Monday, 15 December 2014

KINETIC AND POSITIVE DISPLACEMENT PUMP MAINTENANCE

KINETIC AND POSITIVE DISPLACEMENT PUMP MAINTENANCE

Resistance of the system: head

Pressure is needed to pump the liquid through the system at a certain rate. This pressure has to be high enough to overcome the resistance of the system, which is also called “head”. The total head is the sum of static head and friction head.

A. Static head 

Static head is the difference in height between the source and destination of the pumped liquid. Static head is independent of flow.


Static head consists following types of head 

Static suction head (HS)

The vertical distance between the pump center line and the surface of the liquid in the source tank.
The HS is positive if the liquid level is above pump center line, and negative if the liquid level is below pump center line.

Static discharge head (Hd)

The vertical distance between the pump center line and the surface of the liquid in the destination tank.
 

B. Friction head (hf)

This is the loss needed to overcome that is caused by the resistance to flow in the pipe and fittings. The friction head is proportional to the square of the flow rate.

                                              
Frictional Head versus Flow 


In most cases the total head of a system is a combination of static head and friction head as shown in figure.

                  

System with High Static Head       System with Low Static Head

Pump performance curve

The head and flow rate determine the performance of a pump, which is graphically shown in Figure as the performance curve.



Performance Curve of a Pump

Pump operating point

The pump operating point is determined by the intersection of the system curve and the pump curve as shown in Figure.

              
Pump Operating Point                                
 
Pump suction performance - N.P.S.H
Cavitation is the formation & sudden collapsing of vapor bubbles inside the pump. This may occur when at the fluid’s local static pressure becomes lower than the liquid’s vapor pressure (at the actual temperature). Cavitation have three undesirable effects.
  • Erosion of vane surfaces & unbalancing of impeller
  • Increase of noise and vibration.
  • Reduces the pump performance and can lead to loss of total head in extreme cases.
The N.P.S.H required is the pump suction needed to avoid cavitation



Pump curve with 2 impeller options

Maintenance Types

Pump Alignment 

 

Problems due to misalignment 

To verify alignment, place a straight edge across the coupling as shown in the following illustration; this must rest evenly on both rims at top, bottom and both sides. With a pair of inside calipers or thickness gauge, check distance between coupling halves at points where straight edge was used; distance must be equal at all points.                                             

Qurayyah Power Plant Pumps

There are five the most important pumps used in this plant: Fuel Oil Pump, Boiler Circulation Pump, Boiler Feed Pump Turbine, Circulating Water Pump and Condensate Extraction Pump.

Boiler circulating Pump

Each circulating pump takes suction from the common suction manifold and discharges to the supply drum, which acts as an inlet header for the water wall tubes.

Water pump circulating 

The function of circulating water pump is to deliver cool seawater to the condenser which condenses the steam exhausted from the low pressure turbines.

Too keep pumps operating successfully for long periods of time, a good maintenance practices are required.

Corrective maintenance

Maintenance is performed only when it has failed and you are required to act immediately. This form of maintenance can be supported by spare parts or in some cases a spare pump so that down time is kept to a minimum.

Preventative maintenance

This form of maintenance requires you to act just prior to failure. However, deciding on what & when has to be performed.

Predictive maintenance

By recording selected readings, noting operational conditions you can try to predict when the unit may fail and act prior to this point being reached.

Maintenance Procedure 

Corrective Maintenance Procedure

When corrective maintenance is done, the equipment should be inspected to identify the reason for the failure and to allow action to be taken to eliminate or reduce the frequency of future similar failures. These inspections should be included in the work plan.

Preventative Maintenance Procedure

This type of maintenance divides into many periods of time .In pumps; there are three periods of time: six months PM, one year PM and six years PM. 

Corrective Maintenance Procedure

When corrective maintenance is done, the equipment should be inspected to identify the reason for the failure and to allow action to be taken to eliminate or reduce the frequency of future similar failures. These inspections should be included in the work plan.

Six Months PM: Checklist

  1. Chick the level of oil.
  2. Cleaning the pump.
  3. Visual chick. 

One Year PM: Checklist

  1. Change oil.
  2. Cleaning the pump.
  3. Visual chick.
  4. Chick the seals.
  5. Chick the vibration on the pumps.

Six Years PM: Checklist

  1. Dissemble the pump.
  2. Visually inspect shaft sleeves and bearings for damage and excessive wear.        
  3. Measuring the clearances for suction bell sleeve bearing and for discharge connection sleeve bearing.
  4. Visually inspect wear ring for damage and excessive wear.
  5. Inspection of Casing wear ring for the impeller & also check mechanical seal for damage or excessive wear & Reassemble the pump.
Pump Alignment means that the center line of the pump is aligned with the center line of the driver.
  1. Vibration & noise.
  2. Impeller unbalancing.
  3. Bearing & coupling failure.
  4. Packing could support a misaligned shaft. A mechanical seal cannot.
  5. The misalignment could be severe enough to cause contact between stationary and 
  6. Rotating seal components.
  7. The shaft can contact the restriction bushing often found at the end of the stuffing box.
  8. The shaft or sleeve can contact the stationery face of the mechanical seal.
The impeller could contact the volute or back plate.


                                                                 Misalignment
                                                             




The running cost of a pump is many times its initial cost – selecting an efficient pumping system can save your business money. Almost the pumps in Qurayyah Power Plant are centrifugal pumps because its advantages such as simplicity, compactness, weight saving, adaptability to high-speed prime movers and low maintenance.

Fuel Oil Pump

The function of the main fuel oil pumps is to transport fuel oil from the storage tanks to the Unit 1 and Unit 2 fuel oil burners.

Pump specification
Horizontal, twin screw, positive displacement pumps. 
Capacity = 185 m3/hr
Motor Power = 350 k w 

Fuel oil pump



The function of the boiler circulation pumps is to provide controlled circulation of the boiler water through the boiler water walls.

Pump specification

Vertically mounted, inverted, single stage, end suction, double volute, centrifugal pumps driven by glandless, submerged (water immersed) AC motors.
There are three boiler circulation pumps for each unit.  



Boiler circulating Pump



Boiler Feed Pump Turbine

The function of the boiler feed pump turbine is to convert thermal energy into mechanical energy to drive the associated feed water pump and booster pump. Each unit is equipped with two turbine driven boiler feed pumps.



Boiler Feed Pump Turbine




Condensate Extraction Pumps
The function of the condensate extraction pumps is to supply the defector storage tank with an adequate amount of condensate at the appropriate pressure.

Condensate Extraction Pumps
                                                    
Let us move towards another technical blog, 

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