Operating dry screw compressors

Screw compressors consist of two rotors that are contained in a common casing. Both rotors carry inter meshing helical lobes that rotate against each other with tight clearances between the rotors, and between the rotors and casing. During rotation the lobes and the casing form volumes of varying size. For dry screw compressors, synchronizing gears are used to avoid contact between the rotors. For process gas screw compressors the rotors are supported by hydrodynamic journal and thrust bearings. The rotor shafts may be sealed with a variety of shaft seal types, such as restrictive ring bushings with or without injection of seal gas or liquid, oil cooled mechanical seals, or dry gas seals.

The article contains excerpts from the paper, "Dry screw compressor performance and application range" by Jurgen Winnemar of MAN Turbo at the 38th Turbomachinery Symposium.

A large unloading valve is used for starting and stopping the screw compressor by reducing the pressure difference and lowering torque. Even for speed variable machines a bypass control line is needed if zero flow is required. In most cases a gas cooler between the compressor stages is necessary in order to reduce the suction temperature for the second stage. An additional gas cooler downstream the second stage may be required for process reasons, but is not necessary for the screw compressor itself. If individual bypass control lines are used around both stages, a control of the interstage pressure is possible. This may be necessary for compressors with large variations in suction and/or discharge pressure in order to counter over- or undercompression effects.

The cyclic nature of the working principle of the screw compressor causes gas pulsations that emit noise from the piping. This is especially important for dry screw compressors with high discharge pressures and/or with varying operating conditions.

Therefore it is very important to apply properly designed suction and discharge silencers on dry screw compressor sets. The discharge silencer is especially important because most of the noise is generated by the gas pulsations in the discharge. An unsuitable discharge silencer design may even lead to mechanical damage of the silencer or piping internals. The frequency of the working process is called pocket passing frequency and is calculated by male rotor frequency multiplied by the number of lobes on the male rotor. The gas pulsations occur mainly at pocket passing frequency and its multiples (typically 4, 8, and 12 times male rotor frequency for a 4/6 rotor profile).

Depending on compressor size and application the male rotor speed may range from approximately 1500 rpm up to 25,000 rpm, The corresponding pocket passing frequency range is 6000 cpm (100 Hz) to 100,000 cpm (1667 Hz). Compared to reciprocating compressors these frequencies are much higher and the relevant sound wavelengths much shorter.

The gas pulsations are excited by the difference between internal compression and the pressure in the discharge line. The pulsation excitation increases with gas density and with compressor speed. The gas pulsations may cause mechanical vibrations of the compressor, silencer, and piping. The expertise of the compressor manufacturer should be sought for the silencer design. The silencer must be individually designed for each compressor. A three-part silencer system consists of: a venturi nozzle, a damping plate, and a downstream silencer. The venturi nozzle is a massive casting flanged directly to the compressor discharge. The venturi nozzle reduces the gas pulsations by first accelerating and then decelerating the gas velocity.

Downstream the venturi nozzle a damping plate is placed. This is a massive metal plate with a number of drilled holes through which the gas passes. The free area of the holes is determined by the gas molecular weight, gas mass flow, and the process conditions. The damping plate reduces the pulsations further and isolates the downstream silencer and piping from the gas pulsations emitted from the compressor discharge. Downstream the damping plate a silencer is placed. The silencer may be working by absorption and/or resonance. The typical pressure drop at design conditions for such a silencer system is 2 percent of the discharge pressure. In most cases it will be necessary to add a noise enclosure around the compressor and gear as well.

Starting of dry screw compressors

Screw compressors always start unloaded, i.e., with discharge pressure = suction pressure. The discharge system is separated from the compressor by a check valve. The gas is recycled around the compressor through a large unloading valve with a pressure drop of approximately 7 psi (0.5 bar). After the compressor has reached its minimum speed the unloading valve is closed and the pressure in the discharge line up to the check valve rises. The run-up time with open unloading valve should not exceed 30 seconds. When the pressure in the compressor discharge line exceeds the pressure in the downstream system the check valve opens and the compressor begins to deliver into the downstream system. Prolonged operation at discharge pressure very close to suction pressure must be avoided due to the gas pulsations in the discharge line. Caution: During closing of the unloading valve the compressor rapidly sucks gas out of the suction line. If the volume in the suction line is too small, or if insufficient gas can be supplied by the upstream system, the suction pressure drops rapidly and the compressor will be tripped. Therefore a large volume of the suction line is helpful to ensure smooth starting of the compressor.

Stopping

Screw compressors must be unloaded during shutdown. Upon stop signal of the driver, the unloading valve immediately opens (opening time should be less than 2 seconds) and the gas is recycled around the compressor through the unloading valve. The pressure in the discharge line between compressor and check valve drops rapidly and the check valve closes thus separating the compressor from the downstream system. As the gas is recycled around the compressor the suction pressure may rise. During rundown of the compressor to standstill, the suction and discharge pressures equalize at the settle-out pressure. The run down time depends on machine size and operating conditions. Typical rundown times may range from 5 seconds for small compressors in high pressure applications to 1 minute for large compressors in low pressure applications. Caution: If the settle-out pressure is much higher than the normal suction pressure this may lead to high torques for machines with large built-in volume ratio.

Also the correct operation of the seals may be endangered because the seal supply pressure may be insufficient or the control system may be to slow for the quick rise of suction pressure. Therefore a high settle-out pressure must be avoided by using the following rules: • Volumes at high pressure (interstage volume and volume between the discharge nozzle and check valve) should be minimized. • The volume of the suction line should be as large as possible thus giving the recycled gas space to escape. • Check valves in the suction line close to the compressor must be avoided. If a block valve is placed in the suction line it should be closed only after the compressor has come to standstill.