As the demand for LNG increases, liquefaction process engineers are continually striving to increase process efficiency. Since 1996, LNG liquefaction facilities have utilized LNG liquid expanders to decrease the amount of boil-off gas losses associated with the expansion stage of the liquefaction process, increasing the overall production volume by as much as 5%. Current LNG expanders are only able to reduce the pressure of the LNG to just above the saturation point and are not mechanically able to withstand vapor generation within the machine. Due to this liquid only limitation, the final stage of pressure reduction in a typical liquefaction plant is preformed across a two-phase Joule-Thomson (J-T) valve. To increase plant efficiencies even further, two-phase LNG expanders are being developed which are capable of expanding LNG into the two-phase region, eliminating the need for a down-stream J-T valve, therefore reducing boil-off gas loses even further.
To aid in the development process as well as to verify the final design and manufacturing quality of two-phase expanders, mechanical string tests must be performed at full load, pressure, speed and vapor content. Although testing facilities, such as the one at Ebara International Corporation’s Cryodynamics Division in Sparks, Nevada, are able to perform full string tests on LNG liquid expanders, they are not able to handle the volume of vapor created during two-phase expansion due to limitations of storage and flare capacity. To solve this problem, Ebara International has developed a method to re-condense LNG vapor within the test facility, allowing two-phase expanders to be tested at full load and full vapor generation.
Existing Expander Test Facility
The existing LNG liquid expander test facility at Ebara International is a closed loop system, designed to simulate real world LNG liquefaction plant operation. This state of the art facility allows for the testing of expanders in LNG, while measuring all aspects of the expander performance. In addition to the closed loop system, on-site LNG storage of 100 m3, 240 m3 of liquid Nitrogen and a 3.5 MW variable frequency electrical supply allow testing of expanders with outputs as large as 3 MW and flow rates as high as 3000 m3/hr. Figure (1) shows a basic flow diagram of this single-phase expander test facility.
The first step in the closed loop test process is to increase the pressure of sub-cooled LNG from atmospheric to the required inlet pressure of the expander. This is achieved by using two submerged LNG pumps (1) operating in parallel powered by a variable frequency drive. Because these pumps are completely submerged, the heat generated by the electric motors also warms the LNG, increasing its temperature to the required inlet temperature of the expander. This high pressure, warm LNG is then routed through control valves and a venturi flow meter and into the expander test vessel (2). As all single phase expanders produced by Ebara are completely submerged vertically in a pressure vessel, the LNG feed enters through the top of the expander, and is discharged into the pressure vessel. The amount of pressure dropped across the expander is controlled by regulating a control valve at on the discharge of the expander vessel to ensure that it remains above saturation pressure and is not expanded into the two-phase region. Additionally, the electrical generator attached to the shaft of the expander is connected to a Variable Speed Constant Frequency (VSCF) drive, allowing full control of the expander speed during operation. By varying the pump and expander vessel control valves as well as the expander and pump speeds, a full performance map can be demonstrated.
Once the pressure and temperature of the LNG is reduced across the expander, the LNG stream is routed to a large tube/shell type heat exchanger (3). Liquid nitrogen at significantly colder temperatures than the LNG is passed through the cooling tubes of the heat exchanger, removing most of the remaining heat energy from the first step of the process that wasn’t removed through the expansion process. Due to additional heat input after the expansion process through the process piping, some of the low pressure LNG vaporizes while entering the heat exchanger. This vapor is routed to a flare system (4) where it is burned and released to the atmosphere. The remaining LNG is then routed back to the inlet of the submersible LNG pumps and the cycle is repeated.
From a thermodynamics point of view, the single phase expander test facility is similar to the Rankine cycle except that the expansion of LNG does not reach the two-phase region. Figure (2) shows the temperature enthalpy diagram of the single phase closed-loop testing cycle, consisting of the following four processes:
1-2 Near isentropic compression is the LNG pump
2-3 Constant pressure heat addition from the LNG pump motor
3-4 Near isentropic expansion through the LNG expander
4-1 Constant pressure heat rejection in the heat exchanger
Figure 2 T-s diagram of single phase expander test process
Two-Phase Testing Challenges
There are several factors limiting the single phase test facility’s ability to be used as a two-phase test facility. The most significant of which is the cooling capacity of the tube/shell type heat exchangers used in the process. Due to the limited surface area of the tubes within the heat exchanger shell and the relatively low liquid nitrogen flow rate through these tubes, LNG that has been expanded beyond the saturation pressure and into the two-phase region cannot be re-condensed into a saturated liquid before being directed back into the LNG pumps. When this occurs, vapor will be generated in the heat exchanger at a higher rate than can be physically vented to the flare system, increasing the pressure within the heat exchanger shell. Eventually, the increasing back pressure at the outlet of the expander from the heat exchanger will become so high that the expander will not be able to operate within the two-phase region.
Another factor limiting the use of a single phase expander test facility as a two-phase test facility is the physical orientation of the expander test vessel piping. As noted previously, a single phase expander is supplied with pressurized LNG from the top of the vessel and is discharged into the vessel at low pressure (figure 3). Because of this, the test vessel inlet piping from the LNG pumps is routed to the top of the expander test vessel and the discharge piping is located on the side of the vessel. Two-phase expander vessels however, operate in the opposite direction (figure 4). These vessels require that the high pressure LNG must be directed first into the side of the test vessel, pass through the expander and discharge at the top of the vessel to allow the entire vapor mass generated to be included in the discharge stream.
Figure 3 Typical single phase expander test vessel
Figure 4 Typical two-phase expander vessel
Proposed Two-Phase Test Facility
To solve the previously defined problems of testing two-phase expanders at a single phase test facility, Ebara International has developed a method to upgrade their existing facility to allow for both single and two-phase testing with only slight physical modifications.
There were two distinct approaches considered to deal with the issue of vapor generation in the two-phase expansion testing process. The first approach was to increase the heat exchanger venting and flare capacity to allow venting and flaring of all vapor produced. This solution would also require that a continuous supply of LNG be injected back into the heat exchangers from outside storage to replace the LNG that is flared, creating a constant loss of LNG throughout the test operation. Due to the cost of LNG and the environmental impact of increasing emissions, this method was ruled out as a solution.
The other approach to dealing with the vapor generation was to re-condense the vapor produced back into a saturated liquid state before it enters the main heat exchanger. By doing this, no venting or flaring is required, lowering overall LNG costs and eliminating emissions. To accomplish the re-condensation, a plate/fin type cross flow heat exchanger is installed at the discharge of the expander vessel with the cooling supplied from existing on-site liquid nitrogen storage. Due to the large surface area and heat transfer characteristics of this type of cross flow heat exchanger, all vapor is condensed back into a saturated liquid before entering the main heat exchanger. A simplified block flow diagram of the modified process can be seen in figure 5.
Figure 5 Modified two-phase expander testing process
As with the single phase test process, the pressure of sub-cooled LNG is increased from atmospheric to the required inlet pressure of the expander using two submerged LNG pumps (1) operating in parallel. This high pressure LNG is then routed through control valves and into the side of the expander test vessel (2). The LNG then enters the expander and exits the top of the expander vessel, allowing all vapor generated to be included in the discharge stream. The amount of pressure dropped across the expander is controlled by regulating a control valve at on the discharge of the expander vessel and can be lowered to below the saturation pressure. Once the LNG’s pressure and temperature is reduced across the expander, the two-phase LNG stream is routed to the plate/fin heat exchanger (3a) where all vapor is re-condensed back into a liquid. This saturated LNG is fed to the main heat exchanger (3b) where the remaining heat energy is removed. Finally, the sub-cooled LNG is routed back to the inlet of the submersible LNG pumps and the cycle is repeated.
Like the single phase test facility, the temperature/enthalpy diagram of the two-phase facility is similar to that of a Rankine cycle. However, in the case of the two-phase expander process, the pressure drop is low enough to allow the fluid to enter the two-phase mixture region. Figure 6 shows the temperature enthalpy diagram of this modified testing cycle, consisting of the following five processes:
1-2 Near isentropic compression is the LNG pump
2-3 Constant pressure heat addition from the LNG pump motor
3-4 Near isentropic expansion through the LNG expander into the mixture region
4-5 Constant pressure heat rejection in the plate/fin condensor to saturation
5-1 Constant pressure heat rejection in the heat exchanger
Figure 6 T-s diagram of two-phase testing cycle
As the demand and development of two-phase LNG expanders increases, full string test facilities must be utilized to verify the performance, manufacturing and design quality of this equipment. This is not only critical to the expander manufacturer, but inspires end user confidence in the product by demonstrating that every aspect of operation has been performed before they ever install it. Through implementation of only minor changes to existing LNG expander test facilities, such as the one at Ebara International’s Cryodynamics division, we now have the ability to perform full performance tests throughout both liquid and two-phase operating regions while reducing test costs and emissions.