Pumps - An Introduction

What is a pump ? How are pumps classified ?

A Pump is a mechanical device which moves liquid from a lower level to a higher level.  The pump draws the liquid inside pressurizes it and discharges it through the outlet.  A pump is driven by a prime mover which is, generally, an electric motor.  IC engines and turbines can also be used as prime movers to drive the pump.

Pumps are usually classified into two broad categories

Rotodynamic pump and
Positive Displacement Pumps

Rotodynamic Pumps

In these pumps, a rotary device with blades, called the impeller drives the liquid.  The liquid gets kinetic energy in the process.  The kinetic energy is converted into pressure by means of the design of the pump.

The rotodynamic pumps can be divided into

Centrifugal pumps : Here, the impeller with blades drives the liquid radially outwards towards the casing.  The liquid gets pressurized as it exits the pump.

Axial Pumps:  In these pumps, the liquid is driven axially by the impeller.  The flow of the liquid is parallel to the axis of the impeller.

Positive Displacement Pumps

Positive Displacement pumps are another major category of pumps.  In positive displacement pumps, the liquid is drawn into a chamber, pressurized and expelled at the discharge side.

These pumps are in turn classified into two types

Reciprocating Pumps: In these pumps, a piston moves inside a cylinder.  The piston creates low pressure when it moves up.  This sucks the liquid inside.  Once inside, the piston moves down and pressurizes the liquid which is discharged through a port.  The handpump used to pump water is a reciprocating pump. Eg. Plunger Pump

Rotary Pump: In these types of pumps, two rotating gears or screws move inside a casing.  As the screws or the gears move, the liquid is progressively taken into the pump.  The cross section of the casing is reduced as the liquid moves.  This causes pressure at the discharge side.  Examples: Screw Pumps, Gear Pumps

Rotodynamic Pump

A rotodynamic pump is a pump in which the impeller imparts kinetic energy to the fluid. The term Rotodynamic is a broad one encompassing all pumps with rotary impellers.

Centrifugal pumps are a type of rotodynamic pumps.  The impeller of the centrifugal pump draws in water from the suction and pushes the water radially giving kinetic energy to the liquid.

Apart from centrifugal pumps, axial flow pumps in which the water flows radially, parallel to the axis of the shaft, are also called rotor dynamic pump.

Positive displacement pump ?

A positive displacement pump is a pump which draws a fixed amount of the liquid from the inlet and discharges it in the outlet at high pressure.

Positive displacement pumps have an expanding cavity in the inlet and a decreasing cavity near the inlet.  Positive displacement pumps have constant volume.  The pumps deliver a constant flow regardless of the discharge pressure.  The pressure depends on the speed of the pump.

Positive displacement pumps can be further classified into reciprocating pumps, rotary pumps, etc.

Positive displacement pumps should never be operated with the outlet closed. Since the pump works on a fixed volume of liquid,  the pump can get seriously damaged if it is accidentally operated with the outlet closed.

A special pressure relief valve is provided for protection against excess pressure.

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A comparison of Centrifugal and Positive Displacement Pumps

Priming
Centrifugal pumps need to be primed separately.  The priming can be manual or through a separate priming arrangments

Positive displacement pumps are self priming as they develop a low pressure which can draw the fluid inside.

Flow Rate
Centrifugal Pumps have a flow rate which is dependent on the discharge pressure.  Positive Displacement pumps have a constant flow rate regardless of the pressure

Viscous Fluids
Centrifugal pumps cannot handle viscous fluids due to increased friction between the impeller and the liquid.  Positive displacement pumps can handle viscous fluids.

Efficiency
Centrifugal pumps have lower efficiency as the viscosity increases. Positive displacement pumps have high efficiency as the viscosity increases

Method of operation
Centrifugal pumps build pressure by imparting velocity to the liquid and then converting it into pressure.  Positive displacement pumps develop pressure by drawing a fixed amount of liquid and pressurizing it.

What are the different parts of the Centrifugal Pump?

The centrifugal pump consists of the following main parts.

The Impeller
The Impeller is the heart of the pump.  The impeller provides kinetic energy to the water entering the pump from the suction pipe.

The Volute
The volute refers to the tubular casing of the pump which increases in size as it approaches the discharge port.  The function of the volute is to convert the velocity of the water from the impeller into pressure.  It achieves this by a gradual increase in volume.

The Suction Pipe
The Suction pipe connects the sump to the pump inlet.

The Foot Valve
The foot valve is a non-return valve which is connected on the suction side.  The foot valve prevents the flow of water from the overhead tank which is at a higher level to the sump when the pump is not running.

The Strainer
The Strainer prevents the entry of debris into the pump

The Delivery Pipe
The Delivery Pipe serves to supply water to the tank from the discharge side of the pump.

The Delivery Valve
The delivery valve is a valve at the output of the pump in the delivery line.  The function of this valve is to control the output of the pump.  The delivery valve is closed when the pump is first started during the priming process.  It is then gradually opened.

Common Boiler Concepts

Carbondioxide in Boiler Water

Carbon Dioxide is a constituent of air. As such it get dissolved in water. Carbondioxide mixes with steam to form carbonic acid. Carbonic acid is an unstable compound. It has a tendency to react with steel and can thus corrode piping. Another way carbon is present in the water is in the form of bicarbonates. These carbonates decompose in the boiler to produce carbon dioxide. This carbon dioxide is usually present in the condensate. 

Carbon dioxide reduces the pH of the water. This turns the water acidic which results in further corrosion. Hence, carbon dioxide has to be removed from the water. One simple way of removing carbondioxide is by heating the water. Heating the water reduces the solubility and thus removes the gas. The water should be externally treated to remove the carbonates. Venting at specific locations of condensation can also reduce the carbon dioxide in the system.

      

Design Pressure and Maximum allowable Working Pressure (MAWP) of the Boiler

The Design Pressure of the boiler is the maximum pressure at which the boiler can be operated under normal operating conditions. It is equal to the highest setting of the safety valves in the boiler. 

For instance, if a boiler has two safety valves, the design pressure will be equal to the setting of the valve with the higher setting. The design pressure is calculated based on the stress that the boiler will undergo during operation across its lifetime.

     

Maximum Allowable Working Pressure

This is the maximum pressure that the boiler can withstand. The maximum allowable working pressure is calculated based on the strength of the material, the thickness of the walls, etc. The Design Pressure of the boiler is lesser than or equal to the Maximum Allowable Working Pressure.

     

Super Heater Outlet Pressure

The Super Heater Outlet Pressure is the pressure at which steam is expelled from the super heater. This pressure is depended on the inlet pressure of the turbine. It is generally maintained at 5 percent over the inlet pressure of the turbine. The excess pressure is to offset the drop in pressure between the boiler outlet and the turbine inlet. 

This drop in pressure is due to the piping losses. In fixed pressure boilers, the SH outlet pressure is constant and the turbine inlet pressure is varied with valves in accordance with the load. In variable pressure boilers, the boiler outlet pressure varies with the load.

     

Peak Rating of a Boiler

The Peak Rating of a boiler is the extra evaporation which the boiler can deliver for a specified period such as 2 to 4 hour a day. In some cases, the boiler will be required to operate above the Maximum Continous Rating (MCR) for short period of time. The efficiency during this temporary overloaded operation will be marginally lower. 

The Peak Rating is usually about 110 percent of the normal operating capacity for about 4 hours a day. Any further increase in the Peak Rating will need redesign of the boiler. While the Peak Rating can be used in a contingency, it is best avoided. This is because operating the boiler at peak rating will result in premature aging of the boiler. It will also result in issues such as slagging, fouling, erosion, etc.

     

Maximum Continuous Rating (MCR) and Normal Continuous Rating of a boiler

The Maximum Continuous Rating (MCR) is the maximum output which the boiler can delivery when operated at a specified set of conditions. Alternatively, it can be understood as the minimum assured production of steam in a boiler. The MCR. 

A well designed and maintained boiler will produce an output equal to the MCR value throughout its life. A new boiler can be operated at 8 to 10% above the Maximum Continous Rating. However, the excess capacity is, usually, lost with age.

     

Normal Continuous Rating

The Normal Continuous Rating (NCR) is the rating at which the boiler will be operated normally. The NCR is about 90 percent of the MCR. The NCR is determined based on the rating of the turbine. The boiler is designed to have maximum efficiency at NCR.

       

Boiler Water Treatment

The water in the boiler should be kept within proper chemical paramaters. The treatment of boiler water is intended to facilitate proper heat exchange, protection from corrosion and the generation of steam. Boiler water treatment can be categorized into two main categories. External Treatment in which the water is taken out of the boiler and treated and Internal Treatment in which the water is treated while still in the boiler External Treatment Some of the processes done in external treatment are softening, evaporation, deaeration, etc. 

Internal Treatment Internal treatment involves conditioning the water inside the boiler through chemicals. Internal treatment is generally done in low or moderate pressure boilers. Internal treatments is intended to prevent water hardness and the formation of scales. to prevent sludge from settling in the boiler walls. To prevent foam carryover by providing anti foam protection. To remove oxygen from the water to maintain water alkalinity to prevent corrosion.

     

Overheating in Boilers

Overheating in boilers occurs usually in the boiler tubes. This problem is seen when the boiler is first commissioned and a short while later. It usually does not appear after the plant has been stabilized. Scale formation in the tubes can be a reason for overheating. Scale formation prevents heat transfer and can cause localized overheating. Overheating can also occur if there are changes in the boiler operation such as a change in fuel or any change in any other significant parameter.

      

Silica in Boiler Water

Ordinary Silica is insoluble in water. But when silica combines with other materials such as lime and soda, it can form scales which are very difficult to remove. Soda and lime are used in softening units. Use of silica based lubricants in the thermal plant as well can also result in silica entering the boiler water. Another source is the presence of unreacted silicon in the feed water. If silica is not removed in time, it forms deposits in the turbine nozzles and change the direction of the steam. 

The velocities and pressure drops are changed inside the turbine resulting in reduced efficiency. Uneven nozzle flow can result in torsional vibration due to uneven loading of the blades. This can result in vibrations. Silica deposits in the boiler are difficult to remove. They equipment has to be dismantled and physically cleaned. Blasting aluminium oxide on the surface is also a method used in the removal of silica deposits.

      

British Thermal Units and Boilers

Boiler Capacities are often denoted in British Thermal Units. One British Thermal Unit is defined as the amount of heat required to raise one pound of water by one degree Fahrenheit. While the BTU has generally been replaced with the more popular unit, the Joule, Boilers and the Heating industry still use the British Thermal Unit. 

One BTU is equal to 1.06 Joule BTUs are also used for indicating the energy in fuels. Oil has a BTU of 138000 per gallon. Natural Gas has a BTU of 1075 per cubic foot. A bigger unit is the MMBTU which stands for one million BTU. The M is the Roman number for thousand. MM stands for a thousand thousand which is one million.