Pump Lose Suction After Some Time

7/30/2019 Pump Lose Suction After Some Time

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SUBJECT: The pump works for a while and then loses suction

A couple of things you must keep in mind when troubleshooting centrifugal pump problems:

The centrifugal pump always pumps the difference between the suction and discharge

heads. If the suction head increases, the pump head will decrease to meet the systemrequirements. If the suction head decreases the pump head will increase to meet thesystem requirements.

A centrifugal pump always pumps a combination of head and capacity. These twonumbers multiplied together must remain a constant. In other words, if the head increasesthe capacity must decrease. Likewise if the head decreases, the capacity must increase.

The pump will pump where the pump curve intersects the system curve. If the pump is not meeting the system curve requirements the problem could be in the

pump, the suction side including the piping and source tank, or somewhere in thedischarge system.

Most pumps are oversized because of safety factors that were added at the time the pump

was sized. This means that throttling is a normal condition in most plants, causing the pump to run on the left hand side of its curve.

Cavitation is a main cause of losing pump suction, but remember that there are several differenttypes of cavitation:

Vaporization of the liquid within the pump caused by a loss of suction head or anincrease in suction temperature.

The "vane passing syndrome" caused by too small an impeller to cutwater clearance. Too high a suction specific speed number will cause internal recirculation problems

resulting in cavitation.

Air ingestion on the suction side of the pump allows air and bubbles into the suction of the pump. Turbulence of the fluid that releases entrained gases into the suction piping.

Each of these cavitations has been addressed in other papers in this site In this paper we will belooking at only the intermittent loss of suction fluid. You will be looking at several possibilities:

A recurring restriction in the suction piping that may or may not be causing a cavitation problem within the pump.

Intermittent cavitation problems as opposed to a design or operation problem that causesa constant cavitation condition.

A repetitive need for an increase in the pump's capacity.

Now we will take a look at each of these possibilities in detail:

A re-occurring restriction in the suction piping that may or may not be causing a cavitation problem within the pump.

A foot valve or any valve in the suction piping is sticking.


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Something is occasionally plugging up the suction piping. If the pump suction is comingfrom a river, pond or the ocean, grass is a strong possibility.

A loose rag is another common cause. A collapsed pipe liner will restrict the piping at higher velocities. The suction is being throttled to prevent heating of the process fluid. This can happen

with some volatile fuel applications. A filter or strainer is gradually clogging up. Air is being introduced into the suction side of the pump to reduce the capacity. This is

sometimes done with low specific gravity fluids to avoid throttling the discharge thatmight overheat and flash the product.

Intermittent incidents that cause cavitation problems

The tank vent partially freezes in cold weather. The sun is heating the suction piping, raising the product temperature close to its vapor

point.

The level in the suction tank increases, decreasing the differential head across the pump.This will increase the pump capacity until the level in the tank drops. The level in the open suction tank decreases causing vortex problems that allow air into

the pump suction. Several pumps in the same sump are running, decreasing the level too much. The suction tank float is stuck. It will sometimes show a higher level than you really

have. A discharge recirculation line, piped to the pump suction, opens and heats the incoming

liquid. Sometimes the suction lift is too high. The increase in pipe friction will reduce the

suction head. The vapor pressure of the product is very close to atmospheric pressure. The pump

cavitates every time it rains because of a drop in atmospheric pressure. The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much. The process fluid specific gravity is changing. This can happen with a change in product

operating temperature, or if a cleaner or solvent is being flushed through the lines. A booster pump is malfunctioning or leaking excessively. The source tank is changing from a positive pressure to a vacuum due to the process. A packed valve in the suction piping is at a negative pressure and air is leaking in through

the packing. The tank is being pumped dry.

A repetitive need for an increase in the pumps capacity.

A bypass line, or relief valve opens, decreasing the discharge resistance, increasing thecapacity.

A break or leak in the line down stream of the pump will increase the capacity of the pump as the head drops.

The pump is supplying many sources and too many valves are open at one time.


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The pump discharge is being directed to several different tank farm locations. Thechanging piping resistance is changing the pump's head and capacity.

Operation practices that cause frequent seal and bearing maintenance problems 6-7

Wouldn't it be wonderful if the plant operation and maintenance departments could work independently? The fact of the matter is that there are three types of problems we encounter withcentrifugal pumps and poor operation is only one of them. If you're curious, the other twoare design problems and poor maintenance practices.

Seals and bearings account for over eighty-five percent (85%) of premature centrifugal pumpfailure. In the following paragraphs we will be looking at only those operation practices that can,and will cause premature seal and bearing failure. Design and maintenance practices will bediscussed in other papers in this series.

When pumps were supplied with jam packing, the soft packing stabilized the shaft to prevent too

much deflection. In an effort to save flushing water and to conserve power, many of these same pumps have since been converted to a mechanical seal and the radial stabilization the packing provided has been lost.

The bad operating practices include:

Running the pump dry will cause over-heating and excessive vibration problems that will shortenseal life. Here are some of the common reasons why a pump is run dry:

Failing to vent the pump prior to start-up. Running the tank dry at the end of the operation cycle.

Emptying the tank for steaming or introduction of the next product. Running on the steam that is being used to flush the tank. Starting the standby pump without venting it. Venting a hazardous product can cause a

lot of problems with the liquid disposal. Many operators have stopped venting for thatreason.

Tank vents sometimes freeze during cold weather. This will cause a vacuum in thesuction tank, and in some cases could collapse the tank.

Sump fluids are often dirty, corrosive or both. The control rods for the float switch willoften "gum up" or corrode and give a false reading to the operator. He may think thatthere is an adequate level, when in fact, the tank is empty.

Dead heading the pump can cause severe shaft deflection as the pump moves off of its bestefficiency point (bep.). This translates to excessive heat that will affect both the seal and the bearings as well as causing the seal faces to open, and the possibility of the impeller contactingthe volute when the shaft deflects.

Starting the centrifugal pump with a shut discharge valve is standard practice with manyoperation departments. The concern is to save power without realizing the damage that is

being done to the mechanical seal, impeller, wear rings and bearings.
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Some pumps are equipped with a recirculation valve that must be opened to lessen the problem, but many times the valve is not opened, or the bypass line is clogged or not of the correct diameter to prevent the excessive head. Another point to remember is that if the bypass line is discharged to the suction side of the pump the increased temperaturecan cause cavitation.

After a system has been blocked out the pump is started with one or more valves notopened.

Discharge valves are shut before the pump has been stopped.

Operating off of the best efficiency point (bep.) causes shaft deflection that can fail themechanical seal and over-load the bearings.

Starting the pump with the discharge valve closed to save power. The level in the suction tank is changing. Remember that the pump pumps the difference

between the discharge and suction heads. If the suction head varies, the pump moves to adifferent point on its curve.

Any upset in the system such as closing, throttling or opening a valve will cause the pump to move to a new point on the curve as the tank fills.

Pumping to the bottom of a storage tank will cause the pump to move to a different pointon the curve as the tank fills. Some systems were designed for a low capacity positivedisplacement pump and have since been converted to a centrifugal design because of aneed for higher capacity. Centrifugal pumps must discharge to the top of the tank to

prevent this problem. If the discharge piping is restricted because of product build up on the inside walls, the

pump will run throttled. This is one of the reasons that it is important to take periodicflow and amperage readings.

Increasing the flow will often cause cavitation problems.

Seal environmental controls are necessary to insure long mechanical seal life. It is important thatoperations understand their function and need because many times we find the controls installed,

but not functioning.

Cooling-heating jackets should show a differential temperature between the inlet andoutlet lines. If the jacket clogs up, this differential will be lost and seal failure will shortlyfollow.

Barrier fluid is circulated between two mechanical seals. There may or may not be adifferential temperature depending upon the flow rate. If a convection tank is installed,there should be a temperature differential between the inlet and outlet lines. The linecoming out of the top of the seal to the side of the tank should be warmer than the linefrom the bottom of the tank to the bottom of the seals, otherwise the system is running

backwards and may fail completely. The level in the tank is also critical. It should beabove the tank inlet line or no convection will occur. Some convection tanks are

pressurized with a gas of some type. Many original equipment (O.E.M.) seal designs willfail if this differential pressure is lost.

Some seal glands (A.P.I. type) are equipped with a quench connection that looks like theseal is leaking water or steam. If there is too much steam pressure on this quench


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connection, the excessive leakage will get into the bearings causing premature failure.The steam is often used to keep the product warm to prevent it from solidifying,crystallizing, getting too viscous, building a film on the faces etc. Operating peoplefrequently shut off the quench to stop the condensate from leaking.

Flushing fluids are used for a variety of purposes, but most of the time they are used to

get rid of unwanted solids. The flush can be closely controlled with a flow meter or throttling valve. The amount of flush is determined by the seal design. As an example,those designs that have springs in the product require more flush.

It is important to check that the stuffing box has been vented in vertical pumps. The ventshould be coming out of the seal gland and not the stuffing box lantern ring connection.

There are some additional things that all operators should know to insure longer rotatingequipment life. As an example :

Mechanical seals have an 85% or more failure rate that is normally correctable. This iscausing unnecessary down time and excessive operating expense. Seals should run until

the sacrificial carbon face is worn away, but in more that 85% of the cases the seal fails before this happens. There are five different causes of cavitation. You should know where the best efficiency point (bep.) is on a particular pump, and how

far it is safe to operate off the bep. with a mechanical seal installed. You should be aware that washing down the pump area with a water hose will cause

premature bearing failure when the water penetrates the bearing case. Learn about the affect of shaf t L3/D4 on pump operation. Know how the pumped product affects the life of the mechanical seal and why

environmental controls are necessary. If you are not using cartridge seals, adjusting the open impeller for efficiency will shorten

the seal life. In most cases the seal will open as the impeller is being adjusted to thevolute. Flowserve (Durco) pumps are the best example of the exception to this rule. The

popular Durco pumps adjust to the back plate causing a compression of the seal faces thatcan create mechanical seal "over heating" problems.

Cycling pumps for testing will often cause a mechanical seal failure unless anenvironmental control has been installed to prevent the failure.

Mechanical seals should be positioned after the impeller has been adjusted for thermalgrowth. This is important on any pump that is operated above 200F (100C) or you willexperience premature seal failure.

Some elastomers will be affected by steaming the system. A great deal of caution must beexercised if a flushing fluid such as caustic is going to be circulated through the lines or used to clean a tank. Both the elastomer and some seal faces (reaction bonded siliconecarbide is a good example) can be damaged. If the elastomer is attacked, the failureusually occurs within one week of the cleaning procedure.

The stuffing box must be vented on all vertical centrifugal pumps or otherwise air will betrapped at the seal faces that can cause premature failure of many seal designs.

Most original equipment seal designs cause shaft damage (fretting) necessitating the useof shaft sleeves that weaken the shaft and restrict pump operation to a narrow range at the

bep.
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Here are a few common misconceptions that cause friction between maintenance and operationdepartments

Shutting the pump discharge valve suddenly, will blow the seal open. All ceramics cold shock.

High head, low capacity consumes a lot of power. The pump must come into the shop to change a mechanical seal. If you use two hard faces or dual mechanical seals in slurry applications, you will not

need flushing water with its corresponding product dilution. If you use metal bellows seals for hot oil applications, you will not need the stuffing box

cooling jacket operating. It's okay to use an oversized impeller because throttling back will save power.

A few more thoughts on the subject

Operators should receive proper schooling on the trouble shooting and maintenance of

pumps. In the military and many modern plants, the operator and the maintenancemechanic are often the same person. If the operator knows how the pump works he willhave no trouble figuring out the solution to his problem. Too often he is told to keep theflow gage at a certain point, or between two values without understanding what isactually happening with the equipment. If the operator recognizes cavitation he can tellthe maintenance department and help them with their trouble shooting.

As you wander around the plant, look out for painters that paint the springs of outside anddouble mechanical seals. There is a trend to putting two seals in a pump for environmental reasons and the painting of springs is becoming a common problem.

If someone is adjusting the impeller make sure he is resetting the seal spring tension atthe same time.

If the pump is getting hot or making excessive noises, report it immediately. After thefailure, it does no good to tell maintenance that it was making noise for two weeks.

If you are the floor operator it is common knowledge that taking temperature and pressure readings is very boring, especially on those gages that are located in hot or awkward locations. Avoid the temptation to "radio" these readings. From hot to failure isa very short trip.

Maintenance's favorite expression is "There is never time to do it right, but there isalways time to fix it." Try to keep this in mind when the pressure is on to get theequipment running again.

Do not let cleaning people direct their "wash down" hoses directly at the pump. Water entering the bearings through the lip or grease seals is a major cause of premature bearingfailure. Most water wash downs are used to dilute and wash away seal leakage. Stop theleak and you have eliminated the reason for the hose.

o A great many motor and electrical problems are caused by these same wash downhoses.

Cooling a bearing outside diameter will cause it to shrink, and the bearing will get hotter as the radial load increases. Keep the water hose and all other forms of cooling off of the

bearing casing.
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The pump is not producing enough head to satisfy the application

Let me begin by pointing out that there are a couple of things you must keep in mind whentroubleshooting centrifugal pump problems:

The centrifugal pump always pumps the difference between the suction and dischargeheads.

A centrifugal pump always pumps a combination of head and capacity. These twonumbers multiplied together must remain a constant. In other words, if the head increasesthe capacity must decrease. Likewise if the head decreases, the capacity must increase.

The pump will pump where the pump curve intersects the system curve. If the pump is not meeting the system curve requirements the problem could be in the

pump, the suction side including the piping and source tank, or somewhere in thedischarge system.

Most pumps are oversized because of safety factors that were added at the time the pumpwas sized. This means that throttling is a normal condition in most plants, causing the

pump to run on the left hand side of its curve.

THE PROBLEM COULD BE IN THE PUMP ITS SELF

The impeller diameter is too small.o The impeller is running at too slow a speedo You are running an induction motor. Their speed is different than synchronous

motors. It's always slower. The pump curve was created using a variablefrequency motor that ran at a constant speed. Put a tachometer on your motor tosee its actual speed.

o Your pulley driven pump is running on the wrong pulley diameter.o

A variable frequency motor is running at the wrong speed.o Check the speed of the driver if the pump is driven by something other than anelectric motor.

There is something physically wrong with the motor. Check the bearings etc. Check the voltage of the electric motor. It may be too low. The impeller is damaged. The damage could be caused by excessive wear, erosion,

corrosion or some type of physical damage.o Physical damage often occurs during the assembly process when the impeller is

driven on or off the shaft with a wooden block and a mallet. Many impeller designs do not have a nut cast into the impeller hub to ease removal.

o Erosion occurs when solids enter the eye of the impeller. The solids can chip off pieces of the ceramic that are passivating the impeller, causing localizedcorrosion.

o Damage can occur if the impeller to volute, or back plate clearance is too smalland the shaft experiences some type of deflection. The original clearance couldhave diminished with thermal growth of the shaft. Keep in mind that some openimpellers adjust to the volute (Goulds) while other designs adjust to the back plate(Duriron).


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In an ANSI and similar design centrifugal pumps, the normal thrust towards the volutehas bent the snap ring designed for bearing retention. This can allow the rotating impeller to hit the stationary volute.

Here are some examples of shaft displacement:o Operating the pump too far off the BEP.o

Pulley driven applications.o Pipe strain.o Misalignment between the pump and driver.o The shaft could be bent.o The rotating assembly was probably not dynamically balanced.

The impeller is clogged. This is a major problem with closed impellers. With theexception of finished product, most of what you will be pumping contains entrainedsolids. Remember also that some products can solidify, or they can crystallize with achange in fluid temperature or pressure.

Impeller balance holes have been drilled between the eye and the wear rings of a closedimpeller. The reverse flow is interfering with the product entering the impeller eye. A

discharge recirculation line should have been used in place of the balance holes to reducethe axial thrust. The double volute casting is clogged with solids or solids have built up on the surface of

the casting. The open impeller to volute clearance is too large. 0.017" (0,5 mm) is typical. This

excessive clearance will cause internal recirculation problems. A bad installation, thermalgrowth, or normal impeller wear could be the cause.

o A large impeller to cutwater clearance can cause a problem called dischargerecirculation. Wear is a common symptom of this condition.

If the impeller is positioned too close to the cutwater you could have cavitation problemsthat will interfere with the head.

The impeller specific speed number is too high. Lower specific speed numberedimpellers are used to build higher heads.

An impeller inducer was left off at the time of assembly. Inducers are almost alwaysneeded with high specific speed impellers. Leaving off the inducer can cause cavitation

problems that will interfere with the head. The impeller is loose on the shaft. The impeller is running backwards The shaft is running backwards because of a wiring problem. The pump is running backwards because the discharge check valve is not holding and

system pressure is causing the reverse rotation. This is a common problem with pumpsinstalled in a parallel configuration. Check valves are notoriously unreliable.

The impeller has been installed backwards. This can happen with closed impellers ondouble ended pumps

The second stage of a two stage pump is wired backwards. The pump reverses when thesecond stage kicks in. You should have heard a loud noise when this happened.

The wear ring clearance is too large.o This is a common problem if the shaft L 3/D4 number is greater than 60 (2 in the

metric system).


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o You should replace the rings when the original clearance doubles. Needless to saythis can only be determined by inspection.

If you are pumping a product at 200F (100C) or more you should use a centerlinedesign volute to prevent excessive wear ring wear as the volute grows from the basestraight up, engaging the wear rings.

A wear ring is missing. It was probably left off during the installation process. A high suction tank level is reducing the differential pressure across the pump increasingits capacity. The pump pumps the difference between the suction and discharge heads.

A bubble is trapped in the eye of the impeller. The eye is the lowest pressure area. Whenthis bubble forms it shuts off all liquid coming into the pump suction. This could causethe pump to lose its prime.

You cannot vent a running pump because centrifugal force will throw the liquid out thevent leaving the air trapped inside.

Air is coming directly into the pump. This happens with a negative pressure at the suctionside. Negative suction happens when the pump is lifting liquid, pumping from acondenser hot well etc.

o Air is coming into the stuffing box through the pump packing.o Air is coming into the stuffing box through an unbalanced mechanical seal. As thecarbon face wears the spring load holding the faces together diminishes.

o If you are using mechanical seals in vacuum service, they should be of the O-ringdesign. Unlike other designs, o-rings are the only shape that seals both pressureand vacuum.

o The pump was not primed prior to start up. With the exception of the self primingversion, centrifugal pumps must be full of liquid at start up.

o Air can enter the stuffing box if the gasket between the two halves of a doubleended pump is defective or does not extend to the stuffing box face. Any smallgaps between the face of the stuffing box and the split at the side of the stuffing

box will allow either air in, or product out.o Air is coming into the suction side of the pump through a pin hole in the casing.o Air is entering the stuffing box between the sleeve and the shaft. This happens if

you convert a double ended pump from packing to a mechanical seal and fail toinstall a gasket or o-ring between the impeller hub and the sleeve.

The open impeller was adjusted backwards and now the close fitting "pump out vanes"are creating a vacuum in the stuffing box.

You need a volute casing instead of a concentric casing. Volute casings are much better for producing head.

You have the wrong size pump. It cannot meet the system curve requirements: The pump was not selected to meet the system curve requirements because no system

curve was given to the pump supplier. At replacement time the same size pump was purchased because no one had calculated

losses in the system. The pump was sized from a piping diagram that was thirty five years old. There have

been numerous piping changes and additions since the original layout. In many instancesadditional pumps have been installed and this pump is running in parallel with them, butnobody knows it.


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THE PROBLEM IS ON THE SUCTION SIDE OF THE PUMP. THE PUMP COULD BECAVITATING.

Air is entering the suction piping at some point.o Air is being pumped into the suction piping to reduce cavitation problemso

Fluid returning to the sump is being aerated by too far a free fall. The return lineshould terminate below the liquid level.o The fluid is vortexing at the pump inlet because the sump level is too low and the

pump capacity is too high.o Air is coming into the system through valves above the water line, or gaskets in

the piping flanges.o The liquid source is being pumped dry. If this is a problem in your application

you might want to consider a self priming pump in the future. The vapor pressure of the fluid is too close to atmospheric pressure. When it rains the

drop in atmospheric pressure causes the inlet fluid to vaporize. There is a problem with the piping layout. It is reducing the head on the suction side of

the pump.o There is too much piping between the pump suction and the source tank. You mayneed a booster pump or an inducer. The higher the pump speed the bigger the

problem.o There is an elbow too close to the pump suction. There should be at least ten

diameters of pipe between the elbow and the pump suction. Suction piping shouldnever run parallel with the pump shaft in a double ended pump installation. Thiscan cause unnecessary shaft thrusting.

o A piece of pipe of reduced diameter has been installed in the suction piping.o Piping was added on the inlet side of the pump to by-pass a piece of equipment

that was installed on the floor.o A piping to pump reducer has been installed upside down causing an air pocket.

Concentric reducers can cause the same problem..o Multiple pump inlets are too close together.

The pump inlet is too close to the tank floor. The suction lift is too high. A gasket with too small an inside diameter has been installed in the suction piping

restricting the liquid flow. A gasket in the suction piping is not centered and is protruding into the product stream. A globe valve has been substituted for a gate valve in the suction piping. The loss of head

in a globe valve is many times that of a gate valve. Two pumps are connected in series. The first pump is not sending enough capacity to the

second pump. The piping inlet is clogged. A filter or strainer is clogged or covered with something. Intermittent plugging of the suction inlet.

o Loose rags can do this.o If the suction is from a pond, river, or the sea, grass can be pulled into the suction

inlet. A foot valve is stuck.


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A check valve is stuck partially closed The foot valve is too small. A small clam or marine animal cleared the suction screen, but has now grown large on

the pump side of the screen. The suction piping diameter has been reduced.

o The suction piping collapsed when a heavy object either hit or ran over the piping.

o Solids have built up on the piping walls. Hard water is a good example of this problem

o A liner has broken away from the piping wall and has collapsed in the piping.Look for corrosion in the piping caused by a hole in the liner.

o A foreign object is stuck in the piping It was left there when the piping wasrepaired.

o The suction is being throttled to prevent the heating of the process fluid. This is acommon operating procedure with fuel pumps where discharge throttling couldcause a fire or explosion.

The pump inlet temperature is too high.o

The tank is being heated to deaerate the fluid, but it is heating the fluid up toomuch. Look for this problem in boiler feed pump applications.o The sun is heating the inlet piping. The piping should be insulated to prevent this

problem.o The operating temperature of the pumped fluid has been increased to

accommodate the process requirements.o A discharge recirculation line is heating the incoming fluid. You should direct this

line to a reservoir rather than the pump suction.o Steam or some other hot cleaner is being circulated through the lines.

The problem is in the tank connected to the suction of the pump.o The pump capacity is too high for the tank volume.o The tank float is stuck, showing a higher tank level that does not exist.o The tank vent is partially shut or frozen, lowering the suction pressure.o There is not enough NPSH available for the fluid you are pumping. Maybe you

can use an inducer or booster pump to increase the suction pressure.o A high suction tank level is reducing the differential pressure across the pump,

increasing its capacity and lowering the head.

PROBLEMS ON THE DISCHARGE SIDE OF THE PUMP INCLUDING THE PIPING

Two pumps are in connected in series. The first pump does not have enough capacity for the second pump. They should be running at the same speed with the same widthimpeller.

The pump discharge is connected to the bottom of the tank. The head is low until thelevel in the tank increases.

Units in the discharge piping should not normally be shut off, they should be by-passed to prevent too much of a change in the pump's capacity.

If too many units are being by-passed in the discharge system the head will decrease asthe capacity increases. This can happen if an extra storage tank farm is being by-passed

because the storage capacity is no longer needed.


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A bypass line has been installed in the pump discharge increasing the capacity andlowering the head.

Piping or fittings have been removed from the discharge side of the pump reducing piping resistance.

Connections have been installed in the discharge piping that have increased the demand

that increases capacity. The pump is acting as an accumulator, coming on when the tank level drops. The headwill be low until the accumulator is recharged.

Consider the possibility of a siphon affect in the discharge piping. This will occur if the pump discharge piping is entering into the top of a tank and discharging at a lower levelThe pump must build enough head initially to take advantage of the siphoning action.

A discharge valve (manual or automatic) is opened too much.

Documents

Pump Lose Suction After Some Time