When the competent electrical personnel restore the electrical power supply, the first step to start the station is starting the ‘instrument air system’.
The Instrument Air compressors are started.
Only after establishing the availability of air system, the plant can be started. The inlet ESD’s, isolation ESD’s and the various control valves will have to be fully operable to bring a stations on line. Therefore it is essential to make sure that air compressor is running with proper back up and the instrument air header pressure is maintained at the specified value. (6.9 Bar).
We know that most of the final control elements operate using pneumatic force, usually termed as “instrument air”. Devices that can move its designed parts using air pressure are called actuators.
Actuators operate emergency shut down valves, fluid flow control valves, and similar equipment. The design of equipment will vary according to its application.
The highest pressure to operate an actuator here in Mukhaizna facilities is not more than 690 kPa or 6.9 Bar. The locations of actuators are scattered around the operating area of station. In order to bring the air pressure to the actuator, proper piping is done to the actuator from a main air “header” or pipeline. The pressure is regulated to the required value of the actuator using a regulator for each actuator or device.
For constant regulation of pressure, the pipeline ” header” must be supplied with a pressure higher than the required value. The highest value being
690 kPa, the header is pressurized to a higher value than 6.9 Bar.
There for the header is connected to a continuous supply source of higher pressure.
|CAUTION: Instrument air “low low” pressure shuts down the plant.|
Instrument air Compressor Skids
A skid containing two air compressors is the source of instrument air supply in most of the stations in Mukhaizna.
Air compressor skids in stations includes a main compressor and a standby compressor at least. The compressor draws atmospheric air and compresses it to a sufficiently higher pressure than the highest pressure required in that station. Most of our stations maintain an air pressure of 8 Bar.
The compressor discharge is sent to a receiver, generally called ” air receiver”, (a vessel) with considerable capacity to store the pressurized air.
This vessel absorbs the pressure fluctuations in the compressor discharge when the compressor gets loaded and unloaded. Then the air from receiver is sent to a drier where moisture in the air is removed and clean dried air is supplied to the header.
The pressure in the header is maintained some where near 8 Bar and this is done using a pressure dependant loading and unloading control of the compressor. A pressure sensing device at the inlet of air receiver controls the load / unload logic
” Loading ” means allowing the compressor to compress the air and pressurize the vessel. ” unloading ” is like bypassing the compressor. The compressor is not stopped, but runs without compressing air, namely “without load”.
In order to perform capacity control (in the case of more than one compressor) different set points are given to each compressor for loading and unloading.
The discharge pressure is continuously measured and when it falls below a set point, the compressor inlet will be opened allowing air to enter compressor and the compressor is “loaded”.
The pressure in receiver increases, and the pressure in header too increases. When the pressure in the header crosses the high set point, the in let to compressor is closed, and compressor stops pressurizing the vessel.
The air pressure in receiver is drawn by the user, pressure drops and when it falls below the low set point, compressor gets loaded.
This cycle continues to maintain header pressure. The air from receiver is sent to the drier system to take all moisture content in the air. The drier is using a drying agent for drying. This drying media (silica gel/alumina) has to be regenerated – remove all moisture it has absorbed. Therefore two dryers are used.
When one is drying, the other will be regenerating and will change over automatically at a timer setting which we can set according to our station requirement.
AIR / OIL SYSTEM
Air is drawn in to the compressor element through filter (AF) and un loader assembly (UA). The air is then compressed in the inter-lobe spaces of the rotors, where oil is injected via the oil stop valve (VS) to absorb the compression heat of the air and to lubricate and seal the rotors. At the discharge port the air/oil mixture flows through check valve (CV) into the air/oil separator (AR) Here most of the oil is centrifuged out of the air and drips down in to the bottom of the receiver. The air then flows through the oil separator element (OS) which retains the remaining oil. The oil collecting in the bottom of separator (OS) returns to the compressor via scavenge line.
Oil present in the bottom of the separator (AR) is forced into the pipe-work system owing to the minimum 4 Bar pressurization. The oil flows into the bottom of the cooler (CO) and passes through to the top during which, the temperature is lowered owing to the air flow absorbing the heat as it passes through the fins. The oil is then directed through a filter (OF), before finally reaching the oil stop valve. In the unloaded position the pressure of the oil pushing against the piston inside the valve causes the valve to make contact with the seat allowing no oil to pass through to the compressor. When the compressor is loaded, a small port behind the piston is pressurized from air that is tapped off from the discharge side of the compressor prior to the check valve (CV), causing the piston to lift the valve off the seat allowing oil to enter the compressor.
AIR SYSTEM (UNLOADED)
Air enters the top of the compressor via the air filter (AF), a vacuum indicator (V1) is present in the intake housing to show when the filter requires changing. Because the compressor is in an unloaded condition air cannot pass into the compressor owing to the unloading valve (UV) piston being in the closed position. To keep the compressor rotors lubricated a small air supply is delivered to the compressor via the vent valve (VV) by-passing the un loader valve and air intake.
Air once again enters the top of the compressor via the air filter and passes through the opened un loader valve (UV) into the compressor casing. As the air passes along the rotors it is compressed increasing the pressure, until it is discharged through the exhaust port. Check valve (CV) is opened, owing to the pressure overcoming the spring tension and the air/oil mixture passes into the receiver. Inside the receiver the air is directed into an oil separator element (OS), as it’s name implies the oil is separated from the air and drops to the bottom of the receiver allowing the hot dry air to exit the top of the receiver via the minimum pressure valve (VP). The air then passes through a cooler reducing the temperature and past a moisture trap (MT) which removes any excess moisture in the air before leaving the system
LOADING / UNLOADING
Compressor runs with air inlet valve open so that air is allowed into compressor and is compressed. Pressure increases and air is discharged to header.
Compressor runs idle, with air inlet valve closed. There is no air to be compressed and pressure is not increasing further.
The system controls the air delivery in relation to the air consumption and maintains the net pressure within the selected upper and lower limits of the working pressure (unloading and loading pressure settings).
The pressure sensing at inlet pipe of receiver controls the loading / unloading cycle.
When the net pressure reaches the unloading pressure, the contact of air pressure switch opens, actuating the solenoid valve. This signals the air inlet valve to close. The air delivery is stopped, the compressor runs unloaded, waiting for demand.
When the net pressure decreases to the loading pressure, the contacts on air pressure switch closes, so that the solenoid valve is actuated. Then the air inlet is opened to allow air to compressor. Compressor is now loaded.