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These small items of equipment have a variety ofimportant applications throughout steam systemsand process equipment. The different typesavailable are studied in this tutorial.
Use the quick links below to take you to the mainsections of this tutorial:
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You are here: Home Resources Steam Engineering Tutorials Pipeline AncillariesGauges, Sight Glasses, Vacuum Breakers
Gauges, Sight Glasses, Vacuum Breakers
GaugesPressure gaugesPressure gauges should be installed in at least the following situations:
Upstream of a pressure reducing valve - To monitor the integrity of the steam supply.Downstream of a pressure reducing valve - To set and monitor the downstream pressure. Variationsin the downstream pressure can lead to reduced plant productivity and product quality. Variations in thedownstream pressure may also indicate problems with the pressure reducing valve.On blowdown vessels - A pressure gauge is used to check the vessel pressure during blowdown.This improves safety, since a higher pressure than normal would give an early indication of pipeworkblockage.Flash steam vessels - To monitor the flash steam pressure.
The Bourdon tube pressure gauge is the most commonly used type in steam systems. It consists of a coiled or'C' - shaped tube that is sealed at one end, and open at the other. The open end of the Bourdon tube isexposed to the process fluid, allowing it to flow into the tube. Any increase in pressure causes elastic distortionof the tube, causing it to unwind. The resulting displacement of the closed end of the tube is translated by aseries of gears to an angular displacement of the pointer. The pointer position is therefore proportional to thepressure applied at the gauge's pressure connector. Typically, the maximum deflection of the Bourdon tubecorresponds to a pointer angular displacement of 270°.
The tube can be constructed out of a number of different materials, depending on the application; generally,brass or bronze is used for higher pressures, whereas stainless steel is used for lower pressures.
Fig. 12.6.1 'C'-shaped (a) and coiled (b) Bourdon tubes
Bourdon tube pressure gauges often have the option of being liquid filled. The area surrounding the Bourdontube is filled with a transparent liquid, normally glycerine. This protects the internal mechanisms againstdamage from severe vibration and to keep out ambient corrosives and condensation. This also damps themovement of the pointer making the gauge less susceptible to small transient pressure fluctuations.
As the Bourdon tube may be damaged by high temperatures, it is common practice on steam systems to
Pipeline AncillariesIsolation Valves - LinearMovement
Isolation Valves - RotaryMovement
Check Valves
Strainers
Separators
Gauges, Sight Glasses,Vacuum Breakers
Related ContentGaugesGauges are usefulmonitoring and diagnostictools that indicate thehealth of your steamsystem.
Sight GlassesSight glasses allow quick& easy visual inspectionthroughout your system.
Vacuum BreakersUsed to break the vacuumin a system when thesystem pressure dropsbelow atmosphericpressure.
DiffusersA diffuser creates a safe &silent environment whendischarging toatmosphere; essential forHealth & Safetycompliance.
The Steam andCondensate Loop BookA comprehensive bestpractice guide to savingenergy and optimisingplant performance, thisbook covers all aspects ofsteam and condensatesystems.
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install the gauge at the end of a syphon tube. The syphon tube is filled with water which transmits the pressureof the working fluid to the Bourdon tube, enabling the gauge to be located some distance from the actual pointwhere the pressure is being measured. The two most common forms of syphon tube are the 'U' and ringtypes. The ring tube is used on horizontal pipelines where there is sufficient space above the pipe, and the 'U'type is used when mounting the gauge on a vertical pipeline, or on horizontal pipelines where there is notsufficient space for a ring type siphon.
Fig. 12.6.2 'U' (a) and ring type (b) syphon tubes
The Bourdon type pressure gauge is not suitable for use on corrosive liquids or fluids containing suspendedsolids alone, as these solids may damage the internal elements of the gauge. In such cases, it is necessary tokeep the process fluid separate from the Bourdon tube.
This is done by mounting a flexible diaphragm on the inlet to the gauge. The pressure element of the gaugeand the space behind the diaphragm form a completely sealed system, which is evacuated and then filled witha suitable filling fluid; in the case of steam this is typically a type of oil. The system pressure causes thediaphragm to deflect, and the pressure is transmitted through the filling fluid to the Bourdon tube.
Diaphragm seals should also be used on 'clean steam' applications where no 'dead space' is allowed.
In addition to the Bourdon tube pressure gauge, several other types of pressure gauge are available whichinclude; Diaphragm type pressure gauges, Piezoresistive pressure gauges and Temperature gauges.
Diaphragm type pressure gaugesA metal diaphragm is clamped between two flanges, and is exposed to the pressure medium on one side.Pressure exerted by the fluid causes elastic deflection of the diaphragm. The amount of deflection isproportional to the pressure applied on the diaphragm and it causes the linear displacement of a linkage rodattached to the internal side of the diaphragm. The movement of the linkage rod is in turn translated to angularmovement of the gauge's pointer by a series of gears. Thus, the pointer movement is proportional to thepressure exerted on the diaphragm.
The diaphragm also serves to isolate the fluid from the internals of the gauge; therefore, diaphragm typepressure gauges are suitable for use on most fluid types.
Fig. 12.6.3 Schematic diagram of a diaphragm pressure gauge
Piezoresistive pressure gauges
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These pressure gauges consist of a diaphragm made from a ceramic substrate; piezoresistive type straingauges are bonded to the diaphragm and together with the necessary circuitry, they are integrated on a siliconchip. The diaphragm deflects with changes in pressure, causing a change in the balance of the strain gaugebridge. This is converted by the integrated circuit tutorial to an electronic signal that is proportional to thepressure. The output signal can be fed into a local digital display or further converted into a 4-20 mA signaloutput for remote transmission.
These gauges are very sensitive and are used where precise measurement of pressure is required. Since theyproduce an electrical output signal, it is possible to incorporate them into building management systems.
Temperature gaugesAlthough there are a multitude of different temperature gauges available, five major types are likely to beencountered in steam systems, namely, the bimetallic type, the filled system type, thermistors, thermocouplesand resistance temperature devices (RTDs).
The bimetallic type temperature gauge - Consists of a coiled bimetallic element. The gauge is basedon the principle of the bimetallic strip, which consists of two metal strips, made from different materials,bonded to each other. The two materials are selected so that they have different thermal coefficients ofexpansion. The two metals expand by different amounts when heated, and since they cannot moverelative to each other, the bimetallic strip bends.
Fig. 12.6.4 Principle of a bimetallic strip
When the temperature of the coiled element rises, it tends to unwind. The degree to which this occursis indicative of the temperature. A pointer is connected to the coil by a series of linkages, in a similarway to that in the bourdon tube.
Bimetallic gauges tend to be inexpensive, robust and easy to install. They are used where a simple,quick visual indication of temperature is required.
Fig. 12.6.5 A bimetallic temperature gauge
Other methods of temperature measurement - are dealt with in Tutorial 6.7, Controllers andSensors. These types of temperature sensors are used when a higher level of accuracy is required inmeasuring temperature, or when this function is to be automated or incorporated into a buildingmanagement system.
It is common to place a temperature-measuring probe into a pocket when installed into an item ofplant. This enables the sensor to be removed from pipework or equipment without disturbing theintegrity of the system. A heat conducting paste is used in the pocket to provide good heat transfer
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qualities.
One area of concern when installing a temperature-measuring device is ensuring that it takes arepresentative reading. It is common, particularly in liquid containing vessels, for there to be some kindof thermal layering of the fluid, and measuring the temperature of the vessels at different levels mayproduce different results.
Common applications of temperature-measuring devices include boiler feedtanks, measuring producttemperatures and measuring the steam temperature after de-superheating.
Sight glassesA sight glass, or sight flow indicator, provides a method of observing fluid flow in a pipeline. It has two mainfunctions:
Indication - Sight glasses are used to indicate if fluid is flowing correctly. They are used to detectblocked valves, strainers, steam traps and other pipeline equipment, as well as to detect if a steamtrap is leaking steam.Inspection - Sight glasses can be used to observe the colour of a product at different stages of theproduction process.
When sight glasses are used to indicate the correct functioning of blast discharge type steam traps, theyshould be positioned at least 1 m downstream from the trap. For other traps, the sight glass should bepositioned immediately after the trap.
Sight glasses do not provide an exact method of monitoring the functioning of steam traps. In practice, athorough knowledge of the upstream steam system is required and the diagnosis is often subjective,depending on the experience of the observer. For example, depending on the condensate flowrate, pressureand trap discharge pattern, it can be difficult to differentiate if the steam trap is leaking steam or if flash steamis being generated after the steam trap. Sight glasses have generally been replaced by electrical devices suchas conductivity sensors, which detect flooding upstream of the steam trap, or leaking traps. These devices donot require steam trap expertise and produce a consistently accurate result.
Sight glassesThe sight glass has a smooth concentric reduction in the inlet connection, which promotes turbulence in thesight glass when fluid is flowing through it. The turbulent flow inside the sight glass permits any fluid to bedetected. Sight glasses are available with single, double or multi-viewing windows.
Fig. 12.6.6 Single (a), double (b) and multiple (c) window sight glasses
Some sight glasses may be fitted with a light source, these are useful when the sight glass is fitted in an areaof low ambient lighting, or where a single window sight glass has to be used, such as in tanks.
Sight checkThe sight check (see Figure 12.6.7) is a combination of a sight glass and a check valve. A ball in the top of theflow tube is lifted off its seat by the fluid as it flows through the cylindrical window to the outlet connection.When there is reverse flow, the ball is forced back onto its seat on the inlet. The ball movement makes the floweasy to see, as well as providing shut-off on reverse flow.
As with sight glasses, the sight check is used to observe the discharge of steam traps. In the sight check, theposition of the ball check indicates whether condensate is flowing. Where condensate rises after the trap, thesight check eliminates the need for a separate check valve, thus simplifying installation. The sight check isparticularly useful for commissioning steam traps fitted with a steam lock release (SLR).
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Fig. 12.6.7 A sight check
Vacuum breakersVacuum breakers protect plant and process equipment against vacuum conditions, typically associated withcooling.
Fig. 12.6.8 Vacuum breaker and a cut section of a vacuum breaker
The vacuum breaker consists of a spherical stainless steel ball that rests on its seat during normal operatingconditions. At the point of vacuum, the valve is lifted off its seat and air is drawn into the system.
Fig.12.6.9 Operation of a vacuum breaker
In some cases, the valve may be spring loaded, which means that the vacuum is only broken when there is a
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further pressure decrease. This helps to ensure that the shut-off at near vacuum conditions remains bubbletight.
One of the most common applications of a vacuum breaker is on process equipment such as jacketed pansand heat exchangers. When these items are turned off, they still contain a certain amount of steam. Thesteam condenses as the vessel cools down, and since condensate occupies a much smaller volume than thesteam, vacuum conditions are generated. The vacuum can damage the plant and it is therefore necessary toinstall a vacuum breaker on the steam inlet to such equipment or onto the plant body. The same situation canoccur on steam mains and boilers.
A common application of vacuum breakers is on temperature-controlled heat exchangers that are likely tosuffer from stall (see Block 13). On smaller heat exchangers draining to atmosphere, the stall condition can beavoided by installing a vacuum breaker on the steam inlet to the heat exchanger. When the vacuum isreached in the steam space, the vacuum breaker opens to allow condensate to drain down to the steam trap.
Fig. 12.6.10 The use of a vacuum breaker to prevent stall
In general, it is not desirable to introduce air into the steam space, since it acts as a barrier to heat transferand reduces the effective steam temperature (refer to Tutorial 2.4). This becomes a problem on larger heatexchangers, where it is not advisable to use a vacuum breaker to overcome stall. Furthermore, if thecondensate is lifted after the steam trap, for example, into a raised condensate return main, the vacuumbreaker cannot assist drainage. In both these cases, it is necessary to use an active method of condensateremoval such as a pump-trap (refer to Tutorial 13.8).
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