KOBELCO SCREW COMPRESSORS IN PROCESS GAS APPLICATIONS
Preface
There are two types of screw compressors;
Oil-free and Oil-injected.
KOBE STEEL is producing both types of screw compressors in process gas applications as well as standard package type air compressors and refrigeration compressors. KOBELCO Oil-free and Oil-injected Screw Compressors have wide ranges in pressure and capacity for process gas applications in various fields such as oil refineries, petrochemical plants, steel making industries, general chemical industries, energy/power plants and oil & gas fields.
In this HOME PAGE, KOBELCO Oil-free and Oil-injected Screw Compressors in process gas applications are introduced.


Applications of Oil-free and Oil-injected Screw Compressors
KOBELCO Screw Compressors lie somewhere between reciprocating and centrifugal compressors, but positive displacement compression is obtained by smooth operation of twin rotating rotors. It is commonly recognized that unspared continuous operation for 3 years is expected.
With accumulated experience with more than 1500 sets of oil-free screw compressors since 1955 and 330 sets of oil-injected screw compressors since 1974 in process gas applications, KOBE STEEL has proven that screw compressors are suitable for processes which have the following problematic features :
(1) Dirty Gases
(2) Polymer Forming Gases
(3) Liquid Entrainment Containing Gases
(4) Corrosive Gases
(5) Process Fluctuation in Molecular Weight, Pressures and Flow Rate
(6) Low Molecular Weight Gases


Oil-free Screw Compressors :
Centrifugal compressors are affected by foreign matter sticking to impellers. The foreign matter on the impellers causes excessively high vibration due to unbalance and performance deterioration due to contracted gas passage area. Reciprocating compressors used with problematic gases would require excessively frequent overhaul, and eventually a spare machine would need to be installed to perform continuous plant operation. On the contrary, oil-free screw compressors can tolerate foreign matter in the casing or on the rotors. Screw compressor rotors have rigid shaft feature and operate at lower speeds than the first lateral critical speed, and are insensitive to vibration due to unbalance. Foreign matter sticking to the rotors after operation even increases efficiency owing to decreased inter-lobe leakage.


Oil-injected Screw Compressors :
Another example is taken from applications where suction pressure fluctuates. Centrifugal compressors are sized to obtain the rated gas flow whilst normal operating point has a different gas analysis and pressures. Performance of oil-injected screw compressors is hardly affected by gas analysis, and even power saving is brought by the slide valve in turndown operation. One of remarkable applications for oil-injected screw compressors is fuel gas boosting for gas turbines. Especially, when the inlet and/or discharge gas pressures fluctuate, a centrifugal compressor designed for the largest capacity and highest head results in inefficient operation, when operated at normal conditions with less flow rate and smaller head. On the other hand, an oil-injected screw compressor equipped with an slide valve brings energy saving when the pressures fluctuates. Figure 1 shows this economical operation of oil-injected screw compressors by stepless control of the slide valve.

Thus, for difficult requirements which could not be tolerated by other types of compressors, oil-free and/or oil-injected screw compressors bring benefit to customers.

Figure 1
Economical Operation of
KOBELCO Oil-Injected Screw Compressors
The suitable usage of screw compressors is shown in Table 1 and the major applications are in Table 2.
For customers’ convenience, the major differences between the oil-free and oil-injected types are summarized in Table 3.
Design and Construction of KOBELCO Screw Compressors
KOBELCO Oil-free and Oil-injected Screw Compressors handle an inlet volume flow up to as large as 70,000m3/h (41,200 acfm) and a required discharge pressure up to 60 barg (870 psig).
The typical constructions of KOBELCO Oil-free and Oil-injected Screw Compressors in process gas applications are shown in Figure 2.
OIL-FREE SCREW OIL-INJECTED SCREW


Figure 2Typical Construction of KOBELCO Screw Compressors




Figure 3Slide Valve Control System


KOBELCO Oil-free and Oil-injected Screw Compressors have the following remarkable design features :

1. Reliability
API619 Construction
KOBELCO Oil-free Screw Compressors are designed to satisfy the latest API619 (Rotary Type Positive Displacement Compressors for Petroleum, Chemical, and Gas Industry Services) for servicing in heavy and continuous duties. The intended duration of uninterrupted operatuion is 3 years.
KOBE STEEL has oil-injected screw compressors designed to API619 as well as standard type used for refrigiration systems.

Consideration Against Thermal Problem - Oil-free Screw Compressors
One of the critical issues for oil-free screw compressors is excessive thermal expansion of the casings and rotors. The following measures are employed to provide mechanical reliability at high discharge temperatures :
1) Casing Cooling Jacket - Prevents Thermal Distortion
2) Internal Rotor Cooling - Maintains Inter-lobe Clearances at High Temperatures

Attainable Oil Carryover - Oil-injected Screw Compressor
An oil mist carryover of 5 ppm by weight is standard with coalescer type oil separators and 0.1 ppm is attainable by using additional oil separators.

Selection of Oil - Oil-injected Screw Compressor
Selection of oil is a major concern for oil-injected screw compressors. KOBE STEEL can select most compatible oil with the process gas from the accumulated experience.

2. Options Prepared for Customers
We are preparing many options to comply with customers ‘ requirements :

Material Options
Casings :
Oil-free Screw
Oil-injected
Gray Cast Iron
Cast Carbon Steel
Cast Carbon Steel
+ Stainless Steel Weld-Overlay
Cast Stainless Steel
(304 SS, 316SS, 13Cr-4Ni SS, etc.)
 Gray Cast Iron
Cast Carbon Steel
Rotors :
Oil-free Screw
Oil-injected
Forged Carbon Steel
Forged Stainless Steel
(13Cr SS, 13Cr-4Ni SS, etc.)		 Forged Carbon Steel
Ductile Iron

Shaft Seal Options
Oil-free Screw Compressors
The shaft seal system for KOBELCO Oil-free Screw Compressors is selected in consideration of the following factors :
1) Availability of Seal/Buffer Gas in Utility System
2) Seal/Buffer Gas Consumption
3) Process Gas Contamination by Invasion of Seal/Buffer Gas Injected to Seal Sections
4) Lube Oil Contamination in Contact With Process Gas
5) Optimum Cost of Equipment Installation and Operation
(Seal/Buffer Gas Consumption, Leaking Oil Amount, Overhaul Intervals etc.)

The comparison summary of the shaft seal systems is presented in Table 4.

Oil-injected Screw Compressors
The shaft seal system for KOBELCO Oil-injected Screw Compressors is mechanical contact seals. Dual seal designs with independent seal fluid system, as specified in API619, are available.

The mechanical seals used in KOBELCO Oil-injected Screw Compressors allow high differential pressure by adopting specially designed faces and high grade materials. This brings high allowable suction pressures which could not be achieved by other compressor manufacturers.

3.What is the Difference in KOBELCO Screw Compressors ?
Advanced Rotor Profile for Larger Capacity and Higher Pressures
Screw compressors were introduced into market with symmetrical rotor profile developed by Svenska Rotor Maskiner (SRM), a Swedish company, in the 1940’s. In the 1970’s, asymmetric rotor profile developed by SRM spread as an improved profile bringing higher efficiency. KOBE STEEL was one of the first screw compressor manufacturers to change the rotor profile from symmetric to asymmetric in the 1970’s. In the 1980’s, KOBE STEEL developed its own rotor profiles by using the Screw Compressor Rotor Profile Developing Computer Program. This technology was introduced at the SRM Conference in 1988. Various types of asymmetric rotor profiles have been developed to accommodate the applications of KOBELCO Oil-free and Oil-injected Screw Compressors. The rotor profile affects performance and strength (rigidity) of screw rotors and bearing loading. Generally speaking, screw compressor rotors having more lobe number have larger shaft diameter and higher rigidity. On the other hand, the displacement volume with a fewer lobe number rotors is larger.

Oil-free Screw Compressors :
To satisfy demand for reliable machines at reasonable cost, KOBE STEEL developed the oil-free screw compressors EX-Series (KS40EX, KS50EX, KS63EX) which utilize their own asymmetric rotor profile with “3(Male) + 5(Female)” rotor lobes. This provides a larger displacement volume than conventional “4(Male) + 6(Female)” rotors for applications where high discharge pressure is not required. (Figure 4) Thus, machine size can be smaller than conventional types. A prototype machine, Model KS50EX, was produced and ran in the shop for more than 100 hours. Various data were collected before the compressor was put into the market. It was marvellous that the compressor was smoothly operated at a rotor tip speed of as high as 183 m/s (600 fps) which had been believed unattainable formerly. The EX-Series compressors are designed for a capacity ranging from 15,000 m3/h (8,830 acfm) to 70,000 m3/h (41,200 acfm) (depending on gas properties) and up to a discharge pressure of 4.0 barg (58 psig).
Major applications of the EX-Series are, but not limited to, Styrene Monomer Offgas/Vent Gas, Lime Kiln Gas for Soda Ash Plants, Recycle Gas for LAB and Lower Stage of Butadiene Gas Recycling. (Picture 1)
KS63EXA
Picture 1Styrene Monomer Offgas Compressor KS63EXA Steam Turbine Driven, Water Injected
Conventional "4 (Male) + 6 (Female)" Asymmetric Rotor Configuration Advanced "3 (Male) + 5 (Female)" Asymmetric Rotor Profile of "EX" Series


Figure 4Screw Roter Profiles

Oil-injected Screw Compressors :
Recent demand for oil-injected screw compressors is higher pressure. KOBE STEEL has enabled to increase the discharge pressure of its oil-injected screw compressors EH Series up to 60 barg (870 psig). This has been brought by employing “5(Male)+7(Female)” rotor profile. The capacity is expanded to 20,000m3/h (11,770 acfm). A prototype compressor KS31SEH(single stage) and KS3122SEH (two stage, tandem arrangement) ran in a closed loop shop piping system. The test showed completely satisfactory results including disassembling inspection after the tests. Another remarkable feature is its high allowable suction pressure of 55 barg (797 psig) which could not be obtained by other manufacturers. This feature brings good fit to a typical application of Gas Turbine Fuel Gas Boosting where the suction pressure fluctuates widely. (Picture 2)
KS3122SEH
Picture 240MW / 53,620HP Gas Turbine Fuel Gas Booster
KS3122SEH (Tandem Model)
Suction pressure : 3.90barG / 56.6psiG
Discharge pressure : 48.1barG / 698psiG
Capacity : 12,840Nm3/h / 7,989SCFM
Driver : 1,700kW / 2,279HP

Combined “Oil-free + Oil-injected” Arrangement
- For Large Capacity and High Pressure :
When a large capacity and high discharge pressure are required at a time, KOBE STEEL can offer a large oil-free screw compressor as the lower stage boosting up to a certain pressure , being followed by an oil-injected screw compressor reaching the final discharge pressure. With this arrangement of KOBELCO “Oil-free + Oil-injected” Screw Compressor, an inlet volume of 70,000 m3/h (41,200 acfm) and a discharge pressure of 60 barg (870 psig) are achievable. KOBE SEEL has actual installations of the “ Oil-free + Oil-injected” arrangement. (Picture 3)
KS63LAZ-50LX
Picture 3KS63LAZ-50LX Gas Turbine Fuel Gas Booster
Suction pressure : 0barG / 0psiG
Capacity : 40,000Nm3/h / 24,890SCFM
Discharge pressure : 16.2barG / 235psiG
Gas : Ladle Gas
Driver : 6,800kW / 9115HP