Pressure gauges have been a reliable source of accurate
pressure measurement for more than 100 years. The operation and design of
pressure gauges dates back to the mid 1800s when Eugene Bourdon invented the
Bourdon tube. This same principle is used today, albeit with enhancements to
last longer and resist corrosion.
Many factors affect proper gauge selection, and if WWD
readers follow this seven-step process, the task can become simpler and more
exact.
Step 1: Process Media
While all the factors regarding proper pressure gauge
selection are important, consideration for the actual process media that the
gauge will be exposed to is vital. Because the pressure-sensing element most
commonly used in a typical pressure gauge is a thin-wall Bourdon tube, materials
must be selected to prevent corrosion.
Restrictions do exist in selecting Bourdon tube materials.
The materials used must be limited to those that provide good spring
properties; because the tube will be required to coil and uncoil, this function
has to happen without the tube losing its preset memory. The materials must
also be easy to form into shapes and be easy to join by welding, soldering or
brazing. From an economic sense, these materials must also be readily available
and reasonably priced. Therefore, Bourdon tubes are most often manufactured
from phosphor bronze, 316 stainless steel, Monel and Inconel. When the
available Bourdon tube materials can accommodate a particular process, there
are generally no other complications in the selection process. However, at
times an application cannot be satisfied by available Bourdon tubes materials.
In these cases a diaphragm seal can be used in conjunction with a pressure
gauge to prevent the process media from contacting the Bourdon tube.
The design and nature of a diaphragm seal allow for the
introduction of many more materials than can be used in manufacturing of
Bourdon tubes. By selecting the proper diaphragm and bottom housing for the
diaphragm seal--these are the only parts that are exposed to the
process--WWD readers can satisfy nearly all applications. The diaphragm
seal not only protects a pressure gauge from corrosion attack but also prevents
viscous or extremely dirty media from clogging the Bourdon tube. The diaphragm
seal will typically add to the gauge accuracy an additional 0.5% of the full
scale.
If the process temperature affects the gauge performance a
capillary line can be used to reduce the temperature.
Step 2: Process Pressure
It is important to know if the process pressure is static or
dynamic in nature. If a pressure gauge sees a constant pressure continuously,
then typically, that pressure gauge will last for years without degradation of
performance. However, most pressure gauge applications are dynamic in nature,
and will cause gauge failure unless appropriate measures are taken.
The system operating pressure generally determines the
selection of the proper pressure gauge range. The pressure gauge range should
be twice the normal operating pressure. If fluctuating pressures are present,
as in the case of pulsation caused by a pump or compressor, it is also
suggested that gauges in severe service be liquid filled and dampened to reduce
Bourdon tube stresses. In order to minimize the stresses and extend the life of
the pressure gauge, various product features and accessories can be used. These
include internal throttle screws, pulsation dampeners, pressure snubbers, and
liquid filling the pressure gauge.
Step 3: Environment
We need to be aware of the conditions surrounding the
pressure gauge such as vibration and ambient temperature.
Vibration can cause excessive movement wear, which results
in pressure gauge failure. When vibration is present it becomes difficult to
accurately read the pressure gauge, because the pointer is rapidly oscillating.
To help prevent the movement-wear and readability problem, pressure gauges are
often filled with a dampening fluid such as glycerin. Many plant maintenance
and safety engineers have adopted liquid-filled gauges with throttle screws as
a standard for all applications. Remote mounting the gauge off the process
utilizing a capillary line assembly will also remove the effects of vibration.
Ambient temperature affects pressure gauge accuracies in at
least two ways. First, for all pressure gauges that are dry or liquid filled,
as the temperature increases the Bourdon tube material modulus decreases. This
results in a lighter spring rate of the tube. Therefore, the gauge will tend to
indicate higher pressure as temperature increases. The converse is also true:
as the temperature decreases, the tube material modulus increases, and the
gauge reading will tend to read lower than actual. Practically all pressure
gauges are affected by this phenomenon, except those manufactured with constant
modulus materials that tend to be expensive and more difficult to work with.
Step 4: Accuracy
Before selecting a pressure gauge for its accuracy, WWD
readers need to first understand what accuracy means in a mechanical pressure
gauge. Accuracy is the conformity of a pressure gauge reading to an accepted
standard, e.g., deadweight tester. Inaccuracy is the difference (error) between
the true value and the indication expressed as a percent of the span. Total
accuracy error includes hysteresis and repeatability errors.
It is important to know that accuracy is not a percentage of
the gauge reading. For a mechanical pressure gauge, accuracy is a percentage of
the full range, full scale, or span of the gauge. For example, if a 0-100 psi
pressure gauge with an ASME Grade 1A 1.0% full-scale accuracy is being used,
one can expect a maximum tolerance of ±1 psi anywhere within the entire
scale of the gauge. Another example of accuracy that is sometimes confusing is
a range like 30-in. hgvac/0/30 psi. This is a compound scale, and there is a
tolerance in the vacuum and pressure portions of this range. Let’s
presume the requirement is for an ASME grade 2A accuracy, which is ±0.5%
full scale. To calculate the tolerance or inaccuracy, multiply the
accuracy--0.5% or 0.005--times the full scale range, which consists
of the vacuum and pressure portions of the range; in this case, 30-in. hgvac
(equal to 15 psi) + 30 psi = 45 psi. The calculation to determine accuracy is
0.005 x 45 psi = ±0.225 psi. This is the maximum allowable tolerance anywhere
throughout the entire full range. To take the calculation one step further, in
order to define tolerance in proper units of measurement, on the vacuum side of
the range 0.225 psi is equal to 0.458-in. hg.
Step 5: Size
Dial sizes of pressure gauges range from 11/2-in. to
16-in. diameter. The dial size of a pressure gauge is generally determined by
readability requirements. If the gauge is remote from the operator, a larger,
more legible dial is necessary. For gauges mounted on a panel close to an operator,
smaller dial sizes are often acceptable.
More accurate pressure gauges generally have larger dials,
since more dial graduations are needed to read the higher degree of accuracy.
Therefore, for most test gauges having accuracies of 0.25% or better, the
41/2-in. dial is the smallest selected, and the 6-in. and larger are more
common.
Step 6: Connections
Generally in North America, 1/4 and 1/2 NPT
tapered pipe threads are the most common connections used for pressure gauges.
Pressure gauges with 41/2-in. dial sizes or larger are most often
supplied with a 1/20 NPT connection. This is particularly true when these
gauges are stem-mounted directly and liquid-filled, because the larger size
gauge socket will support the pressure gauge better. For dial sizes smaller
than 41/2-in., a 1/4 NPT is typical, while in some cases a
1/8 NPT will be used. For pressure gauge dial sizes smaller than 2-in., a
1/8 NPT is most common.
Step 7: Mounting
Pressure gauges are mounted in these forms:
* Direct
stem mount lower or back connect;
* Remote
wall/surface mount lower connect;
* Panel
surface mount back connect;
* Panel
hole U-clamp flush mount back connect; and
* Panel
hole front flange flush mount back connect.
While the majority of pressure gauges are supplied with
lower connections for stem mounting, it is still important to know how a
pressure gauge will be mounted when installed; for example, when a pressure
measurement is needed for a piece of equipment that has very severe vibration,
it may be necessary to remotely mount the pressure gauge. This can be done
easily, but it must ensured that the gauge has a back mounting flange. In some
cases when the gauge is inherently designed to be surface mounted, it will also
be necessary to use a length of capillary to attach the gauge and equipment.
Conclusion
Selection of pressure gauges appropriate to an application
is not necessarily as simple as it may appear. Considerations for temperature,
vibration, process conditions, pulsation, corrosion, and many other variables
can complicate the process. However, by reviewing this seven-step process, the
chances of an appropriate and safe selection increase significantly.
Additional information and assistance can be obtained from
specification ASME B40.1 entitled “Gauges -- Pressure indicating Dial
Type - Elastic Element.”
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