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Gauges Pressure Gauges Bourdon Tube


Gauges Pressure Gauges Bourdon Tube working

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Pressure gauges
Pressure 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. Variations in the downstream pressure can lead
       to reduced plant productivity and product quality. Variations in the
       downstream 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 pipework
      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 is exposed to the
process fluid, allowing it to flow into the tube. Any increase in pressure causes
elastic distortion of the tube, causing it to unwind. The resulting displacement
of the closed end of the tube is translated by a series of gears to an angular
displacement of the pointer. The pointer position is therefore proportional to
the pressure applied at the gauge's pressure connector. Typically, the
maximum deflection of the Bourdon tube corresponds 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 Bourdon tube is filled with a transparent liquid, normally
glycerine. This protects the internal mechanisms against damage from severe
vibration and to keep out ambient corrosives and condensation. This also
damps the movement 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 install the gauge at the end of a syphon tube.
The syphon tube is filled with water which transmits the pressure of the
working fluid to the Bourdon tube, enabling the gauge to be located some
distance from the actual point where the pressure is being measured. The two
most common forms of syphon tube are the 'U' and ring types. 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 not sufficient space for a ring type siphon.

                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 suspended solids alone, as these solids may damage the
internal elements of the gauge. In such cases, it is necessary to keep 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 gauge and the space behind the diaphragm form a
completely sealed system, which is evacuated and then filled with a suitable
filling fluid; in the case of steam this is typically a type of oil. The system
pressure causes the diaphragm 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 which include; Diaphragm type pressure
gauges, Piezoresistive pressure gauges and Temperature gauges.

Diaphragm type pressure gauges
A 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 is proportional to the
pressure applied on the diaphragm and it causes the linear displacement of a
linkage rod attached to the internal side of the diaphragm. The movement of
the linkage rod is in turn translated to angular movement of the gauge's
pointer by a series of gears. Thus, the pointer movement is proportional to the
pressure             exerted           on            the          diaphragm.

The diaphragm also serves to isolate the fluid from the internals of the gauge;
therefore, diaphragm type pressure gauges are suitable for use on most fluid

                                                   Fig. 12.6.3 Schematic
                  diagram of a diaphragm pressure gauge
Piezoresistive pressure gauges
These pressure gauges consist of a diaphragm made from a ceramic
substrate; piezoresistive type strain gauges are bonded to the diaphragm and
together with the necessary circuitry, they are integrated on a silicon chip. The
diaphragm deflects with changes in pressure, causing a change in the balance
of the strain gauge bridge. This is converted by the integrated circuit tutorial to
an electronic signal that is proportional to the pressure. The output signal can
be fed into a local digital display or further converted into a 4-20 mA signal
output                 for                  remote                 transmission.

These gauges are very sensitive and are used where precise measurement of
pressure is required. Since they produce an electrical output signal, it is
possible to incorporate them into building management systems.

Temperature gauges
Although there are a multitude of different temperature gauges available, five
major types are likely to be encountered in steam systems, namely, the
bimetallic type, the filled system type, thermistors, thermocouples and
resistance temperature devices (RTDs).
      The bimetallic type temperature gauge - Consists of a coiled
       bimetallic element. The gauge is based on 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 of expansion. The two metals expand by
       different amounts when heated, and since they cannot move relative 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 occurs is indicative of the temperature. A
       pointer is connected to the coil by a series of linkages, in a similar way
       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

                           Fig. 12.6.5 A bimetallic temperature gauge
      Other methods of temperature measurement - are dealt with in
       Tutorial 6.7, Controllers and Sensors. These types of temperature
       sensors are used when a higher level of accuracy is required in
       measuring temperature, or when this function is to be automated or
       incorporated into a building management system.

       It is common to place a temperature-measuring probe into a pocket
       when installed into an item of plant. This enables the sensor to be
       removed from pipework or equipment without disturbing the integrity of
       the system. A heat conducting paste is used in the pocket to provide
       good heat transfer qualities.

       One area of concern when installing a temperature-measuring device is
       ensuring that it takes a representative reading. It is common, particularly
       in liquid containing vessels, for there to be some kind of thermal
       layering of the fluid, and measuring the temperature of the vessels at
       different levels may produce different results.

       Common applications of temperature-measuring devices include boiler
       feedtanks, measuring product temperatures and measuring the steam
       temperature after de-superheating.

Sight glasses
A sight glass, or sight flow indicator, provides a method of observing fluid flow
in a pipeline. It has two main functions:
      Indication - Sight glasses are used to indicate if fluid is flowing
       correctly. They are used to detect blocked valves, strainers, steam traps
       and other pipeline equipment, as well as to detect if a steam trap is
       leaking steam.
      Inspection - Sight glasses can be used to observe the colour of a
       product at different stages of the production process.
When sight glasses are used to indicate the correct functioning of blast
discharge type steam traps, they should be positioned at least 1 m
downstream from the trap. For other traps, the sight glass should be
positioned         immediately         after          the          trap.

Sight glasses do not provide an exact method of monitoring the functioning of
steam traps. In practice, a thorough 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,
pressure and trap discharge pattern, it can be difficult to differentiate if the
steam trap is leaking steam or if flash steam is being generated after the
steam trap. Sight glasses have generally been replaced by electrical devices
such as conductivity sensors, which detect flooding upstream of the steam
trap, or leaking traps. These devices do not require steam trap expertise and
produce a consistently accurate result.

Sight glasses
The sight glass has a smooth concentric reduction in the inlet connection,
which promotes turbulence in the sight glass when fluid is flowing through it.
The turbulent flow inside the sight glass permits any fluid to be detected. 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 area of low ambient lighting, or where a single
window sight glass has to be used, such as in tanks.

Sight check
The sight check (see Figure 12.6.7) is a combination of a sight glass and a
check valve. A ball in the top of the flow 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 flow easy 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, the position of the ball check indicates
whether condensate is flowing. Where condensate rises after the trap, the
sight check eliminates the need for a separate check valve, thus simplifying
installation. The sight check is particularly useful for commissioning steam
traps fitted with a steam lock release (SLR).
                                                          Fig. 12.6.7 A sight

Vacuum breakers
Vacuum breakers protect plant and process equipment against vacuum
conditions, typically associated with cooling.

   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 operating conditions. 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 further pressure decrease. This helps to ensure
that the shut-off at near vacuum conditions remains bubble tight.

One of the most common applications of a vacuum breaker is on process
equipment such as jacketed pans and heat exchangers. When these items
are turned off, they still contain a certain amount of steam. The steam
condenses as the vessel cools down, and since condensate occupies a much
smaller volume than the steam, vacuum conditions are generated. The
vacuum can damage the plant and it is therefore necessary to install a
vacuum breaker on the steam inlet to such equipment or onto the plant body.
The same situation can occur on steam mains and boilers.

A common application of vacuum breakers is on temperature-controlled heat
exchangers that are likely to suffer from stall (see Block 13). On smaller heat
exchangers draining to atmosphere, the stall condition can be avoided by
installing a vacuum breaker on the steam inlet to the heat exchanger. When
the vacuum is reached 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 transfer and reduces the effective steam temperature
(refer to Tutorial 2.4). This becomes a problem on larger heat exchangers,
where it is not advisable to use a vacuum breaker to overcome stall.
Furthermore, if the condensate is lifted after the steam trap, for example, into
a raised condensate return main, the vacuum breaker cannot assist drainage.
In both these cases, it is necessary to use an active method of condensate
removal such as a pump-trap (refer to Tutorial 13.8).

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