Great Eastern Energy Corporation Limited (GEECL) is a leading Indian coal bed methane (CBM) company focusing on monetizing natural gas from coal seams with a total volume of ~350,000 SCMD gas. Raw Gas is collected from well head and transferred to main Gas gathering Station (GGS) through 128 km of MDPE pipelines. Raw gas is compressed using a gas compression system and a gas treatment facility at the gas gathering stations. The gas compression system uses both a single-stage and a two-stage compressor to compress the gas. The gas is then delivered from Gas collection stations to Central gathering station via a 12-inch dedicated steel pipeline, approx. 77 km from Central gathering station. Additional dedicated 4 and 6-inch steel spur pipeline is connected to all customers to the main 12” trunk line.
GEECL is experiencing erratic customer’s consumption i.e. the consumption increases due to sudden demand from downstream and therefore, inability to maintain steady state or constant pressure condition in the natural gas pipeline grid. In a day, when the instant sale exceeds the net dispatched gas, resulting pipeline pressure or line pack. GEECL lately introduced flow control valve at each customer battery limit controlled and operated through a SCADA system, however, the problem is still persisting.
The Objective of this study is to address the following issues, analyze the same with site inspection and give technical resolutions and also to perform Pipeline hydraulic analysis for coal bed methane (CBM) distribution pipeline network
- Erratic reading at Flow meters
- Analyses the Pressure drop & velocity at the major consumers
- Recommendation for maximum allowable loss through pipeline network at different pipeline network pressure
- Maximum allowable flow in Standard Cubic Meter (SCM) per hour considering maximum allowable velocity
- Volumetric quantification due to consequence of pipeline network pressure
- Review of the existing flow meters, Pressure Relief Valve (PRV) & Flow control valve (FCV) installed at SAIL ISP & Super smelter in respect of erratic consumption, Pressure drop & high velocity and recommendations for any up gradation/modification required in any instrument mentioned above.
Gas from well heads is brought to a Gas manifold in the Gas Gathering Station (GGS). The gas then passes through the Group Separators to remove free moisture and is taken into the suction of the compressors. The outlet gas from the compressors passes through respective surge vessels to remove the condensates formed during the compression process and thereafter the gas flows through the respective dehydration units to achieve the desired dew point before flowing into the steel trunk pipeline to Central Gathering Station (CGS).
The major observations from the site are as follows,
- Various type of flow meter (Orifice, V-Cone, Turbine meter) used in the custody service having difference in accuracy level
- Sudden drop in pressure leads to turbulence in the line which creates uncertainty in flow measurement (Linearity and Accuracy)
- The FCV installed at all line is manually operated and fixed open. There is no flow control action
- The sudden pressure drop makes the line empty or low gas volume which affects other consumers
- The Pressure difference (Pressure variance) used in the calculation sheets having error and needs to be validated with average pressure in measurement period
- The flow meter used in inlet of GGS (South) is Annubar which is a coarse measurement
The Errors in Existing Flow meters system was analyzed and was found that, the pressure within the pipeline experiences a dip in pressure, while the temperature of the pipeline is contingent upon surrounding environmental temperature. These two parameters exert an influence in the flow rate parameter. Hence, it is recommended to calibrate the flow meters once in every 6 months
There are possibilities to reduce the uncertainty of these sources by using dedicated actions such as,
- Long Term stability
- Installation effects
The following assumptions/basis were considered for hydraulic analysis
- Model developed based on the Pipeline routing sketch
- Analysis performed for various, gas supply pressure at GGS to find the maximum gas flow to the consumers
- All lines shown in the Pipeline routing considered as buried
- Model did not include any control valves at user end for steady state analysis
- End of pipeline also assumed as consumer for dynamic analysis purpose.
- Additional uncertainty due to long term stability could be reduced by implementing in site verification/Validation with use of portable ultrasonic flow meters (Prover) can be used to determine if the existing flow meter is still operating within its specification.