Working Mechanism of a Centrifugal Pump

Working Mechanism of a Centrifugal Pump

A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose is to convert energy of a prime mover (a electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Note: All of the forms of energy involved in a liquid flow system are expressed in terms of feet of liquid i.e. head.

Generation of Centrifugal Force

The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string.

Conversion of Kinetic Energy to Pressure Energy

The key idea is that the energy created by the centrifugal force is kinetic energy . The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle.

Introduction to Centrifugal Pumps

Introduction

The operating manual of any centrifugal pump often starts with a general statement,“Your centrifugal pump will give you completely trouble free and satisfactory service only on the condition that it is installed and operated with due care and is properly maintained.” Despite all the care in operation and maintenance, engineers often face the statement “the pump has failed i.e. it can no longer be kept in service”. Inability to deliver the desired flow and head is just one of the most common conditions for taking a pump out of service. There are other many conditions in which a pump, despite suffering no loss in flow or head, is considered to have failed and has to be pulled out of service as soon as possible. These include seal related problems (leakages, loss of flushing, cooling, quenching systems, etc), pump and motor bearings related problems (loss of lubrication, cooling, contamination of oil, abnormal noise, etc), leakages from pump casing, very high noise and vibration levels, or driver (motor or turbine) related problems.

The list of pump failure conditions mentioned above is neither exhaustive nor are the conditions mutually exclusive. Often the root causes of failure are the same but the symptoms are different. A little care when first symptoms of a problem appear can save the pumps from permanent failures. Thus the most important task in such situations is to find out whether the pump has failed mechanically or if there is some process deficiency, or both. Many times when the pumps are sent to the workshop, the maintenance people do not find anything wrong on disassembling it. Thus the decision to pull a pump out of service for maintenance / repair should be made after a detailed analysis of the symptoms and root causes of the pump failure. Also, in case of any mechanical failure or physical damage of pump internals, the operating engineer should be able to relate the failure to the process unit’s operating problems.

Any operating engineer, who typically has a chemical engineering background and who desires to protect his pumps from frequent failures must develop not only a good understanding of the process but also thorough knowledge of the mechanics of the pump. Effective troubleshooting requires an ability to observe changes in performance over time, and in the event of a failure, the capacity to thoroughly investigate the cause of the failure and take measures to prevent the problem fro m re-occurring.

labyrinth piston compressor

LABYRINTH PISTON

These are vertical type reciprocating. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. In labyrinth piston compressors, an extremely large number of throttling points provide the sealing effect around pistons and piston rods. No contact seals are used. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also
having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. This increases the durability, reliability and availability of the compressor along with its economic operation.

These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant. This is unique application where lubricants are not allowed in the cylinders, which is true in case of oxygen compressor, where safety is the most important. It is also employed for applications where the process gas is heavily contaminated with the impurities.

Piston and piston rod are guided by the crosshead and the guide bearing which are located in the oil lubricated crankcase. The oil is supplied by the crankshaft driven lube oil pump
The distance piece separates the gas compressing section from the oil lubricated crankcase. Where process gas can be permitted in the distance piece, then open distance piece type compressors are used. When strict separation of cylinder from the crankcase is essential and at the same time no ambient air is also allowed in the distance piece, then Nitrogen purge is provided. Crankshaft is provided with mechanical seal to prevent the gas leaking in to the atmosphere.

ADVANTAGES:
1. Reliable operation
2. Safe for operation , environment
3. Economical
4. High availability
5. Less floor area is required.
6. As rider rings, piston rings and lubrication is not required the valve life is more.

classification of reciprocating compressor

Reciprocating Compressors can be classified as follows based on

I. Cylinder lubrication:
1. Lubricated type & Non-lubricated type
2. Dry running piston rings
3. Ringless or labyrinth type.

II. Cylinder cooling:
1. Air cooled
2. Water cooled
III Cylinder loading
1. Single acting
2. Double acting
IV Cylinder arrangement,
1. Vertical inline or V-type
2. Horizontal opposed balanced.

CYLINDER LUBRICATION:

Lubricated Compressors :
Generally big reciprocating compressor cylinders are lubricated to avoid wear and tear of liner, piston rings, rider rings and stuffing box. Lubricants are injected in drops and are lost with the process gas.

Lubrication of cylinder reduces wear of parts, enhances life of parts and also reduces gas discharge temperature. It is necessary for the lubricant to be compatible with the process gas and down stream system. Generally recip compressors above 100 KW are lubricated type.

Non-Lubricated Type :
There are many services in which oil in any form in the compressed gas, is not acceptable such as instrument air compressor and some process gas compressors. In such services oil is not injected in the cylinders, instead wear parts are made of soft material with low co-efficient of friction such as PTFE,CFT etc. The wear rate of stuffing box packing, rider rings piston rings and cylinder/liner surface may be comparatively more.

Process requirements and gas to be compressed dictate whether to use a compressor with lubricated or non-lubricated cylinder. Some chemical processes do not permit use of lubricant in the system, due to quality problems, catalyst-poising etc.

Following factors must be considered,
1. Non-lubricated compressors cost more than lubricated compressors
2. Non-lubricated compressors require more power
3. Non-lubricated compressors require more maintenance, labyrinth compressors
being exception.

TYPES OF COOLING:

Air Cooled type :
This type of compressors has fins cast as part of the cylinder to dissipate some of the heat generated by the compression of the gas. In some compressors vanes are cast as part of the flywheel or sheave to act as fan to help remove the heat from the cylinder surface. This type of cylinder cooling is used in small portable compressors.

Water-cooled type:
Water-cooled compressors are most common in industry. It is impossible to sustain cooling with air in big compressors where heat generation is very high. Water jackets are cast as part of the cylinder. Water is circulated in the cylinder jacket from external source.

In some case the cylinder jacket temperature is to be maintained few degrees higher than the process gas temperature to avoid condensation during standstill, this is possible with water-cooled cylinders by maintaining the water temperature with use of heaters.

TYPES OF CYLINDER LOADING:

Single acting
In single acting cylinder compression takes place only on one side of the piston and valves are installed only on that side.

Double acting
In double acting cylinder compression takes place on both sides of the piston. When one side is in compression, the other is in suction.

CYLINDER ARRANGEMENT:

Horizontal Type:
This type of arrangement is most common in industries. Multi cylinder reciprocating compressors with horizontal cylinders are very often designed as balanced opposed type. Balanced opposed frame is characterized by adjacent pair of crank 180 ° out of phase and separated by crank webs only. With this configuration inertia forces are balanced. The balanced-opposed design is separable frame.

Vertical Inline or V-type:
Vertical in line or V-type are used for small or moderate compression ratio and duty. Normally this type of compressors cylinder arrangement are non-lubricated type and occupy less space.

Labyrinth piston compressors
These are vertical type. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant.

multistaging with inter cooling

It is not always desirable or possible to achieve the required rise in pressure in a single compression stage. In multistaging gas discharge from the first stage is cooled in the inter cooler up to suction temperature of the first stage before going to the second stage. This is called as perfect intercooling.

Advantages of multistaging:

1. Good volumetric efficiency as compression is done in more than one stage and hence compression ratio is controlled.
2. Lower discharge temperature and hence selection of material of construction for cylinder and its components and results in smaller size of subsequent stages.
3. Reduced work of compression, as due to intercooling, compression is closer to isothermal (gives rise to minimum work of compression). This results in to saving of power and smaller sizes of subsequent stages.
4. Limits pressure differential. This reduces excess strains in the frame.

schematic of reciprocating compressor

Suction:
When the piston moves towards BDC (backward stroke), the pressure within cylinder on the top of the piston drops below suction pressure in the header, thereby forcing suction valve to open and allows gas in the cylinder.

Compression:
When the piston moves from BDC towards TDC (forward stroke) pressure on the top side of the piston start increasing, thereby closing the suction valve.

Discharge:
As the piston approaches TDC, the discharge valve opens as the pressure inside the cylinder on top side of the piston is higher than that in the discharge pipeline. Thus gas is discharged in to the header.

Expansion:
During backward stroke the trapped gas (between piston & cylinder end cover) called clearance volume, expands.

thermodynamic Process

Compression of gas is a thermodynamic process, and can be done in many ways; some of them are listed below.

• Isobaric Process – a process wherein pressure remains constant.
• Isothermal Process – a process in which there os no change in temperature.
• Isentropic Process – a process in which there is no change in entropy.
• Adiabatic Process – a process during which there is no external exchange of heat. PVk = Constant
• Polytropic Process – a process for perfect gases, follows the law PVn = Constant n=1 for isothermal process.