Boiling Point ElevationThe boiling point of water depends on the pressure, however in a sugar factory it is most often not water that is boiling, but juice or syrup. The boiling point of a solution of sugar in water, or of a juice, under a given pressure, increases with the concentration of the solution, or the brix of the juice. Furthermore decreasing purity also increases the boiling point

Bagasse Calorific ValueGross calorific value, also known as the higher calorific value (HCV) of bagasse, is calculated from the following formula:

HCV=[19 605 - 196,05(moisture % sample) - 196,05(ash % sample) - 31,14(brix % sample)]kJ.kg-1

The net calorific value, also known as the lower calorific value (LCV), assumes that the water formed by combustion and also the water of constitution of the fuel remains in vapour form. In industrial practice it is not practicable to reduce the temperature of the combustion products below dew point to condense the moisture present and recover its latent heat, thus the latent heat of the vapour is not available for heating purposes and must be subtracted from the HCV. By ASTM standards the HCV is calculated at atmospheric pressure and at 20°C. LCV of bagasse is calculated by the formula:

LCV=[18 309 - 207,6 (moisture % sample) - 196,05 (ash % sample) - 31,14 (brix % sample)] kJ.kg-1

Cavitation in Centrifugal PumpsThere may be, on the low-pressure side of the runner, regions in which the pressure falls to values considerably below atmospheric. In a liquid, however, the pressure cannot fall below the vapour pressure at the temperature concerned. If at any point the vapour pressure is reached, the liquid boils and small bubbles of vapour form in large numbers. These bubbles are carried along by the flow, and on reaching a point where the pressure is higher they suddenly collapse as the vapour condenses to liquid again. A cavity results and the surrounding liquid rushes in to fill it. The liquid moving from all directions collides at the centre of the cavity, thus giving rise to very high local pressures (up to 1 GPa). Any solid surface in the vicinity is also subjected to these intense pressures, because, even if the cavities are not actually at the solid surface, the pressures are propagated from the cavities by pressure waves similar to those encountered in water hammer. This alternate formation and collapse of vapour bubbles may be repeated with a frequency of many thousand times a second. The intense pressures, even though acting for only a very brief time over a tiny area, can cause severe damage to the surface. The material ultimately fails by fatigue, aided perhaps by corrosion, and so the surface becomes badly scored and pitted. Parts of the surface may even be torn completely away. Associated with cavitating flow there may be considerable vibration and noise; when cavitation occurs in a turbine or pump it may sound as though gravel were passing through the machine.

Not only is cavitation destructive: the larger pockets of vapour may so disturb the flow that the efficiency of a machine is impaired. Everything possible should therefore be done to eliminate cavitation in fluid machinery, that is, to ensure that at every point the pressure of the liquid is above the vapour pressure. When the liquid has air in solution this is released as the pressure falls and so air cavitation also occurs. Although air cavitation is less damaging than vapour cavitation to surfaces, it has a similar effect on the efficiency of the machine.

Since cavitation begins when the pressure reaches too low a value, it is likely to occur at points where the velocity or the elevation is high, and particularly at those where high velocity and high elevation are combined.

Cavitation is likely to occur on the inlet side of a pump particularly if the pump is situated at a level well above the surface of the liquid in the supply reservoir. For the sake of good efficiency and the prevention of damage to the impeller, cavitation should be avoided.

Applying the energy equation between the surface of liquid in the supply reservoir and the entry to the impeller (where the pressure is a minimum) we have, for steady conditions

p0 /ρg + z1 - hf = pmin /ρg + v12 /2g

where

v1

is the fluid velocity at the point where the static pressure has its least value

pmin

is the minimum static pressure

z1

the elevation of the surface of the liquid in the reservoir above this point where the static pressure has its least value

p0

the absolute pressure at that surface

p0 = pgauge + patm

ρ

is the density of the fluid at its operating temperature

hf

is the head loss due to friction in the suction line, care must be taken to include the effect of all devices such as strainers and valves in the suction line.

Re-arranging the above equation gives

pmin /ρg = p0 /ρg - hf - v12 /2g + z1

For cavitation not to occur

pmin > pv

where

pv

is the vapour pressure of the liquid. These equations can be rearranged to give the criterion for no cavitation in the pump suction line.

p0 /ρg - pv /ρg - hf - v12 /2g + z1 > 0

A parameter called Nett Positive Suction Head (NPSH) is defined as

NSPHa = p0 /ρg - pv /ρg - hf + z1

The NPSH available at the inlet flange of the pump can be calculated from the above equation. The pump curves in the pump catalog generally give the NPSH required at each volume flow the pump is required to do. For good pump operation

NPSHavailable > NPSHrequired

Electric Motor Absorbed Power EstimationVery often one needs to know the power absorbed by say, a pump or a carrier or fan or some other equipment driven by an electric motor. If one has a three-phase electric power meter and one measures the three phase currents and voltages then the power can be calculated directly.

Most often however all one has is the current reading from the MCC panel ammeter, the method outlined below can be used to estimate the power absorbed by the driven machine.

P = η·√3·V·I·cosφ

whereP is absorbed power in wattsη is motor efficiencyV is applied voltageI is absorbed current in ampscosφ is the power factor

The applied voltage is usually known or can easily be measured, the current we have, our problems are the efficiency and the power factor. Electric motor catalogs usually state the efficiency and power factor at full load and at various part loads(for

example at no load, 25%, 50%, and 75%).

Curves can be fitted to these points (as shown), but our problem is that we don't know the absorbed power and in fact that is precisely what we want to calculate: an iterative solution is required; ie.

guess the absorbed power

calculate efficiency and power factor from fitted curves

calculate power from

P = η·√3·V·I·cosφ

use this as new estimate of power to recalculate efficiency and power factor

repeat until solutions converge

Steam TrapsA steam trap is a device which distinguishes between water and steam and automatically opens a valve to allow water to pass out but which closes to steam and traps it. Traps are of three broad kinds.

Those which distinguish water from steam owing to the difference in density of the two-these are the mechanical traps;

those which distinguish by means of temperature-these are the thermostatic or expansion traps and the

The thermodynamic type traps

Saturated steam and the condensate it is forming have the same temperature. A steam trap which makes its choice by temperature must impose a delay on removing the condensate, because the condensate must cool to below steam temperature before the trap can make up its mind that it must open. On the face of it this seems to put the thermostatic traps at a great disadvantage. This is only true up to a point. Thermostatic traps have certain advantages for certain applications which will appear in subsequent sections.

Pipe Stress Analysis

Request a QuoteWe have over ten years experience with CAESAR II and in sugar factory piping design. Ask us for a quote.

Why ?The reasons one does a pipe stress analysis on a piping system are as follows

to comply with legislation

to ensure the piping is well supported and does not sag or deflect in an unsightly way under its own weight

to ensure that the deflections are well controlled when thermal and other loads are applied

to ensure that the loads and moments imposed on machinery and vessels by the thermal growth of the attached piping are not excessive

to ensure that the stresses in the pipework in both the cold and hot conditions are below the allowables

How ?The piping system is modelled using analysis software such as CAESAR II, available from Chempute Software. The model is constructed from piping general arrangement drawings, piping isometric drawings and piping and valve specifications. Once the system is accurately modelled, taking care to set the boundry conditions, comprehensive stress analysis calculations are done, modifications to the model are made to ensure compliance with the above requirements.

The modifications may include one or more of the following tools

RestraintsA device which prevents, resists or limits the free thermal movement of the pipe. Restraints can be either directional, rotational or a combination of both.

AnchorsA rigid restraint which provides substantially full fixety, ie encastre or built-in, ideally allowing neither movements nor bending moments to pass through them.

True anchors are usually difficult to achieve. A seemingly solid gussetted bracket welded to a house column does not qualify as an anchor if the column does not have the strength to resist the loads applied to it.

Expansion LoopsA purpose designed device which absorbs thermal growth; usually used in combination with restraints and cold pulls.

Neutral Planes of MovementThis refers to the planes on the 3 axes of a turbo machine or pump from where expansion of the machine starts eg the fixed end of a turbine casing. This information is normally provided by the equipment manufacturer. If not available from this source, the fixed points of the machine must be determined by inspection and an estimation of the turbine growths calculated.

A pipe restraint positioned in line with a neutral plane prevents differential expansion forces between the pipe and the machine.

Cold Pull or Cold SpringThis is used to pre-load the piping system in the cold condition in the opposite direction to the expansion, so that the effects of expansion are reduced. Cold pull is usually 50% of the expansion of the pipe run under consideration. Cold pull has no effect on the code stress, but can be used to reduce the nozzle loads on machinery or vessels.

Spring HangersUsed to support a piping system that is subjected to vertical thermal movements. Commercially available single coil spring units are suitable for most applications. Supplier's catalogues adequately cover the selection of these springs. According to Hooke's law, the spring's supporting capacity will vary in direct proportion to the amount of displacement the spring undergoes due to thermal movement. This variation between cold and hot should be between 25 and 50% of the hot loaded condition.

Solid Vertical SupportIn places where vertical thermal movement does not create undesirable effects, or where vertical movement is intentionally prevented or directed, solid supports in the form of rollers, rods or slippers are used.

It is important that free horizontal movement of the pipe is not impeded unless horizontal restraint is desired. Slipppers and rollers must be well designed and lubricated.

Maximum Pipe SpanIt is obvious that piping needs to be supported, the question is, what is the maximum span between supports for a particular sized pipe with carrying a fluid of a given density. The important considerations are:

The maximum deflection of the pipe

The maximum bending stress in the pipe

It is usual to limit the maximum deflection between spans to about 2 mm and the maximum stress should not exceed about 50MPa. The allowable stress will depend on a number of factors including:

The design code applicable

The piping material

The design temperature

The presence of other forces and moments

Any stress intensifiers in the region

Boiler Evaporation CoefficientThe evaporation coefficient is the quantity of steam in tons generated per ton of fuel consumed. The gross evaporation coefficient

xis given by

x = ηB·HCV / (hst - hfw)

where

ηB

is the boiler efficiency, based on HCV

HCV

is the Higher Calorific Value of the fuel

hst

is the enthalpy of the high pressure steam

hfw

is the enthalpy of the feed water

If the fuel is bagasse then the quantity of steam in tons generated per ton of cane crushed is given by

y = x · (F%C - FibreLost%C) / F%B

where

F%C

is the fibre content of the cane in percent less the fibre lost in juice and the fibre lost in the mud filters and

FibreLost%C

is the fibre lost in juice and

F%B

is the fibre content of the bagasse in percent

Pipe SizingUse these pages to calculate pipe sizes and pressure drops due to friction in the pipes, for the following products:

Water

Steam

Factory Sugar Solution

The pressure drop is calculated from the following formula

hf = 4·f·le / d · v2/ 2·g

where

hf = head loss due to friction

f = friction factor calculated from the formula below

le = equivalent pipe lenth taking into account valves and fittings

d = bore of pipe

v = average flow flow velocity

g = acceleration due to gravity 9.81m/s2

f = 0.001375 · (1 + (20000· k / d + 106 / Re)1/3)

where

k = relative roughnessof the bore of the pipe

Re = Reynolds Number = ρ·v·d / μ

ρ = density

μ = dynamic viscosity

Project Management ServicesThe Sugar Engineers will be very happy to offer you a quote for project management services of the design, fabrication and erection of plant and equipment for your factory. Please send an email to

stating your requirements.

The Scope of Services detailed below is a typical example of the kind project management services we offer.

Scope of ServicesThe Services are broken down into three categories:

Design and Engineering

Project Management and Administration

Construction Management

Design and EngineeringThe following services will be carried out by the Consultant:

A complete mechanical, structural, and piping design of the equipment installation will be done

Terms of Reference (ToR) for the Client’s geotechnical engineer, civil engineer, electrical and instruments engineer and third party inspector will be prepared.

A complete set of documents including technical, commercial, and contractual sections will be prepared so that Requests for Tender (RfT) can be issued to the Client's preferred contractors.

Project Management and Administration

A thorough Work Breakdown Structure (WBS) will be prepared.

A Preliminary Cost Estimate (accuracy ±5% to ±10%) will be prepared.

A Project Plan will be prepared, this project plan will consist of the following sections

o Integration Plan

o Scope Management Plan

o Cost Management Plan

o Time Management Plan

o Quality Management Plan

o Risk Management Plan

o Communications Management Plan

o Human Resources Management Plan

o Procurement Management Plan

Tender adjudication of the piling, civil mechanical and electrical contracts

A monthly report will be prepared.

Payment certification for the various contracts will be done.

Construction Management

Construction management of the various contractors (civil, structural, mechanical, E&I).

Receive, review and approve the contractors’ programs.

Receive, review and approve the contractors’ quality control plans (QCPs).

Quality surveillance and monitoring of the contractors’ QCPs

Chair the monthly meetings with the various contractors.

Facilitating commissioning and handover by the contractors

Facilitating Employer’s Taking Over of the project.

Time for CompletionThe design phase of this work will be completed within 999 days of the date of commencement. The date of Commencement is 99th Zofrember 9999

.

The duration of the construction management phase is dependent on the contractors’ skill and resources and so cannot be explicitly stated here. However it is estimated that the Consultant will be occupied for about 99 months on this project.

DeliverablesThe following documents will be delivered to the Client

WBS

Preliminary Cost Estimate

Process Flow Diagram (PFD)

Piping and Instrumentation Drawing (PID)

Equipment design basis and code calculation report

Equipment data sheet

Plot Plan

Equipment General Arrangement

Civil Footprint Loading Drawing

Equipment Detail Drawing Sheet 1

Equipment Detail Drawing Sheet 2

Project Plan

ToR for Third Party Inspector

ToR for Geotechnical Engineer

ToR for Civil Designer

ToR for Electrical/Instruments Engineer

RfT Documents for Piling Contract

RfT Documents for Civil Contract

RfT Documents for Mechanical, Structural and Piping Contract

RfT Documents for Electrical and Instrumentation Contract

Tender Adjudication Reports for; piling, civil, mechanical and electrical contracts

Monthly Report, including four payment certificates

Commissioning and Employer’s Taking-Over Document Pack