X
Installation and Maintenance Manual
P/N MMI-20018377, Rev. AC
April 2012
Micro Motion® 7812 Gas Density Meter
INMETRO Certificate No. AEX-10971-
IMPORTANT NOTICE
DO NOT drop the meter. HANDLE WITH CARE
DO NOT use liquids incompatible with MATERIALS OF CONSTRUCTION
DO NOT operate the meter above its RATED PRESSURE
DO NOT PRESSURE TEST above the specified TEST PRESSURE
DO NOT expose the meter to excessive vibration (>0.5g continuous)
ENSURE all ELECTRICAL SAFETY requirements are met
ENSURE all EXPLOSION PROOF requirements are applied
ENSURE meter and associated pipework are PRESSURE TESTED to 1½ times the maximum operating pressure
after installation
Chapter 1 Introduction
1.1 General ............................................................................................................................... 1-1
1.2 Principle of operation ........................................................................................................... 1-1
1.3 Design features ................................................................................................................... 1-3
1.3.1 Accuracy ............................................................................................................... 1-3
1.3.2 Repeatability ......................................................................................................... 1-3
1.3.3 Stability ................................................................................................................. 1-3
1.4 The 7812 versions ............................................................................................................... 1-3
1.5 Frequency relationship ........................................................................................................ 1-3
1.6 Safety .................................................................................................................................. 1-4
Chapter 2 Applications
2.1 General ............................................................................................................................... 2-1
2.2 Orifice plate metering .......................................................................................................... 2-1
2.3 Volumetric flow meters ........................................................................................................ 2-1
2.4 Other applications ............................................................................................................... 2-2
Contents
Chapter 3 General Installation
3.1 Delivery component list ....................................................................................................... 3-1
3.2 General installation considerations ...................................................................................... 3-1
3.3 Density equilibrium .............................................................................................................. 3-2
3.4 Temperature equilibrium ..................................................................................................... 3-2
3.5 Pressure equilibrium ............................................................................................................ 3-2
3.6 Flow rate ............................................................................................................................. 3-3
3.7 Response time .................................................................................................................... 3-4
3.8 Deposition, corrosion, condensation and vibration .............................................................. 3-5
3.9 Recommended installations for 78121/2/3/4 pocket unit ..................................................... 3-6
3.9.1 Pressure recovery method .................................................................................... 3-6
3.9.2 Installation procedure ............................................................................................ 3-7
3.9.3 Other methods ...................................................................................................... 3-8
3.10 Recommended installations for 78125 cross-pipe units ...................................................... 3-9
3.10.1 Cross-pipe method ................................................................................................ 3-9
3.10.2 Installation procedure .......................................................................................... 3-10
Chapter 4 Mechanical Installation
4.1 General ............................................................................................................................... 4-1
4.2 Physical dimensions ............................................................................................................ 4-1
4.3 Liners .................................................................................................................................. 4-4
4.4 Filtration .............................................................................................................................. 4-4
4.5 Pocket installation ............................................................................................................... 4-4
4.5.1 Anti-vibration installation ............................................................................... 4-5
4.6 External pocket installation .................................................................................................. 4-8
4.7 Post-installation mechanical checks .................................................................................... 4-8
Cont-1
Chapter 5 Electrical Installation
5.1 General ................................................................................................................................ 5-1
5.2 EMC cabling and earthing ................................................................................................... 5-1
5.3 Use with Signal Converters ................................................................................................. 5-2
5.4 System connections (7950/7951) ........................................................................................ 5-2
5.4.1 Connections to 7950 Signal Converter .................................................................. 5-2
5.4.2 Connections to 7951 Signal Converter .................................................................. 5-7
5.5 System connections (Customer’s own equipment) .............................................................. 5-9
5.5.1 Non-hazardous areas ............................................................................................ 5-9
5.5.2 Hazardous areas ................................................................................................. 5-10
5.6 Post-installation checks ..................................................................................................... 5-12
Chapter 6 Interpretation of Calibration Certificate
6.1 Calibration certificate ........................................................................................................... 6-1
6.2 Instrument serial numbers ................................................................................................... 6-1
6.3 Pressure test ....................................................................................................................... 6-1
6.4 General density equation ..................................................................................................... 6-1
6.5 Temperature corrections ..................................................................................................... 6-1
6.6 User gas offset data ............................................................................................................ 6-2
Chapter 7 Calibration and Performance
7.1 Factory calibration ............................................................................................................... 7-1
7.2 Calibration of transfer standards .......................................................................................... 7-1
7.2.1 Calibration gas....................................................................................................... 7-1
7.2.2 Calibration temperature ......................................................................................... 7-1
7.2.3 Pressure measurement ......................................................................................... 7-1
7.2.4 Evaluation of density ............................................................................................. 7-1
7.2.5 Derivation of constants .......................................................................................... 7-1
7.2.6 Computed data ...................................................................................................... 7-1
7.3 Calibration using transfer standards .................................................................................... 7-2
7.3.1 Preparation ............................................................................................................ 7-2
7.3.2 Calibration ............................................................................................................. 7-2
7.3.3 Computation .......................................................................................................... 7-2
7.4 Temperature coefficient evaluation ...................................................................................... 7-2
7.5 Calibration check methods .................................................................................................. 7-2
7.5.1 Ambient air test ..................................................................................................... 7-2
7.5.2 Atmospheric pressure test ..................................................................................... 7-2
7.5.3 Vacuum test point .................................................................................................. 7-3
7.5.4 Pressure/temperature of known gas test ............................................................... 7-3
Cont-2
Chapter 8 Maintenance
8.1 General ............................................................................................................................... 8-1
8.2 Calibration check methods .................................................................................................. 8-1
8.3 Mechanical maintenance ..................................................................................................... 8-1
8.4 Electrical maintenance ........................................................................................................ 8-2
8.5 De-mounting the 7812 ......................................................................................................... 8-3
8.5.1 Removing the 7812 from the pipeline .................................................................... 8-3
8.5.2 Removing the electronic housing .......................................................................... 8-4
8.5.3 Removing the spigot ............................................................................................. 8-6
8.5.4 Removing the cylinder, spoolbody and filters ........................................................ 8-6
8.6 Post maintenance tests ....................................................................................................... 8-9
8.7 Fault finding ......................................................................................................................... 8-9
8.8 Spare parts list .................................................................................................................. 8-10
Appendix A Specification
Appendix B Calibration Certificate
Appendix C Orifice Metering
Appendix D Velocity of Sound Effect
Appendix E Ethylene Measurement
Appendix F Reference Data
Appendix G Returns Forms
Appendix H Certified System Drawings
Cont-3
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7812 Gas Density Meter Installation and Maintenance Manual
Chapter 1
Introduction
1.1 General
The 7812 Gas Density Meter was specifically designed for metering gas in pipelines. Measurements are made
continuously with the accuracy equal to that normally associated with the best lab oratory methods. The 7812 is
a replacement for the 7810, 7811, and 3093 Series, and is fully interchange able with them, incorporating the
following design improvements:
• Single sensing element for all density ranges up to 400 kg/m
• New design of vibrating cylinder sensing element, which offers less sensitivity to changes in gas composition
and a lower Velocity of Sound Effect.
• Improved temperature equilibrium performance.
• Improved accuracy.
• Improved maintenance features, including new amplifier electronics and a more serviceable gas filter
arrangement.
• A 4-wire PT100 temperature sensor has been incorporated for installation and check purposes.
This meter is suitable for most types of installation. Aspects such as performance, response characteristics, filtration
and servicing vary from application to application and require careful consideration as described in this manual.
The vibrating cylinder sensing element is sensitive to changes in density and, since it is unstressed and is
manufactured from Ni-span C steel, it has very stable char acteristics. The influence of other variables such as
temperature, line pressure, flow rate and gas composition are minimized and c arefully defined so that, where
necessary and for high precision measurements, suitable corrections can b e applied.
Only one low voltage supply is required for the density m easurement and the power consumption is low thus
minimising self-heat generation. The output signal is a square wave, the frequency depending on the gas
density. This type of signal can be transmitted over long distances and easily measured without any loss in
accuracy. The PT100 temperature sensor may be used in the conventional manner.
3
.
1.2 Principle of operation
The density-sensing element consists of a thin metal cylinder, which is activated so that it vibrates in a hoop
mode at its natural frequency. The gas is passed over the outer and inner surfaces of the cylinder and is thus in
contact with the vibrating walls. The mass of gas, which vibrates with the cylinder, depends upon the gas density
and, since increasing the vibrating mass decreases the natural frequency of vibration, the gas density is simply
determined by measuring this frequency.
An amplifier, magnetically coupled to the sensing element, maintains the conditions of vibration and provides
the output signal (see Figure 1-1 and Figure 1-2). The amplifier and signal output circuits are encapsulated in
epoxy resin.
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Installation and Maintenance Manual 7812 Gas Density Meter
Figure 1-1: Schematic block diagram of meter circuit (2-wire system)
1-2
Figure 1-2: Schematic block diagram of meter circuit (3-wire system)
7812 Gas Density Meter Installation and Maintenance Manual
1.3 Design features
1.3.1 Accuracy
The instrument design achieves high accuracy by minimizing the effects of the variables such as pressure,
temperature, sound velocity and viscosity, whilst providing insensitivit y to plant vibratio n and variations in power
supply. Since the power consumption is extremely small, the self-induced heat may also be neglected. The
absolute accuracy is therefore mainly defined by the accuracy of calibration and correction applied.
1.3.2 Repeatability
The repeatability of measurement is within ±0.01% of full scale density.
1.3.3 Stability
The long-term stability of this density sensor is mainly governed by the stability of the vibrating cylinder sensing
element. This cylinder is manufactured from one of the most stable metals and, being unstressed, will maintain
its properties for many years. However, corrosion and deposition on the cylinder will degrade the long-term
stability and care should be taken to ensure that the process gas is suitable for use with materials of
construction. The possibility of deposition is reduced by the use of filters but, should depos ition take place, the
sensing element can be removed and cleaned.
1.4 The 7812 versions
The following meter types are available, covering the basic density range s:
Type No. Range (kg/m3)1 Calibration Gas
78121x
78122x
78123x
78124x
78125x
If x = A: Fluorocarbon (FPM/FKM) 0 rings are used for the gas path ways.
if x = B: Ethylene Propylene 0 rings are used for the gas path ways.
Nitrogen calibration should be used for low density and natural gas a pplications. Argon calibration should be
used for high density and heavy hydrocarbon applications.
1.5 - 10 Nitrogen
9 - 90 Nitrogen
25 - 250 Nitrogen
40 - 400 (pocket)
40 – 400 (cross pipe)
Argon
Argon
1.5 Frequency relationship
The relationship between gas density and the output frequency follows a well-defined law:
ρ =
or τ =
Where: ρ = Density (kg/m
τ = Meter Time Period (μs)
K0, KI, K2 = Calibration coefficients
1
An additional option for low density range measurement (0 to 3 kg/m3), ± 0.5% of full scale, is available as a special
purchase. Contact your local Micro Motion sales office for more information.
2
2K1K0K τ+τ+
2
2K2
3
)
)0K(2K41K1K
ρ−−+−
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Installation and Maintenance Manual 7812 Gas Density Meter
Range selection and linearization are normally introduced within the readout system. In addition, there is an
influence on the measurement performance from changes in gas temperature and composition. These are as
specified on the calibration certificate of an instrument and should form the basis of manual or automatic
corrections if the full performance potential is to be achieved.
1.6 Safety
The 7812 meters have been subjected to the necessar y safety regulations and have qualified for ATEX/ IECEx
certification to Class Ex ia lIC T5.
For ATEX/IECEx safety information, refer to the safety instructions booklet MMI-78125010/SI.
For Pressure Equipment Directive (PED) safety information, refer to safety instructions booklet 78128012/SI.
For CSA safety information, refer to Appendix H.
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7812 Gas Density Meter Installation and Maintenance Manual
Chapter 2
Applications
2.1 General
The 7812 Gas Density Meter provides a continuous and accurate measurement of gas density. This measurement
can be made at the actual flowing conditions of temperature and pressure and, in consequence, is ideally suited for
high-performance gas flow metering tasks.
2.2 Orifice plate metering
The orifice meter is probably the most widely used meter type for gas measurement. It has the advantage that it
does not require flow calibration, as this is defined from dimensional measurements and application of
International Standards (ISO 5167 and AGA3). For flow measurements in either mass units or volume units, it is
necessary to determine the fluid density in addition to the differential pressure.
The 7812 gas density meter offers a direct measurement of density and is an alternative to density calculation
using pressure, temperature and composition measurements. It offers low measurement uncertainty and is
therefore of prime use in major gas metering stations where best accuracy is required. Orifice metering systems
are discussed in more detail in Appendix C.
2.3 Volumetric flow meters
Positive displacement meters or turbine flow meters can be converted to mass flow meters using the 7812 gas
density meter and a simple readout system. Please note that the 7950/51 Signal Converter cann ot accept a flow
meter input.
Since both flow meter and density sensor signals are in frequency form, the readout system need use only digital
techniques (see Figure 2-1).
Figure 2-1: Typical volumetric flow metering system
Signal
Converter
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Installation and Maintenance Manual 7812 Gas Density Meter
The combined uncertainties of the density measurement and signal converter are considerably less than that of
volumetric flow meters. Therefore, the overall accuracy of mass flow measurement will be almost entirely
determined by the accuracy of the volumetric flow meter.
2.4 Other applications
Other applications include process monitoring and control in chemical and petrochemical plants where density or
specific gravity of a gas is required as a control variable.
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7812 Gas Density Meter Installation and Maintenance Manual
Chapter 3
General Inst allation
3.1 Delivery component list
Check that the following items have been included on delivery:
• 7812 Gas Density Meter.
• Nitrogen or Argon Gas Calibration Certificate.
• User Gas Calibration Certificate (if requested).
• Thermal conductor plus silicone fluid (except for 78125).
• Housing blanking plug.
• Cable gland adapter.
3.2 General installation considerations
The basic objective of an installation is to pass a representative sample of gas through the 7812 in a co ntrolled
manner such that the temperature and pressure are at known conditions. Typically, this means that they need to
be the same as the line conditions.
It is worth remembering that the 7812 will always read the correct density for the gas that is inside it. Installation
errors result from the sample gas in the 7812 not being what the installer believes it to be in terms of
composition, temperature or pressure.
The following points should be considered when planning the installation of the 7812:
(a) All necessary mechanical and electrical safety standards MUST be applied.
(b) The effects of the following on the 7812:
• Density Equilibrium.
• Temperature Equilibrium.
• Pressure Equilibrium.
• Sample flow rate and response time.
• Deposition, Corrosion and Condensation.
• Vibration.
• Accuracy of calibration.
• Effects of velocity of sound.
(c) When installing the 7812 in a pipeline, we recommend you do not exceed a 10% reduction of the cross-
sectional area at the point of insertion to ensure minimal effect on pressure.
(d) Adequacy of sample extraction, filtration and conditioning for preventing dirt or condensates from causing
non-operation of the 7812.
(e) Interaction between the 7812 installation and the flow meter.
(f) Unregistered gas, which passes through the 7812 but not the flow meter.
(g) The proving system (e.g. vacuum systems, calibration gas, etc.)
(h) The use of duplicate 7812 meters for performance comparison and for provision of automatic alarm.
(i) Accessibilit y to the system components for proving and maintenance.
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Installation and Maintenance Manual 7812 Gas Density Meter
These points are considered in more detail in the paragraphs below and in subsequent chapters.
3.3 Density equilibrium
Three factors affect the equality of density of the sample gas and the pipeline gas:
(a) The gas in the density sensor should be representative of the main flow with regard to the proportions of
different gas constituents. This is normally best achieved by ensuring that there is a small flow rate of
sample gas.
(b) The pressure of the sample gas MUST be approximately equal to the pipeline pressure as density varies
proportionately with absolute pressure for an ideal gas.
(c) The temperature of the sample gas MUST be approximately equal to the pipeline gas temperature as
density varies inversely with absolute temperature.
3.4 Temperature equilibrium
The major installation consideration is that of temperature equilibrium. If the required density is that of the main
pipeline at the pipeline temperature, it is important that the 7812 is at the same temperature.
A temperature difference of 1°C will cause an error of between 0.3% and 0.6% depending on the gas
composition. Good temperature equilibrium between the 7812 and pipe is therefore essential and may be
achieved by:
(a) Using thermal insulation over the 7812 and associated pipework.
(b) Using a short well-insulated inlet sample pipe.
(c) Using the silicone fluid and pocket cylinder in the recommended way. This will significantly increase the
temperature equilibrium and decrease the response time of the 7812.
(d) Using the smallest acceptable sample flow rate.
(e) Using the 78125 variant mounted in a cross pipe installation; this gives direct contact with the in-line gas,
thus minimizing temperature errors.
The temperature in the 7812 can be checked using the Class A PRT mounted in the spool bod y.
For an internal pocket installation, the recommendations in Chapter 4 of this manual should be followed. Using
the silicone fluid and aluminium cylinder improves the thermal performance by more than 90% and in order for
the silicone to remain in the base of the pocket, the installation should be vertical. If a non-vertical installation is
preferred then the silicone fluid should be replaced with a heat sink compound.
For installations where an external pocket is used or the temperature cannot be maintained at that of the
pipeline, the 7812 PRT can be used to correct the measured density to the conditions of the pipeline. To do
this, some form of density referral method will need to be employed.
3.5 Pressure equilibrium
It is first necessary to define whether the gas in the 7812 should be at the same pressure as that at the gas
take-off tapping point. It is then necessary to ensure that the pressure difference between the 7812 and the
required tapping point is kept to a minimum by ensuring that there is low sample flow rate and that relevant
filters are not causing excessive restriction. It is normally recommended that the gas flow is controlled by a
needle valve which can be mounted before or after the 7812 depending on the chosen installation method. It is
common to install a flow meter to monitor this flow, and is very useful for ensuring that filters are not blocking,
which can cause errors in some installations.
The usual recommended density measurement is taken from the gas return point (or density point). This
reduces the significance of the pressure build up across the fine gauge filters.
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7812 Gas Density Meter Installation and Maintenance Manual
The state of the filters and any resultant excessive pressure drop can be determined b y var ying the sample
flow rate and monitoring the magnitude of the resultant density changes. The 7812 filters can be easily
exchanged without disconnecting the associated pi pe work. For further details, see Chapter 8.
Note: The 78125 meter, the direct replacement of the 3093, is a filterless unit that is installed in a filtered crosspipe installation and is thus held, by definition, at the line pressure.
3.6 Flow rate
The recommended flow rate is 5 ±1 Litres per hour, but anywhere in the region of 1 to 10 Litre(s) per hour is
acceptable. (At flow rates above 10 Litre(s) per hour, the density reading will start to become slightly unstable
and a small density error may be introduced.)
To maintain this flow rate, a pressure differential is required across the 7812. If the filters are clean, the flow rate
will be approximately:
Δ=P
5.0Q
ρ
(About 85% of this differential pressure is across the 2 micron filter, and the remainder is across the 90 micron filter.)
This equation indicates that for natural gas with a typical application density of about 60 kg/m
differential of approximately 1.66 mbar is required to maintain a flow rate of 5 Litres per hour. Figure 3-1 shows
the pressure drop across the 7812 for a typical natural gas application.
The flow rate is dependent on the gas density, which is affected by gas composition, temperature, and pressure.
The first two parameters should not change suddenly, as this would leave the mixture in the pipe inhomogeneous
and render measurements invalid for other reasons. Pressure may chan ge rapidly but this change will be
transmitted to the density meter very rapidly irrespective of the sample gas flow rate.
For most systems, the available differential pressure would creat e too high a flow for accurate density
measurement and maximum 7812 reliability. A low flow rate helps to achieve pressure and temperature
equilibrium, as well as extending the life of filters and minimising carry over of any condensate. It is therefore
normal to include a flow control valve in the sample pipeline.
where Q = sample gas flow rate in litres/minute
ΔP = differential pressure across the
3
ρ = density of gas, in kg/m
7812, in mbar
3
, a pressure
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Installation and Maintenance Manual 7812 Gas Density Meter
Figure 3-1: Pressure drop through 7812
3.7 Response time
Several different response times need to be considered:
(a) Response to pressure changes is instant.
(b) Response to temperature changes is the same response time as the pipework. For a faster response, it
would be necessary to use the 7812 PRT.
(c) The response to composition changes: this depends on flow rate and dead volume.
For example: 7812 volume 40cc
Filter volume 60cc
Flowmeter volume (if upstream of 7812) 40cc
Pipe volume 60cc
Response time at 4 l/hr = 3 minutes
In order to improve the response time, the inlet pipe should be short and of small diameter and any addit ional
inlet filters should be of low volume.
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7812 Gas Density Meter Installation and Maintenance Manual
3.8 Deposition, corrosion, condensation and vibration
The prevention of deposition, corrosion, and condensation on the vibrating cylinder sensing element is essential
if drift in calibration and/or malfunction is to b e avoided. It is also necessary to restrict the level of vibr ation
experienced by the
DEPOSITION
Deposition of solids on the cylinder will cause an increase in vibrating mass and thus a decrease in vibrating
frequency. The
cause oscillation to stop altogether by filling the gaps between the coils and cylinder walls.
A filter of the appropriate size is fitted on all
to remove any solid particles that may be present in the sample gas. The outlet filter is incorporated for protection
should a reverse flow occur. These filters can be exchanged in the f i eld without the removal of any of the
associated pipework.
If the gas is known to be excessively dirty, it is recommended that an e xternal filter is installed in the inlet section
of the sample line. This should be of sufficient area to cause only a negligible pressure drop at the maximum rate
of flow, but of small volume to provide adequate response t ime to gas composition changes.
CORROSION
Corrosion of the cylinder element will reduce both its stiffness and its mass per unit length, but since its stiffness
is of greater significance, the corrosion will cause a reduction in the resonant frequency. The
reading will therefore be high with respect to the actual gas density. Massive corrosion may cause oscillat ion to
stop due to corroded particles blocking the gap as mentioned above.
In order to prevent corrosion of the sensing element and its maintaining system, the constituents of the process
gas should be compatible with Ni-Span-C 902. Other materials that come into contact with the gas flow are
stainless steel 316 S13, stainless steel AMS5643, Emmerson & Cummins Type Stycast 2850FT, Catalyst 11,
Permendur Iron, and AISI316 stainless steel filters.
It is preferable that any traces of corrosive elements present in the process gas that are likely to attack these
materials should be removed by an appropriate absorbent trap, or other method, before the sample g as passes
through the
these, in the presence of certain gases, will cause corrosion.
As a general guide, the properties of Ni-Span-C 902, with regard to corrosion resistance, lie between those of
stainless iron and stainless steel. If there is doubt about the corrosion properties of a gas, advice should be
sought and, if necessary, tests conducted.
CONDENSATION
Condensation of water or other liquid vapours on the sensing element will cause effects similar to deposition of
solids except that the effects will disappear if re-evaporation takes place.
If the gas flow is wet or near dew point, condensation within the
sensing element will cause the effects described above. As the vibrating cylinder is very thin, its thermal capacity
is very low in comparison to the heavy body of the unit. It is very likely that the sensing element will take up the
gas flow temperature very quickly and condensation is more likely to occur on the outer casing and other
component parts.
For certain applications, it may be necessary to use a sample tube at the gas take off point to prevent
condensate carry over or to include a condensate trap in the sample line.
VIBRATION
The 7812 can tolerate vibration up to 0.5g, but levels in excess of this may affect the accuracy of the re adings.
In situations where this is likely to be encountered, anti-vibration gaskets (part number
used, as detailed in section 4.5.1. This will reduce the effects of vibration by at least a factor of 3, at levels up to
10g and 2200Hz.
7812. In general, it is important that the gas is sufficiently dry so that water droplet s cannot form as
7812.
7812 density reading will be high with respec t to the actual gas density. Massive deposition may
7812 units (except the 78125) in both the inlet and outlet gas paths
7812 density
7812 may occur and any condensation on the
78123723A) should be
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Installation and Maintenance Manual 7812 Gas Density Meter
3.9 Recommended installations for 78121/2/3/4 pocket unit
3.9.1 Pressure recovery method
The pressure recovery method is the most common installation method for orifice metering and is recommended
in the ‘Institute of Petroleum measurement manual, Part XV, Metering Systems’.
Figure 3-2 is recommended as a convenient method for obtaining an optimum flow rate, as well as providing a means
for checking the condition of the
downstream tapping of the orifice plate for which the relevant ‘expansion factor’ must be used.
It is recommended that the sample input pipework and the heat conduction coil are made from 6mm instrument
tubing, and the sample return pipework (from the
firmly clamped to the external surface of the meter run to ensure that the temperature of the gas being measured
is as close as possible to that of the main gas flow. The whole arrangement should be enveloped in thermal
cladding at least 100mm thick.
The installation in Figure 3-2 has the following features:
• No bypass of the orifice plate.
• Flow is achieved because the pressure after the orifice is lower than that further downstream.
• Pressure drops through the valves and filters do not affect the reading as the pressure at the outlet, and
hence inside the
flow will decrease but the density reading will not be in error.
• The correct expansion factor for the downstream point must be used in the orifice flow calculations.
• The measured density at the Density point is used in the mass flow calculation as specified by ISO 5167 and
AGA3.
7812, is identical to the orifice downstream point. Hence if the filters are not maintained, the
7812’s filters and the calibration of the sensor. Density is measured at the
7812), from 12mm tubing. The heat conduction coil should be
Figure 3-2: Pressure recovery method
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7812 Gas Density Meter Installation and Maintenance Manual
3.9.2 Installation procedure
SETTING-UP
All the procedures described in this section should be carried out when the gas flow rate in the main pipeline is at
the Nominal Design flow rate. If this condition cannot be conveniently obtained then the density percent offsets
should be adjusted accordingly.
1. Close vent valve S3. Fully open flow control valve T1. Slowly and carefully, fully open valve S1 and then S4.
2. Allow at least 15 minutes for the purging and temperature stabilisation of the 7812.
3. Close isolating valve S1. This will result in a change of indicated density. Immediately record the value of
density thus obtained which should be the density at the ‘density point’.
4. Re-open valve S1 and immediately adjust the f l ow control valve to give a density reading 0.02% above that
observed in operation 3.
5. Close valve S1 again and check that the density decreases by 0.02%.
6. Return valve S1 to the fully open position.
Notes:
• The 0.02% value is considered the optimum value for most systems. However, if a quicker response is
required, opening the flow control valve T1 would create a higher % value.
• Should the density increase by more than 0.02% with valve T1 at its minimum setting, then partially closing
valve S1 should reduce the flow rate and the resultant density increase. However, this situation would
suggest the flow control valve is too large for the application and ideally should be replaced by a more
suitable type.
• All isolating valves should be of the FULL-BORE type to prevent unnecessary restriction.
FILTER CHECK
1. Record the present density reading without dist urbing any valve setting.
2. Close isolating valve S1 and note the density reading.
(a) If the reading has decreased by more than 0.01%
7812’s filters are reasonably clean.
(b) If the reading has decreased by less than 0.01% of that recorded in operation 1, it is recommended that the
7812’s filters are replaced or thoroughly cleaned and then the system optimised as detailed in Section 3.7.
3. After the filter check, return valve S1 to the fully open position.
RESPONSE TIME CHECK
1. Close valves S1 and S4 to isolate the 7812 and open valve S3 to vent it.
2. Pressurise the 7812 through valve S3 with a different gas to that within the pipeline and at a similar
pressure to that of the pipeline. Close valve S3 and allow 15 minutes for temperature stabilisation.
3. Open valve S4 to expose the 7812 to pipeline pressure. Now open S1 to enable gas to flow through the
7812 at the set rate and measure the time taken for the 7812 to stabilise. This should be a good indication
7812’s response to a change in gas composition.
of the
of that recorded in 1, then it can be assumed that t he
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Installation and Maintenance Manual 7812 Gas Density Meter
GENERAL
An additional filter to the
85 filter coalescer (accessory/spare part no. 450600770) or a Balston 95S-4 (accessory/spare part no. 450600810)
is used, but any equivalent filter can be used.
Isolating valves should be included in the installation so that the
filter replacement without the need to shut down the pipeline.
The gas inlet and outlet points should be designed so that they do not collect any liquid that might have
condensed on the pipe wall.
7812 meter filter is normally fitted to ensure the gas is clean and dry. Typically a Balston
7812 meter can be isolated from the pipeline for
3.9.3 Other methods
For most other methods, the density is required at the pressure conditions of the 7812 inlet. In these cases, any
pressure drop through the filters and pipes will caus e a small offset. These are minimised by putting the control
valve downstream of the meter and controlling the flow to the recommended level.
Figure 3-3: Differential Pressure Method
The installation shown in Figure 3-3 can be used with orifice metering or gas turbines. With gas turbines, it is
common to have a tapping point on the turbine body in which case this would be used rather than the upstream
tapping. The important features are:
• The sample flow bypasses the meter but should be low enough (5 l/hr) not to be of significance.
• The measured density is the upstream density that is the most commonly required point.
• Pressure drops through the filters will cause density errors if they become large.
• The control valve and the flow meter can be mounted on either side of the
dependent on where the density point is. For example if the upstream density point is required on an orifice
application, the needle valve and flow meter would be downstream of the
before the measurement.
If a convenient pressure drop is not available to generate the sample flow, the installation in Figure 3-4 can be
used. The gas can be vented to flare or, in some cases to atmospher e. In this installation, the full pipe pressure is
available as a pressure drop. Therefore, caution n eeds to be taken to ensure the flow is adequately controlled by
the control valve. For high-pressure applications, a two-stage let down system may be required to prevent icing.
7812 to suit the installation and
7812 to reduce the pressure loss
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7812 Gas Density Meter Installation and Maintenance Manual
Other methods for generating the required DP (differential pressure) for the required flow can be used such as
pitot tubes or natural bends or changes in section of the main pipework. For any of these other methods
calculations need to be performed to check that the ava ilable DP will be sufficient to achieve an adequate sample
flow rate.
Figure 3-4: Vented gas method
3.10 RECOMMENDED INSTALLATION FOR 78125 CROSS PIPE UNITS
3.10.1 CROSS PIPE METHOD
Figure 3-5 below is recommended as a convenient way of measuring line density with minimal temperature and
pressure effects. The sensing element is held in direct contact with pre-filtered on-line gas held in temperature
equilibrium. Flow rates are governed by the flow path of the cross pipe and the filters held within it.
Figure Chapter 3-5: Cross pipe installation
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Installation and Maintenance Manual 7812 Gas Density Meter
3.10.2 INSTALLATION PROCEDURE
A small diameter pipe configuration is fitted across the interior of the main pipeline. It should be isolated from the
main gas flow and be capable of accepting the threa ded mounting base of the 78125 transducer. The pipe
configuration should incorporate an isolating valve and a filtration system as shown in Figure 3-5. Minimal
thermal lagging is required, mainly involving the sample gas pipeline, since the temperature of the gas
surrounding the sensing element is being kept at main line gas temperature.
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7812 Gas Density Meter Installation and Maintenance Manual
Chapter 4
Mechanical Installation
4.1 General
The 7812 is a sample by-pass meter which can be inserted into the main gas stream. This ensures good thermal
equalisation yet allows the gas to be adequately filtered for reliable measurement. Gas density meters are
normally used as part of a mass metering exercise and in consequence the location of the density meter, with
regards to the flow measuring element, is most important.
4.2 Physical dimensions
The physical dimensions of the 78121/2/3/4 meter variants are shown in Figure 4-1. This unit has been designed
to be a direct replacement for the 7810 and 7811 gas density meters. The 78125* meter is a direct replacement
for the 3093 gas density meter and its physical dimensions are shown in Figure 4-2.
Note: For additional information regarding the physical dimensions of the meter, contact Micro Motion.
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Installation and Maintenance Manual 7812 Gas Density Meter
Figure 4-1: Physical dimensions of 78121/2/3/4 meters (with typical pocket shown)
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7812 Gas Density Meter Installation and Maintenance Manual
Figure Chapter 4-2: 78125 transducer - physical dimensions shown in inches (mm)
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Installation and Maintenance Manual 7812 Gas Density Meter
4.3 Liners
The liners incorporate a replacement inconel spring which takes up tolerances of the specific cylinder assembly
fitted. This enhances the long-term stability of the sensing element.
Note: The instrument is unaffected by the normal strip do wn and re-assem bly operation. However, should a
spool body or cylinder require replacing, then a new liner spring is recommende d and a recalibration is
imperative.
4.4 Filtration
For 78121/2/3/4 meters, two identical filter housings are provided adjacent to their respective gas connection
ports. The ports are identified by a label and should always be correctly connected. The in-line filters used are a
2 micron (inlet) and 90 micron (outlet), the outlet filter affording some protection should a reverse gas flow occur.
This filter arrangement is optimised for best protection of the sensing element and is best suited for density
measurement at the sample gas return point.
4.5 Pocket installation
An aperture is required in the pipeline to receive the pocket, which is inserted to the correct depth and welded
into position without distortion. Ideally, the pocket should be installed on a vertical diameter at the top of the
pipeline. Figure 4-1 shows a typical pocket; a detailed drawing of the pocket dimensions i s shown in Figure 4-4.
If high levels of vibration are likely encountered, fit two 78123723A anti-vibration gaskets as detailed in Section 4.5.1.
To enhance the temperature equalisation:
(a) Pour the supplied silicone fluid (20cc) into the pocket.
(b) Slip the aluminium cylinder over the lower end of the 7812’s main housing.
The main housing can now be installed:
1. Fit one gasket to the pocket and insert the main housing complete with its 2” OlD ‘O’ ring into the pocket.
2. Clamp the main housing securely in position by the clamp ring and its securing screws.
3. Connect the sample lines to their respective gas inlet and outlet ports, ensuring a relaxed pipeline run with
the correct 7812 orientation.
4. Complete the electrical connections as detailed in Chapter 5.
A typical type of installation is shown in Figure 4-4. The advantages of this t ype of installation are as follows:
• Good temperature equalisation.
• Suitable for high pressure installations.
• Best anti-vibration arrangement.
• 7812 can be changed without pipeline closure.
The sophistication of the system employed rests with the customer but should include isolating valves in the
sample by-pass line and preferably a flow control valve and a means of checking the sensor cal ibration in situ.
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7812 Gas Density Meter Installation and Maintenance Manual
4.5.1 Anti-vibration installation
In cases where vibration levels are above the recommended maximum of 0.5g, two optional anti-vibration
gaskets (accessory/spare part no. 78123723A) can be fitted to improve the vibration performance of the 7812.
These anti-vibration gaskets are manufactured from 0.2” (5mm) thick Neoprene and are fitted to either side of the
main body of the 7812 to isolate it from any pipeline vibration. This installation has been evaluated at levels up to
10g maximum and 2200 Hz and shown to reduce the effects of vibration by at least a factor of 3 over the
standard installation.
It should be noted that fitting these gaskets will raise the 7812 main body by approximately 0.4” (9mm) which will
need to be taken into account when re-fitting the gas pipe connections.
The gaskets are fitted as follows (see Figure 4-3 for details):
1. Pour the supplied silicone fluid (20cc) into the pocket. Slip the aluminium cylinder over the lower end of the
7812’s main housing.
2. Place one 5mm gasket between the pocket and the main body of the 7812, and insert the main housing
complete with its 2” o/d ‘O’ ring into the pocket, checking that the gasket is centrally positioned over the bolt
holes.
3. Position the second gasket between the 7812 body and the clamping ring, again placing it centrally, such that
the clamping ring does not touch the spigot and the bolts are located centrally in the 7812 main body holes.
4. Screw in six M8 x 65mm bolts (accessory/spare part no. 409601420) ensuring that the assembly stays
concentric, that the bolts are clear from the main body and that the clamping ring is not touching the central
spigot. It is important that there is no metal to metal contact between the 7812 and the clamping ring and
bolts that hold it in position. This is how vibration isolation is achieved.
(In most cases the bolts should be lightly greased to ensure they can be removed in the future, although if
there is concern about the bolts coming loose and creating any sort of hazard, then Loctite Screw Lock
should be used. An alternative to this is placing grease on the threads and a sealant around the bolt heads to
prevent any risk of vibration loosing.)
5. Tighten the M8 bolts to a torque of 15 ± 5 lb/in (1.7 ± 0.6 Nm). This is equivalent to lightly finger tight with a 3”
(80mm) long Allen key.
Electronics Housing
Clamping Ring
Anti-vibration Gasket
7812
Aluminium Cylinder
Anti-vibration Gasket
Pocket
Figure 4-3: Exploded view of 7812 anti vibration installation
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Installation and Maintenance Manual 7812 Gas Density Meter
Figure 4-4: Pocket drawing
Figure 4-5: Typical pocket installation
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7812 Gas Density Meter Installation and Maintenance Manual
4.6 External pocket installation
In this arrangement, the sensing element of the 7812 is enclosed in a robust housing. The housing can be either
bonded directly on to the pipeline, using Thermon Heat Transfer Cement or equivalent. Figure 4-6 depicts this
arrangement.
It is essential that temperature equalisatio n is maintained between the pipeline gas and the sample gas at the
In order to achieve this, the sample gas pipework must be kept to a minimum and, in conjunction with the
the external pocket housing, must be adequately lagged for thermal insulation.
For installation of the
the sample pipelines are ready for connection.
See Section 4.5 for installation instructions.
7812, it has been assumed that the housing has been assembled to its pipeline and that
7812.
7812 and
4.7 Post-installation mechanical checks
After installation, the 7812 should be pressure tested, with gas only, to 1½ times the maximum working pressure
of the system.
CAUTION: The 7812 pressure test figure should NOT be exceeded to avoid possible changes to the
calibration characteristics. Check around all joints for signs of leakage of gas.
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Installation and Maintenance Manual 7812 Gas Density Meter
10.6DIA
115
140
If required, a suitable spacer may be fitted in order to prevent the electronics
Note:
housing from fouling during wall mounting.
Figure 4-6: External pocket installation
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