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DEVELOPMENT OF A GAS STATION WITHOUT TRADITIONAL HEATING

G.J. van Essen, J.W.F. Janssen and P.J.M.M. van Wesenbeeck Gasunie Research The Netherlands

C.J. Mooy Gasunie

The Netherlands

ABSTRACT

Gasunie is interested to reduce the energy and maintenance cost of its city gate stations in the Netherlands. In the current city gate stations the gas is heated first in order to avoid the formation of hydrocarbon liquids (condensate) as a consequence of the temperature drop of the gas (typical from 40 to 8 bar (g)).

In the new design city gate station the equipment is installed underground and the gas

is not preheated in a traditional way upstream of the pressure regulator. The liquid (condensate) that is possibly formed will be caught in a cyclone separator. Under typical operating conditions ice will be formed around the outlet pipe of the city gate station and subsequently a permafrost situation will occur outside the installation when no further measures are taken.

Gasunie Research has investigated by order of Gasunie how to solve this problem and

has carried out a performance test in the High Pressure Test Laboratory in Groningen. As a result in the new design city gate station the gas will be heated downstream of the cyclone separator making use of ground warmth.

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INTRODUCTION

In the Dutch high-pressure gas transport system of Gasunie there are approximately 1100 city gate stations located where gas is delivered to local gas distribution companies, large industrial customers and power plants. In a city gate station the inlet gas is reduced in pressure, typically from 40 bar (g) to 8 bar (g). Before the actual pressure reduction takes place, the inlet gas, with a typical temperature of + 7 C, is heated. Heating is necessary in order to avoid a decrease in the gas temperature to values far below zero, because of the Joule Thomson effect (1 bar pressure drop causes about 0.5 degree drop in gas temperature).At such conditions, the formation of liquids (condensate) can not be excluded. The presence of liquids is undesirable for the consumers.

The standard layout of the city gate stations of Gasunie consist of at least two runs.

Each run contains a dust filter (to prevent the entrance of solid particles to penetrate into the installation), a heat exchanger, pressure safety equipment, a pressure reducer, and a fiscal metering system (determining the flow, pressure and temperature of the gas stream).

In order to reduce energy costs for gas heating and to minimize maintenance costs for gas heating equipment, Gasunie has investigated the possibility of an alternative design for their city gate stations. This has resulted in a new design characterized by the following properties:

- from an environmental point of view the station is placed underground, resulting in a reduction of the noise emitted by the pressure regulator and makes it easier to fit the station into the landscape (horizon pollution)

- the inlet gas is not heated; so there is no need to install a gas heater (lowering of investment costs). An additional advantage of the absence of the heater is the complete absence of the emission of NOx and SO2

- a gas - liquid cyclone separator (GLC separator) is placed in the outlet section of the station in order to catch liquids (condensate) which are possibly formed in the cold gas downstream of the pressure reducer. Since these separators are also able to catch solids, dust filters are not needed any more

- only one central fiscal metering system is placed at the inlet (high pressure part) of the station

Obviously, in each run, the pressure reducer and pressure safety equipment are

maintained. The principle of this design is illustrated in the figure below (two runs, only the

typical parts are shown)

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Fiscal flowmetering

40 bar 8 bar

Pressure Reducer

Pressure Reducer

Cyclone separator

Figure 1. Concept of the new design city gate station of Gasunie For a typical city gate station operating at a gas inlet temperature of +7 C and with a

pressure reduction from 40 to 8 bar(g), ice will be formed on the installation downstream of the pressure reducer and the ground around the installation will freeze up during a long period (permafrost).

For a city gate station, the amount of formed ice depends particularly on the pressure

drop, the inlet temperature of the gas and the type of ground (wet / dry). Due to the increase in volume, the transition of water into ice causes undesired forces on the installation and can cause damage to the surroundings since the ground will be moved upwards. Formation of ice on the installation itself can simply be solved by isolating the low-pressure part of the installation (from the pressure reducer up to the cyclone separator). However without further measures, ice will be formed on the outlet pipe. Consequently, the ground around the outlet pipe will be frozen during a long-term period of the year (permafrost) and the temperature of the gas will be (far) below zero over a certain distance and possibly contains liquid condensate. A solution had to be found for this off spec situation.

Concerning the location of the fiscal flow metering equipment in this new design, it is

important that the gas temperature is within the range specified by the manufacturer of this equipment. Most of the equipment (flow, pressure, temperature meter) has a specified minimum gas temperature of above 10 C. Thermal calculations have shown that for a typical city gate station, with the maximum pressure reduction (40 => 8 bar) and an inlet gas temperature of +7 C, the gas temperature will still be well above 0 C only a few meters upstream of the pressure reducer. Thus by positioning the fiscal flow metering equipment at a suitable location upstream of the pressure reducer, the performance of this equipment is not expected to be affected. This important point was also confirmed by the experimental tests carried out by Gasunie Research (described below).

From the aforementioned discussion, it follows that the main questions to be answered

before deciding to build a city gate station according to the new design by Gasunie are: - Is there a solution for the problem of ice formation around the outlet pipe of

the installation? In particular, it is important to minimize the amount of ice

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formed on the outlet pipe but also to minimize the distance over which the ice is formed. A number of alternatives have been investigated by carrying out thermal, so called permafrost calculations. Two possible solutions are explained in more detail below.

- Is it possible to test the performance of a new design city gate station under practical conditions? This has turned out to be possible without building a full scale model. Gasunie Research has carried out tests at her High Pressure Flow Laboratory in Groningen. The set-up of these test and the results are described below.

In addition to these two main questions, an investigation has been carried out to the

possible consequences of the low gas temperature on the behaviour of the odorant, which is added to the gas upstream of the city gate stations in the Gasunie grid.

THE BEHAVIOUR OF ODORANT

The odorless natural gas needs to be odorized before it can be distributed to the end consumers. Typical odorants used in the natural gas business are tetrahydrothiophene (THT) and tertiary butyl mercaptan (TBM). Under normal circumstances, the odorant is injected in a gas stream, which is free of liquids and with a temperature equal or close to ground temperature. However, in the new concept, low temperatures occur in the installation, which could result in the condensation of a part of the injected odorant. Furthermore, in the new concept, the formation of hydrocarbon liquids can not be excluded. The presence of hydrocarbon liquids could also dissolve part of the injected odorant. To investigate the extent of these possible effects, Gasunie Research carried out process simulations. The simulations were carried out with THT and three different commercially available odorant blends containing TBM. The inlet pressure was varied between 15 bar(g) and 40 bar(g) , and the gas temperature at the inlet of station varied between +5 C and +13 C. For these simulations, it is important that the condensation behavior of the natural gas is modeled correctly. Therefore, a extended natural gas analyses up to n-pentadecane (n-C15) was used.

The results for THT and the three different TBM blends were comparable: even in the worst-case scenario (40 bar (g) inlet pressure and a gas temperature of +5 C) no condensation will occur. In those cases where hydrocarbon liquids are formed, only a maximum of 4 % of the injected amount of odorant will be dissolved in the condensate.

THE PERMAFROST PROBLEM

As indicated above permafrost simulations were carried out in order to find a solution for the problem of minimizing the amount of ice on the outlet pipe and the distance over which the ice is formed.

As a starting point for the simulations, a city gate station with typical dimensions was taken (6 inlet pipe, 10outlet pipe, 2 runs, typical yearly flow pattern) and with the low-pressure part (pressure reducer cyclone) isolated as described above. The outlet pipe is not surrounded with isolation. The calculations were carried out for two types of soil: dry

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soil (sand), typical for the eastern part of the Netherlands, and wet soil (clay), typical for the western part of the Netherlands.

First solution The permafrost calculations showed that, without taking further measures, a not

acceptable amount of ice of about 0.8 - 1 m would be formed in the winter period around the outlet pipe.

By isolating the outlet pipe in a special manner, the process of ice formation can be controlled. In this configuration, the layer of isolation around the outlet pipe gradually decreases to zero over a distance L1 downstream of the city gate station (the isolation distance). With this configuration, the maximum amount of ice occurring in the winter period can be reduced to an acceptable value of about 30 - 40 cm. Downstream of this point (no isolation present anymore), the amount of ice will eventually decrease to zero and the gas temperature will increase to 0 C (say at a distance L1 + L2 downstream of the city gate station). By isolating in this manner, the stress in the pipe material caused by the formation of ice can be reduced to acceptable values. The principle is shown in figure 2.

Figure 2. Effect of isolating the outlet pipe The distances L1 and L2 are dependent of the minimum gas temperature (just

downstream of the pressure reducer), the type of ground, the supply of environmental warmth (season) and from the gas flow rate (at low gas velocity the gas warms up quickly). The amount of isolation and the isolation distance (L1) can be chosen in such a way that the ice, formed during winter time, will disappear during summer time.

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Additional permafrost calculations have been carried out to determine the distance L1 and L2 for a typical winter period and for gas temperatures of -8 C, -13 C and -18 C directly downstream of the pressure reducer. The results are shown in table 1.

Table 1. Effect of Isolating the Outlet Pipe

Gas temperature after pressure reduction

-18 C -13 C -8 C

WET GROUND isolation length (L1) 630 m 427 m 230 m de-icing length (L1+L2) 1700 m 1180 m 710 m

DRY GROUND isolation length (L1) 440 m 290 m 120 m de-icing length (L1+L2) 1390 m 1000 m 600 m

From these results it may be concluded that, with a gas temperature of -8 C or lower

downstream of the pressure reducer, at least a distance of 120 m is needed to warm the gas up to 0 C. This distance is not acceptable for Gasunie, so a design without any heating is not viable. Convinced of the advantages of a city gate station without traditional heating, Gasunie searched for an alternative solution to prevent the excessive ice formation at the outlet. This solution is described below.

Final Solution Making use of the ground warmth, the permafrost problem can be solved quite

easily. The principle of this solution is based on pumping around a biologically produced and environment-friendly heat transfer medium like betaine (to be assumed further on in this paper) by a circulation pump in a closed loop, thereby passing the following parts of the system:

- A ground heat exchanger, consisting of a number of vertical plastic tubes (length about 40 m, diameter about 25 mm) mounted in loops. The ground temperature, which is constant and about + 11 C at a depth of 40 m, is picked up by the betaine (typical rating is about 1 kW per tube of 40 m length). For a typical city gate station, 50 70 tubes are sufficient to fulfil the heat requirement. Putting these tubes into the ground is an existing technique, applied frequently in the Netherlands.

- A heat exchanger (modular design, consisting of a number of betaine gas heat exchangers), where the gas is heated in a countercurrent way. At the moment of writing this paper, the type of heat exchanger has not been specified yet, but Gasunie probably will make use of a shell & tube heat exchanger, typically consisting of 4- 6 modules with a length of 6 m each. The inlet and outlet temperature of the betaine in the heat exchanger are typically + 5 C respectively. -5 C for an outlet gas temperature of -13C downstream of the pressure reducer.

- After heat exchanging with the gas, the betaine passes an air betaine heat exchanger, with a regulation system. When the ambient temperature of the air is above 0 C the betaine flows through the heat exchanger and will be heated by the air. When the ambient temperature is below 0 C, the heat exchanger will be by-passed.

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The principle is shown in figure 3 .

Figure 3. Solution of the Permafrost Problem (only the pressure reducer in the city gate station is shown)

With this layout, the heat exchanger will bring energy into the soil after the winter

period. The soil can thus be considered as a buffer of energy. At the moment of writing this paper, a final choice for the type of air betaine heat

exchanger has not been made by Gasunie. One possibility is the use of an asphalt collector in which betaine is flowing through short tubes, mounted only a few centimetres under the ground level in a bed of asphalt. These heat exchangers are commercially available and used in The Netherlands.

PERFORMANCE TEST OF THE NEW DESIGN CITY GATE STATION

Test Set Up At the High Pressure Test Laboratory of Gasunie Research in Groningen, the new

design of a city gate station was tested in 2003. At this laboratory, it is possible to perform measurements under realistic conditions. The inlet pressure can be varied between 8 and 40 bar (g), the outlet pressure is fixed at 8 bar (g). All tests and calibrations can be carried out making use of the Dutch high-pressure flow standards, which are present at this laboratory. All materials used during the tests had a minimum specified temperature of -20o C.

In the laboratory a city gate station according to the new design was build up

(aboveground) with one run. The low-pressure part of the installation was isolated. A gas

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temperature of +7 C at the inlet of the station could be created by switching off the inlet heat exchanger of the laboratory. This resulted at maximum pressure reduction from 40 bar (g) to 8 bar (g) in an outlet gas temperature of -7 C in the (isolated) 8 bar part of the installation.

In the 4 high-pressure inlet part of the installation (maximum flow rate 650 m3/h) three flow meters were installed: a reference turbine meter, and two test meters: a 3-path ultrasonic meter and a coriolismeter. In figure 4 the set-up is illustrated.

turbine meter(reference)

Pressure Reducer(type Baai)

Cyclone separator

coriolis meter ultrasonicmeter

Liquid filter Heat exchanger

to consumers

Low pressure part - 8 bar

40 bar

Figure 4. Schematical test set up

The low pressure part (not isolated) is shown in figure 5.

Figure 5. Schematic test set up

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A heat exchanger at the outlet of the installation was installed in order to secure the delivery of on-spec gas to the consumers of Gasunie. The performance of the Gasunie gas liquid cyclone separator was tested (this is explained further on in this paper) and a filter separator was installed to determine the separator performance of the Gasunie cyclone separator and to guarantee the delivery of dry gas to the consumers downstream of the installation. The gas composition of the gas at the inlet and outlet was measured continuously by a gas chromatograph (not shown in figure 4).

The following tests were carried out: - Calibration of the ultrasonic and coriolis flow meter at an inlet pressure of 15,

25 and 40 bar (g). By switching of the heat exchanger at the gas inlet of the laboratory, a minimum inlet gas temperature of about +7 C could be reached. The flow meters turned out to fulfil the Gasunie specifications for custody transfer under these conditions.

- During these calibrations, acoustic measurements were carried out to detect the presence of resonance frequencies that are possibly induced in the new design city gate station by the combination of the pressure reducer and the GLC separator. It is well known that the presence of high frequencies above 100 kHz could affect the performance of a ultrasonic meter. The results of the acoustic measurements are explained further on in this paper.

- Liquid tests were carried out to determine the performance of the GLC separator. These tests consisted of injecting oil with different droplet size distributions. Since the GLC separator plays an important role in the new concept, these tests are described further on in this paper extensively.

- Pressure reducer tests at 15 and 40 bar inlet pressures were carried out in which the flow rate was varied both quickly and in a stepwise way. The reducer performed well at all tests. The internals of the pressure reducer were inspected several times during the test period. No deposits (like for example sulphur) were found. It should be marked that the gas supply line for the pilot was heated by means of electrical heat tracing. In a future real application, the electrical heat tracing might be substituted by a heat pipe (making use of ground warmth).

- The pressure safety equipment was also tested at inlet pressures between 15 and 40 bar and it was found that all equipment performed well.

- It should me remarked that no extreme stress or shrinkage of the installation parts was noticed during the tests that might occur at low temperatures (special measuring equipment was installed to detect the mechanical stresses). By making use of bends in the installation this problem can be avoided.

Results of Acoustic Measurements High frequency measurements were carried out in order to investigate the presence of

ultra sonic noise (100 200 kHz) in the installation possibly generated by the pressure reducer or the GLC separator. High frequency noise might disturb the performance of the ultrasonic flow meter. During the tests, special sensors (range 10 500 kHz) were placed in the high-pressure part of the installation, upstream and downstream of the ultrasonic flow meter.

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For the worst case situation, i.e. at maximum pressure drop (40 => 8 bar) and at maximum flow rate (650 m3/h), the maximum ultrasonic sound pressure over the full spectrum turned out to be about 35 dB (dB with respect to 1 Pa). These results were comparable to results obtained earlier with a similar configuration but without the presence of a cyclone separator, where the temperature of the gas flowing through the flow meter was +20 C .

From this it may be concluded that no additional high frequency ultrasonic sound is generated by the installation and the performance of an ultrasonic flow meter in the high pressure part is not effected (as was confirmed by the calibration results).

Pulsation measurements, carried out in order to investigate the possible presence of

low-frequency pulsations (0 20 kHz) and, if so, if these pulsations are within the acceptable range specified in the API 618 standard. This standard concerns the mechanical integrity of pipelines and specifies maximum acceptable pulsation levels, depending on the pipe diameter, design pressure, and frequency. During the tests, pulsation sensors were placed at several locations in the installation. For the worst-case situation, i.e. at maximum pressure drop (40 => 8 bar) and maximum flow rate (650 m3/h), the pulsation values fulfilled the API 618 norm, as is shown in figure 6.

Figure 6. Result of pulsation measurements

PERFORMANCE OF THE GLC SEPARATOR

The Gasunie Cyclone Separator In the oil and gas industry, gas-liquid separation is frequently applied. The most

commonly used principle for separation is based on settling due to the density difference between gas and liquid. However, with the large flows at high pressures typically handled by this industry, the volume, weight and cost for the separation vessels may become very large. Therefore, another concept was chosen: the cyclone. With the cyclone separator concept, it is possible to obtain a compact design with good separation performance. Gasunie has developed a mono-cyclone design that has been improved several times in the past in terms of catch efficiency at high gas loads [1-3].

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Below a description of this separator is given, then the set up and the results of the tests to determine the catch performance are presented, followed by some general conclusions.

For the separation of liquid and gas, the

upper half of the cyclone is most important. The gas liquid mixture enters the cyclone via the central positioned inlet. Further downward, blades with a small angle with the horizontal, bring the gas/liquid into a rotating movement. This section is called the rotation section. During the centrifugal movement, the separation of gas and liquid takes place due to the density difference between both phases. The liquid leaves the cyclone downwards via the vessel wall to the liquid/solid drain. The clean gas flow returns upwards via the central outlet pipe. The used cyclone has a 0.373 m internal diameter (full scale prototype within the existing range of diameters 0.25 - 1.25 m).

Figure 7 : The Gasunie cyclone. Set up of liquid Performance Tests

The aim of the liquid tests was to determine the catch efficiency of the Gasunie

cyclone separator for different liquid droplet sizes. In addition, the influence of liquid atomisation in a pressure regulator on the catch performance was investigated as well as the catch efficiency for condensate droplets formed due to low temperatures in the pressure regulator.

The tests have been carried out in three phases.

Test 1 The performance of the Gasunie cyclone separator was measured for different liquid

droplets sizes using atomization of oil with a high-pressure nozzle. A typical example of such nozzle is presented in figure 9. Tests were carried out for liquid with droplet sizes of 30 and 100 m, injected between the pressure reducer and the cyclone separator. The difference in oil droplet size was created by using different types of nozzles and varying the pressure drop during atomisation of the liquid. The measured liquid catch performance data is used as a reference for the tests in the second part of the test program.

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Figure 8 : High pressure injection nozzle. Figure 9 : Details of injection nozzle

Test 2 In the second part the influence of liquid atomisation in the pressure regulator on the

catch performance of the cyclone separator was investigated. For this purpose an injection nozzle facility (see figure 8) was installed upstream of the pressure regulator. During these tests the liquid load and the injected oil droplet size was equal to the conditions used in test-1. The measured liquid catch performance was compared to the reference data collected in the first part.

Test 3

During a long duration test condensate droplets were formed due to a suitable combination of pressure and temperature in the pressure regulator and the liquid catch efficiency of the cyclone separator was determined.A schematic overview of the liquid injection system is presented in figure 10 and is explained below.

Figure 10. Liquid injection system for the performance tests of the cyclone separator A gas liquid mixture was made by injecting small liquid droplets into the gas stream

by an injection nozzle (L1). A special mobile injection skid has been designed and

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constructed for this purpose by Gasunie Research. Most of the liquid is separated by the cyclone and then transported to a liquid vessel (L2) from where it can be transported back to the mobile injection skid. The total carry over is separated in a special filter separator which was installed downstream of the cyclone separator (L3). The filter separator is a demister system to remove small liquid droplets, which passes the cyclone separator. The catch efficiency () of the liquid catcher was calculated as

432

2LIQUID LLL

L++=

where L2 is the separated liquid by the cyclone and (L3+L4) is the carry over that is

separated in the filter demister. To determine a reliable carry over it is of major importance that minimal liquid will leave the system by evaporation. For this reason a compressor oil type is chosen as a model liquid. From process calculations (with PRO-II) it was concluded that the liquid loss due to evaporation of oil was negligible (L4 = 0). During and afterwards of the tests no detectable oil presence was observed downstream of the filter demister.

The uncertainty of the calculated liquid catch efficiency is estimated as 0.2% for the 30 m droplets and 0.3% for the 100 m droplets.

Typical process conditions during the experiments were : Groningen composition natural gas and compressor oil as liquid. Gas pressure 15 25 and 40 bar for catch efficiency measurements. Gas flow rates up to 26.000 m3(n)/hr. Liquid flow rates 119 l/h at droplet size 30 m and 94 l/h at droplet size 100 m The Liquid over gas ratio (LGR), the ratio between the actual liquid and gas volume

flow rates, varied between 0.004% and 0.11 % for 30 m droplets and between 0.01% and 0.09 % for 100 m droplets. The LGR was limited by the capacity of the used spray nozzles. However, the standard Gasunie cyclone has been shown to give a good performance for LGRs up to 2%.

Results of Liquid Performance Tests

Influence of liquid droplet size (test 1) The liquid catch efficiency curve for the mono-cyclone separator at different droplet

sizes is above 99,7 % as illustrated in figure 11. The tests were carried out at a pressure of 9 bar in the separator.

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99,0

99,2

99,4

99,6

99,8

100,0

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0

Superficial gas velocity v0 [m/s]

Cat

ch e

ffic

ienc

y [%

]

Droplet size : 100 micron

Droplet size : 30 micron

Figure 11 : Liquid catch efficiency of the Gasunie cyclone separator Liquid Atomising in the Pressure Regulator (test 2)

The performance of a cyclone separator is partly dependent on the liquid droplet size distribution in the feed gas stream of the separator. During the tests a pressure regulator of Baai type R 100 S was used (often used in the transport grid of Gasunie), which operated at inlet pressures of 15 and 40 bar during these tests (outlet pressure 9 bar). In both conditions oil droplets with a droplet size of 30 micron were injected in the gas upstream of the pressure regulator. The results mentioned in figure 12 show the influence of liquid atomisation in the pressure regulator on the catch efficiency of the cyclone separator for oil droplets.

99,0

99,2

99,4

99,6

99,8

100,0

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0

Superficial gas velocity v0 [m/s]

Cat

ch e

ffic

ienc

y [%

]

Droplet size 30 micron : Inlet cyclone

Droplet size 30 micron : P-reducton 15 => 9 bar

Dropletsize 30 micron : P-reduction 40 => 9 bar

Figure 12 : Influence of liquid atomization on catch performance for oil droplets

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The results presented in figure 12 show no dependency in liquid catch performance of the cyclone separator on the presence of the pressure regulator or the process conditions used during the tests.

For a pressure reduction from 40 to 9 bar, the catch efficiency turned out to be independent of the flow rate, both at low and high flow rates a catch efficiency of 99,6% is found. Based on this observation, it was decided to carry out the other performance measurements at one flow rate only and to assume the measured value to be representative for the whole flow range.

The test results mentioned above show the high liquid performance of the Gasunie

cyclone separator for droplets of 30 and 100 micron. No significant reduction of the liquid catch performance was observed if the droplet size was reduced by atomizing in a pressure regulator operation under 40 and 15 bar inlet pressure an 9 bar outlet pressure.

Liquid Formation (Condensate) in the Pressure Regulator (test 3)

During a durability test, the outlet temperature of the gas downstream of the pressure regulator was reduced to 7 C by by-passing the heat exchanger at the inlet of the laboratory as described before. Simultaneous during this experiment the hydrocarbon dewpoint and the potential hydrocarbon liquid content (PHLC-value) were measured. From these measurements it was observed that the PHLC-value of the gas after the pressure reduction was 5 10 mg/m3(n). Most of the condensate formed during the test was separated in the cyclone separator, but some condensate was also found in the filter separator downstream. The calculated catch efficiency of the Gasunie cyclone separator is presented in figure 13.

80,0

82,0

84,0

86,0

88,0

90,0

92,0

94,0

96,0

98,0

100,0

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0

Gas velocity v0 [m/s]

catc

h ef

ficie

ncy

[%]

Droplet size : 30 micron

Condensate

Figure 13 : Catch efficiency of the GU cyclone separator for oil and condensate. For hydrocarbon droplets formed in the pressure regulator (condensate) the measured

catch efficiency was 84%. The measured performance for condensate is thus lower in

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comparison to the catch efficiency for oil droplets of 30 micron. The reason for the reduction in the catch performance is probably caused by the very small liquid droplet size of condensate in the pressure regulator. Measurements carried out in the past in the Gasunie Research laboratory showed a typical liquid droplet size for hydrocarbon compounds in a pressure regulator of less than 0,1 micron.

General Conclusions from the Liquid Tests

Liquid catch performance

For hydrocarbon droplets (oil) with a droplet size of 30 micron or more, a high liquid catch performance of the Gasunie mono-cyclone separator was observed at superficial gas flow velocities up to 6,8 m/s. This range covers the complete working area of the cyclone separator for application in the new generation city gate station.

During liquid atomisation in a pressure regulator type Baai no significant decrease in liquid catch performance was observed. As these tests were executed with a type of pressure regulator frequently used in Gasunie installations and under process conditions that are comparable to the practical situation in the Gasunie transmission grid, the results can be used directly.

If condensation of hydrocarbon compounds in the pressure regulator occurs due to the low temperatures and pressure in the regulator a reduction of the catch performance to values of 84 % was observed.

Condensate formation

During the durability tests hydrocarbon condensation in the pressure regulator occurred because of the existing pressure and temperature conditions. Because the durability tests were executed after the tests with oil injection, a mixture of oil and condensate was separated in the cyclone separator and in the filter separator.

Based on experiences in the Gasunie transmission grid, it is expected that liquid condensation in our test facility was accelerated by the presence of residuals of oil in the separator, the piping of the test facility and in the filter separator. In the past, it is observed that in the presence of oil, the mass flow rate of liquid (condensate) can increase with a factor 2-3.

Entrained condensate can evaporate in the shell & tube heat exchanger, which will be installed in the new generation of pressure reduction stations downstream of the cyclone separator. The formation of condensate is strongly dependent on the gas composition and on the presence of oil. The risk of penetration of condensate into the transmission grid downstream of the city gas station is evaluated using gas/liquid phase equilibrium calculations. Conclusion from these calculations (PRO II) using a typical natural gas composition are:

1. Hydrocarbon dewpoint : A minimum liquid catch efficiency of the cyclone separator for condensate of 75 %

is necessary to result in a calculated hydrocarbon dewpoint of 0 C for this type of natural gas at 8 bar (g) pressure . Thus heating this the natural gas at 8 bar (g) up to at least 0 C will prevent the formation of condensate given the fact that an actual catch efficiency of 84 % was measured. 2. Potential Hydrocarbon liquid content (PHLC-value):

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Many gas delivery contracts between gas suppliers and Gasunie include a specifica-tion for the maximum allowable PHLC-value of 5 mg/m3(n). A minimum catch effi-ciency of the cyclone separator of 40 % is required to guarantee a PHLC-value of 5 mg/m3(n) at 8 bar (g) and 0 C has . Assuming a liquid catch performance of 90 %, the PHLC-value of 5 mg/m3(n) is reached at a temperature of 7 C and a pressure of 8 bar. From these results it may generally concluded that the measured performance of the

Gasunie cyclone separator of 84 % for condensate is high enough for a practical application of this type of separator in the new type of gas reduction stations

CONCLUSIONS

Based on the results of the feasibility study for the new design city gate station (permafrost problem) and the results of the performance test carried out by Gasunie Research at the High Pressure Test Laboratory in Groningen the following conclusions can be drawn:

The permafrost problem can be solved by isolating the low pressure part of the

installation, and by making use of earth warmth by means of a closed circuit in which a biological and environmental friendly heat transfer medium (like betaine) is circulated. In this system, the betaine picks up the energy from the soil at a depth of about 40 m and is subsequently used to heat the outlet gas up to a temperature of at least 0 C. After the winter period (ambient temperature above 0 C) the betaine is preheated before injection into the soil by an air betaine exchanger. Operated in this way, the soil is operated as a heat buffer.

The tests carried out at the High Pressure Test Laboratory showed a good performance of the new design city gate station. Under worst case conditions a typical gas temperature downstream of the pressure reducer of 7 C could be created (when the pressure was reduced from 40 to 8 bar (g)). The Gasunie cyclone separator showed a catch efficiency better than 99,6% for oil droplets of 30 micron and larger. This catch efficiency was maintained for even smaller droplets after atomizing in the pressure reducer. The catch efficiency was 84% for liquid condensate (typical droplet size smaller than 0,1 micron) which was formed from the gas during a long duration test.

Because of the good performance of the test installation, and in particular the performance of the Gasunie cyclone separator, the design which guarantees an outlet gas temperature of minimal 0 C (no condensate and no permafrost outside the installation) and the result of calculations showing that the odorant in the gas will not condense, Gasunie has decided to build a new design city gate station in the near future.

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REFERENCES

(1) L. Oranje, How good are gas-liquid separators, 8th International Conference on offshore Mechanics and Arctic Engineering, The Hague, March 19 23, 1989, p. 397 403.

(2) L. Oranje, Cyclone type separators score high in comparative tests, Oil & Gas

Journal, Januari 22, 1990, p. 54 57.

(3) N. Nanninga, J.W.F. Janssen, Gasunie/CDS Improvement of the Gasunie cyclone Gas-Liquid separator, IGRC 2001, Amsterdam.

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ABSTRACTINTRODUCTIONTHE BEHAVIOUR OF ODORANTTHE PERMAFROST PROBLEMFirst solutionFinal Solution

PERFORMANCE TEST OF THE NEW DESIGN CITY GATE STATIONTest Set UpResults of Acoustic MeasurementsThe Gasunie Cyclone SeparatorResults of Liquid Performance TestsGeneral Conclusions from the Liquid Tests

CONCLUSIONS