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The Steam and Condensate Loop 2.4.1

Block 2 Steam Engineering Principles and Heat Transfer Steam Quality Module 2.4

Module 2.4

Steam Quality S C

- G C M

- 0 8

C M

I s s u e 1

C o p y r i g h t 2 0 0 5

S p i r a x - S

a r c o L i m i t e d

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The Steam and Condensate Loop2.4.2


Fig. 2.4.1 Steam process equipment with an automatic air vent and strainers

Steam

Strainer

Condensate

Automaticair vent

Steam heatedcooking vessel

Strainer

Air vented tosafe location

Steam Quality

Steam should be available at the point of use:o In the correct quantity.o At the correct temperature and pressure.o Free from air and incondensable gases.o Clean.o Dry.

Correct quantity of steamThe correct quantity of steam must be made available for any heating process to ensure that asufficient heat flow is provided for heat transfer.

Similarly, the correct flowrate must also be supplied so that there is no product spoilage or dropin the rate of production. Steam loads must be properly calculated and pipes must be correctlysized to achieve the flowrates required.

Correct pressure and temperature of steamSteam should reach the point of use at the required pressure and provide the desired temperaturefor each application, or performance will be affected. The correct sizing of pipework and pipelineancillaries will ensure this is achieved.

However, even if the pressure gauge is correctly displaying the desired pressure, the corresponding saturation temperature may not be available if the steam contains air and/or incondensablegases.

Air and other incondensable gases Air is present within the steam supply pipes and equipment at start -up. Even if the system werefilled with pure steam the last time it was used, the steam would condense at shutdown, and airwould be drawn in by the resultant vacuum.When steam enters the system it will force the air towards either the drain point, or to the point furthest from the steam inlet, known as the remote point. Therefore steam traps with sufficient airventing capacities should be fitted to these drain points, and automatic air vents should be fittedto all remote points.

However, if there is any turbulence the steam and air will mix and the air will be carried to theheat transfer surface. As the steam condenses, an insulating layer of air is left behind on thesurface, acting as a barrier to heat transfer.

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[ EDU D EDU D

Equation 2.4.1(IIHFWLYH VWHDP $PRXQW RI VWHDP DV D SURSRUWLRQ ,QGLFDWHG SUHVVXUH[SUHVVXUH EDU D RI WRWDO E\ YROXPH EDU D

Steam and air mixturesIn a mixture of air and steam, the presence of air will cause the temperature to be lower thanexpected. The total pressure of a mixture of gases is made up of the sum of the partial pressuresof the components in the mixture.

This is known as Dalton s Law of Partial Pressures. The partial pressure is the pressure exerted byeach component if it occupied the same volume as the mixture:

Note: This is a thermodynamic relationship, so all pressures must be expressed in bar a.

Example 2.4.1Consider a steam/air mixture made up of steam and air by volume. The total pressure is4 bar a.

Determine the temperature of the mixture:

Therefore the steam only has an effective pressure of 3 bar a as opposed to its apparent pressureof 4 bar a. The mixture would only have a temperature of 134 C rather than the expected saturationtemperature of 144 C.

This phenomena is not only of importance in heat exchange applications (where the heat transferrate increases with an increase in temperature difference), but also in process applications wherea minimum temperature may be required to achieve a chemical or physical change in a product.For instance, a minimum temperature is essential in a steriliser in order to kill bacteria.

Other sources of air in the steam and condensate loop Air can also enter the system in solution with the boiler feedwater. Make-up water and condensate,exposed to the atmosphere, will readily absorb nitrogen, oxygen and carbon dioxide: the maincomponents of atmospheric air. When the water is heated in the boiler, these gases are releasedwith the steam and carried into the distribution system.

Atmospheric air consists of 78% nitrogen, 21% oxygen and 0.03% carbon dioxide, by volumeanalysis. However, the solubility of oxygen is roughly twice that of nitrogen, whilst carbon dioxidehas a solubility roughly 30 times greater than oxygen!

This means that air dissolved in the boiler feedwater will contain much larger proportions of carbon dioxide and oxygen: both of which cause corrosion in the boiler and the pipework.

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Fig. 2.4.2 A pipeline strainer

A

C

D

B

The temperature of the feedtank is maintained at a temperature typically no less than 80 C sothat oxygen and carbon dioxide can be liberated back to the atmosphere, as the solubility of these dissolved gases decreases with increasing temperature.

The concentration of dissolved carbon dioxide is also kept to a minimum by demineralising anddegassing the make-up water at the external water treatment stage.

The concentration of dissolved gas in the water can be determined using Henry s Law. This statesthat the mass of gas that can be dissolved by a given volume of liquid is directly proportional tothe partial pressure of the gas.

This is only true however if the temperature is constant, and there is no chemical reaction betweenthe liquid and the gas.

Cleanliness of steamLayers of scale found on pipe walls may be either due to the formation of rust in older steamsystems, or to a carbonate deposit in hard water areas. Other types of dirt which may be found ina steam supply line include welding slag and badly applied or excess jointing material, which mayhave been left in the system when the pipework was initially installed. These fragments will havethe effect of increasing the rate of erosion in pipe bends and the small orifices of steam traps andvalves.For this reason it is good engineering practice to fit a pipeline strainer (as shown in Figure 2.4.2).This should be installed upstream of every steam trap, flowmeter, pressure reducing valve andcontrol valve.

Steam flows from the inlet A through the perforated screen B to the outlet C. While steam andwater will pass readily through the screen, dirt will be arrested. The cap D can be removed,allowing the screen to be withdrawn and cleaned at regular intervals.

When strainers are fitted in steam lines, they should be installed on their sides so that theaccumulation of condensate and the problem of waterhammer can be avoided. This orientationwill also expose the maximum strainer screen area to the flow.

A layer of scale may also be present on the heat transfer surface, acting as an additional barrier toheat transfer. Layers of scale are often a result of either:o Incorrect boiler operation, causing impurities to be carried over from the boiler in water droplets.o Incorrect water treatment in the boiler house.

The rate at which this layer builds up can be reduced by careful attention to the boiler operationand by the removal of any droplets of moisture.

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Fig. 2.4.3 A steam separator

Air and incondensable gases vented

Dry steam out

Wet steam in

Moisture to trap set

Dryness of steamIncorrect chemical feedwater treatment and periods of peak load can cause priming and carryoverof boiler feedwater into the steam mains, leading to chemical and other material being depositedon to heat transfer surfaces. These deposits will accumulate over time, gradually reducing theefficiency of the plant.

In addition to this, as the steam leaves the boiler, some of it must condense due to heat loss

through the pipe walls. Although these pipes may be well insulated, this process cannot becompletely eliminated.

The overall result is that steam arriving at the plant is relatively wet.

It has already been shown that the presence of water droplets in steam reduces the actual enthalpyof evaporation, and also leads to the formation of scale on the pipe walls and heat transfersurface.

The droplets of water entrained within the steam can also add to the resistant film of waterproduced as the steam condenses, creating yet another barrier to the heat transfer process.

A separator in the steam line will remove moisture droplets entrained in the steam flow, and alsoany condensate that has gravitated to the bottom of the pipe.

In the separator shown in Figure 2.4.3 the steam is forced to change direction several times as it flows through the body. The baffles create an obstacle for the heavier water droplets, while thelighter dry steam is allowed to flow freely through the separator.

The moisture droplets run down the baffles and drain through the bottom connection of theseparator to a steam trap. This will allow condensate to drain from the system, but will not allowthe passage of any steam.

Waterhammer As steam begins to condense due to heat losses in the pipe, the condensate forms droplets onthe inside of the walls. As they are swept along in the steam flow, they then merge into a film.The condensate then gravitates towards the bottom of the pipe, where the film begins to increasein thickness.

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Steam

Steam

SteamCondensate

Condensate

Condensate

Fig. 2.4.5 Potential sources of waterhammer

This slug of water is dense and incompressible, and when travelling at high velocity, has aconsiderable amount of kinetic energy.

The laws of thermodynamics state that energy cannot be created or destroyed, but simply convertedinto a different form.

When obstructed, perhaps by a bend or tee in the pipe, the kinetic energy of the water is convertedinto pressure energy and a pressure shock is applied to the obstruction.

Condensate will also collect at low points, and slugs of condensate may be picked up by the flowof steam and hurled downstream at valves and pipe fittings.

These low points might include a sagging main, which may be due to inadequate pipe support ora broken pipe hanger. Other potential sources of waterhammer include the incorrect use of concentric reducers and strainers, or inadequate drainage before a rise in the steam main. Someof these are shown in Figure 2.4.5.

The noise and vibration caused by the impact between the slug of water and the obstruction, isknown as waterhammer.

Waterhammer can significantly reduce the life of pipeline ancillaries. In severe cases the fitting may fracture with an almost explosive effect. The consequence may be the loss of live steam at the fracture, creating a hazardous situation.

The installation of steam pipework is discussed in detail in Block 9, Steam Distribution.

Fig. 2.4.4 Formation of a solid slug of water

Steam

Steam

Steam

Condensate

Slug

Incorrect use of a concentric reducer

Incorrect installation of a strainer

Inadequate drainage before a rise

The build up of droplets of condensate along a length of steam pipework can eventually form aslug of water (as shown in Figure 2.4.4), which will be carried at steam velocity along the pipework(25 - 30 m/s).

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Questions

1. Steam supplied at 6.5 bar g contains 20% air by volume. What is the temperature of themixture ?

a| 165 C

b| 127 C

c| 167 C

d| 159 C

2. Why is a boiler feedtank heated to approximately 85C ?

a| To reduce the energy required to raise steam

b| To reduce the content of total dissolved solids in the water supplied to the boiler

c| To reduce the gas content of the water

d| To reduce the content of suspended solids in the water

3. What is used to dry steam ?

a| A separator

b| A strainer

c| A steam trap

d| A tee piece

4. What causes waterhammer ?

a| Suspended water droplets

b| An air /water mixture

c| Strainers fitted on their sides

d| Slugs of water in the steam

5. How does air enter a steam system ?

a| Through joints, on shut down of the steam system

b| With make-up water to the boiler feedtank

c| With condensate entering the boiler feedtank

d| All of the above

6. Why should strainers installed on steam lines be fitted on their sides ?

a| To prevent the build-up of water in the strainer body

b| To trap more dirt

c| To reduce the frequency of cleaning

d| To provide maximum screening area for the steam

1 : d , 2 : c , 3 : a , 4 : d , 5 : d , 6 : a Answers

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