vapor e con dens ado

8/3/2019 Vapor e Con Dens Ado

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Introduction to Condensate Recovery

An introduction to the reasons for condensate recovery and return, including energy costs, watercharges, effluent restrictions and water treatment costs. Includes sample calculations for potentialsavings.

Use the quick links below to take you to the main sections of this tutorial:

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View the complete collection ofSteam Engineering Tutorials

Contact UsSteam is usually generated for one of two reasons:

To produce electrical power, for example in power stations or co-generation plants.

To supply heat for heating and process systems.

When a kilogram of steam condenses completely, a kilogram of condensate is formed at the same pressureand temperature (Figure 14.1.1). An efficient steam system will reuse this condensate. Failure to reclaim andreuse condensate makes no financial, technical or environmental sense.

Fig. 14.1.1 1 kg ofsteam condenses completely to 1 kg of condensate

Saturated steam used for heating gives up its latent heat (enthalpy of evaporation), which is a large proportionof the total heat it contains. The remainder of the heat in the steam is retained in the condensate as sensibleheat (enthalpy of water) (Figure 14.1.2).

Fig. 14.1.2After giving up its latent heat to heat the process, steam turns to water containing only sensible heat

As well as having heat content, the condensate is basically distilled water, which is ideal for use as boilerfeedwater. An efficient steam system will collect this condensate and either return it to a deaerator, a boilerfeedtank, or use it in another process. Only when there is a real risk of contamination should condensate notbe returned to the boiler. Even then, it may be possible to collect the condensate and use it as hot processwater or pass it through a heat exchanger where its heat content can be recovered before discharging thewater mass to drain.

Condensate is discharged from steam plant and equipment through steam traps from a higher to a lower
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pressure. As a result of this drop in pressure, some of the condensate will re-evaporate into 'flash steam'. Theproportion of steam that will 'flash off' in this way is determined by the amount of heat that can be held in thesteam and condensate. A flash steam amount of 10% to 15% by mass is typical (see Tutorial 2.2). However,the percentage volumetric change can be considerably more. Condensate at 7 bar g will lose about 13% of itsmass when flashing to atmospheric pressure, but the steam produced will require a space some 200 timeslarger than the condensate from which it was formed. This can have the effect of choking undersized trapdischarge lines, and must be taken into account when sizing these lines.

Example 14.1.1 Calculating the amount of flash steam from condensateHot condensate at 7 bar g has a heat content of about 721 kJ/kg. When it is released to atmospheric pressure(0 bar g), each kilogram of water can only retain about 419 kJ of heat. The excess energy in each kilogram ofthe condensate is therefore 721 - 419 = 302 kJ. This excess energy is available to evaporate some of thecondensate into steam, the amount evaporated being determined by the proportion of excess heat to theamount of heat required to evaporate water at the lower pressure, which in this example, is the enthalpy ofevaporation at atmospheric pressure, 2258 kJ/kg.

The subject of flash steam is examined in greater depth in Tutorial 2.2, 'What is steam?' A simple graph

(Figure 14.1.3) is used in this Tutorial to calculate the proportion of flash steam.

Example:Proportion of flash steam using Figure 14.1.3:

The amount of flash steam in the pipe is the most important factor when sizing trap discharge lines.


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Fig. 14.1.3Quantity of Flash Steam Graph

Steam produced in a boiler by the process of adding heat to the water is often referred to as live steam. Theterms live steam and flash steam are only used to differentiate their origin. Whether steam is produced in aboiler or from the natural process of flashing, it has exactly the same potential for giving up heat, and each isused successfully for this purpose. The flash steam generated from condensate can contain up to half of thetotal energy of the condensate. An efficient steam system will recover and use flash steam. Condensate andflash steam discharged to waste means more make-up water, more fuel, and increased running costs.

This Tutorial will look at two essential areas - condensate management and flash steam recovery. Some of theapparent problem areas will be outlined and practical solutions proposed.

Note: The term 'trap' is used to denote a steam-trapping device, which could be a steam trap, a pump-trap, ora pump and trap combination. The ability of any trap to pass condensate relies upon the pressure differenceacross it, whereas a pumping trap or a pump-trap combination will be able to pass condensate irrespective ofoperational pressure differences (subject to design pressure ratings).

Condensate returnAn effective condensate recovery system, collecting the hot condensate from the steam using equipment andreturning it to the boiler feed system, can pay for itself in a remarkably short period of time. Figure 14.1.4shows a simple steam and condensate circuit, with condensate returning to the boiler feedtank.


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Fig. 14.1.4 A typical steam and condensate circuit

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Why return condensate and reuse it?

Financial reasonsCondensate is a valuable resource and even the recovery of small quantities is often economically justifiable.The discharge from a single steam trap is often worth recovering.

Un-recovered condensate must be replaced in the boiler house by cold make-up water with additional costs ofwater treatment and fuel to heat the water from a lower temperature.

Water chargesAny condensate not returned needs to be replaced by make-up water, incurring further water charges from thelocal water supplier.

Effluent restrictionsIn the UK for example, water above 43C cannot be returned to the public sewer by law, because it isdetrimental to the environment and may damage earthenware pipes. Condensate above this temperature mustbe cooled before it is discharged, which may incur extra energy costs. Similar restrictions apply in mostcountries, and effluent charges and fines may be imposed by water suppliers for non-compliance.

Maximising boiler outputColder boiler feedwater will reduce the steaming rate of the boiler. The lower the feedwater temperature, themore heat, and thus fuel needed to heat the water, thereby leaving less heat to raise steam.

Boiler feedwater qualityCondensate is distilled water, which contains almost no total dissolved solids (TDS). Boilers need to be blowndown to reduce their concentration of dissolved solids in the boiler water. Returning more condensate to thefeedtank reduces the need for blowdown and thus reduces the energy lost from the boiler.
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Summary of reasons for condensate recovery:

Water charges are reduced.

Effluent charges and possible cooling costs are reduced.

Fuel costs are reduced.

More steam can be produced from the boiler.

Boiler blowdown is reduced - less energy is lost from the boiler.

Chemical treatment of raw make-up water is reduced.

Figure 14.1.5 compares the amount of energy in a kilogram of steam and condensate at the same pressure.The percentage of energy in condensate to that in steam can vary from 18% at 1 bar g to 30% at 14 bar g;clearly the liquid condensate is worth reclaiming.

Fig. 14.1.5 Heatcontent of steam and condensate at the same pressures

The following example (Example 14.1.2) demonstrates the financial value of returning condensate.

Example 14.1.2

A boiler produces:10000 kg/h of steam 24 hours/day, 7 days/week and 50 weeks/year (8400 hours/year).

Raw make-up water is at 10C. Currently all condensate is discharged to waste at 90C.

Raw water costs 0.61/m, and effluent costs are 0.45/m

The boiler is 85% efficient, and uses gas on an interruptible tariff charged at 0.01/kWh (2.77/GJ).

Determine the annual value of returning the condensatePart 1 - Determine the fuel costEach kilogram of condensate not returned to the boiler feedtank must be replaced by 1 kg of cold make-upwater (10C) that must be heated to the condensate temperature of 90C. ( T = 80C).

Calculate the heat required to increase the temperature of 1 kg of cold make-up water by 80C, by usingEquation 2.1.4.

Equation 2.1.4Where:

Q = Quantity of energy (kJ)

m = Mass of the substance (kg)


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cp = Specific heat capacity of the substance (kJ/kg C)

T = Temperature rise of the substance (C)

m is unity; T is the difference between the cold water make-up and the temperature of returned condensate;cp is the specific heat of water at 4.19 kJ/kg C.

1 kg x 4.19 kJ/kg C x 80C = 335 kJ/kg

Basing the calculations on an average evaporation rate of 10 000 kg/h, for a plant in operation 8 400 h/year,

the energy required to replace the heat in the make-up water is:

10 000 kg/h x 335 kJ/kg x 8 400 h/year = 28 140 GJ/year

If the average boiler efficiency is 85%, the energy supplied to heat the make-up water is:

With a fuel cost of 2.77/GJ, the value of the energy in the condensate is:

Annual fuel cost = 33 106 GJ/year x 2.77/GJ = 91 704

Part 2 - Determine the water costWater is sold by volume, and the density of water at normal ambient temperature is about 1 000 kg/m. Thetotal amount of water required in one year replacing non-returned condensate is therefore:

If water costs are 0.61 per m, the annual water cost is:

Annual water cost = 84 000 m/year x 0.61/m = 51 240

Part 3 - Determine the effluent costThe condensate that was not recovered would have to be discharged to waste, and may also be charged by

the water authority.

Total amount of water to waste in one year also equals 84000 m

If effluent costs are 0.45 per m, the annual effluent cost is:

Annual effluent cost = 84000 m/year x 0.45/m = 37 800

Part 4 - Total value of condensateThe total annual value of 10 000 kg/h of condensate lost to waste is shown in Table 14.1.1:

Table 14.1.1 The potential value of returning condensate in Example 14.1.2On this basis, it follows that for each 1% of condensate returned per 10 000 kg/h evaporated as in Example14.1.2, a saving of 1% of each of the values shown in Table 14.1.1 would be possible.

Example 14.1.3


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If it were decided to invest 50 000 in a project to return 80% of the condensate in a similar plant to Example14.1.2, but where the total evaporation rate were only 5 000 kg/h, the savings and simple payback term wouldbe:

This sample calculation does not include a value for savings due to correct TDS control and reducedblowdown, which will further reduce water losses and boiler chemical costs. These can vary substantially fromlocation to location, but should always be considered in the final analysis. Clearly, when assessing condensatemanagement for a specific project, such savings must be determined and included.

TDS control and water treatment have already been discussed in Block 3.

The routines outlined in Examples 14.1.2 and 14.1.3 may be developed to form the basis of a forced pathcalculation to assign a monetary value to projects intended to improve condensate recovery.

Equation 14.1.1 can be used to calculate the fuel savings per year:


X = Expected improvement in condensate return expressed as a percentage between 1 and 100

A = Cost of fuel to provide 1 GJ of energy:

If gas on an interruptible tariff costs 0.01/kWh (1 kWh = 3.6 MJ)

Similarly, if oil has a calorific value of 42 MJ/l, and costs 0.15/l)

B = Energy required per kilogram of make-up water to reach condensate temperature (kJ/kg).

This is determined by Q in Equation 2.1.4 (Q = m cpT))

C = Average evaporation rate (kg/h)

D = Operational hours per year (h/year)

E = Boiler efficiency (%)

Savings in water costs can be determined using Equation 14.1.2:

Equation14.1.2

Savings in effluent costs can be determined using Equation 14.1.3:


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Equation 14.1.3

Where:

X = Expected improvement in condensate return expressed as a percentage between 1 and 100

C = Average evaporation rate (kg/h)D = Operational hours per year (h/year)

Example 14.1.4A major condensate management project costing 70 000 expects to recover an additional 35% of thecondensate produced at a plant.

The average boiler steaming rate is 15 000 kg/h, and the plant operates for 8 000 h/year.

The fuel used is gas on a firm tariff of 0.011/kWh, and the boiler efficiency is estimated as 80%.

Make-up water temperature is 10C and insulated condensate return lines ensure that condensate will arriveback at the boiler house at 95C.

Consider the water costs to be 0.70/m and the total effluent costs to be 0.45/m.

Determine the payback period for the project.

Part 1 - Determine the fuel savingsUse Equation 14.1.1:


X = Expected improvement in condensate return = 35

B = Energy required per kilogram of make-up water to reach condensate temperature (kJ/kg).

This is determined by Q in Equation 2.1.4 (Q = m cpT)

Q = m x cp x T

Q = 1 x 4.19 x (95C - 10C)

Q = 356.15 kJ/kg

B = Qn Equation 2.1.4 = 356.15kJ/kg

C = Average evaporation rate = 15 000 kg/h

D = Steaming hours per year = 8 000 h

E = Boiler efficiency = 80%

Substituting the values for X, A, B, C, D, and E into Equation 14.1.1

Part 2 - Determine the water and effluent savingsUse Equation 14.1.2 to calculate the savings in water costs/year:


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Equation14.1.2

Substituting values into Equation 14.1.2:

Use Equation 14.1.2 to calculate the savings in effluent costs/year:

Equation 14.1.3

Substituting values into Equation 14.1.3:

Part 3 - Determine the payback period

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Introduction to Condensate Recovery

Layout of Condensate Return Lines

Sizing Condensate Return Lines

Pumping Condensate from Vented Receivers

Lifting Condensate and Contaminated Condensate

Flash Steam

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This calculator allows you to size your condensate lines correctly.

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Sizing Condensate Return Lines

A guide to sizing condensate lines to and from steam traps, including examples and calculations usingthe condensate pipe sizing chart.





Contact UsThe four main types of condensate line, as mentioned in Tutorial 14.2, are shown in Table 14.3.1:

Table 14.3.1 The four basic types of condensate line
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Sizing of all condensate lines is a function of:

Pressure - The difference in pressure between one end of the pipe and the other. Thispressure difference may either promote flow, or cause some of the condensate to flash to steam.

Quantity - The amount of condensate to be handled.

Condition - Is the condensate predominately liquid or flash steam?

With the exception of pumped return lines which will be discussed in Tutorial 14.4, the other three main typesof condensate line and their sizing, will be covered in this Tutorial.

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Sizing drain lines to traps

It should not be assumed that the drain line (and trap) should be the same size as the plant outlet connection.The plant may operate at a number of different operating pressures and flowrates, especially when it istemperature controlled. However, once the trap has been correctly sized, it is usually the case that the drainline will be the same size as the trap inlet connection, (see Figure 14.3.1).

Fig. 14.3.1 The drain line should not be sized on the plant connectionRegarding the conditions inside the drain line, as there is no significant pressure drop between the plant andthe trap, no flash steam is present in the pipe, and it can be sized to carry condensate only.

When sizing the drain line, the following will need consideration:

The condensing rate of the equipment being drained during full-load.

The condensing rate of the equipment at start-up.

At plant start-up, the condensing rate can be up to three times the running load - this is where thetemperature difference between the steam and colder product is at its maximum.

The drain line, trap, and discharge line also have to carry the air that is displaced by the incomingsteam during this time.

The sizing routine for the steam trap will have to consider both of these variables, however, in general:
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For steam mains drainage, the condensate load for each drain trap is typically 1% of thesteam capacity of the main based on drain points at 50 m intervals, and with good insulation.

For most drain points, sizing the trap to pass twice the running load at the working pressure (minusany backpressure) will allow it to cope with the start-up load.

On constant steam pressure processes such as presses, ironers, unit heaters, radiant panelsand boiling pans, sizing the traps on approximately twice the running load at the working pressure

(less any backpressure) will provide sufficient capacity to cope with the start-up load.

On temperature controlled applications, the steam pressure, the plant turndown, the settemperature and steam trap location need to be considered in detail, and the trap needs to be sizedto cater for both the full and minimum load conditions. If these conditions are not known it isrecommended that the steam trap be sized on 3 x the running load at the running differentialpressure. This should satisfy the start-up condition and provide proper drainage at minimum loads.

When the trap is sized in this way, it will also cater for the start-up load. Consequently, if the drain lineto the trap is sized on the trap size, it will never be undersized.

For practical purposes, where the drain line is less than 10 m, it can be the same pipe size as the steam trapselected for the application. Drain lines less than 10 m long can also be checked against Appendix 14.3.1 anda pipe size should be selected which results in a pressure loss at maximum flowrate of not more than 200 Paper metre length, and a velocity not greater than 1.5 m/s. Table 14.3.2 is an extract from Appendix 14.3.1.

On longer drain lines (over 10 m), the pressure loss at maximum flowrate should not be more than 100 Pa/m,and a velocity not greater than 1 m/s.

Table 14.3.2 Flow of water in heavy steel pipes

Example 14.3.1An item of plant, using steam at constant pressure, condenses 470 kg of steam an hour at full-load. Thepipework between the plant item and the steam trap has an equivalent length of 2 m.

Determine the size of pipe to be used.

Revised load allowing for start-up = 470 kg/h x 2 = 940 kg/h.

As the pipe length is less than 10 metres, the maximum allowable pressure drop is 200 Pa/m.


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Using Table 14.3.1, by looking across from 200 Pa/m it can be seen that a 25 mm pipe has a capacity of 1 141kg/h, and would therefore be suitable for the expected starting load of 940 kg/h.

Checking further up the 25 mm column, it can be seen that a flowrate of 940 kg/h will incur an actual pressuredrop of just less than 140 Pa/m flowing through a 25 mm pipe.

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Sizing discharge lines from traps

The section of pipeline downstream of the trap will carry both condensate and flash steam at the samepressure and temperature. This is referred to as two-phase flow, and the mixture of liquid and vapour will havethe characteristics of both steam and water in proportion to how much of each is present. Consider thefollowing example.

Example 14.3.2An item of plant uses steam at a constant 4 bar g pressure. A mechanical steam trap is fitted, and condensateat saturation temperature is discharged into a condensate main working at 0.5 bar g.

Determine the proportions by mass, and by volume, of water and steam in the condensate main.

Part 1 - Determine the proportions by massFrom steam tables:

At 4.0 bar g hf = 640.7 kJ/kgAt 0.5 bar g hf = 464.1 kJ/kg hfg = 2225.6 kJ/kg

Equation 2.2.5 is used to determine the proportion of flash steam:


P1 = Initial pressure

P2 = Final pressure

hf = Specific liquid enthalpy (kJ/kg)

hfg = Specific enthalpy of evaporation (kJ/kg)

Clearly, if 7.9% is flashing to steam, the remaining 100 - 7.9 = 92.1% of the initial mass flow will remain aswater.

Part 2 - Determine the proportions by volumeBased on an initial mass of 1 kg of condensate discharged at 4 bar g saturation temperature, the mass of flashsteam is 0.079 kg and the mass of condensate is 0.921 kg (established from Part 1).

Water:The density of saturated water at 0.5 bar g is 950 kg/m,


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Steam:From steam tables, specific volume (vg) of steam at 0.5 bar g = 1.15 m/kgThe volume occupied by the steam is 0.079 kg x 1.15 m/kg = 0.091 mThe total volume occupied by the steam and condensate mixture is:0.001 m (water) + 0.091 m (steam) = 0.092 m

By proportion (%):

From this, it follows that the two-phase fluid in the trap discharge line will have much more in common withsteam than water, and it is sensible to size on reasonable steam velocities rather than use the relatively smallvolume of condensate as the basis for calculation. If lines are undersized, the flash steam velocity andbackpressure will increase, which can cause waterhammer, reduce the trap capacity, and flood the process.

Steam lines are sized with attention to maximum velocities. Dry saturated steam should travel no faster than40 m/s. Wet steam should travel somewhat slower (15 to 20 m/s) as it carries moisture which can otherwisehave an erosive and damaging effect on fittings and valves.

Trap discharge lines can be regarded as steam lines carrying very wet steam, and should be sized on similarly

low velocities.

Condensate discharge lines from traps are notoriously more difficult to size than steam lines due to the two-phase flow characteristic. In practice, it is impossible (and often unnecessary) to determine the exact conditionof the fluid inside the pipe.

Although the amount of flash steam produced (see Figure 14.3.2) is related to the pressure difference acrossthe trap, other factors will also have an effect.


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Fig. 14.3.2Quantity of flash steam graph

Factors having a bearing on two-phase flow inside a pipe, include

If the condensate on the upstream side of the trap is cooler than the saturation temperature(for example: a thermostatic steam trap is used), the amount of flash steam after the trap is reduced.This can reduce the size of the line required.

If the line slopes down from the trap to its termination, the slope will have an effect on theflow of condensate, but to what magnitude, and how can this be quantified?

On longer lines, radiation losses from the line may condense some of the flash steam,reducing its volume and velocity, and there may be a case for reducing the line size. But at what pointshould it be reduced and by how much?

If the discharge line lifts up to an overhead return line, there will be times when the lifting linewill be full of cool condensate, and times when flash steam from the trap may evaporate some or allof this condensate. Should the rising discharge line be sized on flash steam velocity or the quantity ofcondensate?

Most processes operate some way below their full-load condition for most of their runningcycle, which reduces flash steam for most of the time. The question therefore arises: is there a need


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for the system to be sized on the full-load condition, if the equipment permanently runs at a lowerrunning load?

On temperature controlled plant, the pressure differential across the trap will itself changedepending on the heat load. This will affect the amount of flash steam produced in the line.

Recommendations on trap discharge linesBecause of the number of variables, an exact calculation of line size would be complex and probablyinaccurate. Experience has shown that if trap discharge lines are sized on flash steam velocities of 15 to 20

m/s, and certain recommendations are adhered to, few problems will arise.

Recommendations:

1. Correctly sized trap discharge lines which slope in the direction of flow and are open-endedor vented at a receiver, will be non-flooded and allow flash steam to pass unhindered above thecondensate (Figure 14.3.3). A minimum slope of 1 in 70 (150 mm drop every 10 m) is recommended.A simple visual check will usually confirm if the line is sloping - if no slope is apparent it is not slopingenough!

Fig. 14.3.3 Discharge line sloping 1:70 in the direction of flow

2. If it is unavoidable, non-pumped rising lines (Figure 14.3.4) should be kept as short aspossible and fitted with a non-return valve to stop condensate falling back down to the trap. Risersshould discharge into the top of overhead return lines. This stops condensate draining back into theriser from the return main after the trap has discharged, to assist the easy passage of flash steam up

the riser.

It is sensible to consider using a slightly larger riser, which will produce a lower flash steam velocity.This will reduce the risk of waterhammer and noise caused by steam trying to force a path throughthe liquid condensate in the riser.

Important: A rising line should only be used where the process steam pressure is guaranteed to behigher than the condensate backpressure at the trap outlet. If not, the process will waterlog unless apumping trap or pump-trap combination is used to provide proper drainage against the backpressure.


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Fig. 14.3.4 Keep rising lines short and connect to the top of return lines

3. Common return lines should also slope down and be non-flooded (Figure 14.3.4). To avoidflash steam occurring in long return lines, hot condensate from trap discharge lines should drain intovented receivers (or flash vessels where appropriate), from where it can be pumped on to its finaldestination, via a flooded line at a lower temperature.

Condensate pumping is dealt with in more detail in Tutorial 14.4.

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The condensate pipe sizing chart

The condensate pipe sizing chart (Figure 14.3.5) can be used to size any type of condensate line, including:

Drain lines containing no flash steam.

Lines consisting of two-phase flow, such as trap discharge lines, which are selectedaccording to the pressures either side of the trap.

The chart (Figure 14.3.5):

Works around acceptable flash steam velocities of 15 - 20 m/s, according to the pipe sizeand the proportion of flash steam formed.

Can be used with condensate temperatures lower than the steam saturation temperature, aswill be the case when using thermostatic steam traps.

Is used to size trap discharge lines on full-load conditions. It is not necessary to consider anyoversizing factors for start-up load or the removal of non-condensable gases.


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May also be used to estimate sizes for pumped lines containing condensate below 100C.This will be discussed in Tutorial 14.4.

Fig. 14.3.5 Condensate pipe sizing chart

Using the condensate pipe sizing chart (Also available in Appendix 14.3.2)Establish the point where the steam and condensate pressures meet (lower part of the chart, Figure 14.3.5).From this point, move vertically up to the upper chart to meet the required condensate rate. If the dischargeline is falling (non-flooded) and the selection is on or between lines, choose the lower line size. If the dischargeline is rising, and therefore likely to be flooded, choose the upper line size.


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Note: The reasoning employed for the sizing of a steam trap is different to that used for a discharge line, and itis perfectly normal for a trap discharge line to be sized different to the trap it is serving. However, when the trapis correctly sized, the usual ancillary equipment associated with a steam trap station, such as isolation valves,strainer, trap testing chamber, and check valve, can be the same size as the trapping device selected,whatever the discharge line size.

A steam trap passing a full-load of 1000 kg/h at 6 bar g saturated steam pressure through a falling dischargeline down to a flash vessel at 1.7 bar g.

As the discharge line is non-flooded, the lower figure of 25 mm is selected from the chart (Figure 14.3.5).

Fig. 14.3.6 A non-flooded pressurised trap discharge line (refer to Example 14.3.3)

A steam trap passing a full-load of 1000 kg/h at 18 bar g saturated steam pressure through a discharge linerising 5 m up to a pressurised condensate return line at 3.5 bar g.

Add the 0.5 bar static pressure (5 m head) to the 3.5 bar condensate pressure to give 4 bar g backpressure.

As the discharge line is rising and thus flooded, the upper figure of 32 mm is selected from the chart, (Figure14.3.5).


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Fig

. 14.3.7 A flooded trap discharge line (refer to Example 14.3.4)

A steam trap passing a full-load of 200 kg/h at 2 bar g saturated steam pressure through a sloping dischargeline falling down to a vented condensate receiver at atmospheric pressure (0 bar g).

As the line is non-flooded, the lower figure of 20 mm is selected from the chart, (Figure 14.3.5).

Fig.14.3.8 A non-flooded vented trap discharge line (refer to Example 14.3.5)


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A pump-trap passing a full-load of 200 kg/h at 4 bar g saturated steam space pressure through a dischargeline rising 5 m up to a non-flooded condensate return line at atmospheric pressure.

The 5 m static pressure contributes the total backpressure of 0.5 bar g.

As the trap discharge line is rising, the upper figure of 25 mm is selected from the chart, (Figure 14.3.5).

Fig.14.3.9 A flooded trap discharge line (refer to Example 14.3.6)

Consider a condensate load of 200 kg/h to a receiver and pump. The pump discharge rate for this mechanicaltype pump is taken as six times the filling rate, hence, the condensate rate taken for this example is 6 x 200 =1 200 kg/h.

Because the condensate will have lost its flash steam content to atmosphere via the receiver vent, the pumpwill only be pumping liquid condensate. In this instance, it is only necessary to use the top part of the chart inFigure 14.3.5. As the line from the pump is rising, the upper figure of 25 mm is chosen.

Note: If the pumped line were longer than 100 m, the next larger size must be taken, which for this examplewould be 32 mm. A useful tip for lines of 100 m or less is to choose a discharge pipe which is the same size asthe pump. For further details refer to Tutorial 14.4 'Pumping condensate from vented receivers'.


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Fig.14.3.10 A discharge line from the condensate pump (refer to Example 14.3.7)

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Common return lines - falling lines

It is sometimes necessary to connect several trap discharge lines from separate processes into a commonreturn line. Problems will not occur if the following considerations are met:

The common line is not flooded and slopes in the direction of flow to an open end or a ventedreceiver, or a flash vessel if the conditions allow.

The common line is sized on the cumulative sizes of the branch lines, and the branch linesare sized from Figure 14.3.5.

Example 14.3.8

Figure 14.3.11 shows three heat exchangers, each separately controlled and operating at the same time. Thecondensate loads shown are full loads and occur with 3 bar g in the steam space.

The common line slopes down to the flash vessel at 1.5 bar g, situated in the same plant room. Condensate inthe flash vessel falls via a float trap down to a vented receiver, from where it is pumped directly to the boilerhouse.

The trap discharge lines are sized on full-load with steam pressure at 3 bar g and condensate pressure of 1.5bar g, and as each is not flooded, the lower line sizes are picked from the graph.

Determine the condensate line sizes for the falling discharge lines and common lines.


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Fig. 14.3.11 Refer to Example 14.3.8

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Common return lines - rising lines

It is sometimes unavoidable for condensate discharge and common lines to rise at some point between thetrap and the point of final termination. When this is the case, each discharge line is sized by moving up to thenext size on the chart, as previously discussed in this Tutorial.

Example 14.3.9Figure 14.3.12 shows the same three heat exchangers as in Example 14.3.8.

However, in this instance, the common line rises 15 m and terminates in an overhead non-flooded condensatereturn main, giving the same backpressure of 1.5 bar as in Example 14.3.8. Each of the discharge lines issized as a rising line.

Determine the condensate line sizes for the discharge lines and common lines.

Example 14.3.9


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Figure 14.3.12 shows the same three heat exchangers as in Example 14.3.8.

However, in this instance, the common line rises 15 m and terminates in an overhead non-flooded condensatereturn main, giving the same backpressure of 1.5 bar as in Example 14.3.8. Each of the discharge lines issized as a rising line.

Determine the condensate line sizes for the discharge lines and common lines.

Fig. 14.3.12 Refer to Example 14.3.9

Example 14.3.10 - Falling common lineCalculating the common line sizes for the application shown in Fig. 14.3.12 which falls to a final terminationpoint:


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Fig. 14.3.13

Example 14.3.11 - Rising common lineCalculating the common line sizes for the application shown in Fig. 14.3.14 which rises to a final terminationpoint:

Note that the steam loads are the same as Example 14.3.10, but the discharge lines are one size larger due tothe rising common line.


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Fig. 14.3.14The procedure shown in Examples 14.3.10 and 14.3.11 can be simplified by using Appendix 14.3.3.

For example, where pipes A and B (20 mm and 50 mm) join, the minimum required pipe diameter is shown as54 mm. Clearly, the user would fit the next largest size of commercial pipe available, unless the calculated boreis close to a nominal bore size pipe.

Appendix 14.3.1 Flow of water in heavy steel pipes


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Appendix 14.3.2 Condensate pipe sizing chart


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Appendix 14.3.3 Common pipe sizing tableD1 = Connecting branch size (N.B.)D2 = Common pipe size


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Layout of Condensate Return Lines

Considerations surrounding the design and layout of condensate return pipework, including drainlines to steam traps, discharge lines from traps, common return lines and pumped return lines.Includes the effect of trap types used, the effect of different pressures and discharging condensateinto flooded mains.





Contact UsNo single set of recommendations can cover the layout of condensate pipework. Much depends on theapplication pressure, the steam trap characteristics, the position of the condensate return main relative to theplant, and the pressure in the condensate return main. For this reason it is best to start by considering whathas to be achieved, and to design a layout which will ensure that basic good practice is met.

The prime objectives are that:

Condensate must not be allowed to accumulate in the plant, unless the steam usingapparatus is specifically designed to operate in this way. Generally apparatus is designed to operatenon-flooded, and where this is the case, accumulated condensate will inhibit performance, andencourage the corrosion of pipes, fittings and equipment.

Condensate must not be allowed to accumulate in the steam main. Here it can be picked upby high velocity steam, leading to erosion and waterhammer in the pipework.

The subject of condensate piping will divide naturally into four basic types where the requirements andconsiderations of each will differ. These four basic types are defined and illustrated in Figure 14.2.1.
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Fig. 14.2.1 A steam main trap set discharging condensate into a common return lineTop

Drain lines to steam traps

In the drain line, the condensate and any incondensable gases must flow from the drain outlet of the plant to

the steam trap.

In a properly sized drain line, the plant being drained and the body of the steam trap are virtually at the samepressure and, because of this, condensate does not flash in this line. Gravity is the driving force and is reliedupon to induce flow along the pipe. For this reason, it makes sense for the trap to be situated below the outletof the plant being drained, and the trap discharge pipe to terminate below the trap. (An exception to this is thetank heating coils discussed in Tutorial 2.10).

The type of steam trap used (thermostatic, thermodynamic or mechanical) can affect the piping layout.

Thermostatic steam trapsThermostatic traps will cool condensate below saturation temperature before discharging. This effectivelywaterlogs the drain line, often allowing condensate to back-up and flood the plant.

There are some applications where the sub-cooling of condensate has significant advantages and isencouraged. Less flash steam is produced in the trap discharge line, and the introduction of condensate intothe condensate main is gentler.

Thermostatic traps discharging via open-ended pipework will waste less energy than mechanical trapsbecause more of the sensible heat in the waterlogged condensate imparts its heat to the process; a typicalexample is that of a steam tracer line.

Thermostatic traps should not be used to drain steam mains or heat exchangers, unless proper considerationis given to a longer and/or larger drain line to act as a reservoir and dissipate heat to atmosphere. The extralength (or larger diameter) of drain line required to do this is usually impractical, as shown in Example 14.2.1.
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Example 14.2.1A 30 kW air heater is to be fitted with a DN15 thermostatic steam trap, which releases condensate at 13Cbelow saturation temperature. The normal working pressure is 3 bar g, the ambient temperature is 15C, andthe heat loss from the drain line to the environment is estimated to be 20 W/m C.

Determine the minimum required length of 15 mm drain line to the thermostatic trap.

From steam tables, at 3 bar g:

Saturation temperature of steam = 144C

Trap discharge temperature = 144 - 13C = 131C

Enthalpy of evaporation (hfg) = 2 133.24 kJ/kg

Equation 2.8.1 can be used to calculate the steam flow from the heat load:

Equation 2.8.1

As the trap discharges at 131C, the drain line has to emit enough heat such that the condensate at the heateroutlet is at saturation temperature, and that condensate will not back-up into the heater. The required heat lossfrom the drain line can be calculated from Equation 2.6.5.

Equation 2.6.5= Mean heat transfer rate (kW)

= Mean secondary fluid flowrate (kg/s)

cp = Specific heat capacity of the secondary fluid (kJ/kg K) or (kJ/kg C) = 4.19 for water

T = Temperature rise of the secondary fluid (K or C)

T in Equation 2.6.5 is the required temperature drop along the drain line of 13C.

This heat loss will be achieved from the mean condensate temperature along the drain line.

The surface area of the drain line to provide the required heat loss can be calculated using Equation 2.5.3.



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= Heat transferred per unit time (W (J/s))

U = Overall heat transfer coefficient (W/m K or W/m C)

A = Heat transfer area (m)

T = Temperature difference between the primary and secondary fluid (K or C)

Note:

will be a mean heat transfer rate ( M) if T is a mean temperature difference (T LM or T AM).

T in Equation 2.5.3 is the difference between the mean condensate temperature and the ambient

temperature = 137.5C - 15C = 122.5C= 0.768 kW

U = 20 W/mCFrom Equation 2.5.3:

0.768 x 10 watts = 20 watts/m C x A x 122.5C

Therefore, A = 0.313 m

The length of pipe required to provide this surface area can be calculated using information from Table 2.10.3.

Table 2.10.3 Nominal surface areas of steel pipes per metre length

This length of pipe (4.7 m) is probably impractical in the field. Two alternatives remain. One is to increase thediameter of the drain line, which is still usually impractical; the other is much simpler, to fit the correct trap forthis type of application; a float-thermostatic trap which discharges condensate at steam temperature andhence requires no cooling leg.

Should a thermostatic trap be considered essential, and fitted no more than 2 metres away from the heateroutlet, it would be necessary to calculate the required diameter of drain line. The heat loss required from thepipe remains the same, along with the total surface area of the pipe, but the surface area per metre lengthmust increase.

From Table 2.10.3, it can be seen that the minimum sized pipe to give this area per metre is a 50 mm pipe,which, again, may be construed as being impractical and expensive to fabricate.

The moral of this is that it is usually easier and cheaper to select the correct trap for the job, than have thewrong type of trap and fabricate a solution around it.

Thermodynamic steam trapsTraps that discharge intermittently, such as thermodynamic traps, will accumulate condensate betweendischarges. However, they are extremely robust, will tolerate freezing ambient temperatures and have arelatively small outer surface area, meaning that heat loss to the environment is minimised. They are notsuitable for discharging condensate into flooded return lines, as will be explained later in this Block.

Mechanical steam traps


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Mechanical steam traps with a continuous discharge characteristic, for example float-thermostatic traps, oftenprove to be the best option, and have the additional advantage of being able to vent air.

Most float traps are available in two basic flow configurations, either horizontal or vertical flow through the trap.Some inverted bucket traps have bottom inlet and top outlet connections. Clearly, the trap connections willaffect the path of connecting pipework.

The drain line should be kept to a minimum length, ideally less than 2 metres. Long drain lines from the plantto the steam trap can fill with steam and prevent condensate reaching the trap. This effect is termed steam

locking. To minimise this risk, drain lines should be kept short (see Figure 14.2.2). In situations where longdrain lines are unavoidable, the steam locking problem may be overcome using float traps with steam lockrelease devices. The problem of steam locking should be tackled by fitting the correct length of pipe in the firstplace, if possible.

Fig. 14.2.2 Keep drain lines short

The detailed arrangements for trapping steam-using plant and steam mains drainage are different as isexplained in the following paragraphs.

With steam-using plant, the pipe from the condensate connection should fall vertically for about 10 pipediameters to the steam trap. Assuming a correctly sized ball float trap is installed, this will ensure that surges ofcondensate do not accumulate in the bottom of the plant with its attendant risks of corrosion andwaterhammer. It will also provide a small amount of static head to help remove condensate during start-upwhen the steam pressure might be very low. The pipework should then run horizontally, with a fall in the

direction of flow to ensure that condensate flows freely (see Figure 14.2.3).

Fig. 14.2.3 Ideal arrangement when draining a steam plant


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With steam mains drainage, provided drain pockets are installed as recommended in Tutorial 10.3, then thedrain line between the pocket and the steam trap may be horizontal. If the drain pocket is not as deep as therecommendation, then the steam trap should be fitted an equivalent distance below it (see Figure 14.2.4).

Fig. 14.2.4 Ideal arrangement when draining a steam mainTop

Discharge lines from traps

These pipes will carry condensate, incondensable gases, and flash steam from the trap to the condensatereturn system (Figure 14.2.5). Flash steam is formed as the condensate is discharged from the high-pressurespace before the steam trap to the lower pressure space of the condensate return system. (Flash steam isdiscussed briefly in Tutorial 14.1, and in more detail in Tutorial 2.2).

These lines should also fall in the direction of flow to maintain free flow of condensate. On shorter lines, the fallshould be discernible by sight. On longer lines, the fall should be about 1:70, that is, 100 mm every 7 metres.

Fig. 14.2.5 Trap discharge lines pass condensate, flash and incondensibles

Discharging into flooded return lines


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Discharging traps into flooded return lines is not recommended, especially with blast action traps(thermodynamic or inverted bucket types), which remove condensate at saturation temperature.

Good examples of flooded condensate mains are pumped return lines and rising condensate lines. They oftenfollow the same route as steam lines, and it is tempting to simply connect mains drainage steam trap dischargelines into them.

However, the high volume of flash steam released into long flooded lines will violently push the water along thepipe, causing waterhammer, noise and, in time, mechanical failure of the pipe.

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Common return lines

Where condensate from more than one trap flows to the same collecting point such as a vented receiver, it isusual to run a common line into which individual trap discharge lines are connected. Provided the layouts asfeatured in Figures 14.2.6/7/8 and 10 are observed, and the pipework is adequately sized as indicated inTutorial 14.3, this is not a problem.

Blast discharge traps

If blast discharge traps (thermodynamic or inverted bucket types) are used, the reactionary forces andvelocities can be high. Swept tees will help to reduce mechanical stress and erosion at the point where thedischarge line joins the common return line (see Figure 14.2.6).

Fig. 14.2.6 A swept tee connection

Continuous discharge trapsIf, for some reason, swept tees cannot be used, a float-thermostatic trap with its continuous discharge action isa better option (Figure 14.2.7). The flooded line will absorb the dissipated energy from the (relatively small)continuous flow from the float-thermostatic trap, more easily.

If the pressure difference between the steam and condensate mains is very high, then a diffuser will help tocushion the discharge, reducing both erosion and noise.


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Fig.14.2.7 Float trap with a diffuser into a flooded line

Another alternative is to use a thermostatic trap that holds back condensate until it cools below the steamsaturation temperature; this reduces the amount of flash steam formed (Figure 14.2.8).

To avoid waterlogging the steam main, the use of a generous collecting pocket on the main, plus a cooling legof 2 to 3 m of unlagged pipe to the trap is essential. The cooling leg stores condensate while it is cooling to thedischarge temperature.

If there is any danger of waterlogging the steam main, thermostatic traps should not be used.

Fig.14.2.8 Balanced pressure thermostatic trap with cooling leg into a flooded line

Temperature controlled plant with steam traps draining into flooded linesProcesses using temperature control provide an example where the supply steam pressure is throttled acrossa control valve. The effect of this is to reduce steam trap capacity to a point where the condensate flow canstop completely, and the system is said to have stalled. The subject of stall is discussed in greater depth inBlock 13.

Stall occurs as a result of insufficient steam pressure to purge the steam plant of condensate, and is morelikely when the plant has a high turndown from full-load to part load.

Not all temperature controlled systems will stall, but the backpressure caused by the condensate system couldhave an adverse effect on the performance of the trap. This in turn, might impair the heat transfer capability ofthe process (Figure 14.2.9).

Condensate drain lines should, therefore, be configured so that condensate cannot flood the main into whichthey are draining as depicted in Figure 14.2.10.


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Fig. 14.2.9 Discharge from steam traps on temperature controlled equipment into flooded lines

Fig. 14.2.10 Condensate discharging freely via a falling common line

Discharge lines at different pressuresCondensate from more than one temperature controlled process may join a common line, as long as this lineis:

Designed to slope in the direction of flow to a collection point.

Sized to cater for the cumulative effects of any flash steam from each of the branch lines atfull-load.

The concept of connecting the discharges from traps at different pressures is sometimes misunderstood.

If the branch lines and the common line are correctly sized, the pressures downstream of each trap will be

virtually the same. However, if these lines are undersized, the flow of condensate and flash steam will berestricted, due to a build up of backpressure caused by an increased resistance to flow within the pipe.Condensate flowing from traps draining the lower pressure systems will tend to be the more restricted.

Each part of the discharge piping system should be sized to carry any flash steam present at acceptable steamvelocities. The discharge from a high-pressure trap will not interfere with that from a low-pressure trap if thedischarge lines and common line are properly sized and sloped in the direction of flow. Tutorial 14.3, 'Sizing ofcondensate return lines' gives further details.

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Pumped return lines

Flash steam may, at some point, be separated from the condensate and used in a recovery system, or simplyvented to atmosphere from a suitable receiver (Figure 14.2.11). The residual hot condensate from the lattercan be pumped on to a suitable collecting tank such as a boiler feedtank. When the pump is served from avented receiver, the pumped return line will be fully flooded with condensate at temperatures below 100C,which means flash steam is less likely to occur in the line.

Fig. 14.2.11 Condensate recovery from a vented receiver

Flow in a pumped return line is intermittent, as the pump starts and stops according to its needs. The pumpdischarge rate will be higher than the rate at which condensate enters the pump. It is, therefore, the pumpdischarge rate which determines the size of the pump discharge line, and not the rate at which condensateenters the pump.

The pumping of condensate is discussed in further detail in Tutorial 14.4, 'Pumping condensate from ventedreceivers'.
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