Control Valves

October 31, 2016Ken Bean

How do we keep the air flowing where we need it to go? Control valves!

In a centrifugal compressor the unit is controlled by several valves.

Inlet Valve

The inlet valve or Inlet Guide Vanes (IGV) controls the amount of air allowed into the 1st stage of compression. The valve can be as simple as a butterfly valve or more commonly an inlet guide vane which functions as the inlet valve. The advantage to utilizing the IGV is the incoming air can be pre-swirled to assist in getting the air moving in the correct orientation for the 1st stage impeller to pick up and compress the air.

Discharge Valve

The discharge air adjust how much air is allowed to leave the compressor and enter the plant piping system. Personally I prefer the term blow off valve which is simply a valve the blows the compressed air to atmosphere if it is not needed in the plant compressed air piping header. The most efficient compressors will utilize modulating blow off valves rather than an open/closed arrangement which allows for much finer control of the air that blows off to atmosphere. It is important to note that the most inefficient aspect of centrifugal compressors is blowing off air that you have just paid money to compressor!

Discharge Check Valve

The discharge check valve is used on the discharge of the compressor to prevent any opportunity for compressed air from the plant header system to backflow into the compressor while it is running unloaded or off. The backwards flow of air into a centrifugal compressor can spin the impellers in the opposite direction causing massive damage to the unit.

Isolation or Block Valve

A secondary valve on the discharge air line to again prevent any backwards flow of air into the compressor.

All of these valves are controlled by the compressor control system.

Lubrication System

October 18, 2016Ken Bean

The previous discussions have focused on a variety of centrifugal components including bearings and gearing. While a centrifugal compressors meets all of the class requirements for oil free air (just as other types of oil free compressors) there is obviously still a requirement for the bearings and gearing to be lubricated. The centrifugal compressor maintains it’s oil free status by utilizing seals that keep this oil out of the air stream.

The lubrication system is critical to the longevity of the compressor! Most centrifugal compressors utilize 2 oil pumps on each compressors. The primary run oil pump and the auxiliary oil pump.

Prior to compressor startup, the auxiliary oil pump which is electrically driven is started to lubricate the bearings and gears. The control system monitors the oil pressure and temperature to assure proper levels of each before allowing permission for the unit to be started.

Once the compressor is started the auxiliary oil pump continues to run as the compressor ramps up to full speed. As the unit begins turning the primary shaft driven oil pump also begins to add oil pressure to the system. The control system is looking for the additional oil pressure and once this pressure is achieved by both oil pumps running at the same time the control system shuts down the auxiliary oil pump and allows the primary shaft driven oil pump to carry all of the lubrication requirements.

In the event of a shutdown situation, such as a high vibration alarm, high temperature alarm, low oil pressure alarm or any other trip alarm condition the auxiliary oil pump immediately turns back on to assure the unit maintains proper lubrication.

The auxiliary oil pump will also be turned back on when the compressor is put through the shut down sequence. As the compressor is coasting to a stop, the shaft driven oil pump is not sufficient to provide all of the lubrication so the auxiliary oil pump is used to maintain this lubrication. Also, once the compressor has stopped it is standard procedure to continue to run the auxiliary oil pump for a period of time to dissipate the heat and cool the gears and bearings.

The lubrication system should also include the following components:

Oil Cooler:

To dissipate the heat from the oil after the oil flow has left the bearing and gear spray area. The oil is not only used to lubricate the components but also to carry the heat away. Oil leaving bearings and gear spray areas can be in the 150 degree F range and depending on the manufacturers oil weight requirements, the oil must be cooled to approximately 120 degrees F. Normally the oil cooler will be a tube in shell design where cooling water flows through the tubes.

Oil Filter:

To filter any metal contamination from the oil. This is the most critical component (IMO) of the lubrication system and I suggest always using an OEM element for the oil filter. These filters can come in either a spin on design or a cartridge in housing design. It is frequently recommend to utilize a dual oil filter design such that a filter element can be changed while the compressor remains on line.

Oil Mist Collection System:

In all centrifugal compressors, a positive pressure oil mist is created within the gearbox by the meshing gears. The positive pressure will result in oil leaks through the oil seals if not addressed. Typically the appropriate action is a simple vacuum venturi which pulls a vacuum on the oil reservoir to collect the oil mist. This oil laden air is then passed through a mist eliminator filter to remove the oil mist prior to the air venting to atmosphere. Most manufacturers also offer an optional electric motor driven vacuum system to eliminate the use of compressed air for this process.

Oil Pressure Regulating Valve:

A simple pressure regulator to assure the proper oil pressure for bearing and gear lubrication.

Temperature Control Valve:

As previously stated, the returning oil (from bearings and gears) is heated and must be cooled via the oil cooler. To assure oil flowing to bearing and gear spray areas the cool oil is mixed with hot oil to assure the proper temperature.

Routine Operation:

During routine operation, normally a check valve will be used to prevent oil from being pumped back into the reservoir through the auxiliary oil pump. The pressure regulator is used to maintain proper oil pressure to bearings and gearing and returns any excess oil to the reservoir.

The flow can vary between manufacturers but a possible route is oil being returned from the gears and bearings is directed to the oil cooler and then passes through the oil filter. The cool, filtered oil is then directed back to the oil reservoir where it is then picked up by the oil pump and directed to the gear sprays, pinion and bull gear bearings. On some models this same oil is used to lubricate the drive motor bearings.

Bull Gear: The Driving Force

October 4, 2016October 4, 2016Ken Bean

Our focus has been on the integral geared centrifugal compressor. This arrangement calls for several pinions shafts (previously discussed) to mounted around a central drive gear, commonly called the Bull Gear.

The exploded view of a bull gear (left) shows the helical gearing that will mate with the pinion gear located on the pinion shaft.

The advantage of the integral geared centrifugal is where the bull gear drives multiple pinion shafts, each shaft can be speed altered with simple gear changes.

This arrangement is unlike a barrel type centrifugal compressor where all the stages are driven at the same speed. This allows each pinion to run at different speeds thus allowing every impeller pair (or single) to operate at it’s optimum aerodynamic speed. This results in higher efficiencies along with the ability to package the unit in a smaller space.

The bull gear is typically driven by an electric motor at 3600 RPM although various manufacturers may use motors with a rotational speed of 1800 RPM depending on the compressor application. It should also be noted that in many cases the bull gear can be driven by a steam turbine. This is a particularly advantageous situation when a large facility has excess steam or the unit can be used as a steam pressure reduction valve (PRV). In this instance the operational cost of the compressor can be lowered or completely eliminated.