diameter

4. Decrease suction lift requirement

5. Install larger pump running slower which will decrease the Net Positive Suction Head Required by the pump(NPSHR)

6. Increase discharge pressure

7. Fully open Suction line valve

 

Discharge Cavitation

Discharge Cavitation occurs when the pump discharge head is too high where the pump runs at or near shutoff.

Page 23: Centrifugal Pump

Symptoms

1. The pump sounds like it is pumping rocks!

2. High Discharge Gauge reading

3. Low flow

Causes

1. Clogged discharge pipe

2. Discharge line too long

3. Discharge line diameter too small

4. Discharge static head too high

5. Discharge line valve only partially open

Remedies

1. Remove debris from discharge line

2. Decrease discharge line length

3. Increase discharge line diameter

4. Decrease discharge static head requirement

5. Install larger pump which will maintain the required flow without discharge cavitating

6. Fully open discharge line valve

Vo r t e x i n g

Vortexing is caused by insufficient surface tension on the liquid

Symptoms

1. Pump makes a growling sound

2. Whirlpool usually visible on liquid surface

3. Loss of Flow

Causes

Page 24: Centrifugal Pump

1. There is not enough liquid height above the suction line entrance

2. The velocity at the suction line entrance is too high

Remedy

Decrease liquid velocity and/or increase submergence in accordance with the following table

 

Submergence can be increased by resetting the pump shutoff   level higher in the sump or tank.

Velocity can be decreased by enlarging the suction bottom opening by installing a suction bell.

Atmospheric

Atmospheric Pressure/Elevation Chart

Altitude Above Barometric Equivalent Equivalent

Page 25: Centrifugal Pump

Sea Level

Feet

Reading

Inches of Mg

Head

Feet

Pressure

PSI

0 29.92 33.96 14.7

1000 28.86 32.76 14.18

2000 27.82 31.58 13.67

3000 26.81 30.43 13.17

4000 25.84 29.33 12.69

5000 24.89 28.25 12.22

6000 23.98 27.22 11.78

7000 23.09 26.21 11.34

8000 22.22 25.22 10.91

Va p o r C h a r t

Water Vapor Pressure Chart

Temperature Vapor Pressure

F C PSI FT

40 4.4 .1217 .281

50 10 .1781 .4115

60 15.6 .2563 .592

70 21.1 .3631 .815

80 26.7 .5069 1.17

90 32.2 .6982 1.612

100 37.8 .9492 2.191

110 43.3 1.275 2.942

120 48.9 1.692 3.91

130 54.4 2.223 5.145

140 60 2.889 6.675

150 65.6 3.718 8.56

160 71.1 4.741 10.95

170 76.7 5.992 13.84

180 82.2 7.510 17.35

190 87.8 9.339 21.55

200 93.3 11.50 26.65

212 100 14.70 33.96

 

F i t t i n g C h a r t

Fitting Losses

Page 26: Centrifugal Pump

Equivalent Length of Pipe in Feet

 

Pipe

Diameter

Valves

Gate Plug Globe Angle Check Foot

1.5" 0.9 - 45 23 11 39

2" 1.10 6.0 58 29 14 47

3" 1.6 8.0 86 43 20 64

4" 2.1 17 113 57 26 71

6" 3.2 65 170 85 39 77

Pipe

Diameter

ElbowsTube

TurnTee Enlrg Contr

45 90 45 90 Strt Side 1:2 3:4 2:1 4:3

1.5" 1.9 4.1 1.4 2.3 2.7 8.1 2.6 1.0 1.5 1.0

2" 2.4 5.2 1.9 3.0 3.5 10.4 3.2 1.2 1.8 1.2

3" 3.6 7.7 2.9 4.5 5.2 15.5 4.7 1.7 2.8 1.7

4" 4.7 10.2 3.8 6.0 6.8 20.3 6.2 2.3 3.6 2.3

6" 7.1 15.3 5.8 9.0 10.2 31 9.5 3.4 5.6 3.4

F r i c t i o n C h a r t

Friction Drop Chart

Friction Loss of Water in Feet per 100 Feet of Pipe

U.S.

GPM

1"Pipe 2"Pipe 3"Pipe 4"Pipe 5"Pipe 6"Pipe

Vel Loss

Vel Loss

Vel Loss

Vel Loss

Vel Loss

Vel Loss

10 3.72 11.7 1.02 0.50 0.45 0.07 - - - - - -

20 7.44 42.0 2.04 1.82 0.91 0.25 0.51 0.06 - - - -

30 11.15 89.0 3.06 3.84 1.36 0.54 0.77 0.13 0.49 0.04 - -

40 14.88 152 4.08 6.60 1.82 0.91 1.02 0.22 0.65 0.08 - -

50 - - 5.11 9.90 2.27 1.36 1.28 0.34 0.82 0.11 0.57 0.04

60 - - 6.13 13.9 2.72 1.92 1.53 0.47 0.98 0.16 0.68 0.06

70 - - 7.15 18.4 3.18 2.57 1.79 0.63 1.14 0.21 0.79 0.08

80 - - 8.17 23.7 3.65 3.28 2.04 0.81 1.31 0.27 0.91 0.11

90 - - 9.19 29.4 4.09 4.06 2.30 1.00 1.47 0.34 1.02 0.14

100 - - 10.2 35.8 4.54 4.96 2.55 1.22 1.63 0.41 1.13 0.17

110 - - 11.3 42.9 5.00 6.00 2.81 1.46 1.79 0.49 1.25 0.21

120 - - 12.3 50.0 5.45 7.00 3.06 1.72 1.96 0.58 1.36 0.24

130 - - 13.3 58.0 5.91 8.10 3.31 1.97 2.12 0.67 1.47 0.27

Page 27: Centrifugal Pump

140 - - 14.3 67.0 6.35 9.20 3.57 2.28 2.29 0.76 1.59 0.32

150 - - 15.3 76.0 6.82 10.5 3.82 2.62 2.45 0.88 1.70 0.36

Valves and Fittings

 

Page 28: Centrifugal Pump
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Vi s c o s i t y P i p e C h a r t

Pipe Pressure Drop Chart

Page 30: Centrifugal Pump