BLENDAIRE II PACKAGED LPG-AIR MIXING SYSTEM

BLENDAIRE II uses a proprietary control system housed in a SIEMENS PLC for mixing gases in parallel streams. It revolutionizes the conventional passive way of correcting the mixing ratio in a typical mixer. Typically, a correction occurs only when deviation from a setpoint is detected. That is too late! We PREVENT the error from occurring! BLENDARIRE II is a versatile gas mixing system for LPG and air mixing, manufactured gas or natural gas conditioning with LPG (for CV enrichment) or air (for dilution). It replaces conventional piston mixing systems and provides many additional advantages. Typically, there is also a cost savings. This is only possible with our advanced and proven state-of-the-art control technology. The concept of parallel-stream mixing is a premium grade mixing solution, required when stringent requirements are to be achieved. For example, when very accurate CV control is required for an industrial process, or there are changes in the quality of the feedstock.

​

LPG vapor at regulated pressure is sent through a flow meter and next enters the mixing section of the BLENDAIRE II. Here it mixes with compressed air, which is injected at a rate controlled by the airflow control valve. The compressed air is regulated to a pressure above the LPG pressure; the mixed gas (SNG) pressure is the same pressure as the LPG pressure. The flow of LPG is automatically controlled by the upstream regulator at a rate required to meet the current SNG demand. This signal is sent to the PLC where it is converted into standard flowrate using the pressure and temperature information from the transmitters located near the flow meter. The PLC also calculates the necessary actual flow rate of air to achieve the required mixing ratio, again, based on the measured line pressure and temperature and assumed CV value of LPG. The control system issues commands to adjust the air flow control valve to a position to allow this flow rate necessary to pass through. Since the air flow control valve responds o changes in LPG flowrate before the two gases mix, it is a feed forward control system as opposed to conventional feedback systems. The latter style system only responds to mixing errors found by the downstream monitoring instrument, for example, a calorimeter. Any correction occurs minutes after the wrong mix has been generated. Such lag time in conventional mixers causes control oscillation and results in unstable outputs. The BLENDAIRE II makes great improvement on this due to its feed forward control design.

​

In the BLENDAIRE II system has a feedback loop also included as standard. It performs a final check to ensure the gas quality is as desired. Any discrepancy detected is sent back to the feed forward control loop for an overall compensation of ALL errors associated with the measuring instruments and the assumed CV of LPG. The problem of conventional mixing systems using feedback control alone is most obvious at start-up and under low flow conditions. None can deliver an accurate mix immediately upon start-up.

When an off-ratio signal is received by the calorimeter, the control system will try to make adjustments to eliminate this error. However, due to the time lag of the signal the correction will not be appropriate for the time being and hence may even amplify the error! It requires a long time for the system to stabilize close to the setpoint and oscillations may continue due to the phased out feedback signals. Under very low flow, regulators for air and/or LPG tend to keep opening and closing all the time, with intermittent spurts of gas injecting into the downstream. Such conditions cause an imbalance of pressures between the two and result in an unstable mix with the potential of generating a hazardous mixture.

In the BLENDAIRE II, the flow signal of LPG is sent to the PLC which then issues a command for the air flow control valve to act – before mixing takes place. The signals occur instantaneously. Upon receiving a signal, the valve is first positioned in accordance with the flow coefficient, which is defined as the relationship between the valve position and the flowrate based on the current differential pressure across the valve. The actual air flowrate measured by the flow meter is then compared with the required flowrate and necessary fine adjustments to the control valve position are made automatically.

The initial response of the valve is direct-acting and takes into account of the momentary unbalanced downstream pressure at startup. It ensures that the valve is set to the approximate position immediately upon start-up, without having to wait for any feedback signal or a long process of back and forth adjustments as is typically the case required in conventional blenders. The subsequent fine adjustment from this position is fast and accurate. The fast and accurate response of the BLENDAIRE II from startup facilitates the automation of the load tracking process In BLENDAIRE II systems parameters (including pressure and/or flowrate) can be set to shut off the mixed gas outlet valve and reopen it at a lower downstream pressure all automatically, eliminating the concern of turndown ratio. This option is called LINE-PAK™.