PUMPS
What is cavitation, causes and remedies ?
Cavitation can be occurred when the pump suction contains air or when the pump is running faster than the designed speed.
Low pressure regions occur in the flow at points where high local velocities exist.
If vaporisation occurs due to these low pressure areas then bubbles occur, these expand as they move with the flow and collapse when they reach a high pressure region.
Such formation and collapse of bubbles is very rapid and collapse near a surface can generate very high pressure hammer blows which results in pitting, noise, vibration, and fall off in the pump efficiency.
So the pump should be run at designed speed and throttling of the suction valve should be avoided.
What are the reasons for ballast pump not delivering water ?
Incomplete priming
Too high suction lift
Low net positive suction head
Air leaks in suction line
Gas or air lock
Suction filter choke
Screw pump overhaul procedure ?
Put off breaker and remove fuse from motor starter box
Close suction and discharge valve
Remove suction and discharge pipe flange bolt & nut, coupling bolt of motor coupling
Remove pump foundation bolts & take out pump from motor
Remove cover both side & remove driver screw & driven screw with bearing and mechanical seal
Inspect mechanical seal, bearing and renew if required.
Clean all parts and assemble back.
What are the check points in screw pumps ?
Mechanical seal
Bearing
Driver screw and driven screw wear & tear
Pump casing internal surface wear
Clearance between driver and driven screw
Clearance between driven and casing.
Clearance between driver and casing.
What is the centrifugal pump overhaul procedure ?
Put off breaker and remove fuse from motor starter box
Close suction and discharge valve
Remove suction and discharge pipe flange bolt & nut, coupling bolt of motor coupling
Remove pump foundation bolts & take out pump from motor
Remove impeller lock & nut & take out the impeller
Remove mechanical seal.
Remove bearing cover from other side & take out shaft with ball bearing
Renew mechanical seal and bearings if required.
Clean and inspect all parts and assemble.
What are the check points in centrifugal pumps ?
Impeller
Wear ring
Neck bush
Ball bearing
Shaft.
Mechanical seal.
Starting and Running of Screw Pumps
The complete pipe line system must be flushed and pressure tested before installing and operating the pump. If any corrosive liquid is used for the same, then complete system to be properly drained and dried for the protection of the complete system.
Before starting the pump for the first time, it must be completely filled with working liquid as shown in figure below.
Line up all the valves in the pumping system to avoid liquid pressure build up as well as pump losing suction.
Make sure the prime mover motor is isolated electrically and turn the pump-motor coupling by hand to check it is turning smoothly.
When pump is ready to be started, first briefly switch on and off the motor and check the direction of rotation is as per the marking on the pump.
Start the pump and keep checking suction and discharge pressure gauges for rated pressure.
Check prime mover motor ampere and compare with rated current.
Check for any leakages from mechanical seal or other joints of the pump, flanges, etc.
Air in the pumping liquid causes abnormal vibration, noise and over heating of the pump and liquid which may result in a fire hazard.
Periodically inspect foundation bolts, coupling pads, leakages and performance of the pump as condition monitoring to avoid breakdown maintenance.
Use of Pressure Relief Valve in Pump
All systems with screw pumps must be equipped with a pressure relief valve installed immediately adjacent to the pump.
In normal medium capacity pumps, this pressure relief valve is an integral part of the pump to protect the system against excess pressure.
When liquid is circulated through the valve it heats up in proportion to the set pressure level and the percentage of by-passed liquid.
100% bypass can only be tolerated for less than about 3 minutes.
50 %by-pass generally for unlimited periods of time.
If more than 50% recirculation is anticipated, a value specific to each application should be determined by closely monitoring the pump body temperature.
Maintenance of Screw Pumps
Pump to be overhauled at regular intervals, not exceeding 3 years.
Wear of spare parts greatly depends on the pumping medium. Pump screws, liner, etc. are lubricated by the pumping liquid itself. So it is important to avoid presence of any abrasive particles in the pumping medium. Hence pump strainer must be cleaned regularly, by monitoring suction pressure of the pump. A gradual reduction in the suction pressure of the pump indicates that the suction filter is getting chocked.
Indication of pump parts wear can be identified from abnormal noise, vibration, loss of capacity, reduction in discharge pressure, etc.
Inspect internal parts carefully while overhauling. Internal clearances in the pump, which are vital for its proper function, may have been affected by wear of rotors and bores or liner. Acceptable wear can be determined only by experience of the actual application. As a rule of thumb the following max clearance values may apply: Between rotor and bores: 0.2 mm, Between rotor flanks: 0.4 mm.
Inspect for any scratches inside.
Check the condition of the mechanical seal especially the mating faces and o-rings. Discard the seal if mating face is damaged or o-rings hardened. Excessively leaking shaft seals (more than 10 drops per hour) should be changed without delay, as the leakage normally will grow worse and cause additional damage.
Gaskets and o-rings of the pump to be renewed while overhauling.
Check shaft bearing for damages and renew if necessary.
Relief valve, valve seat, springs to checked.
It is advisable to overhaul the motor also along with the pump.
Troubleshooting of Screw Pumps
Problem
Cause
Remedy
Wrong direction of rotation
Electric cables to motor wrongly connected.
Reverse the terminal connection on electric motor.
The pump cannot be primed
Wrong direction of rotation.
Suction line is not open or pressure drop in the suction line is too high.
Major air leakage into the suction line.
The pump cannot evacuate the air through the discharge line due to excessive counter pressure.
See above.
Check all components in suction line. The inlet condition should be checked with a vacuum gauge at the pump inlet.
Check the suction line.
Discharge the air through air purge cock at the discharge of the pump.
No flow
The pump is not primed.
The pressure relief valve is set below the counter pressure.
See above.
Readjust the pressure relief valve to a value above counter pressure.
Flow too low
The coupling is slipping
The pressure relief valve is set too low (Discharge pressure also low).
Something is restricting the flow in the suction line. (This would usually cause noise).
The pumped liquid contains a significant amount of compressible gas, such as free air. (This would usually cause noise).
Check the condition of the coupling. Re-tighten, renew coupling pads.
Readjust the pressure relief valve to get rated discharge pressure.
Check all components in the suction line (strainers, valves etc.).
Go through the system and determine if there are any leaks. Rectify if any.
Pressure too low
The pressure relief valve is set too low.
Counter pressure in the discharge line is too low due to a major leakage.
The valve piston is stuck in open position.
Something is restricting the flow in the suction line. (This would usually cause noise).
The pumped liquid contains a significant amount of compressible gas, such as free air. (This would usually cause noise).
A too small pump has been chosen.
Readjust the pressure relief valve.
Check the components in the discharge line inclusive the recipients.
Check the valve.
Check all components in the suction line (strainers, valves etc.).
Go through the system and determine if there are any leaks. Rectify if any.
Check the capacity of the pump.
Pressure too high
The pressure relief valve is set too high.
The oil is too cold (or has higher viscosity than anticipated).
Counter pressure in the discharge line is too high.
Readjust the pressure relief valve.
Reduce the pressure setting until operational temperature has been reached.
Check the discharge line.
Drive motor difficult to start or tends to stop by tripping the motor overload relay
Counter pressure too high.
Liquid too cold.
Motor is undersized for the prevailing conditions.
Electrical power supply faulty.
Motor overload relay set too low or is faulty.
Incorrect setting of Y/D starter.
See above: Pressure too high.
Readjust the pressure relief valve to a lower value. Thus the power consumption for the pumping is relieved and overloading due to the high viscosity may be avoided. When the liquid has reached normal temperature and thus flows easily, the relief valve is reset to normal pressure.
Check the motor.
Check the motor and motor connection.
Readjust or replace the relay.
Readjust the setting of the starting sequence. The time before the motor overload relay is tripped should not exceed 10-15 seconds.
Noise and vibration
The flow to the pump is insufficient.
Insufficient support of pipe work.
Air leakage into the suction line.
Faulty electrical supply.
See chapter: The flow is too low.
Check for pipe vibrations in the pump connections. Check that the pipes are sufficiently clamped.
Check the suction line for air leakage.
Check all three phases of the supply.
The displacing pumping action is achieved by the reduction or increase in volume of a space causing the liquid (or gas) to be physically moved. The method employed is either a piston in a cylinder using a reciprocating motion, or a rotating unit using vanes, screws or screws.
Reciprocating Piston Pump
A reciprocating positive displacement pump is shown diagrammatically in the figure above to demonstrate the operating principle. The pump is double acting, that is liquid is admitted to either side of the piston where it is alternatively drawn in and discharged. As the piston moves upwards, suction takes place below the piston and liquid is drawn in, the valve arrangement ensuring that the discharge valve cannot open during suction stroke. Above the piston, liquid is discharged and the suction valve remains closed. As the piston travels down, the operations of suction and discharge occur now on opposite sides.
Why an Air Vessel is Fitted?
As indicated in the figure, positive displacement pumps are usually fitted with an air vessel. An air vessel usually fitted in the discharge pipe work to dampen out the pressure variations during discharge. As the discharge pressure rises the air is compressed in the vessel, and as the pressure falls the air expands. The peak pressure energy is thus stored in the air and returned to the system when pressure falls. Air vessels are not fitted on the reciprocating boiler feed pumps since they may introduce air into the de-aerated water.
Working
When starting the pump, the suction and discharge valves must be opened. It is important that no valves in the discharge line are closed, otherwise either the relief valve will lift or damage may occur to the pump when it is started. Positive displacement pumps are self priming, but where possible to reduce wear or the risk of seizure it should be flooded with liquid before starting. An electrically driven pump only need to be switched on, when it will run erratically for a short period until liquid is drawn into the pump. A steam driven pump will require the usual draining and warming through procedure before the steam is gradually admitted.
Use of Relief Valve
A relief valve is always fitted between the pump suction and discharge chambers to protect the pump should it be operated with a valve closed in the discharge line.
Maintenances
Most of the moving parts in the pump will require examination during overhaul. The pump piston, rings and cylinder liner must also be thoroughly checked. Ridges will eventually develop at the limits of the piston ring travel and these must be removed. The suction and discharge valves must be refaced or ground in as required.
Gear Pump
These gear pumps are rotary displacement or rotary positive displacement pumps. Two toothed wheels shown, mesh together and are a close fit in casing. Initially the air or gas is trapped between each pair of consecutive teeth and the same is dragged along the casing from suction to discharge side till no more air is left on the suction side. Liquid from the tank will thus rise up into suction line under atmospheric pressure, subsequently this liquid will now be trapped between each pair of two consecutive teeth and dragged along the casing into the discharge side and pumping of liquid will commence. The working principle just explained is what makes the pump a self priming pump. Further if liquid level on suction side is at a higher level, the liquid will flow into the suction side on its own at first instant itself.
Usually the gear pumps are electric motor driven through a chain or wheel drive. Control of flow rate is achieved by a bypass valve or controlling speed of prime mover.
A number of such pumps in series can be used to develop high pressure. Such pumps are efficient and smooth running.
Applications
Gear pumps are used for duties as a lube oil pump, boiler fuel oil pump, fuel oil transfer pump, main engine driven lube oil pump. As a main engine driven lube oil pump it will have a set of suction and discharge valve to give same side discharge at all times irrespective of ahead or astern movement of the main engine.
From the figure shown, If pressure exerted by atmospheric air (or any other atmosphere, which is surrounding the liquid on suction side) is H0 and is more than the 3 losses mentioned below:
Loss of head because of friction in suction line, H1
Loss of head because of volatility of liquid, H2
Loss of head in rising the liquid into the pump suction, H3
Only then will the liquid rise up to the pump. However the liquid can be discharged effectively and without cavitation of the pump only if this ‘left over head’ called ‘available NPSH (Net Positive Suction Head) is greater than the ‘required NPSH’ of the pump. Former we have calculated as
[H0 – (H1 + H2) + or – H3]
and later is given by pump manufacturer after conducting trials on the pump.
The ‘required NPSH’ curve of a pump is provided by the pump manufacture.
Required NPSH Curve
At lesser flow rate the pump requires lesser NPSH. Therefore when an oil tank of a tanker is being stripped; to prevent cavitation and vibration of the pump and yet strip the cargo tank almost dry, we reduce the flow rate of the pump as the level of the liquid falls.
Though now the pump discharges at slow rate but same time it does not cavitate as value of ‘required NPSH’ is much lesser and is easily provided even by he reduced liquid level of tank. Alternatively if ‘available NPSH’ is less than ‘required NPSH’, increasing the inert gas pressure will delay the time when pump will start cavitating.
Required NPSH and Cavitation
Referring to the figure above, when cargo level in tank is at ‘X’, a flow rate upto ‘X1’ can be maintained without fear of cavitation of the pump. When cargo level drops to say level ‘Y’, the flow rate should be reduced to or below ‘Y1’ to avoid cavitation. In the tankers this is done by throttling the discharge valve of the cargo line.
Centrifugal pumps are not self priming. If initially there is no liquid a the eye, there will be no pumping action for a centrifugal pump. In absence of liquid, air (sometimes vapour) will be present at the eye, and owing to its light density air could be thrown out under centrifugal force only if the speed of the impeller is very very high (like in a Turbocharger Blower). In such a case, where normally a the start of the pump the level of the liquid is below the eye of the impeller, we can make use of a self priming unit.
Operation
Figure above shows an automatic arrangement for pumping out bilges, using a centrifugal pump, wherein the air (vane) pump will get engaged automatically and draw out any air at the start or during running. Once the air is drawn out it will get disengaged automatically.
Discharge side of the pump is connected with one side of the piston (engage / disengage mechanism) as shown in the figure. Consider the pump is started with no liquid at the eye of the impeller. Now the impeller will be rotating but the absence of liquid at the discharge (means no discharge pressure) makes the piston to move forward due to spring pressure and thus the bevel connected to the air pump rotor shaft engages with the rotating shaft of the centrifugal pump. This drives the air pump to remove any vapor or air present inside the pump suction and the liquid rises to prime the pump. Once the pump is primed discharge commences, discharge pressure rises which acts on the piston thereby pushing the piston against the spring pressure. Thus the air pump gets disengaged. Hence whenever there is any ingress of air or vapor in the pump suction, discharge pressure reduces and air pump engages to remove the same.
Centrifugal pump is a device, which adds to the energy of a liquid or gas causing an increase in its pressure and perhaps a movement of the fluid.
A Simple Pumping System
A simple pumping system consists of a suction branch, a pump, and a discharge branch. See the figure above.
Liquid flows into the centrifugal pump under either “GRAVITY & ATMOSPHERIC PRESSURE” (when the liquid to be pumped is above the center line of the pump) or only under “ATMOSPHERIC PRESSURE” (when the liquid to be pumped is below the center line of the pump).
Centrifugal Pump Characteristics
Pump only adds to the energy of the fluid in the system. Energy required to bring the fluid to the pump is an external one and in most practical conditions is provided by the atmospheric pressure.
Types of Pumps
Displacement
Liquid or gas is displaced from suction to the discharge by the mechanical variation of the volume of a chamber or chambers. All displacement pumps are self-priming pumps. These pumps include Reciprocating Pump, Gear Pump,Screw Pump, etc.
Centrifugal
Flow through the pump is induced by the centrifugal force imparted to the liquid by the rotation of an impeller or impellers.Centrifugal pumps are not self-priming pumps. These pumps must be primed by gravity or by priming equipment external or internal with the pump.These pumps are basically radial flow, axial flow or mixed flow type.
CENTRIFUGAL PUMP
Construction and Working of Centrifugal Pumps
The pump consists of a rotating impeller within a stationary casing. The impeller construction has two discs joined at in between surface by a set of internal curved vanes. Impeller has an eye (opening) at the center and is mounted on shaft, which is driven by a suitable prime mover such as an electric motor, steam engine through crank mechanism, or a turbine.
Opening in the sides of the impeller near shaft, called eye, communicates with the suction branch as shown in the figure below.
Assume there is a certain amount of fluid at the eye of the rotating impeller. The fluid will flow radially outwards (because of centrifugal action) along the curved vanes in the impeller, increasing its linear velocity.
The fluid leaves the impeller in a similar manner to sparks shooting from a Catherine wheel. The high velocity fluid is collected in specially shaped casing called volute casing, where some of the kinetic energy of the fluid is converted into pressure energy. Fluid under pressure now leaves the impeller producing a drop in pressure behind it at the eye of the impeller. (Throwing off the water from the eye of the impeller leaves the space with vacuum). This causes the fluid from the suction pipe to flow into the pump under atmospheric pressure and subsequently that fluid also gets discharged like earlier one. This way fluid in the pump acts like a piston moving outward causing drop in pressure behind it. However, if initially there is no liquid at the eye, there will be no pumping action as explained, since there is no vacuum formed at the eye of the impeller. Centrifugal pump therefore is not a self-priming pump. In such case, where normally at the start of the pump the level of the liquid is below the eye of the pump, a self priming unit is normally attached to the pump which helps to create vacuum at the eye of the impeller hence priming the pump. As soon as pump starts taking suction self priming unit is automatically disengaged.
Performance Characteristics of Centrifugal Pumps
Refer the performance characteristics drawn above.
n-Q – Efficiency Vs Flow Rate and
HP/Q – Horse Power (of the prime mover)
Theoretical Discharge Head Vs Flow Rate (H/Q) plot is a straight line as shown. When there is no flow or discharge valve is shut, loss of head is mainly due to shock and eddy losses. As flow rate increases, frictional losses come into picture and it dominates other losses.
From above graphs it is clear that,
If the pump discharge head is lesser the flow rate of the liquid is higher and therefore pumping of the liquid is faster.
Pump if run at normal duty flow rate by maintaining normal duty discharge head the liquid will be pumped utilizing least possible rate of energy by the pump or in other word at this point efficiency of the pump is maximum.