The centrifugal pump is not producing enough capacity

THE CENTRIFUGAL PUMP IS NOT PRODUCING ENOUGH CAPACITY PT003

The low capacity problem could be in the pump its self

  • The impeller diameter is too small.
  • The impeller width is too narrow.
  • For high capacity you will need a double-ended design with a very wide impeller or maybe two pumps running in parallel.
  • The impeller is running at too slow a speed
    • You are running an induction motor. Induction motor speed is different than synchronous motors; it’s always slower. The pump curve was created using a variable frequency motor that ran at a constant speed. Put a tachometer on your motor to record its actual speed.
    • Your pulley driven pump is running on the wrong pulley diameter.
    • A variable frequency motor is running at the wrong frequency.
    • Check the speed of the driver if the pump is driven by something other than an electric motor. A governor could be set incorrectly.
    • 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.
    • 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.
    • Erosion occurs when solids enter the eye of the impeller. The solids can chip off pieces of the oxide (ceramic) that are passivating the impeller, causing localized corrosion.
    • Damage can occur if the impeller to volute, or back plate clearance is too small and the shaft experiences some type of deflection. The original clearance could have diminished with thermal growth of the shaft. Keep in mind that some open impellers adjust to the volute (Goulds) while other designs adjust to the back plate (Duriron).
    • In an ANSI, and similar design centrifugal pumps the normal thrust towards the volute has bent the snap ring designed for bearing retention. This can allow the rotating impeller to move axially and contact the stationary volute slowing down the impeller.
  • There are a variety of reasons why a pump shaft will deflect from the centerline of the pump or move axially an excessive amount. See shaft deflection.
  • The impeller is clogged. This is a major problem with closed impellers, but it happens with open and semi-open impellers also. With the exception of finished product most of what you will be pumping contains entrained solids. Remember that some products can solidify or they can crystallize with a change in fluid temperature or pressure.
  • Impeller balance holes may have been drilled between the eye and the wear rings of a closed impeller. The resultant 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 reduce the 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. 0.015″ to 0.020″ (0,5 mm) is a typical proper clearance. This excessive clearance will cause internal recirculation problems. A bad installation, thermal growth, or normal wear could be the cause of this excessive clearance.
  • A large impeller to cutwater clearance can cause a problem called “discharge recirculation”. Excessive impeller wear is a common symptom of this condition.
  • If the impeller is positioned too close to the cutwater you could have a cavitation problem that will interfere with the final head. See “vane passing syndrome cavitation”.
  • The impeller specific speed number is too low. Lower specific speed impellers are used to build higher heads.
  • An impeller inducer was left off at the time of assembly. Inducers are almost always needed with high specific speed impellers. Leaving off the inducer can cause cavitation problems that will interfere with the capacity.
  • The impeller is loose on the shaft.
  • The wear ring clearance is too large. This can happen if the shaft L3/D4 number is greater than 60 (2 in the metric system). Excessive shaft deflection will erode the wear rings so you should replace them when the original clearance doubles. Needless to say this can only be determined by inspection.
  • If you are pumping a product at 200°F (100°C) or more; you should use a centerline design volute to prevent excessive wear ring wear as the volute grows from the base straight up, engaging the wear rings.
  • The impeller is running backwards. You will notice a slight reduction in head and a larger reduction in capacity.
    • 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 pumps installed in a parallel configuration. Check valves are notoriously unreliable.
    • The impeller has been installed backwards. This can happen with closed impellers on double ended pumps
    • The second stage of a two-stage pump is wired backwards. The pump reverses when the second stage kicks in. You should have heard a loud noise when this happened.
  • A wear ring is missing. It was probably left off during the installation process.
  • The wear ring clearance is too large.
    • This is a problem if the shaft L3/Dnumber is greater than 60 (2 in the metric system). Excessive shaft deflection will erode the wear rings and increase their clearance causing possible internal re-circulation problems.
    • You should replace the wear rings when the original clearance doubles. Needless to say this can only be determined by inspection.
  • A low suction tank level is increasing the differential pressure across the pump decreasing its capacity. The pump always 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. When this bubble forms it shuts off all liquid coming into the pump suction. This could cause the pump to lose its prime.
  • You cannot vent a running pump because centrifugal force will throw the liquid out the vent leaving the air trapped inside.
  • Air is coming directly into the pump. Negative suction happens when the pump is lifting liquid, pumping from a condenser hot well etc.
  • Air is coming into the stuffing box through the pump packing. This is a real problem if the pump has been fitted with a repeller to lower stuffing box pressure.
  • Air is coming into the stuffing box through an unbalanced mechanical seal. As the carbon face wears the spring load holding the faces together diminishes.
  • If you are using mechanical seals in vacuum service they should be of the O-ring design. Unlike other designs, O-rings are the only shape that seals both pressure and vacuum.
  • The pump was not primed prior to start up. With the exception of the self-priming version, centrifugal pumps must be full of liquid at start up.
  • Air can enter the stuffing box if the gasket between the two halves of a double-ended pump is defective or does not extend to the stuffing box face. Any small gaps 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. Bolt on stuffing boxes can have the same problem.
  • Air is coming into the suction side of the pump through a pinhole in the casing. Pumps are manufactured from castings and some of them are porous.
  • Air is entering the stuffing box between the sleeve and the shaft. This can happen if you convert a double-ended pump from packing to a mechanical seal and fail to install 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. Watch out for this problem if the mechanics are familiar with Duriron brand pumps and are now servicing another brand.
  • You need a concentric casing instead of a volute casing. Concentric casings are much better for producing capacity.
  • 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.
  • The pump was specified for a different application. It was in the plant inventory and someone is trying to use it in this application. It is not unusual for a company to purchase someone’s excessive inventory.
  • 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 instances additional pumps have been installed and this pump is running in parallel with them, but nobody knows it.

The problem is on the suction side of the pump; the pump could be cavitating.

  • Air is entering the suction piping at some point.
  • Air is being pumped into the suction piping to reduce cavitation problems
  • Fluid returning to the sump is being aerated by too far a free fall. The return line should terminate below the liquid level.
  • The fluid is vortexing at the pump inlet because the sump level is too low and the pump capacity is too high.
  • Air is coming into the system through valves above the water line or gaskets in the piping flanges.
  • 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.
  • There is too much piping between the pump suction and the source tank. You may need a booster pump or an inducer. The higher the pump speeds the bigger the problem.
  • 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 should never run parallel with the pump shaft in a double-ended pump installation. This can cause unnecessary shaft thrusting.
  • A piece of pipe of reduced diameter has been installed in the suction piping.
  • Piping was added on the inlet side of the pump to bypass a piece of equipment that was installed on the floor. The added piping is causing an excessive loss of head at the pump suction.
  • A piping to pump reducer has been installed upside down causing an air pocket. Concentric reducers can cause the same problem.
  • Multiple pump inlets are too close together.
  • The pump inlet is too close to the tank floor. The increased velocity is causing a pressure drop at the pump suction.
  • 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 was replaced and the center of the gasket was not cut out.
  • 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.
    • Loose rags can do this.
    • If the suction is from a pond, river, or the sea, grass can be pulled into the suction inlet.
  • A foot valve is stuck.
  • 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.
    • The suction piping collapsed when a heavy object either hit or ran over the piping.
    • Solids have built up on the piping walls. Hard water is a good example of this problem
    • 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.
    • A foreign object is stuck in the piping. It was left there when the piping was repaired or a valve was replaced.
    • The suction is being throttled to prevent the heating of the process fluid. This is a common operating procedure with fuel pumps where discharge throttling could cause a fire or explosion.
  • The pump inlet temperature is too high.
    • The tank is being heated to deaerate the fluid, but it is heating the fluid up too much. Look for this problem in boiler feed pump applications.
    • The sun is heating the inlet piping. The piping should be insulated to prevent this problem.
    • The operating temperature of the pumped fluid has been increased to accommodate the process requirements.
    • A discharge recirculation line is heating the incoming fluid. You should direct this line to a reservoir rather than the pump suction.
    • Steam or some other hot cleaner is being circulated through the lines.
    • Some heaters are designed to be on when the pump is stopped and to be shut off when the pump is running. Maybe they are stuck in the “on” position.
  • The problem is in the tank connected to the suction of the pump.
    • The pump capacity is too high for the tank volume.
    • The tank float is stuck, showing a higher tank level that does not exist. A corroded float rod is not that uncommon.
    • The tank vent is partially shut or frozen, lowering the suction pressure.
    • There is not enough net positive suction head available (NPSHA) for the fluid you are pumping. Maybe you can use an inducer or booster pump to increase the suction pressure.
    • 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 width impeller.
  • The pump discharge is connected to the bottom of a tank. The head is low until the level in the tank increases. As the tank fills the head increases and the capacity decreases. Centrifugal pump discharges should be piped to the top of tanks, not the bottom.
  • Equipment in the discharge piping should not be shut off, it 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 as the capacity increases. This can also happen if an extra storage tank farm is being by-passed because the storage capacity is no longer needed.
  • Piping or fittings have been added to the discharge side of the pump increasing piping resistance.
  • The pump is acting as an accumulator, coming on when the tank level drops. The capacity will decrease when the accumulator is recharging.
  • Consider the possibility of a siphon affect in the discharge piping. This will occur if the pump discharge piping is entering above a tank and discharging at a lower level. The pump must build enough head initially to take advantage of the siphoning action. You will see a decrease in capacity until the siphon takes affect.
  • A discharge valve (manual or automatic) is throttled too much.

Posted

  • On February 18, 2018