Pumps


Pumps

A mechanical device or machine that lift liquids against the force of gravity or remove exhaust gases from closed vessels. It is used to lift liquid from a source below the pump to a higher elevation, to move liquid from one location to another, to move liquid from a low pressure area to a higher pressure area. The machine used to withdraw air or any gases from a closed vessel is also classified as a pump.

Pumps are classified with respect to their construction or the service for which they are designed. The three groups of pumps are centrifugal, reciprocating and rotary.

Centrifugal Pumps
This pump derived its name from the type of force that this design of pump utilizes. Centrifugal force acts on a body moving in a circular path, tending to force it farther from the center of the circle. Inside the body of a centrifugal pump, the liquid is force to revolve and so generates the force that enable this style of pump to develop pressure and move liquids. It compose of blades or vanes called the impeller and the surrounding case called housing or volute.

Centrifugal pumps are made in a great variety of types, styles and sizes to suit application requirements and operating conditions. A basic design difference between pumps is the number of stages. Pumps with one impeller are classified as single-stage pumps and those with two or more are called multistage pumps.



Centrifugal Pump Troubleshooting Chart
Problem or Symptom
Cause
Correction
No liquid being pumped
Suction line plugged or clogged
Pump not primed
Impeller clogged

Incorrect rotation
Clear obstruction.

Re-prime pump.
Try back-flushing to clear impeller.
Reverse motor rotation.
Pump not producing rated flow or pressure
Air leak at stuffing box

Air leak at gasket


Impeller partly clogged
Low suction head
Tighten packing or replace mechanical seal.
Replace with new gasket and proper gasket sealant if required.
Back flush.
Open suction valve all the way and check suction piping.
Hot bearings
Improper lubrication

Improper alignment
Check for proper grease or oil and lubricate.
Realign pump and motor.
Pump vibrates
Poor alignment
Worn bearings
Pump with much cavity


Unbalanced impeller


Broken parts on impeller or shaft
Align motor and pump shafts.
Replace bearings.
Recalculate pump or check system attributes and may replace impeller.
If due to foreign material, back flush; if impeller is worn, replace.
Replace impeller.
Pump begins to pump and then stops
Air pocket in suction line

Air leak in suction line
Pump not primed properly
Change piping to eliminate air pocketing.
Plug the leak.
Re-prime pump.
Motor amperage draw is excessive
Rotating parts are binding

Pump is pumping too much liquid
Stuffing box packing is too tight
Check rebuilding procedures and correct interfering parts.
Machine impeller to smaller size.
Readjust.
Leakage at stuffing box
Worn mechanical seal parts
Shaft sleeve is cored or cut
Packing improperly adjusted
Replace seal.
Replace sleeve.
Tighten or replace packing
Material.

Reciprocating Pumps
This pump take their name from the back and forth motion of the pumping element. The reciprocating pump moves liquid by displacing the liquid with a solid. This principle of operation is called positive displacement. Three basic types of positive displacement reciprocating pumps are in common use. They are classified as piston, plunger and diaphragm.

Piston Type
This is relatively short cylindrical part that is moved back and forth in the pump chamber or cylinder. Generally the distance that the piston travels back and forth called the stroke is greater than the piston length. Leakage pass the outside of the piston is usually controlled by packing or piston rings.


Plunger Type
This often confused with the piston type pump, probably because in both cases the pumping element is cylindrical solid moved back and forth within a cylindrical chamber. One major difference is that the piston moves back and forth within a cylinder, whereas the plunger moves into and withdraws from a cylinder. Usually the length of a plunger is greater than its stroke.



Diaphragm Type
A flexible diaphragm is employed as the pumping element. This is secured fastened around its outside and its center is moved back and forth to provide positive displacing action. Packing or seals are not required because there are no sliding fits.


Rotary Pumps
This is like reciprocating that is positive displacement in operation. However, the design much simpler because no check valves required and the flow is continuous. Different designs make use of such elements as vanes, gears, lobes and cams to move liquid. A common style consists of two gears in mesh. The idler is driven by gear which is rotated resulting positive displacement of fluids. This design has close running tolerance and generally self-priming. In operation, liquid fills the space between the gear teeth. The gears rotate and as they mesh, the liquid is literally squeezed out through the discharge opening. As the teeth moves into the suction area, they separate, creating partial vacuum.


Troubleshooting of Rotary Pumps

No Liquid Delivered
  • Stop pump immediately.
  • If pump is not primed, primed according to instructions.
  • Lift maybe too high. Check this factor with a vacuum gauge on the inlet. If the lift is too high, lower the position of the pump and increase the size of the inlet pipes; check the inlet pipe for air leaks.
  • Check for incorrect direction of rotation.

Insufficient Liquid Delivered
  • Check for air leak in the inlet line or through stuffing box. Oil and tighten the stuffing box gland. Paint the inlet pipe joints with shellac or use RTV rubber to seal.
  • Speed is too slow. Check the rpm with manual tach or strobe light. The driver maybe overloaded or the cause may be low voltage or low steam pressure.
  • Left may be too high. Check with vacuum gauge. Small fractions in some liquids vaporize easily and occupy a portion of the pump displacement.
  • There is too much lift for hot liquids.
  • Pump may be worn.
  • Foot valve may not be deep enough.
  • Foot valve may be either too small or obstructed.
  • Piping is improperly installed, permitting air or gas to pocket inside the pump.
  • There are mechanical defects, such as defective packing or damaged pump.

Pump Delivers for Short Time and Quits
  • There is a leak in the inlet.
  • The end of the inlet valve is not deep enough.
  • There is air or gas in the inlet.
  • Supply is exhausted.
  • Vaporization of the liquid in the inlet line has occurred. Check with vacuum gauge to be sure the pressure in the pump is greater than the vapor pressure of the liquid.
  • There are air or gas pockets in the inlet line.
  • Pump is cut by the presense of sand or other abrasives in the liquid.

Rapid Wear
  • Grit or dirt is in the liquid that is being pumped. Install a fine-mesh strainer or filter on the inlet line.
  • Pipe strain on the pump casing causes working parts to bind. The pipe connections can be released and the alignment checked to determine whether this factor is a cause of rapid wear.
  • Pump is operating against excessive pressure.
  • Corrosion roughens surfaces.
  • Pump runs dry or with insufficient liquid.

Pump Requires Too Much Power
  • Speed is too fast.
  • Liquid is either heavier or more viscous than water.
  • Mechanical defects occur, such as bent shaft, binding of the rotating element, stuffing box packing too tight, misalignment of pump and driver, misalignment caused by improper or sprung connections to piping.

Noisy Operation
  • Supply is insufficient. Correct by lowering pump and increasing size of inlet pipe.
  • Air leaks in inlet pipe cause a cracking noise in pump.
  • There is an air or gas pocket in the inlet.
  • Pump is out of alignment, causing metallic contact between rotor and casing.
  • Pump is operating against excessive pressure.
  • Coupling is out of balance.