Liquefied gases are transported by tankers and ships and are stored in specially designed tanks. However, when the temperature of liquefied gasesammonia, ethylene, propylene and liquefied natural gas (LNG), etc.is considerably low, the vaporization occurs very quickly due to ambient temperatures. As the liquid vaporizes, the pressure inside the tank, likewise, increases. The rising pressure must not exceed the design pressure of the tanker/storage tank. To control pressure, the re-liquefaction process was adopted, in which reciprocating compressors are an indispensable part of the whole system.
When the temperature of the liquid gas and its vapor is quite low (160°C for LNG and 100°C for liquefied ethylene and propylene gases), the compressor to handle low-temperature gas has some special features. Accordingly, when selecting reciprocating compressors for low-temperature services, several criteria must be considered:
- Effects of gas temperature on compressor size
- Construction materials of components in contact with the cold gas
- Cylinder cooling systems.
The size of a compressor depends mainly on two factors:
- Gas flow and volume at the inlet to the compressor
- Power required to compress the gas to desired pressures.
For a given suction pressure, the suction gas capacity also depends on the temperature of the gas at the cylinder inlet manifold. In low-temperature applications, the actual temperature of the gas entering the cylinder during the suction stroke is always higher than the gas in the storage tank. Therefore, while calculating the required compressor BKW (brake power in kilowatts) and size of the cylinders, determining the cylinder inlet gas temperature is very important. Cylinder inlet gas temperature is affected by:
- Distance between the storage tank and compressor
- Ambient temperature
- Insulation of piping and vessels.
Ulta-low-temperature (ULT) gas systems will experience greater delta temperature rises. In other words, the temperature rise will be more in the case of LNG, at 162°C, as compared to the gas service involving liquefied ethylene at 100°C. In addition, design engineers should address these system conditions to manage increases in the suction-gas temperature:
In low-temperature applications, the intercooling of the gas does not occur. Therefore, the second-stage inlet gas temperature depends on the first-stage gas discharge temperature. If the overall compression ratio (CR) of the compressor is low, then the CR of the first stage will also be low. Consequently, the discharge gas temperature will be considerably lower. But, in such a case, the second-stage inlet gas temperature will be considerably higher than the first-stage discharge gas temperature.
To understand this processing condition, consider the case of ethylene boiloff (BO) service, in which the suction pressure is 1.02 kg/cm2 A and the suction temperature is 102°C. If the discharge pressure of the compressor is 8 kg/cm2 A, then the first-stage CR is around 3, and the discharge temperature will be approximately 40°C, which is a considerably low temperature. Therefore, at the second-stage cylinder, the inlet gas temperature will be considerably higher than 40°C. However, in case when the compressor discharge pressure is higher (approaching 20 kg/cm2 A), then the CR for the first stage is also higher (approximately 5) and the resulting first-stage discharge temperature is also higher. Result: The gas heating effect is not as much for the second-stage inlet gas. Therefore, with high CR conditions, the average temperature of the gas inside the cylinder will be higher, and the volumetric efficiency of the cylinder will be lower.
For ULT services, e.g., LNG (160°C), the effect of frictional heat, developed inside the cylinder, is more pronounced for the first-stage cylinder, as this cylinder is cooled by the gas itself. The heating of suction gas is related to higher operating pressures, where more frictional heat is generated.
At partial loading, the flow of cold gas is reduced while the frictional heat is almost the same. Therefore, heating of the gas is greater during partial load operations. Moreover, in the case of capacity control by suction-valve unloading, the gas temperature in the suction pulsation dampener increases due to the pressure drop in the unloaded valves during suction and discharge strokes. The net result is higher gas-suction temperatures at the loaded end of the cylinder.
With a higher clearance provided inside the cylinder, more hot gas will be present inside the cylinder. Again, the gas temperature will increase further during the suction stroke.
With a larger cylinder bore, less heating of the gas will occur. The capacity of the gas handled by a cylinder increases as the bore size increases. However, the surface area of the cylinder exposed to the atmosphere (heat source) does not increase in the same proportion. For the same reason, if the bore of the cylinder is smaller, then the temperature of the gas will increase due to absorption of more heat by a lesser amount of gas.
Number of compressors
When gas piping is designed for two or more compressors running in parallel, and not all of the compressors are operating, then the gas temperature at the inlet side will be higher. Cooling occurs as the gas flows through the piping. Consequently, heating of the gas is related to ambient temperatures and the quantity of gas flowing through the system.
The overall effects from the listed process and equipment factors are that the gas temperature at the compressor inlet will increase, and, in turn, gas flow will decrease. Therefore, unless we know the actual temperature of the gas entering the cylinder, then sizing the compressor and its driver is very difficult.
Considering all of the previously mentioned factors, here are several key points that are highly suggested to consider when sizing the compressor cylinders:
- When the discharge temperature of first stage is around 35°C and greater, consider the effect of gas heating only for the first-stage inlet gas; for the second stage inlet gas, it can be ignored.
- When the first-stage discharge gas temperature is below 35°C, consider using higher suction temperatures for both stages.
SELECTION OF MATERIALS
The selection of material for various components in contact with low-temperature gas is done considering the characteristics of the material at low temperatures and type of loading of the components. Based on past experience, these suggestions may be considered:
- For ULT service, e.g., LNG, the material of the first-stage cylinder should be ASTM A351 CF8 or an equivalent.
- In ULT service, since the cylinders distance pieces are covered with ice, the construction materials must be from cast iron ASTM A571 D2M or an equivalent.
- When suction-valve unloaders are to be provided and the unloader length (the distance between the valve and the actuator) cannot be increased sufficiently, then the valve unloader body should also be constructed from ASTM 571 D2M cast iron.
- Although the piston-rod working temperature is considerably higher than the gas temperature, construction material of the first-stage piston rod should be A522 (9% Ni) stainless steel.
- Low-temperature service is more severe than the service involving moisture-free gases. Therefore, special care should be taken while selecting the materials for the piston rings and rider rings. For such services, special materials are available to provide satisfactory performance.
CYLINDER COOLING SYSTEM
While designing the cooling system of the cylinder jackets, ensure that the selected coolant freezing temperature should be sufficiently lower than the average temperature of the gas inside the cylinder. The cooling of cylinders when handling low-temperature gases is done normally by isobutyl alcohol (freezing temperature 120°C). Likewise, here are several other considerations for the cylinder cooling system:
- In the case of LNG service, the discharge gas temperature for the first stage at 100% load is always below 0°C. The whole cylinder will remain covered in ice. Therefore, cooling by an external source of the first-stage cylinder is not carried out. Similarly, cooling of the cylinder packing is also not done as it is cooled by the cylinder itself.
- Operating at partial loading over a long period, even with the assistance of a bypass valve is not desirable. However, if it is necessary to run the machine at partial loading, some arrangements can be made for cooling the first-stage cylinder.
- In LNG service, at 100% load, the average gas temperature inside the first-stage cylinder is normally lower than the coolant freezing temperature (120°C). If the first-stage cylinder is also equipped with cooling by an external source, then a fail-safe arrangement should be provided so that the coolant is drained out before freezing occurs inside the cylinder jacket.
- When intercooling is done in a flash vessel and the compressor is shutdown, the gas temperature at the second-stage inlet will also be quite low. Therefore, design engineers should consider the temperature inside the second-stage cylinder when designing the cylinder cooling system.
Design and process engineers should take extra precautions when selecting and sizing reciprocating compressors in low-temperature gas service. To incorporate all of the required features within the design of such machines, customers should also specify all the necessary information and data including ambient condition, plant layout, type of service (intermittent or continuous,) capacity control requirements and number of working compressors. All of this information will impact the final design and selection of compressor along with auxiliaries to achieve a long, uninterrupted service. HP
Syed Mubarak is the regional manager of Bharat Pumps and Compressors Ltd., New Delhi, India. He joined Bharat Pumps and Compressors Ltd., Naini, Allahabad, India, a government-owned enterprise, engaged in the manufacturing of rotating equipment--compressors and pumps along with welded/seamless gas cylinders. He has over 33 years of experiences and has held various positions of responsibility with Bharat Pumps and Compressors Ltd. He was a compressor application engineer for more than 25 years in the design and engineering department. Before his appointment as regional manager in 2007, he was the deputy general manager of the design and engineering department at the Allahabad office. Mr. Mubarak holds a BS degree in mechanical engineering from the Z. H. College of Engineering and Technology at A. M. University in Aligarh, India.