Efficient use of Steam

Sir Oliver LyleThe Efficient Use of SteamSteam Possesses Many Outstanding Qualities:Very high heat contentGives up heat at constant temperatureProduced from water (cheap and plentiful)Clean, odorless, tastelessIts heat can be used over and over againCan generate power, then be used for heatingCan be readily distributed and easily controlled1947

Latent heat vs pressure

Extract from the steam table

Pressure

(kg/cm2)

Temperature oC

Enthalpy in Kcal/kg

Specific Volume (m3/kg)

Water (hf )

Evaporation (hfg)

Steam (hg)

1

100

100.09

539.06

639.15

1.673

2

120

119.92

526.26

646.18

0.901

3

133

133.42

517.15

650.57

0.616

4

143

143.70

509.96

653.66

0.470

5

151

152.13

503.90

656.03

0.381

6

158

159.33

498.59

657.92

0.321

7

164

165.67

493.82

659.49

0.277

8

170

171.35

489.46

660.81

0.244

Dryness fraction

Steam distribution system

The working pressureThe distribution pressure of steam is influenced by a number of factors, but is limited by:The maximum safe working pressure of the boiler.The minimum pressure required at the plant.

As steam passes through the distribution pipework, it will inevitably lose pressure due to:Frictional resistance within the pipework Condensation within the pipework as heat is transferred to the environment.Therefore allowance should be made for this pressure loss when deciding upon the initial distribution pressure.A kilogram of steam at a higher pressure occupies less volume than at a lower pressure. It follows that, if steam is generated in the boiler at a high pressure and also distributed at a high pressure, the size of the distribution mains will be smaller than that for a low-pressure system for the same heat load.

Dry saturated steam - pressure /specific volume relationship

Generating and distributing steam at higher pressure : AdvantagesThe thermal storage capacity of the boiler is increased, helping it to cope more efficiently with fluctuating loads, minimising the risk of producing wet and dirty steam.Smaller bore steam mains are required, resulting in lower capital cost, for materials such as pipes, flanges, supports, insulation and labour.Smaller bore steam mains cost less to insulate.

Steam Piping : FeaturesSteam pipes should be laid by the shortest possible distance.Provision for proper draining of condensate.For example, a 100mm well lagged pipe of 30-meter length carrying steam at 7 Kg/cm2 pressure can condense nearly 10 Kg. of water in the pipe in one hour unless it is removed from the pipe through traps. The pipes should run with a fall (slope)of not less than 12.5 mm in 3 meter in the direction of flow.

Piping layout

Reverse gradient

Steam piping requirementsLarge pockets in the pipes to enable water to collectDrain pockets should be provided at every 30 to 50 meters and at any low point in the pipe network.Expansion loops are required to take care of the expansion of pipes when they get heated up.Automatic air vents should be fixed at the dead end of steam mains, which will allow removal of air, which will tend to accumulate.

Drain points

Expansion fittings

Branch lines

Water hammer

Water hammer and its effect Waterhammer is the noise caused by slugs of condensate colliding at high velocity into pipework fittings, plant, and equipment. This has a number of implications:Because the condensate velocity is higher than normal, the dissipation of kinetic energy is higher than would normally be expected.Water is dense and incompressible, so the cushioning effect experienced when gases encounter obstructions is absent.The energy in the water is dissipated against the obstructions in the piping system such as valves and fittings.

Potential source of water hammer

Preventing water hammer the possibility of waterhammer is minimised by:Installing steam lines with a gradual fall in the direction of flow, and with drain points installed at regular intervals and at low points.Installing check valves after all steam traps which would otherwise allow condensate to run back into the steam line or plant during shutdown.Opening isolation valves slowly to allow any condensate which may be lying in the system to flow gently through the drain traps, before it is picked up by high velocity steam. This is especially important at start-up.

Steam Pipe Sizing and Design 1. Pipe Sizing Proper sizing of steam pipelines help in minimizing pressure drop.

The velocities for various types of steam are:Superheated 50-70 m/secSaturated 30-40 m/secWet or Exhaust 20-30 m/sec

The steam piping should be sized, based on permissible velocity and the available pressure drop in the line. A higher pipe size will reduce the pressure drop and thus the energy cost. However, higher pipe size will increase the initial installation cost.

Empirical FormulaP1 = 7 bar , P2 = 6.6 bar , Flow, m, = 286 kg/hr , L = 165 m

Find out Diameter ?

Answer : 42 mm

What is the Function of Steam Traps?Steam traps are automatic valves used in every steam system to remove condensate, air, and other non-condensable gases while preventing or minimizing the passing of steam.If condensate is allowed to collect, it reduces the flow capacity of steam lines and the thermal capacity of heat transfer equipment. In addition, excess condensate can lead to "water hammer," with potentially destructive and dangerous results. Air that remains after system startup reduces steam pressure and temperature and may also reduce the thermal capacity of heat transfer equipment. Non-condensable gases, such as oxygen and carbon dioxide, cause corrosion. Finally, steam that passes through the trap provides no heating service. This effectively reduces the heating capacity of the steam system or increases the amount of steam that must be generated to meet the heating demand.

What steam trap does ?

Trap InstallationFigure 2.

Types of steam traps

Group

Principle

Sub-group

Mechanical trap

Difference in density between steam and condensate.

Bucket type

Open bucket

Inverted bucket, with lever, without lever

Float type

Float with lever

Free float

Thermodynamic trap

Difference in thermodynamic properties between steam and condensate

Disc type

Orifice type

Thermostatic trap

Difference in temperature between steam and condensate

Bimetallic type metal expansion type.

Inverted bucket trapsThe operation of a mechanical steam trap is driven by the difference in density between condensate and steam. The denser condensate rests on the bottom of any vessel containing the two fluids. As additional condensate is generated, its level in the vessel will rise. This action is transmitted to a valve via either a "free float" or a float and connecting levers in a mechanical steam trap. One common type of mechanical steam trap is the inverted bucket trap. Steam entering the submerged bucket causes it to rise upward and seal the valve against the valve seat. As the steam condenses inside the bucket or if condensate is predominately entering the bucket, the weight of the bucket will cause it to sink and pull the valve away from the valve seat. Any air or other non-condensable gases entering the bucket will cause it to float and the valve to close. Thus, the top of the bucket has a small hole to allow non-condensable gases to escape. The hole must be relatively small to avoid excessive steam loss.

Inverted Bucket

Float and thermostatic

Thermostatic trapsAs the name implies, the operation of a thermostatic steam trap is driven by the difference in temperature between steam and sub-cooled condensate. Valve actuation is achieved via expansion and contraction of a bimetallic element or a liquid-filled bellows. The nonlinear relationship between steam pressure and temperature requires careful design of the bimetallic element for proper response at different operating pressures.

Thermostatic bellows trap

Thermodynanic trapThermodynamic trap valves are driven by differences in the pressure applied by steam and condensate, with the presence of steam or condensate within the trap being affected by the design of the trap and its impact on local flow velocity and pressure. Disc, piston, and lever designs are three types of thermodynamic traps with similar operating principles. When subcooled condensate enters the trap, the increase in pressure lifts the disc off its valve seat and allows the condensate to flow into the chamber and out of the trap. The narrow inlet port results in a localized increase in velocity and decrease in pressure as the condensate flows through the trap, following the 1st law of thermodynamics and the Bernoulli equation. As the condensate entering the trap increases in temperature it will eventually flash to steam because of the localized pressure drop just described. This increases the velocity and decreases the pressure even further, causing the disc to snap closed against the seating surface. The moderate pressure of the flash steam on top of the disc acts on the entire disc surface, creating a greater force than the higher pressure steam and condensate at the inlet, which acts on a much smaller portion of the opposite side of the disc. Eventually, the disc chamber will cool, the flash steam will condense, and inlet condensate will again have adequate pressure to lift the disc and repeat the cycle.

On start-up, incoming pressure raises disc A from its seat rings C. Air and cool condensate are instantly discharged through outlet B.

Hot condensate releases flash steam as it flows through the trap. The high velocity of the flash steam creates a low-pressure area under the disc and draws it towards the seat. Pressure builds up in chamber D due to the impact of higher velocity flash steam against the cap.

.

The flash steam pressure in chamber D forces the disc down against the pressure of the incoming fluid until it seats on inner ring C and closes the inlet. The disc also seats on the outer ring C, trapping pressure in chamber D.In due course, condensation decreases the pressure in Chamber D. The disc is raised by incoming pressure. The cycle repeats.Disc traps click audibly when operating. This should be taken into consideration when using disc traps, as the noise could be objectionable.

Thermodynamic trap

Strainer

Group trapping

Individual trapping

Basic methods for testing a steam trap Ultrasonic test KitInfrared temperature gun Sight glass evaluation

11Steam Traps are install on Drip Pockets to purge condensate from a distribution line (Steam Main). You will notice the installation has a Drip Pocket the same OD as the Steam Main. If you try to use a reduced pipe size for your Drip Pocket, condensate could roll past the Drip Pocket and cause greater problems. A slug of cool condensate hitting steam turbine blades rotating at 10,000 RPM can have devastating results. Properly placed and working drip traps can help alleviate water hammer problems

TRACE TRAPSSteam Traps are installed on Steam Tracing Lines to purge the condensate and insure the steam trace lines stay hot. Steam Trace Lines consist of copper or stainless steel tubing connected to a steam supply and run next to the pipe or equipment to be heated. The animation on the left shows Thermostatic Steam Traps connected to a Steam Trace system discharging into a closed condensate return system. There is an art to running Steam Tracing. Improperly run and you can cause process lines to freeze or transmitters become over-heated. This is extremely important in colder climates.

PROCESS TRAPSTraps are also installed on the Condensate Boot of heat exchangers. Steam enters the Shell and Tube Heat Exchanger, gives up its heat energy and is condensed into water or condensate. At the same time process fluid is circulated through the heat exchanger to absorb the heat given up by the steam. The condensate flows into the condensate boot and is then purged out through the steam trap. Each application should be sized individually. I have been in facilities where the process traps are either installed or sized improperly and has caused the heat exchanger to become flooded with condensate, which results in a reduced production of the heat exchanger.

The animation is so true. Numerous times I have pointed out the reason for repeated repairs. Work Orders are written, submitted, meetings, approvals, scheduling, repairs completed, but no system changes. So, the same repairs are made over and over. This could be avoided by adding a drip pocket upstream of the elbow. Imagine a 6" steam main with 36" long slug of water traveling at approximately 100 miles per hour. Now, imagine that slug of water slamming into an obstruction. This is equivalent to a cannon ball traveling at about 145 feet per second through a pipe and hitting a partially opened valve, reducer or a 90 pipe elbow. The sudden change of direction of the water will convert the kinetic energy of the high velocity water into a high pressure area. By following good piping practices, it is possible to eliminate Water Hammer. I have read and listened to a number of explanations of Water Hammer. The best explanation is from: "Maxims and Instructions for the Boiler Room" by N. Hawkins, ME Copyright 1897, 1898, 1903 Theo, Audel & Co. page 284 - 285 "It is well known that the presence of condensed water in pipes is a source of danger, but little is known of what exactly goes on in the pipe." . . . "Seeing that the tube or pipe is capable of withstanding all the pressure that the steam can give, it is difficult to account for the tremendous repelling force, which is, undoubtedly, brought into operation in explosions or ruptures of steam pipes carrying what are now comparatively low pressures." . . . "The remedy for this is simple, the pipes must be properly located so as to drain themselves or be drained by rightly located drip cocks. The drip should be on the other side of the throttle valve, and if steam is left on over night this valve should be left open enough to drain out all the water." . . . "During the twelve years between 1879 and 1891 there were recorded 2,159 boiler explosions; these resulted in the death of 3,123 persons, and in more or less serious injury to 4,352 others." Is the trap working correctly or not? If not, has the trap failed in the open or closed position?

Traps that fail open result in a loss of steam and its energy. Where condensate is not returned, the water is lost as well. The result is significant economic loss, directly via increased boiler plant costs, and potentially indirectly, via decreased steam heating capacity.

Traps that fail closed do not result in energy or water losses, but can result in significantly reduced heating capacity and/or damage to steam heating equipment

The best method for checking trap working is visual method. Other methods such as ultrasonic test kit, infrared temperature gun and sight glass evaluation are shown in the photos.