Condensate and Flash Steam Recovery

TECHNICAL REFERENCE GUIDE

Condensate and flash steam recovery

Contents Introduction Condensate return Why return condensate and reuse it? Condensate recovery cost saving example Condensate return lines Drain lines to traps Discharge lines from traps • Discharging into flooded return lines Common return lines • Temperature controlled plant with steam traps draining into flooded lines Discharge lines at different pressures Discharge lines from vented pumps Sizing condensate lines Sizing drain lines to traps • From steam mains • From process applications Sizing discharge lines from traps • Recommendations on trap discharge lines • The condensate pipe sizing chart Sizing common return lines Sizing pumped return lines • Pumping traps and pump-trap installations Condensate pumping from vented receivers Pumping terminology Electrical centrifugal condensate pumps Sizing an electrical condensate recovery unit Sizing the discharge pipework for an electrical condensate recovery unit Mechanical condensate pumps Sizing a mechanical condensate pump Sizing the discharge pipework for a mechanical condensate pump Longer delivery lines Fully loaded pumps and longer lines Consideration of a larger pump and smaller pipeline Lifting condensate from steam mains drain traps Contaminated condensate Stall and the stall point The stall cycle Temperature controlled plant • Condensate drainage to atmosphere • Closed loop condensate drainage Determining the stall point on controlled plant Using the stall chart A typical stall chart Constant pressure plant

3 4 4 6 9 9 10 10 11 13 13 14 14 15 15 16 18 20 21 29 31 33 34 34 37 38 39 41 43 45 45 46 47 50 51 52 52 54 54 55 57 57 60 61 1

Contents

Flash steam What is flash steam and why should it be used ? How much flash steam ? Sub cooled condensate Pressurised recovery The flash vessel Sizing flash steam recovery vessels Requirements for successful flash steam applications Control of flash steam pressure Typical applications for flash steam Flash steam supply and demand in-step Flash steam supply and demand not in-step Boiler blowdown heat recovery applications Spray condensing Steam tables Further information Appendix 1 - Condensate line sizing chart

2

62 63 64 64 65 66 66 68 68 70 70 73 74 76 78 80 81

Introduction Steam is usually generated for one of two reasons : to produce power, as in power stations and co-generation plants. to carry energy for heating and process systems. When a kg of steam condenses, a kg of condensate at the same pressure and temperature is formed. An efficient steam distribution system will make good use of this condensate. Failure to do so makes no financial, technical or environmental sense. Steam, used for heating, gives up its latent heat, which is a large proportion of its total heat. The remainder is held by the condensed water. As well as having heat content, the condensate is also a distilled form of water, which is ideal for use as boiler feedwater. An efficient installation will collect condensate and either return it to the deaerator, boiler feedtank, or use it in another process. Only when there is a real risk of contamination should condensate not be returned to the boiler. But then it may be possible to collect the condensate and use it as hot process water or pass it through a heat exchanger where its heat content can be recovered before discharging to drain. Condensate is discharged through traps from a higher to a lower pressure. As a result of this drop in pressure, some of the condensate will then re-evaporate into 'flash steam'. The proportion that will 'flash off' is determined by the pressure difference between the steam and condensate sides of the system, and a figure of 10 % to 15 % by mass is typical. However, the percentage volumetric change can be considerably more. Condensate at 7 bar g will lose about 13 % of its mass when flashing to atmospheric pressure, but the steam produced will require a space some 200 times larger than the condensate from which it was formed. This can have the effect of choking undersized trap discharge lines, and should be taken into account when sizing these lines. The flash steam generated can contain up to half of the total energy of the condensate, hence flash steam recovery is an essential part of an energy efficient system. Condensate and flash steam discharged to waste means replacement feedwater, more fuel, and increased running costs. This technical reference guide will look at two essential areas condensate management and flash steam recovery. Some of the apparent problem areas will be outlined and solutions offered. Illustrations, together with tables and charts to which reference is made, are included in the text. Basic steam tables can be found at the end of this guide. Note: the term 'trap' is used to denote a steam trapping device which could be a steam trap, a pumping trap, or a pump-trap combination. The ability of any steam trap to pass condensate relies upon the pressure difference across it, whereas a pumping trap or a pump-trap combination is able to remove condensate irrespective of pressure differences across it. 3

Condensate return An effective condensate recovery system, collecting the hot condensate from the steam using equipment and returning it to the boiler feed system, can pay for itself in a remarkably short period of time. Fig. 1 shows a typical steam and condensate circuit, where condensate is returned to the boiler feedtank. Steam Pan

Pan

Process vessel

Space heating system

Steam Vat

Vat

Condensate

Make-up water Condensate Steam Feedtank

Boiler

Feedpump

Fig. 1 A typical steam and condensate circuit Why return condensate and reuse it?

Monetary value. Condensate 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. Unrecovered condensate is replaced by cold make-up water with additional costs of water treatment and fuel to heat the water from a lower temperature. Water charges. Any condensate which is not returned needs to be replaced by make-up water, incurring further water charges from the local water supplier. Effluent restrictions. In the UK for example, water above 43°C cannot be returned to the public sewer because it is detrimental to the environment and may damage earthenware pipes. Condensate above this temperature must be cooled if discharged, which could incur extra energy costs. Similar restrictions apply in most countries and effluent charges and fines may be imposed by water suppliers for non-compliance.

4

kJ/kg

Saturated steam temperature °C 100 2 800

120

134

144

152

159

165

170

175

180

184

188

192

195

198

Total heat of steam

2 600 2 400

Latent heat (entha

2 200 2 000

lpy of evaporatio

n)

1 800 1 600 1 400 1 200 1 000 800

H

600 400

ensate at eat in cond

erature steam temp Heat available for flash steam release to atmospheric pressure

Heat in condensate at atmospheric pressure

200 0

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Pressure bar g

Fig. 2 Heat content of steam and condensate Figure 2 shows the relative amounts of energy in steam and condensate at various pressures. Maximising boiler output. Colder boiler feedwater will reduce the steaming rate of the boiler. The lower the feedwater temperature, the more heat,and thus fuel needed to raise steam. Boiler feedwater quality. Condensate is a distilled water which contains almost no dissolved solids (TDS). Blowdown is used to reduce the concentration of dissolved solids in the boiler. More condensate returned to the feedtank reduces the need for blowdown and thus reduces the energy lost from the boiler. Summary of reasons for condensate recovery. Water charges are reduced. Effluent charges and possible cooling costs are reduced. Fuel costs are reduced. Boiler blowdown is reduced - less energy lost from boiler. Chemical treatment is reduced.

5

Condensate recovery cost saving example

The following example demonstrates how savings are possible by returning condensate to the boiler feedtank. Savings will obviously depend on the cost of fuel and water, and this example gives typical costs in the UK at the time of writing. The fuel used in this example is a heavy fuel oil with a gross calorific value of 42 MJ/litre. Fuel savings based on the following average temperatures Condensate return temperature = 90°C Make-up water temperature = 10°C Temperature difference = 80°C Each kg of condensate not returned must be replaced by 1 kg of cold make-up water that will need heating to the same temperature. Heat required to raise 1 kg of cold make-up water by 80°C: 1 kg x 80°C x 4.19 kJ/kg °C = 335 kJ/kg Basing the calculations on an average of 10 000 kg/h evaporation rate, and where none of the condensate is presently returned, 24 hours a day, 7 days a week, 50 weeks of the year (8 400 h/year), the nett 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 / per year If the average boiler efficiency is 85 %, gross energy needed to heat the make-up water 2 8140 GJ / year = 33106 GJ/year 0.85 With a calorific value of 42 MJ / litre, potential savings on fuel 33106 GJ / year = 788 000 litres / year 42 MJ / litre With fuel at £0.15 / litre, cost savings

= £ 788 000 x 0.15

Therefore, potential annual fuel savings

= £ 118 200

Water savings. Total amount of water required in one year to replace condensate which is not returned: 8 400 h x 10 000 kg/h = 84 000 m³ 1 000 kg/m³ Costed at £0.61 per m³: = £51 240 Therefore potential annual water savings = £51 240 6

Effluent savings. The condensate that was not recovered would have to be discharged to waste which may also be charged by the water authority. Total amount of water to waste in one year also equals 84 000 m³ If effluent costs £0.45 per m³

= £37 800

Therefore, potential annual effluent savings

= £37 800

Total potential savings. The total annual potential savings for 10 000 kg/h evaporated based on none of the condensate presently being returned are : fuel savings water savings effluent savings

= = =

£ 118 200 £ 51 240 £ 37 800

total savings

=

£ 207 240

It follows that for each 1% of condensate returned per 10 000 kg/h evaporated in the above example, a saving of 1% of each of the above values would be possible. To calculate relative savings based on the same reasoning, use the formulae on the next page by putting figures in the blank boxes. Fuel savings

=

£

Water savings

=

£

Effluent savings

=

£

Total

=

£

(on 80°C increase in feedwater)

This sample calculation does not include a value for savings due to correct TDS control and reduced blowdown which will further reduce water loss and boiler chemical costs. These can vary substantially from location to location, but should always be considered in the final analysis. Consult Spirax Sarco for advice regarding any specific installation. Further information on how to calculate savings by automatic TDS control is available in the Spirax Sarco Technical Reference Guide TR-GCM-01, 'Water treatment, storage and blowdown for steam boilers'. Clearly, when assessing condensate management for a specific project, such savings should be determined and included. 7

Fuel savings

Savings in currency used in 'D' = £ 335 x A x B x C x D E x F where: A = average evaporation rate in tonnes/h B = hours per year C = percentage increase in condensate return D = cost per unit of fuel ( £ / litre; £ / therm; £ / kg) E = calorific value of fuel per same unit ( MJ / litre; MJ / therm; MJ / kg) F = boiler efficiency eg, consider the previous example, if a 30 % increase in condensate return is to be made, annual cost savings on fuel: £ 335 x 10 x 8 400 x 30 x 0.15 42 x 85 fuel savings = £ 35 470

Water savings

Savings in currency used in 'C' = A

x

B

x C 100

x

D

where: A = average evaporation rate in tonnes/h B = hours per year C = cost per m³ of water D = percentage increase in condensate return eg, consider the previous example, if a 30 % increase in condensate return is to be made, annual cost savings on water: £ 10 x

8 400 x 0.61 x 30 100

water savings = £ 15 372 Effluent savings

Savings in currency used in 'C' = A x B x C x D 100 where: A = average evaporation rate in tonnes / h B = hours per year C = cost per m³ of effluent D = percentage increase in condensate return eg, consider the previous example, if a 30 % increase in condensate return is to be made, annual cost savings on effluent : £ 10 x 8 400 x 0.45 x 30 100 effluent savings = £ 11 340

8

Condensate return lines The subject of condensate piping will divide naturally into four basic sections where the requirements and considerations of each will differ. They are: Type of line Pipe sized to carry Drain lines to traps condensate

Drain lines to traps

Discharge lines from traps

flash steam

Common return lines

flash steam

Pumped return lines

pumped condensate

The condensate must flow from the steam space outlet to the trap. The steam space and the body of the trap upstream of its orifice will usually be at the same pressure, and flow usually occurs due to the force of gravity. As there is no significant pressure drop between the process and the trap, no flash steam is present in the pipe, and it can be sized to carry condensate only. It should never be assumed that the plant outlet connection indicates the correct size for the trap or condensate pipe, especially in the case of temperature controlled processes where low differentials in pressure can occur across the trap under part-load conditions. Each process will have its own system conditions, and should be treated with these in mind. Refer to the later section 'Stall and the stall point' for further details. Stall is also discussed in Reference Guides: Steam trapping and air venting - TR-GCM-11 Condensate removal from heat exchangers - TR-GCM-23 Long drain lines from plant can fill with steam and prevent condensate getting to the trap. The effect is generally termed 'steam locking'. To minimise this risk, drain lines should be kept short (Fig. 3), first falling vertically wherever possible before any horizontal run, to ensure the trap is below the plant outlet. This also encourages gravitational flow between the outlet and the trap. Float traps are also available with steam lock release devices to alleviate the problem.

✔

✗

Fig. 3 Keep drain lines short

9

Fig. 4 Trap disharge lines pass condensate, flash, and incondensables Discharge lines from traps

These carry condensate, incondensable gases, and flash steam from the trap to the condensate return system (Fig. 4). Flash steam is formed due to the pressure drop across the trap orifice, caused by the difference in pressure between the steam and condensate systems. During start-up of a steam system, condensate will be cool with little or no flash steam, but the condensing rate will be maximum, and air will have to pass with the condensate. Soon, as the system heats up, full steam load may occur, the pressure in the steam space will be at its highest, and the amounts of flash steam released in the discharge line immediately after the trap will be at their greatest. Trap discharge lines are sized on full load conditions because of this. In so doing, the pipe will be adequately sized for start-up loads, including the efficient purging of noncondensable gases.

Discharging into flooded return lines

10

Discharging traps into flooded return mains is best avoided, especially from blast action traps draining steam pipelines at saturation temperature. Pumped and rising condensate lines often follow the same route as steam lines, and it is tempting to simply connect drain trap discharge lines into them. The high volume of flash steam released into long flooded lines will violently push the water along the pipe, causing waterhammer, noise, and in the extreme, mechanical failure of the pipe. The solution is to avoid discharging into flooded lines by returning condensate and flash steam in lines that slope at least 1 in 70 down to a vented collecting receiver, from which it can be pumped.

Common return lines

Where condensate from more than one trap flows to the same collecting point such as a vented receiver, it is feasible to run a common line into which the individual lines can discharge, as long as certain conditions are met, and the pipework is adequately sized. When connecting to the common line, swept tees will help to reduce mechanical stress and erosion at the joint (Fig. 5).

Steam main

Condensate main

Fig. 5 A swept tee connection

11

If this is not possible, use a float trap to discharge into the flooded line (Fig. 6). The energy dissipated from the relatively small continuous flow from the float trap can usually be absorbed by the flooded line, especially when fitted with a diffuser such as the DF2.

Diffuser

Condensate

Steam

Fig. 6 Float trap with diffuser into a flooded line Another alternative is to use a thermostatic trap which holds back condensate until it cools below the steam saturation temperature thus reducing the amount of flash steam formed (Fig. 7). To avoid waterlogging the steam main, the use of a generous collecting pocket on the main, plus a cooling leg of 2 to 3 m of unlagged pipe to the trap is essential. The cooling leg gives storage for condensate while it is cooling to the discharge temperature. If there is any danger of waterlogging the steam main, do not use this method. Always consult expert advice from Spirax Sarco if in any doubt.

Diffuser

Condensate

Steam

Thermostatic trap set with cooling leg

Fig. 7 Thermostatic trap with cooling leg into a flooded line 12

Temperature controlled plant with steam traps draining into flooded lines Take care if condensate from steam traps on temperature controlled plant is discharged into flooded lines. The back pressure could have a derogatory effect on the performance of the trap and the efficiency of the process (Fig. 8).

Heat exchanger

Heat exchanger

✗ Steam trap

Flooded common line

✔ Pumping trap Non - flooded common line

Fig. 8 Discharge from steam traps into non-flooded lines if possible. Discharge lines at different pressures

However, condensate from more than one temperature controlled process may join a common line as long as this line is: a) designed to slope in the direction of flow to a collection point b) sized to cater for the cumulative effects of any flash steam from each of the branch lines at full 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 should be virtually the same. However, if these lines are undersized, the flow of condensate and flash steam will be restricted due to a build up of back pressure caused by the increased friction along the pipe. Condensate flow from traps operating at lower pressures will tend to be restricted first. Each part of the discharge piping system should be sized to carry any flash steam present at acceptable velocities. The discharge from a high pressure trap will not interfere with that from a low pressure trap if the discharge lines and common line are properly sized and sloped in the direction of flow. A later section "Sizing of condensate lines" gives further details. 13

Discharge lines from vented pumps

Flash steam may ultimately be separated from the condensate and used in a recovery system, or vented to atmosphere from a suitable receiver (Fig. 9). The residual hot condensate from the latter can be pumped on to a suitable collecting tank such as a boiler feedtank. When the pump is served from a vented receiver, the return line will be fully flooded with condensate having little or no tendency to create flash steam.

Vent

pumped condensate Plant

Plant

Plant Receiver

Pump

Fig. 9 Condensate recovery from a vented receiver Flow in a pumped return line is intermittent as the pump starts and stops according to needs. The pump discharge rate will be higher than the rate at which condensate enters the pump. It is the pump discharge rate that determines the size of the discharge line. Pumping will be further covered in a later section. Sizing condensate lines

14

As mentioned previously, the four main situations for sizing condensate lines are: Type of line Pipe sized to carry Drain lines to traps

condensate

Discharge lines from traps

flash steam

Common return lines

flash steam

Pumped return lines

pumped condensate

Sizing drain lines to traps

A simple rule is to make the line to the trap the same size as the trap connections. This presupposes, however, that the trap itself has been sized on sound technical reasoning. A brief synopsis follows: Steam traps basically fall into two distinct areas of application, steam mains or process applications.

From steam mains

The condensate load per trap is affected by various factors such as the size of the pipe, pressure, degree of insulation, ambient temperature, number of traps used along a defined length, position and situation of the pipe. The Technical Reference Guide 'Steam Distribution' (TR-GCM-03), gives information for condensate loads with different sized pipes at various pressures. It is sufficient to consider a condensate load for each drain trap based on 1% of the steam capacity of the main and traps placed every 50 m if insulated, and 5% and 25 m if not. Whatever the size of the main and traps, it is important they are served by an adequately sized drain pocket. As a guide, see below (Fig. 10):

Mains diameter - D

Pocket diameter - d1

Pocket depth - d2

Up to 100 mm nb

d1 = D

Minimum d2 = 100 mm

125 - 200 mm nb

d1 = 100 mm

Minimum d2 = 150 mm

250 mm and above

d1 = D / 2

Minimum d2 = D

Steam main

D

d2

d1

Condensate return

Fig. 10

15

The drain line off-take should be at least 25 to 30 mm from the bottom of the pocket for mains up to 100 mm, and roughly a third to centre of the pocket for larger mains. This allows a space below the outlet for dirt and scale to collect, and the bottom may be fitted with a blowdown valve for cleaning purposes. On most drain points, by sizing the trap to pass approx twice the rated design load at the working pressure (minus any back pressure) will allow it to cope with both start-up and running loads. From process applications

The method of selecting and sizing the trap depends on whether the process is temperature controlled or not, but in either case the pipe should be sized as below on the worst condition. i) Applications on constant steam pressure Some applications work on a constant pressure supply, such as presses, ironers, ovens, unit heaters, radiant panels, boiling pans etc. When an adequate steam supply is provided, the working pressure tends to remain fairly constant even under varying load conditions. The worst condition will apply at start-up when the steam pressure will tend to drop and the condensation rate is at its highest due to the large difference in temperature between the steam and cold metal.

Air vent

Reducing valve

Jacketed pan

Trap set

Fig. 11 Typical constant pressure application On most constant pressure applications, sizing the trap to pass approx twice the rated design load at the working pressure (minus any back pressure) will allow it to cope with both start-up and running loads.

16

ii) Applications with temperature control If the process is temperature controlled, the system operating parameters and layout need to be considered in greater detail as the heat load may change during normal operation. The steam pressure and condensate load in the heat exchanger will alter as the steam control valve modulates to meet this change, and as the steam pressure reduces, so does the trap's capacity. Take the case of an air heater battery which is designed to heat air from -5oC to 25oC using steam at 3.2 bar (145oC). If the incoming air temperature rises to 5oC, the DT and heat load will be reduced by 30%. The steam temperature will reduce by ratio, and once established, its pressure can be established from steam tables. Steam temp. at full load Steam temp. at no load ie, steam temperature range 30% of range steam temp. at 30 % reduction steam pressure at 105 oC

= 145 oC (a) = 25 oC (b) = 120 oC (a - b) = 40 oC (c) = ( [a - b] >< 0.3) = 105 oC (a - c) = 0.2 bar g (from steam tables)

The pressure in the heat exchanger has reduced from 3.2 bar g to 0.2 bar g, and will reduce the trap's capacity. If the trapping device were a float trap and sized on the full load at 3.2 bar g, then it is possible that its capacity may be below that needed at the lower pressure. It is for this reason that it is important to size the float trap on the minimum heat load rather than the full load. Should the steam space pressure reduce enough to approach the condensate pressure, stall will occur and the trapping device is selected and sized on the load at stall point. Not all temperature controlled applications will stall. Stall will not occur if the steam space pressure at the minimum heat load is higher than the condensate back pressure. Whether the trapping device is a float trap, a pumping trap, or a mechanical pump and float trap in combination, will depend on the system operating requirements and the piping infrastructure. The drain line can usually be the same size as the trap especially on shorter lines, but on lines over 5 m, should be checked on the table in Fig. 37 on page 48, against a pressure drop of up to 160 Pa / m. The size of the trap discharge line needs to be determined by a different set of rules, and this is considered next. Stall and its implications on trap sizing is discussed in further detail in a later chapter, and in the Technical Reference Guide "Condensate Removal from Heat Exchangers" (TR-GCM-23) 17

The section of pipeline downstream of the trap will carry both condensate and flash steam at the same pressure and temperature. This complex situation is called "two phase flow", where the mixture of fluids will have the characteristics of both steam and water in proportion to how much of each component is present. Consider this by example where 10% of condensate forms flash steam : Flash steam pressures bar

g bar g 1.5 ba 1.0 r g bar g 0.5 bar g 0 ba rg

13

2.5

12 11

2.0

Pressure on traps bar g

Sizing discharge lines from traps

10 9 8 7 6 5

Example

4 3 2 1 0

Fig. 12 Quantity of flash steam from condensate

0 0.02

0.06

0.10 0.14 0.18 0.22 kg Flash per kg condensate

As each kg of condensate at 4 bar g passes through the trap, 0.1 kg will become steam at 100°C, and 0.9 kg will become water at 100°C. However, the respective volumes will depend on the specific volume of each at the pressure in the line (0 bar g). 0.9 kg of condensate will have a volume of = 0.0009 m³ 0.1 kg of flash steam will have a volume of 0.1 kg >< 1.673 m³/kg (spec. vol.at 0 bar g) = 0.1673 m³ Total volume of 1 kg of the mixture = 0.1682 m³ Therefore, 0.0009 >< 100 = 0.5% is volume of water in the line 0.1682 and,

0.1673 >< 100 = 99.5% is the volume of flash steam 0.1682

It follows that the flow of fluid through this line will have more in common with steam than water, and it is sensible to size on reasonable steam velocities rather than the relatively small volume of condensate. If lines are undersized, the flash steam velocity and back pressure will increase which can cause waterhammer and reduced trap capacity. 18

Steam lines are sized with attention to maximum velocities. Dry saturated steam can safely travel up to 40 m/s. Wet steam needs to travel somewhat slower (15 to 25 m/s) as it carries moisture which can have an erosive and damaging effect on fittings and valves if travelling too fast. Similarly, trap discharge lines can be regarded as steam lines carrying very wet steam, and should be sized on similar 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 to determine what is going on inside the pipe with any certainty. Although the amount of flash steam produced is related to the pressure difference across the trap, there are other factors that will have some bearing on what is happening inside the pipe. For example, If, for some reason, the condensate on the upstream side of the trap is cooler than the saturation temperature, the amount of flash formed after the trap is reduced. This can reduce the size of the line needed. If the line slopes down from the trap to its termination, the degree of slope will have an effect on the flow 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, its volume will decrease along with its velocity, and there may be a case for reducing the line size. But at what point should 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 line will be full of cool condensate, and times when flash steam from the trap may evaporate some or all of this condensate. Should the line be sized on flash steam velocity or the quantity of condensate? Most processes operate some way below their full load condition for most of their running cycle, which reduces the amount of flash produced for most of the time. Should the designer size on the full load condition when it may not be warranted due to the frequency and small amount of time it occurs? On temperature controlled plant, the pressure differential across the trap will itself change depending on the heat load. This will affect the amount of flash steam produced in the line. Due to the conflicting nature of all the above, an exact calculation of line size would be complex and probably inaccurate. In practice, experience has shown that if trap discharge lines are sized on comfortable flash steam velocities and certain recommendations are adhered to, few problems will arise.

19

Recommendations on trap discharge lines

Correctly sized trap discharge lines that slope in the direction of flow and are open-ended are non-flooded and allow flash steam to pass unhindered over the condensate, (Fig. 13). A minimum slope of 1 in 70 (150 mm drop every 10 m) is recommmended. A simple visual check will usually confirm if the line is sloping - if no slope is apparent it is not sloping enough! vent easy passage for flash steam

Process easy passage for condensate

pumped condensate

1 : 70 slope = 150 mm per 10 m run

vented receiver and pump

Fig. 13 Discharge line sloping 1 : 70 in the direction of flow If unavoidable, non-pumped rising lines (Fig. 14) should be kept as short as possible and fitted with a non-return valve to stop condensate falling back down to the trap. They should discharge into the top of overhead return lines to allow easy passage of flash steam into them. It is sensible to consider slightly larger pipes having lower flash steam velocities to reduce the risk of waterhammer and noise from the steam trying to find passage through the liquid condensate in the rising line. Important: A rising line should only be used where the lowest steam pressure in the process is guaranteed to be higher than the total condensate back pressure. If not, the process will waterlog unless a pumping trap or pump/trap combination is used to provide proper drainage against the back pressure.

20

vent 1 : 70 slope = 150 mm per 10 m run

pumped condensate

Process

Flash steam has to pass through the condensate vented receiver and pump

Fig. 14 Keep rising lines short and connect to the top of return lines Return lines themselves should also slope down and be nonflooded (Fig. 14). To avoid flash steam occurring in flooded return lines, hot condensate from trap discharge lines should drain into vented receivers (or flash vessels where appropriate), from where it can be pumped on to its final destination via a flooded line at a lower temperature. The Condensate pipe sizing chart (Fig. 15). The condensate pipe sizing chart can be used to size any type of condensate line. Lines containing two-phase flow, such as trap discharge lines, are selected according to the pressures either side of the trap. The chart works around acceptable flash steam velocities according to the pipe size and percentage flash steam formed. The chart can be entered on lower temperatures than the steam saturation temperature, such as may be the case when using thermostatic steam traps for condensate discharge.

21

Pipe sizes can be estimated for pumped lines containing condensate below 100°C, as shown by example 5. Also, short drain lines to traps (less than 5 m) can be determined in a similar way. Note: in the case of pumped lines, the pressure drop and velocity must always be checked by referring the condensate flowrate to the pipe size against the table provided in Fig.37 (pages 48 and 49). The chart is used to size trap discharge lines on full load conditions. It is not necessary to consider any oversizing factors for start-up load or the removal of non condensable gases. Using the chart

On the lower chart, establish the point where the steam and condensate pressures meet. Move vertically up to the upper chart to choose the selected condensate rate. If the discharge line is falling (non-flooded) and the selection is on or between lines, choose the lower line size. If the discharge line is rising (flooded), choose the upper line size, (Fig. 15). Some examples for sizing trap discharge lines follow. Note: The reasoning behind sizing a trap and a discharge line is different, and it is perfectly normal for a trap discharge line to be a different size than the trap it is serving. However, the normal ancillary equipment associated with the steam trap set, such as the isolation valves, strainer, trap testing chamber, and check valve can be the same size as the trapping device whatever the discharge line size. A condensate line sizing chart is provided for photocopying in Appendix 1 (page 81).

22

500

100,000

400

350

300

250

200 150 100

50,000

65

10,000

50

5,000

40 32

2,000

25

1,000

20

500

15

5

Condensate line size mm

Condensate rate kg/h

80

20,000

10

200 100

6

50

20 10

1

Steam temperature °C

200 180 160 140

50

120

1 0.5

100

0

2

20

20 15 10

2

6

30

30

5

4

2

10

4

5 4 3 2

6

1 0.5

1

3

Condensate system pressure bar g

250

Steam system pressure bar g

3

0

Fig. 15 Condensate line sizing chart 23

Example 1

A steam trap passing a full load of 1 000 kg/h at 6 bar g saturated steam pressure through a sloping discharge line down to a flash vessel at 1.7 bar g.

6 bar

H.P. steam Shell and tube heat exchanger

L. P. steam Float trap set 25 Ø

1.7 bar

Discharge line being sized Flash vessel

Fig. 16 Example 1 - non-flooded pressurised trap discharge line As the discharge line is non-flooded, the lower figure of 25 mm is selected from the chart.

24

Example 2

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

3.5 bar 18 bar Air vent

H.P. steam

32 Ø 5m

Float trap SA control valve acting as air vent and condensate drain on start up

Discharge line being sized

Fig. 17 Example 2 - flooded trap discharge line Add the 0.5 bar static pressure (5 m head) to the 3.5 bar condensate pressure to give 4 bar g back pressure. As the discharge line is rising and thus flooded, the upper figure of 32 mm is selected from the chart.

25

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

Example 3

2 bar H.P. steam

Plate heat exchanger To high level condensate return line Discharge line being sized

Vent

20 Ø

25 Ø

Fig. 18 Example 3 - non-flooded vented trap discharge line As the line is non-flooded, the lower figure of 20 mm is selected from the chart.

26

Example 4

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

4 bar H.P. steam

5m Air flow

25 Ø

Discharge line being sized

Fig. 19 Example 4 - flooded trap discharge line The 5 m static pressure contributes the total back pressure of 0.5 bar g. As the trap discharge line is rising, the upper figure of 25 mm is selected from the chart.

27

Example 5

The automatic condensate pump shown in example 3 can also have its discharge line sized by the chart. The pump discharge rate is sized on 6 times the maximum expected inlet rate, in this case 6 >< 200 kg / h = 1 200 kg / h.

Vent

Condensate in

Sloping non-flooded return line

25 Ø Pumped condensate out

Discharge line being sized

Fig. 20 Example 5 - pumped discharge line Because the condensate will have lost its flash steam content to atmosphere via the receiver vent, the pump will only be pumping liquid condensate. In this instance, it is only necessary to use the top graph as shown in the example. As the line from the pump is rising, the upper figure of 25 mm is chosen. A useful tip for lines of 100 m or less is to choose the discharge pipe the same size as the pump. Also refer to the later section on condensate pumping for further details. Example 6

A balanced pressure thermostatic steam trap draining a hot table operating on a constant steam pressure of 2.6 bar g discharges condensate at 20°C below saturation temperature from a 2 metre cooling leg up to an overhead non-flooded condensate line 2 metres above the trap. The full load is 100 kg/h. The saturation temperature of steam at 2.6 bar g is 140°C, so the discharge temperature from the trap will be around 120°C. The chart is then entered on the temperature scale at 120°C rather than the pressure scale. The 2 m back pressure contributes the total back pressure of 0.2 bar g. As the trap discharge line is rising, the upper figure of 15 mm is selected.

28

Sizing common return lines

It is sometimes required to connect several trap discharge lines from separate processes into a common return line. Problems will not occur if the following considerations are met: a) the common line is not flooded and slopes in the direction of flow to an open end or a vented receiver, or a flash vessel if the conditions allow. b) the diameter of the common line is sized on the cumulative sizes of the branch lines.

Example

The common line size downstream of two connected trap discharge lines is the root of the sum of the squares of the connected lines. The example shows three heat exchangers, each separately controlled and each operated at the same time. Loads shown are full condensate loads and occur at 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 in the flash vessel falls via a float trap down to a vented receiver from where it is pumped direct to the boiler house. The trap discharge lines are sized on full load with steam pressure at 3 bar g and condensate pressure of 1.5 bar g, and as each is not flooded, the lower line sizes are picked from the graph. Line 1 picked as 20 mm, 2 picked as 20 mm, 3 picked as 15 mm

3 bar g

HE 1

Full load 750 kg/h

Common line for 1+2

= Ö 20²+20² = 28 mm

Common line for (1+2)+3

= Ö 28²+15² = 32 mm

3 bar g

HE 2

3 bar g

HE 3

Flash steam

Full load 375 kg/h

Full load 750 kg/h 1" FT14HC

1" FT14 1.5 bar g

20 Ø

15 Ø

1" FT14HC 20 Ø 28 Ø

Fig. 21 Calculating the common line size from the discharge lines

32 Ø

To receiver and pump

29

The theoretical dimension of 28 mm for the common line 1+2 does not exist as a nominal bore in commercial pipe sizes. The internal diameters of pipes can be larger or smaller than the nominal bore depending on the pipe schedule. Eg. for a DIN 2448 steel pipe, the internal diameter for a 25 mm nb is about 28.5 mm, while that for a 25 mm nb Schedule 40 pipe is about 26.6 mm. For most practical purposes, a 25 mm nb pipe may be comfortably selected. If in doubt, seek expert advice. Example

40 Ø A

Further example of calculating the common line size (Fig. 22)

15 Ø

15 Ø B

15 Ø

D

15 Ø

F

32 Ø

H

C

E

G

?

?

?

K J ?

L ?

Fig. 22 Trap discharge lines connecting to a common line Line A B C D E F G H J K L

Size (mm) 40

Commercial size (mm) 40

15

Ö 402+152

15 = 42.7

15

Ö 152+42.72

15 = 45.2

15

Ö 152+45.22

= 47.6

50 15

= 49.9

32

Ö 322+49.92

40 15

15

Ö 152+47.62

40

50 32

= 59.3

65

The commercial pipeline size is taken as the nearest available to the calculated size. This may mean downsizing in certain instances, but this will not normally cause problems in practice due to the diversity of loads in the other lines. 30

Sizing pumped return lines

Flash steam, separated from the condensate, will be used in a flash steam recovery system or simply vented to atmosphere. The remaining hot condensate should be pumped to the boiler house where its energy content and purity can be used to good effect. The pumped return line will only carry condensate but at lower velocities (typically 1 - 2 m/s) than those experienced in the trap discharge and common lines. As seen in example 5, the pump discharge line can be selected from the condensate line sizing chart, or often simply sized the same size as the pump outlet. Refer to the following section "Condensate Pumping" for more detail. It is important to remember that the flow in a pumped line is intermittent, as the pump usually cycles. The instantaneous flowrate while the pump discharges is higher than that which enters the pump. It is the instantaneous discharge figure that has to be considered on discharge lines.

Pumped lines longer than 100 m

Water in lines longer than 100 m will develop larger forces of inertia due to the larger mass of water that is moved during the pumping stroke. It is advisable to add the effects of inertia to pressure drop calculations on sizing these longer lines when mechanical pumps are used. Refer to the section 'longer delivery lines' at a later stage in this document for further details. As a general rule, the pipe should be at least one size larger than the pump outlet check valve.

Mechanical pump

Line over 100 m

Additional check valve 1 pipe length from pump

Fig. 23 An additional check valve 1 pipe length from the pump body to reduce the effect of backflow At the end of the pumping stroke, the condensate will tend to keep moving and can often cause a vacuum to be created downstream of the pump outlet check valve. As the momentum of the condensate falls, the vacuum creates a sudden backflow onto the check valve which can, in extreme cases, cause severe waterhammer and noise. An additional check valve fitted 1 pipe length after the pump outlet check valve tends to dampen the effect and protect the pump check valve from damage (Fig. 23). 31

If there is any choice, it is always best to lift immediately after the pump to a height allowing a gravity fall to the end of the line (Fig. 24). If the fall is enough to overcome the frictional resistance of the pipe (Fig. 26), then the only back pressure onto the pump is that formed by the initial lift. A vacuum breaker can be installed at the top of the lift not only to assist the flow along the falling line but also to prevent any tendency for backflow at the end of the stroke.

Vacuum breaker

Automatic air vent

fall fall due to obstruction

Mechanical pump

Fig. 24 best choice - lift after the pump Should the falling line have to fall anywhere along its length to overcome an obstruction, then an automatic air vent fitted at the highest point will assist flow around the obstruction (Fig. 24).

Tank

Mechanical pump

Fig. 25 alternative choice - lift after the pump to a break tank

32

Alternatively, any question of back pressure caused by the horizontal run can be entirely eliminated by an arrangement as in Fig. 25 in which the pump simply lifts into a breaktank. The pipe from the tank should fall in accordance with the table in Fig. 26. Pipefall need to overcome

Pipe size (DN mm) 15

20

25

32

pipe friction

40

50

65

80

100

125

150

Litres of water per hour

25 mm in 15 m

48

140

303

580

907

1 950

3 538

5 806

12 610

22 906

25 mm in 10 m

59

177

381

694

1 134

2 449

4 445

7 257

15 680

28 576

37 284 46 492

25 mm in 8 m

69

204

442

800

1 310

2 834

5 148

8 391

18 159

33 089

53 862

25 mm in 6 m

79

231

503

907

1 487

3 220

5 851

9 525

20 638

37 602

61 223

25 mm in 5 m

86

256

553

1 007

1 642

3 551

6 441

10 568

22 770

41 821

67 538

25 mm in 4 m

93

279

598

1 093

1 778

3 878

7 030

11 521

24 811

45 994

73 571

25 mm in 3 m

113

338

730

1 329

2 168

4 672

8 527

13 925

30 073

54 073

89 356

25 mm in 2 m

140

419

907

1 655

2 694

5 851

10 614

17 327

37 421

68 039

111 128

25 mm in 1.75 m*

152

454

984

1 793

2 923

6 327

11 498

18 756

40 573

73 708

120 426

25 mm in 1.5 m

165

490

1 061

1 932

3 152

6 804

12 383

20 185

43 726

79 378

129 725

25 mm in 1 m

206

612

1 324

2 404

3 923

8 482

15 422

25 174

54 431

99 019

161 476

Fig. 26 Pipe fall to overcome frictional losses Vented pumps, pumping traps and pump-trap installations

*(1:70)

Discharge lines from pumps vented to atmosphere are sized on the discharge rate of the pump. Condensate passing through pumping traps and pump/trap combinations in closed loop applications will often be at higher pressures and temperatures and flash steam will be formed in the discharge line. Because of this, discharge lines from pumping traps (such as the APT14), and pump/trap combinations (such as an MFP14 and FT float trap) are sized on the trapping condition at full load and not the pumping condition, as the line has to be sized to cater for flash steam. Sizing on flash steam will ensure the line is also able to cope with the pumping condition.

33

Condensate pumping from vented receivers In nearly all steam using plants, as much condensate as possible should be returned to the boiler house to use again. Even if gravity drainage can be used from the plant to the boiler house, often the condensate must be lifted into a boiler feedtank. Before looking at the types of pump available for condensate pumping, it may be helpful to discuss some basic pumping terminology. Pumping terminology

Vapour pressure. This term is used to define the pressure corresponding to the temperature at which conversion of a liquid into vapour takes place. In other words, it is the pressure at which a liquid will boil i.e. At atmospheric pressure, water will boil at 100°C At a pressure of 7 bar g, water will boil at 170.5°C At a pressure of 0.75 bar abs, water will boil at 92°C The vapour pressure is a very important consideration when pumping condensate. Condensate is usually close to its boiling point, which may cause difficulties where a centrifugal pump is concerned. This is because as the condensate is drawn into the pump impeller, it is accelerated and so experiences a drop in pressure. If this drop in pressure takes the condensate below the vapour pressure or saturation pressure for its temperature, the condensate will boil and some of the condensate will be released as flash steam bubbles. As the bubbles are carried along within the water, they reach a region of increased pressure, as they leave the pump impeller. This increased pressure brings the steam bubbles back above the saturation pressure causing the bubble to implode rapidly. If this occurs while next to a solid surface, the forces exerted by the liquid rushing in to fill the spaces creates very high localised pressures. This is known as cavitation and is capable of doing a great deal of damage to a pump impeller and housing within a short period of time. It also creates noise, similar to that of gravel rotating in the pump. It is often recommended that electrical pumps are not used to pump condensate at temperatures above 100°C. Some will have limits as low as 94°C or 96°C, depending on the design of the pump and the suction head provided by the receiver. Head (h) Head is a term used to describe the potential energy of a fluid at a given point. There are several ways that head can be measured.

34

Pressure head (hp). Pressure head is simply the fluid pressure at the point in question. e.g. A pump is required to discharge against a pressure head of 3 bar g. The pump fills from a pressure head of 0.1 bar g. Where water is the fluid, a 1 bar pressure head is equivalent to approximately 10 m of static head.

0.1 bar g

Pump inlet

3 bar g

Fig. 27 Pressure head Static head (hs). Static head is the equivalent vertical height of fluid above the point in question. The following example best explains the measure of static head. The pump inlet in Fig. 28 is subjected to a static head (known as the suction or filling head) of 1 m, and discharges against a static head (known as the static delivery head) of 30 m. Note that in this case, the water in the bottom of the header tank is above the pump inlet (this situation is called a flooded suction), With an electrical pump the suction head is subtracted from the static delivery head, to give the net static head against which the pump has to work. With a mechanical displacement pump (Fig.29), the filling head simply provides the energy to fill the pump, and has no effect on the head against which the pump has to operate. Collecting tank

Net static head 29 m Static delivery head 30 m

Header tank

Static suction head 1m

Pump inlet

Fig. 28 Net static head for an electrical pump 35

Collecting tank

Pump receiver

Condensate flow hs = net static head

flooded suction head

Fig. 29 Net static head for a mechanical pump Friction head (hf). The friction head is more accurately defined as the pressure loss due to friction, and is the head required to actually move the liquid along the pipeline, and, in simple terms, increases proportionately to the square of the velocity. Pressure loss can be found from tables showing the liquid flowrate, the pipe diameter and the pipe length. To be precise, the resistance to flow encountered by the various pipe line fittings must also be taken into account. Tables are available to calculate the equivalent length of straight pipe for various pipe fittings. This extra 'equivalent length' for pipe fittings is then added to the actual pipe length to give a 'total equivalent length'. However, in practice, if the pipe is correctly sized, it is unusual for the pipe fittings to represent more than an additional 10 % of the actual pipe length. A general rule which can be applied is: Total equivalent length ( le ) = Actual length + 10 % Within this reference guide, a figure of 10 % will be used as the extra equivalent length considered for calculating pressure loss due to friction. Tables are available which give head loss per metre of pipe for various flowrates, pipe diameters, and velocities. The standard S.I. units are Pascals per metre (Pa/m) or millibars per metre (mbar/ m). An example of such a table is given in Fig. 32 page 40. 36

Total delivery head (hd). The total delivery head hd against which the pump needs to operate is the sum of : Pressure required to raise the water to the desired level hs Pressure required to move the water through the pipes hf Pressure in the condensate system hp ie Total delivery head, hd = hs + hf + hp Electrical centrifugal condensate pumps

Pump operation. Centrifugal pumps utilise centrifugal force, which imparts a high velocity to the liquid (condensate) being pumped. Pressure energy is obtained by the rotation of an impeller fitted within a casing. Liquid enters the pump and is directed to the centre of the rotating impeller vanes. As the impeller rotates, the liquid is passed along the impeller vanes and increases in velocity. Pump application. The electrical pump is well suited to applications where large volumes of liquid need to be moved. Electrical pumps are usually built into a unit, often referred to as a condensate recovery unit (CRU). A CRU will usually include: A receiver. A control system operated by probes or floats. One or two pumps. The instantaneous flow from the CRU can be up to 1.5 times greater than the rate at which condensate returns to the receiver. It is this pumping rate that must be considered when calculating the friction loss in the discharge line. On twin pump units, a 'cascade' control system may also be employed which allows either pump to be selected as the 'lead' pump and the other as a 'stand-by' pump to provide back up if the condensate returning to the unit is greater than one pump can handle. This control arrangement also provides back up in the case of the one pump failing to operate; the condensate level in the tank will increase and bring the second pump into operation. Cascade type units usually pump at a rate of 1.1 times the return rate to the receiver, allowing a smaller discharge line to be considered. It is very important that the manufacturer's literature is read regarding the discharge pumping rate. Failure to do so could result in undersizing the pump discharge pipe work.

37

Vent Condensate in Condensate in

Level sensor

Receiver

Condensate out

Overflow with "U" seal

Electric pump

Fig. 30 A typical electrical condensate recovery unit (CRU) Sizing an electrical condensate recovery unit

To size an electric condensate recovery unit, it is necessary to know: The amount of condensate reaching the receiver in kg/h at running load. The temperature of the condensate. This must be below the manufacturer's specified ratings to avoid cavitation, however, manufacturers usually have different impellers to suit different temperature ranges, eg. 90°C, 94°C and 98°C. The total discharge head required. (Will need to be calculated from the site conditions) The pump discharge rate in order to size the return pipework. (Be sure to read the manufacturer's data properly to determine this).

38

Example Temperature of condensate Condensate to be handled Static lift ( hs ) Length of pipe work Condensate back pressure

= = = = =

94°C 1 800 kg/h 30 m 150 m friction losses only ( hf)

Using the data below, an initial selection of a condensate recovery unit can be made from the manufacturer's sizing chart, such as the one in Figure 31. From the chart, CRU 1 should be the initial choice subject to frictional losses in the delivery pipework. Pump 35 delivery head in metres

CRU 1

30

25 CRU 2 20

15 CRU 3 10

1800

5 100

200

Fig. 31 Typical CRU sizing chart Sizing the discharge pipework for an electric condensate recovery unit

2 000 300 400 500 1 000 Condensate to be handled at 94°C kg/h

From the chart in Fig. 31, it can be seen that CRU 1 is actually rated to handle 2 000 kg/h of condensate. Reading the manufacturer's data shows that the CRU will actually pump 1.5 times the maximum return rate shown on the sizing chart. i.e.: 1.5 x 2 000 kg/h = 3 000 kg/h This ensures start-up loads can be handled without overflowing, and this is what the discharge pipe work must be sized on. As in the earlier example it is now possible to determine the optimum size for the return line. 39

Actual length of pipe work = 150 m Equivalent length of pipe work = 150 m + 10 % = 165 m Fig. 32 Section of typical friction loss table for fully flooded pipelines (flowrates in L/h) Pressure drop Pa/m mbar/m

15

20

25

Pipe size (mm) 32 40 50

65

80

100

95

0.95

176

414

767

1 678

2 560

4 860

9 900

15 372

31 104

97.5

0.975

180

421

778

1 699

2 596

4 932

10 044

15 552

31 500

100

1.00

184

425

788

1 724

2 632

5 004

10 152

15 768

31 932

120

1.20

202

472

871

1 897

2 898

5 508

11 196

17 352

35 100

140

1.40

220

511

943

2 059

3 143

5 976

12 132

18 792

38 160

From the pressure drop table above, using a 40 mm nb. pipe will allow a flowrate of 3 000 kg/h (L/h) and incur a pressure drop of between 120 and 140 Pa per metre. For this example 128 Pa/m is about right. Therefore the head loss to friction can be calculated; Headloss to friction = = =

128 Pa / m x 165 m 21 kPa approx 2.1 metres

The total delivery head required by the pump is: 30 m (hs) + 2.1 m (hf) =

32.1 metres

The figure of 32.1 metres needs to be checked against the manufacturer's sizing chart for the CRU to confirm that there is sufficient head available - there is in this case, but had the allowable head been exceeded, then the options are to re-calculate using a larger pipe or to select a CRU with a greater lift capacity. Alternatively, it can be seen that the selected CRU1 can pump against a total head (hd) of 35 m. With an actual static head (hs) of 30 m, 5 m are "available" for pipe friction loss (hf). It may be possible to install a smaller pipe and take up a larger friction loss. Reference to the pipe sizing table on page 42 will show that, if the next lower sized pipe is used (in this case 32 mm), the unit friction loss (hf) to pass 3000 kg/h (or 3000 L/h) is 300 Pa/m, and the velocity is just over 1 m/s which is suitable for this application. hf is 300 Pa/m x 165 m Therefore, total delivery head

= 49.5 kPa (or 4.95 m) = hs + h f = 30 + 4.95 m = 34.95 m The conclusion is that 32 mm pipe can be used, as the CRU 1 pump can handle up to 35 m total delivery head. 40

Mechanical condensate pumps

Pump operation. Mechanical pumps consist of a body, into which condensate flows by gravity, containing a float and an automatic mechanism, operating a set of changeover valves. Condensate is allowed to build up inside the body, which raises a float. When the float reaches a certain level, it triggers a vent valve to close and an inlet valve to open to allow steam to enter and pressurise the body to push out the condensate. The condensate level and the float both fall. The steam inlet valve then shuts and the vent valve opens allowing the pump body to refill. Check valves are fitted to the condensate ports to ensure correct directional flow. It should be noted that a receiver is needed when using a pump (Fig. 33), due to its cyclical action. When the pump is discharging it is not filling, so there is a need to store the condensate which is being produced between pumping cycles.

Condensate in

Vent

Receiver

Steam supply to pump

Pumped condensate

Fig. 33 A typical mechanical condensate pump 41

Pump application. Generally, mechanical pumps handle smaller amounts of condensate than electrical pumps. They are however, particularly valuable in situations where: Condensate temperature causes cavitation. Condensate is in vacuum. Space is at a premium. Low maintenance is required. The environment is hazardous, humid or wet. Electrical supplies are not at hand (operated by steam, air or any inert gas). Condensate has to be removed from individual items of temperature controlled equipment which may be subjected to stall conditions. As with electrically driven pumps, they are sometimes, but not always, specified as packaged condensate recovery units. A mechanical condensate recovery unit will comprise a condensate receiver and the pump unit. No additional control system is required as the pump is fully automatic and only operates when needed. This means that the pump is self regulating. Mechanical pumps are, however, a little more involved to size because the flow in the return line is intermittent. The pump cycles as the receiver fills and empties. The instantaneous flowrate while the pump is discharging can often be up to six times the filling rate and it is this instantaneous flowrate which must be used to calculate the size of the discharge pipe. Always refer to the pump manufacturer for data on sizing the pump and discharge line.

42

Sizing a mechanical condensate pump

To size a mechanical condensate pump, the following information is required: The maximum condensate flowrate reaching the receiver. The motive pressure of steam or air available. The selection of steam or air depends on the application and site circumstances. The filling head available. The total back pressure of the condensate system. The sizing of mechanical pumps varies from manufacturer to manufacturer, and is usually based on empirical data, which are translated into factors and nomographs. The following is a typical example on how to size a mechanical pump. (The pipe length is less than 100 m and friction loss is taken as being negligible): Example Condensate load Steam pressure available for operating pump Vertical lift from pump to return piping Pressure in the return piping (piping friction negligible) Available filling head on the pump

= = = = =

2 200 kg/h 5.2 bar g 9.2 m 1.7 bar g 0.3 m

1.7 bar g return main pressure

Condensate manifold

Vent

Total plant condensate 2 200 kg/h

Reservoir

9.2 m lift Filling head 0.3 m

Pump

5.2 bar g operating pressure

Fig. 34 Mechanical pump sizing example 43

Calculate the total back pressure (delivery head hd), against which the condensate must be pumped: Total back pressure (hd) = lift (hs)+ condensate pressure (hp) (friction loss neglected as line is shorter than 100 m) lift (hs) cond. pressure (hp) Total

= 9.2 m = 1.7 bar g = 17 m head = 9.2 + 17 m = 26 m

Reference to Fig. 35 below shows that a DN50 pump at 5.2 bar g motive pressure will pump 2600 kg/h against a 26 m head, and will thus be the correct choice for this example. Note: the pump is sized on the filling rate.

10

9

9

8

8

7

7

6

6

5.2 5

5.2 5

4

4

2 1 0 2000 2200 2500 2600 3000

4000

5000 Flowrate kg/h

Fig. 35 - DN50 MFP 14 pump sizing chart

10 m lift

20 m lift

30 m lift

40 m lift

26 m

4 m lift

10

Motive pressure bar g

11

3

80 m lift 50 m lift

4 m lift

11

Motive pressure bar g

12

DN50 size capacities

44

13

12

1000

32 m

14 10 m lift

20 m lift

40 m lift

80 m lift 50 m lift

13

30 m lift

32 m 27 m 26 m

14

3 2 1 0 1000

2000 2500

3000

DN80 x DN50 size capacities

4000

5000 6000 Flowrate kg/h

Fig. 36 - DN80 MFP 14 pump sizing chart

Sizing the discharge pipework for a mechanical condensate pump

Below 100 m long, the discharge pipe from a mechanical pump can usually be taken as the same size as the pump body. The frictional resistance of the pipe is relatively small compared to the back pressure caused by the lift and condensate return pressure, and can usually be disregarded. Above 100 m, a general rule would be to select one pipe size larger than the pump outlet check valve.

Longer delivery lines

On delivery lines over 100 m, and/or where the condensate flow is near to the pump maximum, it is advisable to check the pipe size to ensure that the total friction loss (including inertia loss) does not increase above that which effects the pump's capability (or installation costs). With 5.2 bar g motive steam and 26 m delivery head, from Fig. 35, for a DN50 pump, Maximum pump capacity = 2 600 kg/h Actual condensate flowrate into pump = 2 200 kg/h again, from Fig. 35, for a DN50 pump, Max. back pressure permissible at 2 200 kg/h = 32 m therefore, max. frictional resistance allowable = 32 - 26 m = 6m (60 kPa) Inertia loss On lines over 100 m, a considerable volume of liquid will be held within the pipe. The sudden acceleration of this mass of liquid at the start of the pump discharge can absorb some part of the pump energy, and this needs to be considered within the friction loss calculation by reducing the allowable friction loss by 50%, thus, Total allowable friction loss

= 50 % × 60 kPa = 30 kPa

Consider delivery pipe length to be 250 m + 10% for additional fittings then, max. frictional resistance allowable / m approx. Taking delivery flowrate as 6 times filling rate

= 275 m = 30 kPa 275 m = 109 Pa/m = 6 × 2 200 = 13 200 kg/h

Referring to Fig. 37 (the table), a frictional resistance of 109 Pa/m reveals that an 80 mm pipe is required to give an acceptable flowrate of 13 200 kg/h. In fact, the table shows that this size pipe will pass about 16 500 kg/h with this frictional resistance. By rising up the '80 mm column', it can be seen that, by interpolation, the flowrate of 13 200 kg/h actually induces a frictional loss of about 72 Pa/m in an 80 mm pipe.

45

Fully loaded pumps and longer lines

Should the condensate filling rate have been near the maximum 2 600 kg/h for the above example, say 2 500 kg/h, then less head is available for friction loss, and progressively less so for longer lines. Sizing on a filling rate of 2 500 kg/h, and a 250 m (+10%) line, referring to Fig. 35, for the DN50 pump, it can be seen that a condensate filling rate of 2 500 kg/h equates to a max. back pressure of about 27 m, hence in this instance, available head left for friction losses

= 27 - 26 m = 1 m (10 kPa)

frictional resistance allowable

= 10 kPa 275 m = 36 Pa / m

minus allowance of 50% for inertia loss

= 50 % × 36 Pa/m

therefore, max. frictional resistance allowable = 18 Pa / m As before, the discharge pipework has to be sized on the instantaneous flowrate from the pump outlet, which is taken as 6 × the filling rate. In this instance, the pipe would have been sized on 6 × 2 500 kg/h = 15 000 kg/h with a friction loss of 18 Pa/m. Fig. 37 (the table) reveals that this would require a pipe larger than 100 mm to allow the pump to operate within its capability. Although the system would certainly work with this arrangement, it may be more economical to consider a larger pump with smaller pipework.

46

Consideration of a larger pump and smaller pipeline

Fig. 36 reveals that a DN 80 pump under the same conditions of 5.2 bar g motive steam and 26 m back pressure would allow the following friction losses: Back pressure

= 26 m

At a filling rate of 2 500 kg/h, max. allowed = 35 m head available for friction loss = 35 - 26 m = 9 m (90 kPa) 90 kPa over 250 m and inc. inertia loss max. frictional resistance allowable

= 50 % × 90 250 = 180 Pa/m

Fig. 37 (the table) shows that an 80 mm pipe will accommodate 21420 kg/h with a friction loss of 180 Pa/m. Hence, in this instance, the larger pump will comfortably allow a pipe two sizes smaller than that for the smaller pump. Always check that velocity is within recommendations. The 80 mm pipe will handle the above condition at just under 1 m/s, and is therefore suitable. The DN80 pump would cost about 10% more than the DN50 pump, but these costs could well be recovered with the difference in installation costs on longer delivery lines between an 80 mm and 100+ mm pipe plus fittings and insulation etc.

47

Fig. 37 - Flow of water in heavy steel pipes (with velocities) kg/h 15 mm Pa/m 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35 37.5 40 42.5 45 47.5 50 52.5 55 57.5 60 62.5 65 67.5 70 72.5 75 77.5 80 82.5 85 87.5 90 92.5 95 97.5 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440

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mbar/m 0.1 0.125 0.15 0.175 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5 0.525 0.55 0.575 0.6 0.625 0.65 0.675 0.7 0.725 0.75 0.775 0.8 0.825 0.85 0.875 0.9 0.925 0.95 0.975 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4

50 58 65 68 76 79 83 90 94 97 101 104 112 115 119 122 126 130 130 133 137 140 144 148 151 151 155 158 162 166 166 169 173 176 176 180 184 202 220 234 252 266 281 288 306 317 331 342 353 364 374 385 396 407

20 mm 25 mm