Field Instrumentation

What is a pump?

A pump can be described as a device that can transfer or raise the fluids, this device can also be used to achieve a constant flow rate or a constant pressure rate. The pump can add energy to the fluid this device is capable to convert fuel energy to water energy with the help of combustion engines or electric motors. Pumps are used in many industries to transfer and distribute fluids, the pump can convert the mechanical energy into hydraulic energy. There are different kinds of pumps and electrical energy is used to operate these pumps. Most pumps are used in industries to transfer fluid from one place to another and it is also used to circulate liquid around a system an example of this will be a flow of cooling water or lubricants through machines and equipment.

How to select a pump?

  • It must be selected according to the system requirements
  • System layout
  • Fluid characteristics
  • Material of construction
  • Code requirements
  • It must be selected according to the flow rate

How to improve the performance of a pump?

If the diameter of the impeller is reduced then the pump would run more efficiently at lower flow without throttling. Certain manufacturers offer pumps with different motor options so that one pump can be used over a wide range of duties. By using a variable speed drive the pump can operate efficiently for a wide range of speeds and this would save a lot of energy. A pump can be used to do different duties according to the diameter of the impeller and also because of the speed of the motor. So we must select the motor according to the applications, the efficiency of the pump could decrease if the pump is operated away from its efficiency point.

What are the functions of a pump?

  • It could transfer water to boiler to make steam
  • It can also provide cooling liquids to equipment
  • It can transfer liquid for plant process
  • Potable water to the city
  • It can deliver fluid at higher elevation or at a long distance

What are the characteristics of pumps?

  • A pump is not able to create pressure, it only provides the flow
  • The pump is not able to pump the vapor, it can only pump liquids
  • The flow is created by the pump by reducing the atmospheric pressure in the water
  • The energy of the pump is measured by using head instead of pressure and it is because that the pressure from the pump would change if the specific gravity of the liquid changes

What are the factors that must be considered while doing the electrical connection of the pump?

The electrical connection must be proper at the bolts which are used for earthing. The cable which is used between the motor and the power supply should be of the proper size in order to reduce the voltage drop. The joints in the cable must be avoided, the wires and connections must be properly insulated otherwise it can cause electrical shocks. A proper backup connection must be used. 

How to do the installation of the pump?

The pump must be installed near the water source in order to reduce the suction lift. If the pump is installed then the foundation bolts must be tightened properly and the grouting must be completed. The pipe size must be compatible with the flange size in order to get a good discharge. The piping must be airtight, if there is any leakage in the suction pipe then it would affect the performance of the pump. In order to get a better discharge, the piping should be short. There should be a check valve in the pipeline in order to provide pressure of more than twenty meters. The horizontal length of the suction pipe must be straight in order to avoid the air tapping in the pipe.

What are the types of pumps and how do they operate?

Centrifugal pump

The centrifugal pump would raise water from a lower level to a higher level by using the centrifugal force. The centrifugal pumps are used in places where high flow rates at low pressure are needed. This pump has an impeller and it would be mounted in a housing and the impeller would rotate. A shaft would transfer the torque from the motor to the impeller while the startup.  The water will be drawn into the impeller due to the quick rotation of the impeller pressure and velocity is created and the water would be thrown outward. So the rotating impeller converts the driver energy into kinetic energy and provides centrifugal acceleration to the liquid. So the liquid is accelerated by the impeller and it would flow into a diffuser after that it would reach the piping system.

What are the advantages of a centrifugal pump?

  • It can handle hazardous liquids
  • Less friction
  • No noise
  • No leakage
  • Good efficiency
  • There is a gap between the pump chamber and the motor so there is no heat transfer between them and also because of the gap, water won’t enter the motor
  • The operation of this pump is by using magnetic coupling and it won’t handle high loads and thus the motor damage can be avoided

Positive displacement pump

A positive displacement pump operates by forcing the fluid from the inlet part, of the pump to the discharge section of the pump. These pumps can be considered larger or equal capacity to that of the dynamic pumps. These pumps are mostly used in hydraulic systems at pressure up to five thousand PSI. These pumps can provide fixed displacement per revolution

What are the advantages of the positive displacement pump?

  • It can be used for the constant flow
  • It can be used to pump oils and viscous fluids because of its ability to generate high pressure
  • These pumps can do both the combination of low flow and high pressure and this feature would be useful to handle fluids that have suspended solids

Axial flow pump

These pumps are also called propeller pumps and these pumps develop most of their pressure by propelling or lifting action of the vanes on the liquid. This type of pump would use a screw propeller to axially accelerate the fluid. The outlet passage and the guide vanes are arranged in a way to increase the liquid pressure. The leakage is prevented by a mechanical seal, the axial pump is used where large qualities of water at the low head are needed. These pumps are used in wet pit drainage, low-pressure irrigation, and stormwater applications

Reciprocating pump

It is a type of positive displacement pump, this is a displacement pump so it would create a lift and pressure by displacing the liquid with a moving piston. This type of pump would draw a volume of water into the cylinder with the help of a suction valve. So the water which is drawn will be discharged under positive pressure through the outlet valves on the discharge stroke. The discharging of a reciprocating pump is pulsating and changes only when the pump speed changes and it is because the intake is always a constant volume. These pumps can be used for a highly viscous fluid. The efficiency of this pump would increase if the viscosity is high. This pump has constant flow with varying pressure and also has a pulsating flow.

What are the problems or faults that could happen in a pump and how to troubleshoot it?

If the pump fails to deliver water

This problem could be caused because of the rotation in the wrong direction, and it could also happen if the pump is not primed or filled with water. This fault could happen if there is an air or vapor pocket in the suction line and also could happen if there are air leaks in suction line or stuffing box. If the pump is not up to its rated speed then it could also be a reason for this, so all of this must be considered if the pump fails to deliver water.

Pump does not prime

It could happen if the pump and suction line is not filled with water, or because of air leaks in the suction line or stuffing box. This problem could happen if there is vapor or air in the liquid.

Pump losses water after start

This could happen if there is an air or vapor pocket in the suction line or if there is gas or vapor in the liquid. If the suction lift is too high then it can cause the water to lose.

Overheating of stuffing box

This could happen if there is gas or vapor in the liquid or if the gland packing is too tight. This problem could arise if the gland packing is not lubricated or if there is not enough cooling water.

Pump overloads prime mover

If the pump runs beyond the lower recommended head or due to the low pump efficiency.

Pump does not deliver the rated discharge

This could happen if the foot valve is very small or clogged, or because of the damages of the wear rings or impeller. This could happen if there is no proper power supply, or because of the variation of the speed than it is specified.

Excessive power consumption

So the excessive power consumption is due to mechanical or hydraulic problems. The mechanical problems could be bearing failure, pump seizure, and poor shaft alignments. If the viscosity is high then it could cause the motor overloading.

What are the applications of pumps?

  • It is used in oil-fired boilers to pump fuel oil to the burners
  • A lubricating oil pump is used to circulate oil to the bearings of a machine
  • A circulating water pump is also called a cooling water pump, it is used to pump water through a heat exchanger such as a condenser or an oil cooler
  • A chemical feed pump is used to pump chemicals to boilers
  • A fire pump is used to supply water to plant fire lines
  • Pumps are used for irrigation
  • It is also used for domestic purpose

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