Boilers

Power Plants

Battelle has provided a well-written report that discusses power plant coal utilization in great detail. It gives

a thermal efficiency of 80-83% for steam generation plants and 37-38% thermal efficiency for power generating plants at base load (about 70%). A base load plant designed for about 400 MW and up will run at steam pressures of 2,400 or 3.600psi and 1,000”F with reheat to 1.000”F and regenerative heating of feedwater by steam extracted from the turbine. A thermal efficiency of 40% can be had from such a plant at full load and 38% at high annual load factor. The 3,600psi case is supercritical and is called a once-through boiler, because it has no steam drum. Plants designed folr about 100-350MW run around 1,800 psi and 1,000”F with reheat to 1,000”F. Below 100 MW a typical condition would be about 1,350psi and 950°F with no reheat. Below 60% load factor, efficiency falls off rapidly. The average efficiency for all steam power plants on an annual basis is about 33%.

Pumps

Affinity laws


Dynamic type pumps obey the affinity laws:

1. Capacity varies directly with impeller diameter and speed. 
2. Head varies directly with the square of impeller diameter and speed.
3. Horsepower varies directly with the cube of impellerdiameter and speed.

Horsepower


The handiest pump horsepower formula for a process  engineer is: 


HP = GPM(AP)/1715(Eff)

where:

HP = Pump horsepower
GPM = Gallons per minute
AP = Delivered pressure (discharge minus suction), psi
Eff = Pump efficiency, fraction

Piping

Standard pipe is made in a discrete number of sizes that are designated by nominal diameters in inches, as “inches IPS (iron pipe size).” Table A5 lists some of these sizes with dimensions in inches. Depending on the size, up to 14 different wall thicknesses are made With the same outside diameter. They are identified by schedule numbers, of which the most common is Schedule 40. Approximately,


Schedule number = 1000 PIS,



where:
P = internal pressure, psig
S = allowable working stress in psi.

Ultrasonic Cleaning

The material presented in this section features models of ultrasonic cleaners that are unique to this manufacturer. Other sources’ equivalent models’ features should be considered for relevant applications.


Theory of Ultrasonic Cleaning



Everything that makes a sound vibrates, and everything that vibrates makes a sound; however, not all sounds are audible. Ultrasound literally means beyond sound—sound beyond the audible spectrum. Considering 18,000 Hz (cycles per second) as an approximate limit of human hearing, ultrasonics refers to sound above
18,000 Hz.
The ultrasonic power supply (generator) converts 50/60 Hz voltage to high frequency 20 or 40 kHz (20,000/40,000 cycles per second) electrical energy. This electrical energy is transmitted to the piezoelectric transducer within the converter, where it is changed to high-frequency mechanical vibration. The vibrations from the converter are amplified by the probe (horn), creating pressure waves in the liquid. This action forms millions of microscopic bubbles (cavities) that expand during the negative pressure excursion and implode violently during the positive excursion. It is this phenomenon, referred to as cavitation, that produces the powerful shearing action at the probe tip, and causes the molecules in the liquid to become intensely agitated.

How Desalination Works

There are many different techniques for desalination. Among the main ones are the multistage flash (MSF) process and reverse osmosis. Countries that do not have anabundance of fresh water are prime candidates for the use of this technology. Quatar’s installations include the Ras Abu Fontas B power and desalination plant, which cost $1 billion to build. Dubai in the United Arab Emirates (UAE) has a 60- million-gal/day desalination plant at Jebel Ali. The plant’s eight MSF units are part of a cogeneration power facility. The fresh water would have been at least as much incentive in the Middle East as the increased total thermal efficiency. The desalination equipment in both cases was supplied by Weir Westgarth (WW).



WW designed the MSF process. The principle of the system is simple: Water and steam in a closed system can be made to boil at temperatures lower than at standard temperature and pressure by reduction of the system pressure. MSF plants contain a series of closed chambers—as many as 20—each held at a lower pressure than the preceding one.

Fans, Centrifugal

Besides being used in process plants, centrifugal fans are common in building heating and air-conditioning systems. Vendors for these systems should be consulted for required options

Types of Fans

Three types of centrifugal fans are available: (1) forward curved blade fans, (2) radial blade fans, and (3) backward curved blade fans. The characteristics of the forward curved blade fan make it the most appropriate type of cooling tower service. By virtue of the direction and velocity of the air leaving the fan wheel, the fan can be equipped with a comparatively small size housing, which is desirable from a structural standpoint. Also, because the required velocity is generated at a comparatively low speed, forward curved blade fans tend to operate quieter than other centrifugal types. Note the relatively small rpm and head values. The envelopes on the head versus volume curves each indicate a separate fan model is available. Centrifugal fans are usually of sheet metal construction, with the most popular protective coating being hot-dip galvanization. Damper mechanisms are also available to facilitate capacity control of the cooling tower.

Fan

All propeller-type fans operate in accordance with common laws. For a given fan
and cooling tower system, the following is true:
1. The capacity (cfm) varies directly as the speed (rpm) ratio, and directly as the
pitch angle of the blades relative to the plane of rotation.
2. The static pressure (hs) varies as the square of the capacity ratio.
3. The fan horsepower varies as the cube of the capacity ratio.
4. At constant cfm, the fan horsepower and static pressure vary directly with the
air density.
If, for example, the capacity (cfm) of a given fan were decreased by 50 percent
(either by a reduction to half of design rpm, or by a reduction in blade pitch angle
at constant speed), the capacity ratio would be 0.5. Concurrently, the static pressure
would become 25 percent of before, and the fan horsepower would become 12.5
percent of before. These characteristics afford unique opportunities to combine cold
water temperature control with significant energy savings.

Cooling Towers: Design and Operation Considerations

Cooling towers are a very important part of many chemical plants.  They represent a relatively inexpensive and dependable means of removing low grade heat from cooling water.

Closed Loop Cooling Tower System

The make-up water source is used to replenish water lost to evaporation.  Hot water from heat exchangers is sent to the cooling tower.   The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.

Compressors

A compressor is a device which pressurizes a working fluid. One of the basic purposes of using a compressor is to compress the fluid and to deliver it at a pressure higher than its original pressure. Compression is required for a variety of purposes, some of which are listed below:

1. To provide air for combustion
2. To transport process fluid through pipelines
3. To provide compressed air for driving pneumatic tools
4. To circulate process fluid within a process

Different types of compressors are shown in Fig. below. Positive displacement compressors are used for intermittent flow in which successive volumes of fluid are confined in a closed space to increase their pressures. Rotary compressors provide continuous flow. In rotary compressors, rapidly rotating parts (impellers) accelerate fluid to a high speed; this velocity is then converted to additional pressure by gradual deceleration in the diffuser or volute which surrounds the impeller. Positive-displacement compressors can be further classified as either reciprocating or rotary type. The reciprocating compressor has a piston having a reciprocating motion within a cylinder. The rotary positive-displacement compressors have rotating elements whose positive action results in compression and displacement. The rotary positive-displacement compressors can be further subdivided into sliding vane, liquid piston, straight lobe, and helical lobe
compressors. The continuous flow compressors can be classified as either dynamic compressors or ejectors. Ejectors entrain the in-flowing fluid by using a high-velocity gas or steam jet and then convert the velocity of the mixture to pressure in a diffuser. The dynamic compressors have rotating elements, which accelerate the inflowing fluid, and convert the velocity head to pressure head, partially in the rotating elements and partially in the stationary diffusers or blade. The dynamic compressors can be further subdivided into centrifugal, axial-flow, and mixed-flow compressors. The main flow of gas in the centrifugal compressor is radial. The flow of gas in an axial compressor is axial, and the mixed-flow compressor combines some characteristics of both centrifugal and axial compressors.

It is not always obvious what type of compressor is needed for an application. Of the many types of compressors used in the process industries, some of the more significant are the centrifugal, axial, rotary, and reciprocating compressors.

For very high flows and low pressure ratios, an axial-flow compressor would be best. Axial-flow compressors usually have a higher efficiency but a smaller operating region than does a centrifugal machine. Centrifugal compressors operate most efficiently at medium flow rates and high pressure ratios. Rotary and reciprocating compressors (positive-displacement machines) are best used for low flow rates and high pressure ratios. The positivedisplacement compressors and, in particular, reciprocating compressors were the most widely used in the process and pipeline industries up to and through the 1960s.

In turbomachinery the centrifugal flow and the axial-flow compressors are the ones used for compressing gases. Positive-displacement compressors such as reciprocating, gear-type, or lobe-type are widely used in the industry for many other applications such as slurry pumping

The industrial pressure ratio is low because the operating range needs to be large. The operating range is defined as the range between the surge point and the choke point. The surge point is the point at which the flow is reversed in the compressor. The choke point is the point at which the flow has reached Mach = 1.0, the point where no more flow can get through the unit, a “stone wall.” When surge occurs, the flow is reversed, and so are all the forces acting on the compressor, especially the thrust forces. Surge can lead to total destruction of the compressor. Thus surge is a region that must be avoided. Choke conditions cause a large drop in efficiency, but do not lead to destruction of the unit. Note that with the increase in pressure ratio and the number of stages, the operating range is narrowed in axial-flow and centrifugal compressors.

Please click at picture to enlarge size

Pump Diagnostics

Pump problems vary over a large range depending on the type of pumps and the usage of the pumps. They can be classified in the following manner by the pump type and the service:

1. Positive-displacement pumps—reciprocating pumps problems can be classified into the following categories:
   
   
   • Compressor valve problems: plate valves, feather valves, concentric disk valves, relief valves
     
   • Piston and rod assembly: piston rings, cylinder chatter, cylinder cooling, piston-rod packing
     
   • Lubrication system
     
   
   
2. Positive displacement pumps—gear-type and roots-type problems can be classified into the following categories:
   
   
   • Rotor dynamic problems: vibration problems, gear problems or roots rotor problems, bearing and seal problems
     
   • Lubrication systems
     
   
   
3. Continuous flow pumps such as centrifugal pumps problems can be classified into the following categories:
   
   
   • Cavitation
     
   • Capacity flow
     
   • Motor overload
     
   • Impeller
     
   • Bearings and seals
     
   • Lubrication systems
     

Table below about classifies different types of centrifugal pump-related problems, their possible causes, and corrective actions that can be taken to solve some of the more common issues. These problems in the table are classified into three major categories for these type of pumps:

1. Cavitation
2. Capacity flow
3. Motor overload

The use of vibration monitoring to diagnose pump and compressor problems is discussed at the end of the subsection on compressor problems.

Please click at picture to enlarge size

Introduction of Centrifugal Pumps

The centrifugal pump is the type most widely used in the chemical industry for transferring liquids of all types—raw materials, materials in manufacture, and finished products—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc. These pumps are available through a vast range of sizes, in capacities from 0.5 m3/h to 2 × 104m3/h (2 gal/min to 105 gal/min), and for discharge heads (pressures) from a few meters to approximately 48 MPa (7000 lbf/in2). The size and type best suited to a particular application can be determined only by an engineering study of the problem.

The primary advantages of a centrifugal pump are simplicity, low first cost, uniform (nonpulsating) flow, small floor space, low maintenance expense, quiet operation, and adaptability for use with a motor or a turbine drive.

A centrifugal pump, in its simplest form, consists of an impeller rotating within a casing. The impeller consists of a number of blades, either open or shrouded, mounted on a shaft that projects outside the casing. Its axis of rotation may be either horizontal or vertical, to suit the work to be done. Closed-type, or shrouded, impellers are generally the most efficient. Open- or semiopen-type impellers are used for viscous liquids or for liquids containing solid materials and on many small pumps for general service. Impellers may be of the single-suction or the double-suction type—single if the liquid enters from one side, double if it enters from both sides.

Casings There are three general types of casings, but each consists of a chamber in which the impeller rotates, provided with inlet and exit for the liquid being pumped. The simplest form is the circular casing, consisting of an annular chamber around the impeller; no attempt is made to overcome the losses that will arise from eddies and shock when the liquid leaving the impeller at relatively high velocities enters this chamber. Such casings are seldom used.

Volute casings take the form of a spiral increasing uniformly in cross-sectional area as the outlet is approached. The volute efficiently converts the velocity energy imparted to the liquid by the impeller into pressure energy.

A third type of casing is used in diffuser-type or turbine pumps. In this type, guide vanes or diffusers are interposed between the impeller discharge and the casing chamber. Losses are kept to a minimum in a well-designed pump of this type, and improved efficiency is obtained over a wider range of capacities. This construction is often used in multistage high-head pumps.

Action of a Centrifugal Pump Power from an outside source is applied to shaft A, rotating the impeller B within the stationary casing C. The blades of the impeller in revolving produce a reduction in pressure at the entrance or eye of the impeller. This causes liquid to flow into the impeller from the suction pipe D. This liquid is forced outward along the blades at increasing tangential velocity. The velocity head it has acquired when it leaves the blade tips is changed to pressure head as the liquid passes into the volute chamber and thence out the discharge E.

Pump Specifications

Pump specifications depend upon numerous factors but mostly on application. Typically, the following factors should be considered while preparing a specification.

1. Application, scope, and type
   
2. Service conditions
   
3. Operating conditions
   
4. Construction application-specific details and special considerations
   
   • Casing and connections
     
   • Impeller details
     
   • Shaft
     
   • Stuffing box details—lubrications, sealing, etc.
     
   • Bearing frame and bearings
     
   • Baseplate and couplings
     
   • Materials
     
   • Special operating conditions and miscellaneous items

Table below is based on the API and ASME codes and illustrates a typical specification for centrifugal pumps.

Please click at picture to enlarge size

Terminology of Pumps and Compressors (Part 2)

Static Suction Head The static suction head hss is the vertical distance measured from the free surface of the liquid source to the pump centerline plus the absolute pressure at the liquid surface.

Total Discharge Head The total discharge head hd is the reading hgd of a gauge at the discharge flange of a pump (corrected to the pump centerline*), plus the barometer reading and the velocity head hvd at the point of gauge attachment:

Again, if the discharge gauge pressure is below atmospheric, the vacuum-gauge reading is used for hgd in Eq. (10-45) with a negative sign.

Before installation it is possible to estimate the total discharge head from the static discharge head hsd and the discharge friction head hfd as follows:
Static Discharge Head The static discharge head hsd is the vertical distance measured from the free surface of the liquid in the receiver to the pump centerline,* plus the absolute pressure at the liquid surface. Total static head hts is the difference between discharge and suction static heads.

Velocity Since most liquids are practically incompressible, the relation between the quantity flowing past a given point in a given time and the velocity of flow is expressed as follows:

Q = Av

Velocity Head This is the vertical distance by which a body must fall to acquire the velocity v.

Viscosity In flowing liquids the existence of internal friction or the internal resistance to relative motion of the fluid particles must be considered. This resistance is called viscosity. The viscosity of liquids usually decreases with rising temperature. Viscous liquids tend to increase the power required by a pump, to reduce pump efficiency, head, and capacity, and to increase friction in pipe lines.

Friction Head This is the pressure required to overcome the resistance to flow in pipe and fittings.

Work Performed in Pumping To cause liquid to flow, work must be expended. A pump may raise the liquid to a higher elevation, force it into a vessel at higher pressure, provide the head to overcome pipe friction, or perform any combination of these. Regardless of the service required of a pump, all energy imparted to the liquid in performing this service must be accounted for; consistent units for all quantities must be employed in arriving at the work or power performed.

Classification of pumps. (Courtesty of Hydraulic Institute.)

Please click at picture to enlarge size

Terminology of Pumps and Compressors (Part 1)

Displacement Discharge of a fluid from a vessel by partially or completely displacing its internal volume with a second fluid or by mechanical means is the principle upon which a great many fluidtransport devices operate. Included in this group are reciprocatingpiston and diaphragm machines, rotary-vane and gear types, fluid piston compressors, acid eggs, and air lifts.

The large variety of displacement-type fluid-transport devices makes it difficult to list characteristics common to each. However, for most types it is correct to state that (1) they are adaptable to high-pressure operation, (2) the flow rate through the pump is variable (auxiliary damping systems may be employed to reduce the magnitude of pressure pulsation and flow variation), (3) mechanical considerations limit maximum throughputs, and (4) the devices are capable of efficient performance at extremely low-volume throughput rates.

Centrifugal Force Centrifugal force is applied by means of the centrifugal pump or compressor. Though the physical appearance of the many types of centrifugal pumps and compressors varies greatly, the basic function of each is the same, i.e., to produce kinetic energy by the action of centrifugal force and then to convert this energy into pressure by efficiently reducing the velocity of the flowing fluid.

In general, centrifugal fluid-transport devices have these characteristics: (1) discharge is relatively free of pulsation; (2) mechanical design lends itself to high throughputs, capacity limitations are rarely a problem; (3) the devices are capable of efficient performance over a wide range of pressures and capacities even at constant-speed operation; (4) discharge pressure is a function of fluid density; and (5) these are relatively small high-speed devices and less costly.

A device which combines the use of centrifugal force with mechanical impulse to produce an increase in pressure is the axial-flow compressor or pump. In this device the fluid travels roughly parallel to the shaft through a series of alternately rotating and stationary radial blades having airfoil cross sections. The fluid is accelerated in the axial direction by mechanical impulses from the rotating blades; concurrently, a positive-pressure gradient in the radial direction is established in each stage by centrifugal force. The net pressure rise per stage results from both effects.

Electromagnetic Force When the fluid is an electrical conductor, as is the case with molten metals, it is possible to impress an electromagnetic field around the fluid conduit in such a way that a driving force that will cause flow is created. Such pumps have been developed for the handling of heat-transfer liquids, especially for nuclear reactors.

Transfer of Momentum Deceleration of one fluid (motivating fluid) in order to transfer its momentum to a second fluid (pumped fluid) is a principle commonly used in the handling of corrosive materials, in pumping from inaccessible depths, or for evacuation. Jets and eductors are in this category.

Absence of moving parts and simplicity of construction have frequently justified the use of jets and eductors. However, they are relatively inefficient devices. When air or steam is the motivating fluid, operating costs may be several times the cost of alternative types of fluid-transport equipment. In addition, environmental considerations in today’s chemical plants often inhibit their use.

Mechanical Impulse The principle of mechanical impulse when applied to fluids is usually combined with one of the other means of imparting motion. As mentioned earlier, this is the case in axial-flow compressors and pumps. The turbine or regenerative-type pump is another device which functions partially by mechanical impulse.

Measurement of Performance The amount of useful work that any fluid-transport device performs is the product of (1) the mass rate of fluid flow through it and (2) the total pressure differential measured immediately before and after the device, usually expressed in the height of column of fluid equivalent under adiabatic conditions. The first of these quantities is normally referred to as capacity, and the second is known as head.

Capacity This quantity is expressed in the following units. In SI units capacity is expressed in cubic meters per hour (m3/h) for both liquids and gases. In U.S. customary units it is expressed in U.S. gallons per minute (gal/min) for liquids and in cubic feet per minute (ft3/min) for gases. Since all these are volume units, the density or specific gravity must be used for conversion to mass rate of flow. When gases are being handled, capacity must be related to a pressure and a temperature, usually the conditions prevailing at the machine inlet. It is important to note that all heads and other terms in the following equations are expressed in height of column of liquid.

Total Dynamic Head The total dynamic head H of a pump is the total discharge head hd minus the total suction head hs.

Total Suction Head This is the reading hgs of a gauge at the suction flange of a pump (corrected to the pump centerline∗), plus the barometer reading and the velocity head hvs at the point of gauge attachment:
If the gauge pressure at the suction flange is less than atmospheric, requiring use of a vacuum gauge, this reading is used for hgs in Eq. with a negative sign.

Before installation it is possible to estimate the total suction head as follows:


where hss = static suction head and hfs = suction friction head.