What is the function of a foot valve in a centrifugal pump? Mention principal causes of pump troubles with remedies?by Madan Mohan
Various types of valves are required to maintain optimum function of water and air pumps, and among these valves are foot valves. Foot valves are used to prime up a centrifugal pump; they are known for their efficacy and affordability. Foot valves are a type of check valve and are placed at the pump’s wet well. Unlike other valves, a foot valve is created with a larger flow area than the actual pipe size to make sure that there is less head loss. Foot valves are either made of PVC plastic or stainless steel, and they are known for keeping the continuous presence of suction within the pump.
Foot valves are used to maintain hydraulic pressure to keep the water flow in accordance with the given settings or configurations. There are instances where the pressure can actually pop the valve out and cause major leakage; thus, it is important to use the right kind of material in the tubing to be able to support the force within the valve.
Centrifugal Pump Trouble Shooting
Where Trouble Occurs….. Failure of a centrifugal pump in service may be sudden, as when a shaft breaks, or gradual, as when brackish water causes blistering of the impeller or casing. Fortunately, neither condition is common in well-operated and maintained pumps. Nor is slow corrosion of the impeller vanes too often met.
Failures of the materials used in construction of a pump are usually traceable to incorrect application, instead of poor operating and maintenance procedures. But where pumps are wrongly applied for example, using a bronze-fitted pump for an alkaline solution having a high pH thorough maintenance should quickly show up the error. As a result, wrongly applied pumps are generally replaced before too much trouble occurs in the plant.
Hydraulic troubles, like failure to deliver any liquid, low discharge pressure, and others, are more common and more difficult to solve. One manufacturer estimates that, except for mechanical defects, about 85% of the troubles met with centrifugal pumps occur on the suction side of the unit. So this directive has been specially prepared as a handy troubleshooting guide for all types and sizes of centrifugal pumps handling any liquid at any flow rate, and at any temperature and pressure for which pumps of this type are hr today. Each of the major troubles is listed separately; below it the possible causes are given in italics, followed by the recommended remedy or cure. Horizontal pumps are considered first, vertical pumps second. Some troubles, however, are common to both types of pumps.
A. No Liquid Delivered
Lack of Prime: Fill the pump and its suction pipe completely with the liquid being handled. To rid the casing and piping of air, open all Centrifugal Pumps the vent valves while filling the pump and pipe. Leave the vents open until clear bubble-free liquid flows from them. Close the vents and start the pump.
Speed of Pump Driver Too Low: With a motor drive, check to see that it is connected directly across the line and receives full rated voltage. With alternating-current (AC) motors, check the frequency it may be too low. Or the motor may have an open phase, causing it to run at a speed lower than its rated value. With a turbine-driven pump check the governor setting and throttle valve. A loose or damaged throttle-valve disk may plug the steam passage. causing wire drawing and a loss of pressure. With engine-driven pumps, check the fuel supply, governor setting. intake-air supply, scavenging-air pressure, spark plugs, magneto, carburetor, or fuel-injection pumps. Air-driven centrifugal pumps run at too low a speed when the air pressure is lower than that required at the air-motor inlet. A fall in air pressure may be caused by reduced output of the compressor, excessive use of air in other parts of the system, use of undersize hose or pipe connections, or open drain valves in the system.
Discharge Head Too High: Check all valves in the discharge line to see that they are wide open. Be sure that gage valves are not stuck closed by some obstruction in the pipe. If the discharge head is still too high and no new devices have been installed in the system, check the piping for obstructions, from either solids contained in the liquid, or scale buildup.
Suction Lift Too High: Check the pump inlet for clogging by mud or some other obstruction. If a foot valve is used, cheek it for broken disks or a clogged strainer. When the pump is being started for the first time and it will not draw water from the suction pit, check the maximum suction lift recommended by the manufacturer. With centrifugal pumps this is usually 15 ft. (4.572 m) As Fig. 1-A shows, atmospheric pressure forces liquid into the pump suction when an open suction pit is used. If a perfect vacuum could be produced in the suction eye of a pump handling water at ordinary atmospheric temperatures, the atmospheric pressure could support a column of water 34 ft (10.3632 m) high (Fig. 1-B). But a standard suction lift of only 15 ft (4.572 m) is usually recommended for centrifugal pumps, giving a 19-ft reserve (Fig. 1-C). Remember, suction lift is made up of the static lift, friction head, and velocity head. So check the vertical distance between the liquid surface and pump inlet, as well as any possible clogging of the suction pipe by dirt or scale.
Impeller Plugged: Solids in the liquid may accumulate on the impeller, preventing it from discharging liquid. Open the casing and clean all parts of the impeller.
Wrong Direction of Rotation: Despite all the warnings of pump manufacturers, wrong rotation still occurs when some pumps are first started. See that the pump turns in the direction of the arrow on its casing.
Other, less common causes of no liquid being delivered are an air or vapor pocket in the pump suction line, suction pipe not sufficiently submerged, available net positive suction head (npsh) not high enough, and the total head against which the pump works higher than that for which the pump is designed.
B. Not Enough Liquid Delivered
With this trouble the pump delivers some liquid but the amount is less than the rated capacity of the unit at the head at which it is operating. This can be almost as serious as no delivery at all because valuable units served by the pump may be endangered. Possible causes, covered above, include wrong direction of rotation, speed too low, discharge head too high, impeller clogged, suction lift too high, air or vapor pocket in suction line, suction pipe not sufficiently submerged, available npsh insufficient, and clogged foot valve. The remedies to be used are the same as those recommended above. Other causes are given below.
Air Leaks: These may occur in two places the suction line or the pump stuffing boxes. Check the flanges and screwed joints with a flame or match only if the liquid handled is nonexclusive and no explosive dust or gas is present in the atmosphere. The flame will be drawn toward any leaks, if it is held close to the pipe and flanges. Where an explosive liquid is handled, or an open flame cannot be used around the pump, check for leaks by shutting the suction valve or plugging the inlet of the suction pipe and putting the line under pressure from another source like a small hand pump. Attach a pressure gage to the suction line and observe it to see if the pressure falls over a period of time say 30 minute. Plug all leaks found in the suction piping.
If the suction piping has no leaks, check the pump stuffing boxes. The box serving the pressure side of the main shaft should leak a small amount of liquid during pump operation. Adjust the gland to give a suitable flow from the box. If this adjustment does not give the desired results, stop the pump and check the packing. New packing is probably needed. Install as directed in the maintenance manual. Recheck leakage from the box. If it is still unsatisfactory, disconnect the water-seal piping at the box and check liquid flow. Clean out plugged piping. Next, check the position of the seal cage. It should be directly under the seal-liquid inlet. Inspect shaft sleeve for excessive wear. Deep grooves and pits allow air to leak into the suction side of the casing. Replace the shaft sleeve.
Another way of testing for stuffing-box or mechanical-seal air leaks requires somewhat more work but is effective. Disconnect the suction line at the pump and screw a plug into the suction coupling, or fit a blind flange to the suction flange. Dope the threads or gasket. Drill and tap a ¼-in (6.35 mm), hole into the plug or flange and connect a vacuum gage to it. Fill the pump completely with liquid, start its drive, and run it at the normal operating speed. The pump should develop a vacuum of 20 in (508 mm) or more within 1 minute after starting, if there is no leak in the stuffing box or mechanical seal.
Low NPSH: Connect a compound pressure gage to the suction pipe. If the needle fluctuates rapidly, the liquid in the suction pipe is flashing into vapor. Safest remedy is to check with the pump manufacturer.
Worn Wearing Rings: Inspect the rings visually. If they are badly worn, permitting leakage in the pump, replace all rings.
Damaged impeller: Remove casing and inspect the impeller. Replace with a new one if vanes or other parts are damaged or worn.
Undersize Foot Valve: Area of foot-valve openings should be at least equal to that of the suction pipe, and preferably 1.5 to 2 times as large. The net clear area of the strainer should be 3 to 4 times that of the suction pipe.
Wrong Direction of Rotation: A pump turning in the wrong direction may deliver some liquid, but its head will be low and the driver will be overloaded. The pump usually will not deliver more than about one-third of its rated capacity. Check the directional arrow on the casing.
Suction-bay Disturbances: Vertical centrifugal pumps must often be located a specified distance from the walls of the suction bay. If the pump does not deliver its rated capacity, check the location against the manufacturer’s recommendations.
Viscosity Too High: A pump designed to handle water will deliver less head and capacity when pumping a thick oil or other viscous liquid. So if the pump is moved from one job to another, or the liquid it handles has been changed, check the viscosity to see that it is not too high for the unit.
Worn Gaskets: Replace all gaskets during a pump overhaul. Tighten the hold-down bolts to the desired or recommended value, using a torque wrench, if necessary.
Impeller Eye Too Small: The larger the diameter of the impeller eye, the greater its capacity (Fig. 2). Incorrect choice of a pump or moving it from one job to another may cause this trouble. The only solution is installation of a pump having a suitable capacity for the job.
C. Pump Discharge Pressure Low
Typical causes of this trouble include too low a speed, worn wearing rings, damaged impeller, worn packing, gas or vapor in the liquid. too viscous a liquid, wrong direction of rotation, and worn gaskets. Other possible causes are given below. Gas or Air in Liquid: Bubbles will form imi the liquid when it enters the suction pipe. Check for this condition by reducing the pressure on the surface of a small amount of liquid and observe if any bubbles form. A gas-separation chamber on the suction line near the pump may be needed to remove the gas or air from the liquid.
Pump Water Passages Obstructed: Open the casing and check the water passages for freeness. Remove any obstructions and replace the casing.
Impeller Diameter Too Small: This traces back to the assumptions made in the selection of the pump. Check with the pump manufacturer. It may be possible to use a larger-diameter impeller in the same casing. Or by reducing the pipe friction losses or static suction or discharge head, or increasing the speed of the pump, a higher pressure can be obtained. But be extremely careful when changing the system head or pump speed not to overload the driver beyond its safe rated limit.
D. Pump Loses Prime After Starting
There are a number of common causes of this trouble. These are incomplete priming, too high a suction lift, air leaks in the suction pipe or packing glands, gas or air in the liquid, suction line not filled with liquid, air or vapor pockets in the suction line, inlet not sufficiently submerged, low available npsh, plugged seal-liquid piping, or a misplaced lantern ring in the stuffing box. Various remedies are discussed above under these or similar headings.
E. Pump Overloads Driver
Discharge Head Low: With too low a discharge head the pump delivers too much liquid, overloading the driver. It may be possible to turn down the outside diameter of the impeller, thereby reducing its capacity. But never do this without complete advice from the pump manufacturer. Serious damage can occur otherwise.
Wrong Liquid: If either the specific gravity or viscosity of the liquid is different from that for which the pump is rated, there is a chance of the driver’s being overloaded. Use a larger driver, after consulting the manufacturer for the recommended size. Be sure to check the viscosity and specific gravity of the liquid before ordering a larger driver. Another difficulty may be the cause.
Speed Too High, or Wrong Direction of Rotation: Correct as described above.
Packing Too Tight: Release the gland pressure; then retighten reasonably. Check the leakage of the seal liquid from the packing. If there is no leakage while the pump runs, replace the packing as directed in the maintenance manual. Check for a scored shaft sleeve if the packing wears rapidly. Replace worn sleeves with new or refinished ones.
Distorted Casing: Poorly aligned suction and discharge piping can distort the pump casing, causing excessive friction between the impeller and casing. Check the piping and drive alignment. Inspect for worn wearing rings; replace damaged or worn parts.
Bent Shaft: Thermal distortion damage during pump overhaul, or wrong assembly of the rotating element can cause bending of the shaft. Check the shaft deflection by means of a dial gage by turning the shaft between lathe centers. (Refer Manual) Recommended average runout varies from one manufacturer and pump design to another, but 0.003 in. (0.9144 mm) is usually the maximum runout allowed on the shaft of a high speed pump. On a slow-speed pump it should not exceed 0.006 in (1.8288 mm).
Mechanical Failures: These can increase the drag on the shaft, raising the power input. Check all rotating and stationary parts for failure, including the pump bearings, wearing rings, packing gland, bushing, and impeller. Replace, as necessary.
Misalignment: Realign the pump and its driver.
Pump Speed Too High: Power input to a pump increases as the cube of the speed. So a slight increase in pump speed can mean a measurable rise in power input. Check the line voltage on motor-driven pumps and the governor on turbine and engine-driven units. Also check the speed of the driver to see that it is correct.
F. Stuffing Boxes Overheat
Common causes of this trouble include packing that is too tight, not enough packing lubricant, wrong grade of packing, not enough seal liquid flowing to the packing, and incorrect installation of the packing. Remember, there is a correct way of installing each type of packing. Check with the manufacturer manual.
G. Excessive Vibration
Gas or air in the liquid leads to a starved suction, as do insufficient npsh, not enough submergence of the end of the suction pipe, and gas or vapor pockets in the suction line. Other causes of vibration include pump misalignment, worn or loose bearings, rotor unbalanced because of a plugged or damaged Impeller, bent shaft, Improper positioning of a control valve in the discharge, and a non rigid foundation.
H. Bearings Overheat
Many lubricating troubles cause overheated bearings. These include too low an oil level, a poor or wrong grade of oil, dirt in the bearings or the oil, moisture in the oil, a clogged or scaled oil cooler, failure of the oiling system, not enough bearing cooling water, bearings too tight, misalignment, or oil seals fitted too closely on the shaft.
I. Bearings Wear Rapidly
Excessive wear of sleeve, ball, or roller bearings may be caused by misalignment, a bent shaft, vibration, excessive thrust from a mechanical failure inside the pump, lack of lubrication, wrong bearing installation procedures, dirt in the bearings, moisture in the oil, and excessive cooling of the bearings.
Vertical Centrifugal Pumps. Though subject to many of the same troubles as horizontal pumps, vertical units are likely to have different causes of their troubles. This is because of differences in construction, installation, and operating conditions. Typical troubles, causes, and remedies are given below.
J. Pump Will Not Start
Impeller Locked: Sand causes’ many locks. Try to raise and lower the impeller adjusting nut this may free them. If it does not, backwash the pump, using clear water or whatever other liquid the pump normally handles. Try turning the shaft at its top, using a small pipe wrench. Be careful the shaft is easily damaged by a wrench. If the impellers cannot be freed, pull the pump and tear down the bowl assembly to get at the rotating parts.
Trash in Casing: Rags, wood, Or metal jammed, in the pump may prevent it from turning. Tear down the pump and remove the obstruction. Fit the suction with a strainer to keep trash out of the pump
Corrosion or Growths: Pumps that are out of service for long periods may be locked tight. Use acid or other recommended chemicals to remove corroded matter or growths from the pump.
Packing Too Tight: Adjust so there is enough leakage for cooling and lubrication of the pump shaft.
Too Much Bearing Friction: Use the right oil; consult the pump builder for the correct viscosity range. Check the tube tension nut for tightness. See if the pump shaft is bent; replace, if necessary. Check the anchoring of the pump head to see that it has not caused bending and distortion of the pump. Return bent shafts and columns to the builder’s factory for new ones. See that water-lubricated rubber bearings are wetted and free of sand. The wrong tension on the shaft enclosing tube of oil-lubricated pumps may throw the bearings out of line. If the well in which the pump is installed is so crooked that it causes misalignment of the pump, have it reamed to a larger diameter or install a smaller pump.
Motor or Wiring Faulty: Check the circuit breaker or fuses for an open line. If the starter overload relays have tripped, reset them. Disconnect the motor from the pump and see if it starts. If it does not, have an electrician look it over or check it as directed in the manual.
Impellers Not Adjusted Properly: Set the impellers high enough so there is room for the shaft stretch caused by hydraulic thrust. This adjustment should allow the shaft to turn freely; then the stretch caused by the rotor and shaft weight will not bind the pump.
Well Cave-in: Outside help is needed to correct this condition.
K. Pump Does Not Deliver Liquid
Wrong Rotation: Change the rotation of the motor. With a 3-phase motor, just switch any two power leads.
Speed Too Low: Check voltage and frequency of the power supply. See if excessive bearing friction, corrosion, or obstruction of the impellers slows the pump. Check the gear ratio and motor speed if the pump is being operated for the first time. Look over belt-driven units for slippage or wrong pulley size.
Pump Not Primed: Vent the well to the atmosphere so there is not a vacuum at the pump suction. All impellers of vertical turbine pumps must be under water or the liquid handled because these units will not ordinarily start discharging against a suction lift. A 4 to 10-ft (1.2192 to 3.048 m) npsh is needed for good operation. Have enough head on the pump to allow it to discharge at rated capacity.
Well Over pumped: With excessive draw down, the pump may break suction and fail to deliver water. Reduce the pump capacity by throttling the discharge.
Two other common causes of trouble of this type are failure of pump parts and too high a pumping head. Check for a broken shaft, broken bowl assembly, and loose column-pipe joints. Tighten loose impellers. Check discharge valves to see that they are open and that the check valves do not stick. If the water table has fallen, the suction lift may be too high for the unit. Clear a clogged suction pipe or impeller by back washing. If the well screen is plugged, help from an experienced well driller is probably needed.
L. Pump Uses Too Much Power
Causes of this trouble include over speeding, wrong lubricant, tight packing, impeller rub, wrong rotation, misalignment, tight bearings, excessive vibration of the pump or piping, incorrectly chosen pump, and high discharge pressure. Correct as directed. On vertical pumps fitted with water-lubricated shaft bearings, an air vent or air-relief valve may be needed on the column to allow water to enter the bearings. This is true of all vertical pumps where the liquid handled acts as the bearing lubricant.
M. Pump Capacity Low
Low Liquid Level: Vent the well. Check the pump inlet for excessive turbulence, vortexing, or eddies. The velocity of the liquid entering the pump must be that recommended by the manufacturer and the suction submergence sufficient. Check the bowls and well screen for sand, rust, or bacterial blocking.
Impeller Wear: Metal loss from the outer tips of the impeller vanes reduces pump capacity. Loss at the inner or suction end has not much effect. If fully enclosed impellers have the usual wearing rings, the trouble may be in them. Look for excessive clearance. With semi-open impellers not having bottom shroud or wearing rings, a close running clearance is needed at the bottom of the vanes.
Faulty Instruments: Make sure that the water-level reading is correct. See that flow meters for measuring pump capacity are adjusted and read properly. Check the pressure gages.
Other causes include too great a head on the pump, piping and pump leaks, and too low a speed. Check and correct as given above.
N. Pump Vibrates Excessively
Rough Operation: Check to see that the impeller and bowl passages are free of wood, rags, sand, and other material that might throw the pump out of balance. Check the driver by disconnecting it and operating it alone. Look for excessive wear in the rotating parts.
Pump Taking Air: Check the water velocity at the pump inlet. See if there are any leaks in the well vent. Check if the suction head on the pump is sufficient. Over pumping a well so the water level is intermittently drawn down may cause the pump to “grab air” and is a common source of severe vibration.
Bearing Troubles: Check the lube oil or grease for grade and quantity. Look for too much sand in the water or liquid handled. Check pumps alignment.
Comments are closed.