A measurement problem that is mostly undetected with serious consequences for the user is that caused by pulsation flow. It is, therefore, important to grasp the effects of pulsations on gas flow meters to be able to formulate control techniques to effectively mitigate pulsation effects.
To clearly explain the effects of pulsations on gas flow metering, Hendrik van Huyssteen, Managing Director of Energas Technologies, a leading supplier of high-end and specialised equipment to the oil and gas industries in Southern Africa, cites a closer-to-home instance. He says that natural gas from Mozambique, for instance, is supplied in Gauteng, South Africa, through a network of distribution and reticulation pipework.
He notes that the American Gas Association gives recommendations for installation of flow meters to minimise the negative effect of line conditions that could result in metering errors such as non‐uniform flow and swirl along the distribution pipework. However, the installation requirements cannot compensate for the effect of pulsations in the line. Pulsations can be caused by piston compressors, often used to increase the pressure of the gas as in Compressed Natural Gas (CNG) applications.
“Custody certified meters used here must have errors not more than +/- 1% when compared to a certified master meter,” notes Van Huyssteen. “Such meters are equipped with volume correctors which calculate totalised gas flow corrected to standard pressure (101.325 kPA) and temperature (15 °C) and compensate for the compressibility of gas.”
The types of meters used and ranges at typical operating pressures used in the industry are:
- Diaphragm meters for small flow rates less than 16 Sm3/h at 50 kPa
- Positive displacement meters up to about 1500 Sm3/h at 100 kPa
- Turbine meters up to 200 000 Sm3/h at 3000 kPa
- Ultrasonic meters in excess of 500 000 Sm3/h at 3000 kPa
Pulsating gas flow
In turbine meters, for example, pulsating gas flow causes a cyclical slowing and speeding of the main rotor. This can result in considerable over-reading of the meter, resulting in the client being billed for much more than the gas used.
“Most meter types are adversely affected by pulsations. A pulsation is a periodic fluctuation in local pressure and velocity that occurs throughout a piping system or network. Pulsations typically move through a piping system as travelling waves. The travelling waves can be effected by closed and open ends of a piping network,” explains Van Huyssteen, adding that pulsating gas flow can cause severe damage to pressure components and supports in the pipework and should, therefore, be minimised.
To reduce the effect of pulsations, pulsation dampeners could be installed at inlet and outlet of reciprocating compressors. A well designed pulsation dampener would consider the frequency of pulsations, design pressure, gas properties and the pipe network lay‐out. The pulse frequency is determined by the number of pistons and the rotating speed of the compressor.
“The pulsation dampener is a pressure vessel with a series of carefully positioned baffle plates. The pulsation dampener must be designed for the line pressure and temperature and comply with the PER requirements,” says Van Huyssteen. “Normally, it is recommend to put the dampener as close to the compressor as possible, to avoid resonances in the pipeline. In a case where you have two compressors, it would be safer to put a dampener on each compressor.”
Using one dampener with two compressors will work from a mathematical standpoint, but you have to connect it further from the pulsation source, which risks having a harmonic length of pipe in the line before the dampener.
The pulsation dampener reduces the amplitude of the pulsation as it travels through the vessel. The reduction is dependent on the frequency and can be reduced by as much as 97%.
Auto‐Adjust Turbine meters
The AGA report No. 7 notes that pulsations cause a positive error in turbine meter output that is dependent on pulsation velocity amplitude at the meter, flow rate, gas density and meter and pulsation properties. This error can be as large as 50%.
According to Van Huyssteen, the Auto‐Adjust Turbine (AAT) meter supplied by Sensus is the only one of its nature that can detect and compensate for pulsating flow. Energas Technologies is the sole representative of Sensus Meters in South Africa. Energas has supplied AAT meters to the local natural gas industry since 2001.
“The AAT meter has two rotors, a main rotor and a second sensing rotor, typically running at 10% of the speed of the main rotor,” he explains. “The speed of the rotors is monitored by a special volume corrector using complex algorithms. Any deviations of accuracy relative to the originally calibrated accuracy are detected and displayed.”
Inaccuracies at the meter could be due to various conditions such as bearing failure, dirt build up in the meter, swirl of the gas after bends, pulsations or non‐uniform flow. The meter can compensate 100% for these deviations up to deviations of 5% of the correct flow rate.
“Even small deviations in accuracy of as little as 0,2% could result in losses of millions of Rands for large meters over a 12 month period. A new meter must be supplied with a factory calibration certificate, showing the meter is accurate to 99% or better over the meter’s operating range. To achieve higher accuracies, meters in high pressure gas applications are often tested at the specified operating pressure,” says Van Huyssteen.
With a normal single rotor turbine meter, pipeline disturbances, bearing drag or dirt is not detected once the meter is in the line. As meters are recalibrated typically once in two years or longer, meter problems are not detected and meter inaccuracies goes undetected for up to a period of two years.
“The AAT meter solves this problem by showing a delta A, a deviation relative to the original calibration from the moment it is first operated. Deviations can be investigated immediately. As long as the delta A stays below 5%, the meter technically does not need to be recalibrated,” concludes Van Huyssteen.