Operation Manual - RITTER Drum-type Gas Meters

General

The meter must be filled with the same packing liquid which the meter was calibrated with. Otherwise major measurement errors will occur!

The Gas Meter (which is shipped dry) must be approximately half-filled with a suitable »Packing Liquid« before first use. The measuring drum which rotates in the packing liquid forms the actual measuring unit in conjunction with the liquid.

The packing liquid has two functions: Firstly, it seals off the active measuring chamber (inside the measuring drum which is being filled with gas). Secondly, the packing liquid level inside the measuring chamber defines the volume of the measuring chamber. The latter function is the basis for calibration of the gas meter’s measurement accuracy which is performed at the factory. Therefore, the measurement accuracy is directly dependent on the packing liquid level and so an incorrectly set level at time of installation is bound to cause incorrect measurements (see »Adjusting the packing liquid level«).

Mutual influence of packing liquid and gas

Irrespective of the chosen packing liquid, the packing liquid and the flowing gas inevitably affect each other with respect to evaporation and dissolving:

• Absorption of evaporated particles of the packing liquid by the gas.
• Dissolving the gas in the packing liquid up to the saturation limit.

Generally valid figures and limit values for the mutual influence of gas and packing liquid cannot be stated, because they depend to a great extent on the particular gas and its state. For example, when water is used as a packing liquid, a dry, warm gas absorbs significantly more evaporated water particles than a moist and cool gas. The solubility of gases in the packing liquid also varies greatly. The gas can only dissolve in the packing liquid up to the saturation limit. A measuring error caused by solubility can be avoided if the gas can dissolve in the packing liquid up to the saturation limit during test operation before the experiments are subsequently carried out.

Selection of packing liquid

The criterion for choosing a packing liquid should be that any mutual influence between the packing liquid and the flowing gas should be as small as possible, or that the effects can be ignored. In most cases, water can be used as the packing liquid. No special requirements for the water are necessary – that means that normal clean tap water can be used. When water is not suitable as packing liquid, oils or synthetic liquids can be used. Generally speaking, a thin-bodied fluid (ideal: viscosity of water) with a low vapour pressure (ideally: < 0.1 mbar/hPa) should be selected. A thin-bodied fluid causes a small friction resistance of the rotating measuring drum only, and hereby a small pressure difference between gas inlet and outlet of the meter. This results in a better (flatter) calibration curve. A low vapour pressure reduces the (unavoidable) evaporation of the packing liquid. A better long term stability of the packing liquid level is obtained and thus more stable measuring results.[/wpml-string]

RITTER recommends and can supply the following packing liquids:

• »Pionier 4281«, a medical paraffinic mineral White Oil containing aromates in trace elements. It is colourless, odourless, and clear.
• »Silox«, a silicone oil belonging to the group of polydimethyl siloxane. It is colourless and clear with a weak odour.
• »CalRiX«, a completely synthetic fluid on a fluorine base. It is almost totally inert, even to the most aggressive gases. It can also be used without difficulty under the most demanding and critical application situations. Further advantages of CalRiX are a low evaporation rate, a viscosity similar to that of water, 1.8 times the density of water and very low surface tension, which result in a more even rotation of the measuring drum. Dry gases remain dry.

High-Pressure Gas Meters (> 1 bar)

• Installation and set-up may only be performed by qualified personnel.
• The Gas Meter must not be dropped when it is being unloaded, moved or set up. Due to its heavy weight, there is a risk of personal injury and the possibility of damage to the unit.
• If the Gas Meter is installed in an elevated position, e.g. on a rack or similar, the device must be firmly secured so that it cannot move or slip to avoid personal injury or property damage due to the Meter falling. The feet of the Meter should be secured with tension clamps, screws or similar.
• When connecting the pipes, a gas-tight connection must be ensured. All caps and plugs on the gas meter must be sealed to maintain a leak-proof operation.
• The gas pressure in the inlet line must be increased slowly to avoid a pressure surge and the possibility of a hose rupturing. Otherwise, personal injury might occur.
• The Gas Meter may only be used within the over-pressure and temperature operating limits listed on the Meter label and in the data sheet.
• The Gas Meters do not have an over-pressure safety valve installed. In order to completely avoid exceeding the permissible operating pressure (listed on the Meter label and in the Data Sheet), an over-pressure safety valve must be installed in the connected gas pipes.
• When refilling with packing liquid, the gas meter must be depressurised before opening the filling-level indicator. Otherwise, packing liquid will be forcefully ejected.
• Before any disassembly, the Gas Meter must be pressure-free.
• Only original parts should be used to rebuild the Gas Meter if it has been disassembled.
• Disassembly of the Meter, i.e. removal and replacement of the measuring drum, can alter the calibration results (please refer to »Cleaning of casing and measuring drum from sediments«)

Positioning

Place the Drum-type Gas Meter onto a solid, vibration-free support. Align the gas meter precisely horizontally by means of the integrated level (at top of casing) and the levelling feet.

Filling with the packing liquid

It is essential to use the same packing liquid which the gas meter was calibrated with. The packing liquid is stated on the calibration certificate as well as at the calibration label on the Gas Meter.

Using a different packing liquid other than that used with the calibration will cause a significant measuring error!

Standard-pressure Gas Meters:

• Open the filling-level indicator located at the rear plate by turning the sealing screw 2 to 3 times anti-clockwise. Please do not remove the screw from the thread. Open the filling nozzle located at the rear plate by turning the sealing screw anticlockwise until it is unscrewed. Pour the packing liquid into the gas meter through the filling nozzle.

High-pressure Gas Meters:

• Loosen and remove the sealing screw of the filling nozzle (hexagonal screw). Pour the selected packing liquid into the Gas Meter via the filling nozzle. After correctly setting the packing liquid level as described in »Adjusting the packing liquid level«, replace the sealing screw and firmly tighten it with a spanner.

Adjusting the packing liquid level

The amount of packing liquid depends on the size/type of the Gas Meter and on the individual setting for each unit. The approximate quantity is shown in the data sheet enclosed with every Gas Meter. This quantity does not take into account individual differences based on the calibration performed in the factory.

Fine adjustment of the packing liquid level is essential for measurement accuracy, since the measurement accuracy/display depends directly on the packing liquid level and reacts very strongly to an incorrectly set level!

For Gas Meters with the High Precision Packing Liquid Level Indicator (HPLI), please refer to the HPLI Data Sheet.

For Gas Meters with the standard Packing Liquid Level Indicator (located at the rear plate of the Meter):

When the Packing Liquid Level Indicator (1) is opened by turning the sealing screw (2) with Plastic Meters (or stop cock (2) with Stainless Steel Meters), the rising pipe of the level indicator is connected to the packing liquid in the Gas Meter housing in accordance with the communicating pipes principle. When the level in the Gas Meter housing rises when filling with packing liquid, the level in the level indicator rises accordingly. The correct packing liquid level is reached when the surface of the liquid column in the level indicator is flush with the upper edge of the level indicator (3) and neither forms a convex or concave shape (figure 2). The liquid column can be read (if water is used as the packing liquid) more easily by reducing the surface tension with the addition of a small amount of detergent. If too much packing liquid has been added, it will escape through the level indicator causing the level to adjust by itself to a certain extent. Any resulting liquid dome at the upper edge of the level indicator must, however, be remedied by draining off liquid via the drainage nozzles. Following an extended downtime, the liquid in the level indicator is mostly evaporated, whereas it may not have evaporated on the inside of the (closed) housing. If the level indicator’s screw plug is opened, the packing liquid escapes from the housing into the level indicator. Even if the packing liquid level was previously correctly set inside the housing, packing liquid must now be added to readjust the level. This can be avoided by filling the level indicator with packing liquid before the screw plug is opened (see figure 2 for the correct amount of packing liquid). If the level does not change after the screw plug is opened, the packing liquid level was and still is correct.

After filling and adjustment of the packing liquid

• Close the filling-level indicator and filling nozzle by turning the respective sealing screws clockwise.
• If the Meter is equipped with the HPLI: When closed, the HPLI can indicate a slightly lower filling level than after the correct filling of the Meter. This can also occur during operation. This is system-related and does not indicate any error. The given correct filling level will only be indicated when the Meter is not in operation and simultaneously, when the HPLI is open and not connected to the gas supply so that it is pressure-free.

Grounding of Gas Meters made out of electrically conductive material (stainless steel or PE-el)

Gas Meters with a casing and/or measuring drum made out of electrically conductive material (stainless steel or PE-el) must be grounded in order to discharge a potential static charge. For this purpose, the feeder clamp at a flange screw of the meter’s rear plate must be connected to ground (earth).

Connection of gas pipes

Connect the gas pipe to the inlet nozzle marked »gas inlet« at the rear plate. Allow the meter to perform one or two revolutions in order to remove any possible air bubbles within the measuring drum. Then disconnect the gas pipe again and repeat the steps from: »Filling with the packing liquid« and »Adjusting the packing liquid level«.

Moving of the filled meter

If the Gas Meter has to be moved after having been filled (for example carried into another room), it must be kept in a horizontal position. This is to avoid packing liquid getting into the gas inlet. If this occurs, the Gas Meter should be tipped 90° forwards (until the dial faces the floor). The packing liquid can then drain out of the gas inlet again (and back into the Meter).

Determination of Flow Rate

Due to their design, Drum-type Gas Meters measure the volume of flowing gases and indicate the measured volume by means of a needle and counter. A flow rate cannot be measured directly and must be calculated via the measured volume per time unit. This can be done by software or external hardware. For this purpose RITTER recommends these products: Data Acquisition Software »RIGAMO« and Electronic Display Unit »EDU 32 FP«. Note: For both options, the Gas Meter must be equipped with a Pulse Generator.

When graphing a calculated flow rate, the corresponding curve will not be linear but wavy, even if the original gas flow rate was constant. This is physically unavoidable due to the design principle of Drum-type Gas Meters:

The measuring drum of the drum gas meter consists of four individual chambers that are opened and closed cyclically. The preceding chamber must be closed before the following chamber opens. This »forced measurement« is the reason for the high measuring accuracy of Drum-type Gas Meters, on the other hand, the opening/closing causes a slight pressure change inside the chamber. Furthermore, the surface tension of the packing liquid used causes an increase in pressure when a measuring chamber emerges from the liquid. The resulting pressure change causes a change in the rotational speed of the measuring drum, which causes the described wavy curve of the volume flow. This effect is called »periodic error of drum-type gas meters«. The smaller the Gas Meter, the greater the effect: for a TG05 the effect can be ±20 – 30%. The effect is avoided when using integer drum revolutions as a time base for the volume flow calculation. The Data Acquisition Software »RIGAMO« can be parameterised to calculate the volume flow using integer drum revolutions and avoid any periodic error. With the Electronic Display Unit »EDU 32 FP«, the volume flow is calculated by means of a moving average, which reduces the periodic error.

Initial set-up of measurement

To simplify the readout at the end of measurements, the large needle of the dial plate can be set manually to zero prior to each measurement. For units equipped with a totalizing roller-type counter (standard version), noting the counter reading is necessary. With a resettable roller-type counter (optional), the counter can be set to zero with the reset button. The Gas Meter is now ready for operation.

Examination of packing liquid level

Prior to each subsequent measurement, the fluid level has to be checked again in accordance with the steps from: »Filling with the packing liquid« and »Adjusting the packing liquid level«.

Review of performance data

When taking measurements, the performance data of the respective Gas Meter (refer to the attached data sheet) must be observed. The maximum pressure load is 50 mbar for standard Drum-type Gas Meters made from plastic and 500 mbar with Meters with a stainless steel casing.

Measurements with oxygen

The mixture of some gases with oxygen can cause explosive reactions. Therefore, before and after measuring with oxygen, ensure that no gas used in the previous measurement is still within the measuring drum or the inside the housing. To minimize the danger of explosion, the Gas Meter has to be purged with an inert gas (e.g. nitrogen or any noble gas). This can be performed by operating the Gas Meter with an inert gas for at least five revolutions of the measuring drum (= five revolutions of the large needle on the dial plate).